tor-browser

calc.rs (74784B)

---

/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at https://mozilla.org/MPL/2.0/. */

//! [Calc expressions][calc].
//!
//! [calc]: https://drafts.csswg.org/css-values/#calc-notation

use crate::derives::*;
use crate::values::generics::length::GenericAnchorSizeFunction;
use crate::values::generics::position::{GenericAnchorFunction, GenericAnchorSide};
use num_traits::Zero;
use smallvec::SmallVec;
use std::convert::AsRef;
use std::fmt::{self, Write};
use std::ops::{Add, Mul, Neg, Rem, Sub};
use std::{cmp, mem};
use strum_macros::AsRefStr;
use style_traits::{CssWriter, NumericValue, ToCss, ToTyped, TypedValue};

use thin_vec::ThinVec;

/// Whether we're a `min` or `max` function.
#[derive(
   Clone,
   Copy,
   Debug,
   Deserialize,
   MallocSizeOf,
   PartialEq,
   Serialize,
   ToAnimatedZero,
   ToResolvedValue,
   ToShmem,
)]
#[repr(u8)]
pub enum MinMaxOp {
   /// `min()`
   Min,
   /// `max()`
   Max,
}

/// Whether we're a `mod` or `rem` function.
#[derive(
   Clone,
   Copy,
   Debug,
   Deserialize,
   MallocSizeOf,
   PartialEq,
   Serialize,
   ToAnimatedZero,
   ToResolvedValue,
   ToShmem,
)]
#[repr(u8)]
pub enum ModRemOp {
   /// `mod()`
   Mod,
   /// `rem()`
   Rem,
}

impl ModRemOp {
   fn apply(self, dividend: f32, divisor: f32) -> f32 {
       // In mod(A, B) only, if B is infinite and A has opposite sign to B
       // (including an oppositely-signed zero), the result is NaN.
       // https://drafts.csswg.org/css-values/#round-infinities
       if matches!(self, Self::Mod)
           && divisor.is_infinite()
           && dividend.is_sign_negative() != divisor.is_sign_negative()
       {
           return f32::NAN;
       }

       let (r, same_sign_as) = match self {
           Self::Mod => (dividend - divisor * (dividend / divisor).floor(), divisor),
           Self::Rem => (dividend - divisor * (dividend / divisor).trunc(), dividend),
       };
       if r == 0.0 && same_sign_as.is_sign_negative() {
           -0.0
       } else {
           r
       }
   }
}

/// The strategy used in `round()`
#[derive(
   Clone,
   Copy,
   Debug,
   Deserialize,
   MallocSizeOf,
   PartialEq,
   Serialize,
   ToAnimatedZero,
   ToResolvedValue,
   ToShmem,
)]
#[repr(u8)]
pub enum RoundingStrategy {
   /// `round(nearest, a, b)`
   /// round a to the nearest multiple of b
   Nearest,
   /// `round(up, a, b)`
   /// round a up to the nearest multiple of b
   Up,
   /// `round(down, a, b)`
   /// round a down to the nearest multiple of b
   Down,
   /// `round(to-zero, a, b)`
   /// round a to the nearest multiple of b that is towards zero
   ToZero,
}

/// This determines the order in which we serialize members of a calc() sum.
///
/// See https://drafts.csswg.org/css-values-4/#sort-a-calculations-children
#[derive(
   AsRefStr, Clone, Copy, Debug, Eq, Ord, Parse, PartialEq, PartialOrd, MallocSizeOf, ToShmem,
)]
#[strum(serialize_all = "lowercase")]
#[allow(missing_docs)]
pub enum SortKey {
   #[strum(serialize = "")]
   Number,
   #[css(skip)]
   #[strum(serialize = "%")]
   Percentage,
   Cap,
   Ch,
   Cqb,
   Cqh,
   Cqi,
   Cqmax,
   Cqmin,
   Cqw,
   Deg,
   Dppx,
   Dvb,
   Dvh,
   Dvi,
   Dvmax,
   Dvmin,
   Dvw,
   Em,
   Ex,
   Ic,
   Lh,
   Lvb,
   Lvh,
   Lvi,
   Lvmax,
   Lvmin,
   Lvw,
   Ms,
   Px,
   Rcap,
   Rch,
   Rem,
   Rex,
   Ric,
   Rlh,
   S, // Sec
   Svb,
   Svh,
   Svi,
   Svmax,
   Svmin,
   Svw,
   Vb,
   Vh,
   Vi,
   Vmax,
   Vmin,
   Vw,
   #[css(skip)]
   ColorComponent,
   #[css(skip)]
   Other,
}

/// `anchor()` function used in math functions.
pub type GenericCalcAnchorFunction<L> =
   GenericAnchorFunction<Box<GenericCalcNode<L>>, Box<GenericCalcNode<L>>>;
/// `anchor-size()` function used in math functions.
pub type GenericCalcAnchorSizeFunction<L> = GenericAnchorSizeFunction<Box<GenericCalcNode<L>>>;

/// A generic node in a calc expression.
///
/// FIXME: This would be much more elegant if we used `Self` in the types below,
/// but we can't because of https://github.com/serde-rs/serde/issues/1565.
///
/// FIXME: The following annotations are to workaround an LLVM inlining bug, see
/// bug 1631929.
///
/// cbindgen:destructor-attributes=MOZ_NEVER_INLINE
/// cbindgen:copy-constructor-attributes=MOZ_NEVER_INLINE
/// cbindgen:eq-attributes=MOZ_NEVER_INLINE
#[repr(u8)]
#[derive(
   Clone,
   Debug,
   Deserialize,
   MallocSizeOf,
   PartialEq,
   Serialize,
   ToAnimatedZero,
   ToResolvedValue,
   ToShmem,
)]
pub enum GenericCalcNode<L> {
   /// A leaf node.
   Leaf(L),
   /// A node that negates its child, e.g. Negate(1) == -1.
   Negate(Box<GenericCalcNode<L>>),
   /// A node that inverts its child, e.g. Invert(10) == 1 / 10 == 0.1. The child must always
   /// resolve to a number unit.
   Invert(Box<GenericCalcNode<L>>),
   /// A sum node, representing `a + b + c` where a, b, and c are the
   /// arguments.
   Sum(crate::OwnedSlice<GenericCalcNode<L>>),
   /// A product node, representing `a * b * c` where a, b, and c are the
   /// arguments.
   Product(crate::OwnedSlice<GenericCalcNode<L>>),
   /// A `min` or `max` function.
   MinMax(crate::OwnedSlice<GenericCalcNode<L>>, MinMaxOp),
   /// A `clamp()` function.
   Clamp {
       /// The minimum value.
       min: Box<GenericCalcNode<L>>,
       /// The central value.
       center: Box<GenericCalcNode<L>>,
       /// The maximum value.
       max: Box<GenericCalcNode<L>>,
   },
   /// A `round()` function.
   Round {
       /// The rounding strategy.
       strategy: RoundingStrategy,
       /// The value to round.
       value: Box<GenericCalcNode<L>>,
       /// The step value.
       step: Box<GenericCalcNode<L>>,
   },
   /// A `mod()` or `rem()` function.
   ModRem {
       /// The dividend calculation.
       dividend: Box<GenericCalcNode<L>>,
       /// The divisor calculation.
       divisor: Box<GenericCalcNode<L>>,
       /// Is the function mod or rem?
       op: ModRemOp,
   },
   /// A `hypot()` function
   Hypot(crate::OwnedSlice<GenericCalcNode<L>>),
   /// An `abs()` function.
   Abs(Box<GenericCalcNode<L>>),
   /// A `sign()` function.
   Sign(Box<GenericCalcNode<L>>),
   /// An `anchor()` function.
   Anchor(Box<GenericCalcAnchorFunction<L>>),
   /// An `anchor-size()` function.
   AnchorSize(Box<GenericCalcAnchorSizeFunction<L>>),
}

pub use self::GenericCalcNode as CalcNode;

bitflags! {
   /// Expected units we allow parsing within a `calc()` expression.
   ///
   /// This is used as a hint for the parser to fast-reject invalid
   /// expressions. Numbers are always allowed because they multiply other
   /// units.
   #[derive(Clone, Copy, PartialEq, Eq)]
   pub struct CalcUnits: u8 {
       /// <length>
       const LENGTH = 1 << 0;
       /// <percentage>
       const PERCENTAGE = 1 << 1;
       /// <angle>
       const ANGLE = 1 << 2;
       /// <time>
       const TIME = 1 << 3;
       /// <resolution>
       const RESOLUTION = 1 << 4;
       /// A component of a color (r, g, b, h, s, l, alpha, etc.)
       const COLOR_COMPONENT = 1 << 5;

       /// <length-percentage>
       const LENGTH_PERCENTAGE = Self::LENGTH.bits() | Self::PERCENTAGE.bits();
       // NOTE: When you add to this, make sure to make Atan2 deal with these.
       /// Allow all units.
       const ALL = Self::LENGTH.bits() | Self::PERCENTAGE.bits() | Self::ANGLE.bits() |
           Self::TIME.bits() | Self::RESOLUTION.bits() | Self::COLOR_COMPONENT.bits();
   }
}

impl CalcUnits {
   /// Returns whether the flags only represent a single unit. This will return true for 0, which
   /// is a "number" this is also fine.
   #[inline]
   fn is_single_unit(&self) -> bool {
       self.bits() == 0 || self.bits() & (self.bits() - 1) == 0
   }

   /// Returns true if this unit is allowed to be summed with the given unit, otherwise false.
   #[inline]
   fn can_sum_with(&self, other: Self) -> bool {
       match *self {
           Self::LENGTH => other.intersects(Self::LENGTH | Self::PERCENTAGE),
           Self::PERCENTAGE => other.intersects(Self::LENGTH | Self::PERCENTAGE),
           Self::LENGTH_PERCENTAGE => other.intersects(Self::LENGTH | Self::PERCENTAGE),
           u => u.is_single_unit() && other == u,
       }
   }
}

/// For percentage resolution, sometimes we can't assume that the percentage basis is positive (so
/// we don't know whether a percentage is larger than another).
pub enum PositivePercentageBasis {
   /// The percent basis is not known-positive, we can't compare percentages.
   Unknown,
   /// The percent basis is known-positive, we assume larger percentages are larger.
   Yes,
}

macro_rules! compare_helpers {
   () => {
       /// Return whether a leaf is greater than another.
       #[allow(unused)]
       fn gt(&self, other: &Self, basis_positive: PositivePercentageBasis) -> bool {
           self.compare(other, basis_positive) == Some(cmp::Ordering::Greater)
       }

       /// Return whether a leaf is less than another.
       fn lt(&self, other: &Self, basis_positive: PositivePercentageBasis) -> bool {
           self.compare(other, basis_positive) == Some(cmp::Ordering::Less)
       }

       /// Return whether a leaf is smaller or equal than another.
       fn lte(&self, other: &Self, basis_positive: PositivePercentageBasis) -> bool {
           match self.compare(other, basis_positive) {
               Some(cmp::Ordering::Less) => true,
               Some(cmp::Ordering::Equal) => true,
               Some(cmp::Ordering::Greater) => false,
               None => false,
           }
       }
   };
}

/// A trait that represents all the stuff a valid leaf of a calc expression.
pub trait CalcNodeLeaf: Clone + Sized + PartialEq + ToCss + ToTyped {
   /// Returns the unit of the leaf.
   fn unit(&self) -> CalcUnits;

   /// Returns the unitless value of this leaf if one is available.
   fn unitless_value(&self) -> Option<f32>;

   /// Return true if the units of both leaves are equal. (NOTE: Does not take
   /// the values into account)
   fn is_same_unit_as(&self, other: &Self) -> bool {
       std::mem::discriminant(self) == std::mem::discriminant(other)
   }

   /// Do a partial comparison of these values.
   fn compare(
       &self,
       other: &Self,
       base_is_positive: PositivePercentageBasis,
   ) -> Option<cmp::Ordering>;
   compare_helpers!();

   /// Create a new leaf with a number value.
   fn new_number(value: f32) -> Self;

   /// Returns a float value if the leaf is a number.
   fn as_number(&self) -> Option<f32>;

   /// Whether this value is known-negative.
   fn is_negative(&self) -> Result<bool, ()> {
       self.unitless_value()
           .map(|v| Ok(v.is_sign_negative()))
           .unwrap_or_else(|| Err(()))
   }

   /// Whether this value is infinite.
   fn is_infinite(&self) -> Result<bool, ()> {
       self.unitless_value()
           .map(|v| Ok(v.is_infinite()))
           .unwrap_or_else(|| Err(()))
   }

   /// Whether this value is zero.
   fn is_zero(&self) -> Result<bool, ()> {
       self.unitless_value()
           .map(|v| Ok(v.is_zero()))
           .unwrap_or_else(|| Err(()))
   }

   /// Whether this value is NaN.
   fn is_nan(&self) -> Result<bool, ()> {
       self.unitless_value()
           .map(|v| Ok(v.is_nan()))
           .unwrap_or_else(|| Err(()))
   }

   /// Tries to merge one leaf into another using the sum, that is, perform `x` + `y`.
   fn try_sum_in_place(&mut self, other: &Self) -> Result<(), ()>;

   /// Try to merge the right leaf into the left by using a multiplication. Return true if the
   /// merge was successful, otherwise false.
   fn try_product_in_place(&mut self, other: &mut Self) -> bool;

   /// Tries a generic arithmetic operation.
   fn try_op<O>(&self, other: &Self, op: O) -> Result<Self, ()>
   where
       O: Fn(f32, f32) -> f32;

   /// Map the value of this node with the given operation.
   fn map(&mut self, op: impl FnMut(f32) -> f32) -> Result<(), ()>;

   /// Canonicalizes the expression if necessary.
   fn simplify(&mut self);

   /// Returns the sort key for simplification.
   fn sort_key(&self) -> SortKey;

   /// Create a new leaf containing the sign() result of the given leaf.
   fn sign_from(leaf: &impl CalcNodeLeaf) -> Result<Self, ()> {
       let Some(value) = leaf.unitless_value() else {
           return Err(());
       };

       Ok(Self::new_number(if value.is_nan() {
           f32::NAN
       } else if value.is_zero() {
           value
       } else if value.is_sign_negative() {
           -1.0
       } else {
           1.0
       }))
   }
}

/// The level of any argument being serialized in `to_css_impl`.
enum ArgumentLevel {
   /// The root of a calculation tree.
   CalculationRoot,
   /// The root of an operand node's argument, e.g. `min(10, 20)`, `10` and `20` will have this
   /// level, but min in this case will have `TopMost`.
   ArgumentRoot,
   /// Any other values serialized in the tree.
   Nested,
}

impl<L: CalcNodeLeaf> CalcNode<L> {
   /// Create a dummy CalcNode that can be used to do replacements of other nodes.
   fn dummy() -> Self {
       Self::MinMax(Default::default(), MinMaxOp::Max)
   }

   /// Change all the leaf nodes to have the given value. This is useful when
   /// you have `calc(1px * nan)` and you want to replace the product node with
   /// `calc(nan)`, in which case the unit will be retained.
   fn coerce_to_value(&mut self, value: f32) -> Result<(), ()> {
       self.map(|_| value)
   }

   /// Return true if a product is distributive over this node.
   /// Is distributive: (2 + 3) * 4 = 8 + 12
   /// Not distributive: sign(2 + 3) * 4 != sign(8 + 12)
   #[inline]
   pub fn is_product_distributive(&self) -> bool {
       match self {
           Self::Leaf(l) => l.unit() != CalcUnits::COLOR_COMPONENT,
           Self::Sum(children) => children.iter().all(|c| c.is_product_distributive()),
           _ => false,
       }
   }

   /// If the node has a valid unit outcome, then return it, otherwise fail.
   pub fn unit(&self) -> Result<CalcUnits, ()> {
       Ok(match self {
           CalcNode::Leaf(l) => l.unit(),
           CalcNode::Negate(child) | CalcNode::Invert(child) | CalcNode::Abs(child) => {
               child.unit()?
           },
           CalcNode::Sum(children) => {
               let mut unit = children.first().unwrap().unit()?;
               for child in children.iter().skip(1) {
                   let child_unit = child.unit()?;
                   if !child_unit.can_sum_with(unit) {
                       return Err(());
                   }
                   unit |= child_unit;
               }
               unit
           },
           CalcNode::Product(children) => {
               // Only one node is allowed to have a unit, the rest must be numbers.
               let mut unit = None;
               for child in children.iter() {
                   let child_unit = child.unit()?;
                   if child_unit.is_empty() {
                       // Numbers are always allowed in a product, so continue with the next.
                       continue;
                   }

                   if unit.is_some() {
                       // We already have a unit for the node, so another unit node is invalid.
                       return Err(());
                   }

                   // We have the unit for the node.
                   unit = Some(child_unit);
               }
               // We only keep track of specified units, so if we end up with a None and no failure
               // so far, then we have a number.
               unit.unwrap_or(CalcUnits::empty())
           },
           CalcNode::MinMax(children, _) | CalcNode::Hypot(children) => {
               let mut unit = children.first().unwrap().unit()?;
               for child in children.iter().skip(1) {
                   let child_unit = child.unit()?;
                   if !child_unit.can_sum_with(unit) {
                       return Err(());
                   }
                   unit |= child_unit;
               }
               unit
           },
           CalcNode::Clamp { min, center, max } => {
               let min_unit = min.unit()?;
               let center_unit = center.unit()?;

               if !min_unit.can_sum_with(center_unit) {
                   return Err(());
               }

               let max_unit = max.unit()?;

               if !center_unit.can_sum_with(max_unit) {
                   return Err(());
               }

               min_unit | center_unit | max_unit
           },
           CalcNode::Round { value, step, .. } => {
               let value_unit = value.unit()?;
               let step_unit = step.unit()?;
               if !step_unit.can_sum_with(value_unit) {
                   return Err(());
               }
               value_unit | step_unit
           },
           CalcNode::ModRem {
               dividend, divisor, ..
           } => {
               let dividend_unit = dividend.unit()?;
               let divisor_unit = divisor.unit()?;
               if !divisor_unit.can_sum_with(dividend_unit) {
                   return Err(());
               }
               dividend_unit | divisor_unit
           },
           CalcNode::Sign(ref child) => {
               // sign() always resolves to a number, but we still need to make sure that the
               // child units make sense.
               let _ = child.unit()?;
               CalcUnits::empty()
           },
           CalcNode::Anchor(..) | CalcNode::AnchorSize(..) => CalcUnits::LENGTH_PERCENTAGE,
       })
   }

   /// Negate the node inline.  If the node is distributive, it is replaced by the result,
   /// otherwise the node is wrapped in a [`Negate`] node.
   pub fn negate(&mut self) {
       /// Node(params) -> Negate(Node(params))
       fn wrap_self_in_negate<L: CalcNodeLeaf>(s: &mut CalcNode<L>) {
           let result = mem::replace(s, CalcNode::dummy());
           *s = CalcNode::Negate(Box::new(result));
       }

       match *self {
           CalcNode::Leaf(ref mut leaf) => {
               if leaf.map(std::ops::Neg::neg).is_err() {
                   wrap_self_in_negate(self)
               }
           },
           CalcNode::Negate(ref mut value) => {
               // Don't negate the value here.  Replace `self` with it's child.
               let result = mem::replace(value.as_mut(), Self::dummy());
               *self = result;
           },
           CalcNode::Invert(_) => {
               // -(1 / -10) == -(-0.1) == 0.1
               wrap_self_in_negate(self)
           },
           CalcNode::Sum(ref mut children) => {
               for child in children.iter_mut() {
                   child.negate();
               }
           },
           CalcNode::Product(_) => {
               // -(2 * 3 / 4) == -(1.5)
               wrap_self_in_negate(self);
           },
           CalcNode::MinMax(ref mut children, ref mut op) => {
               for child in children.iter_mut() {
                   child.negate();
               }

               // Negating min-max means the operation is swapped.
               *op = match *op {
                   MinMaxOp::Min => MinMaxOp::Max,
                   MinMaxOp::Max => MinMaxOp::Min,
               };
           },
           CalcNode::Clamp {
               ref mut min,
               ref mut center,
               ref mut max,
           } => {
               if min.lte(max, PositivePercentageBasis::Unknown) {
                   min.negate();
                   center.negate();
                   max.negate();

                   mem::swap(min, max);
               } else {
                   wrap_self_in_negate(self);
               }
           },
           CalcNode::Round {
               ref mut strategy,
               ref mut value,
               ref mut step,
           } => {
               match *strategy {
                   RoundingStrategy::Nearest => {
                       // Nearest is tricky because we'd have to swap the
                       // behavior at the half-way point from using the upper
                       // to lower bound.
                       // Simpler to just wrap self in a negate node.
                       wrap_self_in_negate(self);
                       return;
                   },
                   RoundingStrategy::Up => *strategy = RoundingStrategy::Down,
                   RoundingStrategy::Down => *strategy = RoundingStrategy::Up,
                   RoundingStrategy::ToZero => (),
               }
               value.negate();
               step.negate();
           },
           CalcNode::ModRem {
               ref mut dividend,
               ref mut divisor,
               ..
           } => {
               dividend.negate();
               divisor.negate();
           },
           CalcNode::Hypot(ref mut children) => {
               for child in children.iter_mut() {
                   child.negate();
               }
           },
           CalcNode::Abs(_) => {
               wrap_self_in_negate(self);
           },
           CalcNode::Sign(ref mut child) => {
               child.negate();
           },
           CalcNode::Anchor(_) | CalcNode::AnchorSize(_) => {
               wrap_self_in_negate(self);
           },
       }
   }

   fn sort_key(&self) -> SortKey {
       match *self {
           Self::Leaf(ref l) => l.sort_key(),
           Self::Anchor(..) | Self::AnchorSize(..) => SortKey::Px,
           _ => SortKey::Other,
       }
   }

   /// Returns the leaf if we can (if simplification has allowed it).
   pub fn as_leaf(&self) -> Option<&L> {
       match *self {
           Self::Leaf(ref l) => Some(l),
           _ => None,
       }
   }

   /// Tries to merge one node into another using the sum, that is, perform `x` + `y`.
   pub fn try_sum_in_place(&mut self, other: &Self) -> Result<(), ()> {
       match (self, other) {
           (&mut CalcNode::Leaf(ref mut one), &CalcNode::Leaf(ref other)) => {
               one.try_sum_in_place(other)
           },
           _ => Err(()),
       }
   }

   /// Tries to merge one node into another using the product, that is, perform `x` * `y`.
   pub fn try_product_in_place(&mut self, other: &mut Self) -> bool {
       if let Ok(resolved) = other.resolve() {
           if let Some(number) = resolved.as_number() {
               if number == 1.0 {
                   return true;
               }

               if self.is_product_distributive() {
                   if self.map(|v| v * number).is_err() {
                       return false;
                   }
                   return true;
               }
           }
       }

       if let Ok(resolved) = self.resolve() {
           if let Some(number) = resolved.as_number() {
               if number == 1.0 {
                   std::mem::swap(self, other);
                   return true;
               }

               if other.is_product_distributive() {
                   if other.map(|v| v * number).is_err() {
                       return false;
                   }
                   std::mem::swap(self, other);
                   return true;
               }
           }
       }

       false
   }

   /// Tries to apply a generic arithmetic operator
   fn try_op<O>(&self, other: &Self, op: O) -> Result<Self, ()>
   where
       O: Fn(f32, f32) -> f32,
   {
       match (self, other) {
           (&CalcNode::Leaf(ref one), &CalcNode::Leaf(ref other)) => {
               Ok(CalcNode::Leaf(one.try_op(other, op)?))
           },
           _ => Err(()),
       }
   }

   /// Map the value of this node with the given operation.
   pub fn map(&mut self, mut op: impl FnMut(f32) -> f32) -> Result<(), ()> {
       fn map_internal<L: CalcNodeLeaf>(
           node: &mut CalcNode<L>,
           op: &mut impl FnMut(f32) -> f32,
       ) -> Result<(), ()> {
           match node {
               CalcNode::Leaf(l) => l.map(op),
               CalcNode::Negate(v) | CalcNode::Invert(v) => map_internal(v, op),
               CalcNode::Sum(children) | CalcNode::Product(children) => {
                   for node in &mut **children {
                       map_internal(node, op)?;
                   }
                   Ok(())
               },
               CalcNode::MinMax(children, _) => {
                   for node in &mut **children {
                       map_internal(node, op)?;
                   }
                   Ok(())
               },
               CalcNode::Clamp { min, center, max } => {
                   map_internal(min, op)?;
                   map_internal(center, op)?;
                   map_internal(max, op)
               },
               CalcNode::Round { value, step, .. } => {
                   map_internal(value, op)?;
                   map_internal(step, op)
               },
               CalcNode::ModRem {
                   dividend, divisor, ..
               } => {
                   map_internal(dividend, op)?;
                   map_internal(divisor, op)
               },
               CalcNode::Hypot(children) => {
                   for node in &mut **children {
                       map_internal(node, op)?;
                   }
                   Ok(())
               },
               CalcNode::Abs(child) | CalcNode::Sign(child) => map_internal(child, op),
               // It is invalid to treat inner `CalcNode`s here - `anchor(--foo 50%) / 2` != `anchor(--foo 25%)`.
               // Same applies to fallback, as we don't know if it will be used. Similar reasoning applies to `anchor-size()`.
               CalcNode::Anchor(_) | CalcNode::AnchorSize(_) => Err(()),
           }
       }

       map_internal(self, &mut op)
   }

   /// Convert this `CalcNode` into a `CalcNode` with a different leaf kind.
   pub fn map_leaves<O, F>(&self, mut map: F) -> CalcNode<O>
   where
       O: CalcNodeLeaf,
       F: FnMut(&L) -> O,
   {
       self.map_leaves_internal(&mut map)
   }

   fn map_leaves_internal<O, F>(&self, map: &mut F) -> CalcNode<O>
   where
       O: CalcNodeLeaf,
       F: FnMut(&L) -> O,
   {
       fn map_children<L, O, F>(
           children: &[CalcNode<L>],
           map: &mut F,
       ) -> crate::OwnedSlice<CalcNode<O>>
       where
           L: CalcNodeLeaf,
           O: CalcNodeLeaf,
           F: FnMut(&L) -> O,
       {
           children
               .iter()
               .map(|c| c.map_leaves_internal(map))
               .collect()
       }

       match *self {
           Self::Leaf(ref l) => CalcNode::Leaf(map(l)),
           Self::Negate(ref c) => CalcNode::Negate(Box::new(c.map_leaves_internal(map))),
           Self::Invert(ref c) => CalcNode::Invert(Box::new(c.map_leaves_internal(map))),
           Self::Sum(ref c) => CalcNode::Sum(map_children(c, map)),
           Self::Product(ref c) => CalcNode::Product(map_children(c, map)),
           Self::MinMax(ref c, op) => CalcNode::MinMax(map_children(c, map), op),
           Self::Clamp {
               ref min,
               ref center,
               ref max,
           } => {
               let min = Box::new(min.map_leaves_internal(map));
               let center = Box::new(center.map_leaves_internal(map));
               let max = Box::new(max.map_leaves_internal(map));
               CalcNode::Clamp { min, center, max }
           },
           Self::Round {
               strategy,
               ref value,
               ref step,
           } => {
               let value = Box::new(value.map_leaves_internal(map));
               let step = Box::new(step.map_leaves_internal(map));
               CalcNode::Round {
                   strategy,
                   value,
                   step,
               }
           },
           Self::ModRem {
               ref dividend,
               ref divisor,
               op,
           } => {
               let dividend = Box::new(dividend.map_leaves_internal(map));
               let divisor = Box::new(divisor.map_leaves_internal(map));
               CalcNode::ModRem {
                   dividend,
                   divisor,
                   op,
               }
           },
           Self::Hypot(ref c) => CalcNode::Hypot(map_children(c, map)),
           Self::Abs(ref c) => CalcNode::Abs(Box::new(c.map_leaves_internal(map))),
           Self::Sign(ref c) => CalcNode::Sign(Box::new(c.map_leaves_internal(map))),
           Self::Anchor(ref f) => CalcNode::Anchor(Box::new(GenericAnchorFunction {
               target_element: f.target_element.clone(),
               side: match &f.side {
                   GenericAnchorSide::Keyword(k) => GenericAnchorSide::Keyword(*k),
                   GenericAnchorSide::Percentage(p) => {
                       GenericAnchorSide::Percentage(Box::new(p.map_leaves_internal(map)))
                   },
               },
               fallback: f
                   .fallback
                   .as_ref()
                   .map(|fb| Box::new(fb.map_leaves_internal(map)))
                   .into(),
           })),
           Self::AnchorSize(ref f) => CalcNode::AnchorSize(Box::new(GenericAnchorSizeFunction {
               target_element: f.target_element.clone(),
               size: f.size,
               fallback: f
                   .fallback
                   .as_ref()
                   .map(|fb| Box::new(fb.map_leaves_internal(map)))
                   .into(),
           })),
       }
   }

   /// Resolve this node into a value.
   pub fn resolve(&self) -> Result<L, ()> {
       self.resolve_map(|l| Ok(l.clone()))
   }

   /// Resolve this node into a value, given a function that maps the leaf values.
   pub fn resolve_map<F>(&self, mut leaf_to_output_fn: F) -> Result<L, ()>
   where
       F: FnMut(&L) -> Result<L, ()>,
   {
       self.resolve_internal(&mut leaf_to_output_fn)
   }

   fn resolve_internal<F>(&self, leaf_to_output_fn: &mut F) -> Result<L, ()>
   where
       F: FnMut(&L) -> Result<L, ()>,
   {
       match self {
           Self::Leaf(l) => leaf_to_output_fn(l),
           Self::Negate(child) => {
               let mut result = child.resolve_internal(leaf_to_output_fn)?;
               result.map(|v| v.neg())?;
               Ok(result)
           },
           Self::Invert(child) => {
               let mut result = child.resolve_internal(leaf_to_output_fn)?;
               result.map(|v| 1.0 / v)?;
               Ok(result)
           },
           Self::Sum(children) => {
               let mut result = children[0].resolve_internal(leaf_to_output_fn)?;

               for child in children.iter().skip(1) {
                   let right = child.resolve_internal(leaf_to_output_fn)?;
                   // try_op will make sure we only sum leaves with the same type.
                   result = result.try_op(&right, |left, right| left + right)?;
               }

               Ok(result)
           },
           Self::Product(children) => {
               let mut result = children[0].resolve_internal(leaf_to_output_fn)?;

               for child in children.iter().skip(1) {
                   let right = child.resolve_internal(leaf_to_output_fn)?;
                   // Mutliply only allowed when either side is a number.
                   match result.as_number() {
                       Some(left) => {
                           // Left side is a number, so we use the right node as the result.
                           result = right;
                           result.map(|v| v * left)?;
                       },
                       None => {
                           // Left side is not a number, so check if the right side is.
                           match right.as_number() {
                               Some(right) => {
                                   result.map(|v| v * right)?;
                               },
                               None => {
                                   // Multiplying with both sides having units.
                                   return Err(());
                               },
                           }
                       },
                   }
               }

               Ok(result)
           },
           Self::MinMax(children, op) => {
               let mut result = children[0].resolve_internal(leaf_to_output_fn)?;

               if result.is_nan()? {
                   return Ok(result);
               }

               for child in children.iter().skip(1) {
                   let candidate = child.resolve_internal(leaf_to_output_fn)?;

                   // Leaf types must match for each child.
                   if !result.is_same_unit_as(&candidate) {
                       return Err(());
                   }

                   if candidate.is_nan()? {
                       result = candidate;
                       break;
                   }

                   let candidate_wins = match op {
                       MinMaxOp::Min => candidate.lt(&result, PositivePercentageBasis::Yes),
                       MinMaxOp::Max => candidate.gt(&result, PositivePercentageBasis::Yes),
                   };

                   if candidate_wins {
                       result = candidate;
                   }
               }

               Ok(result)
           },
           Self::Clamp { min, center, max } => {
               let min = min.resolve_internal(leaf_to_output_fn)?;
               let center = center.resolve_internal(leaf_to_output_fn)?;
               let max = max.resolve_internal(leaf_to_output_fn)?;

               if !min.is_same_unit_as(&center) || !max.is_same_unit_as(&center) {
                   return Err(());
               }

               if min.is_nan()? {
                   return Ok(min);
               }

               if center.is_nan()? {
                   return Ok(center);
               }

               if max.is_nan()? {
                   return Ok(max);
               }

               let mut result = center;
               if result.gt(&max, PositivePercentageBasis::Yes) {
                   result = max;
               }
               if result.lt(&min, PositivePercentageBasis::Yes) {
                   result = min
               }

               Ok(result)
           },
           Self::Round {
               strategy,
               value,
               step,
           } => {
               let mut value = value.resolve_internal(leaf_to_output_fn)?;
               let step = step.resolve_internal(leaf_to_output_fn)?;

               if !value.is_same_unit_as(&step) {
                   return Err(());
               }

               let Some(step) = step.unitless_value() else {
                   return Err(());
               };
               let step = step.abs();

               value.map(|value| {
                   // TODO(emilio): Seems like at least a few of these
                   // special-cases could be removed if we do the math in a
                   // particular order.
                   if step.is_zero() {
                       return f32::NAN;
                   }

                   if value.is_infinite() {
                       if step.is_infinite() {
                           return f32::NAN;
                       }
                       return value;
                   }

                   if step.is_infinite() {
                       match strategy {
                           RoundingStrategy::Nearest | RoundingStrategy::ToZero => {
                               return if value.is_sign_negative() { -0.0 } else { 0.0 }
                           },
                           RoundingStrategy::Up => {
                               return if !value.is_sign_negative() && !value.is_zero() {
                                   f32::INFINITY
                               } else if !value.is_sign_negative() && value.is_zero() {
                                   value
                               } else {
                                   -0.0
                               }
                           },
                           RoundingStrategy::Down => {
                               return if value.is_sign_negative() && !value.is_zero() {
                                   -f32::INFINITY
                               } else if value.is_sign_negative() && value.is_zero() {
                                   value
                               } else {
                                   0.0
                               }
                           },
                       }
                   }

                   let div = value / step;
                   let lower_bound = div.floor() * step;
                   let upper_bound = div.ceil() * step;

                   match strategy {
                       RoundingStrategy::Nearest => {
                           // In case of a tie, use the upper bound
                           if value - lower_bound < upper_bound - value {
                               lower_bound
                           } else {
                               upper_bound
                           }
                       },
                       RoundingStrategy::Up => upper_bound,
                       RoundingStrategy::Down => lower_bound,
                       RoundingStrategy::ToZero => {
                           // In case of a tie, use the upper bound
                           if lower_bound.abs() < upper_bound.abs() {
                               lower_bound
                           } else {
                               upper_bound
                           }
                       },
                   }
               })?;

               Ok(value)
           },
           Self::ModRem {
               dividend,
               divisor,
               op,
           } => {
               let mut dividend = dividend.resolve_internal(leaf_to_output_fn)?;
               let divisor = divisor.resolve_internal(leaf_to_output_fn)?;

               if !dividend.is_same_unit_as(&divisor) {
                   return Err(());
               }

               let Some(divisor) = divisor.unitless_value() else {
                   return Err(());
               };
               dividend.map(|dividend| op.apply(dividend, divisor))?;
               Ok(dividend)
           },
           Self::Hypot(children) => {
               let mut result = children[0].resolve_internal(leaf_to_output_fn)?;
               result.map(|v| v.powi(2))?;

               for child in children.iter().skip(1) {
                   let child_value = child.resolve_internal(leaf_to_output_fn)?;

                   if !result.is_same_unit_as(&child_value) {
                       return Err(());
                   }

                   let Some(child_value) = child_value.unitless_value() else {
                       return Err(());
                   };
                   result.map(|v| v + child_value.powi(2))?;
               }

               result.map(|v| v.sqrt())?;
               Ok(result)
           },
           Self::Abs(ref c) => {
               let mut result = c.resolve_internal(leaf_to_output_fn)?;

               result.map(|v| v.abs())?;

               Ok(result)
           },
           Self::Sign(ref c) => {
               let result = c.resolve_internal(leaf_to_output_fn)?;
               Ok(L::sign_from(&result)?)
           },
           Self::Anchor(_) | Self::AnchorSize(_) => Err(()),
       }
   }

   /// Mutate nodes within this calc node tree using given the mapping function.
   pub fn map_node<F>(&mut self, mut mapping_fn: F) -> Result<(), ()>
   where
       F: FnMut(&CalcNode<L>) -> Result<Option<CalcNode<L>>, ()>,
   {
       self.map_node_internal(&mut mapping_fn)
   }

   fn map_node_internal<F>(&mut self, mapping_fn: &mut F) -> Result<(), ()>
   where
       F: FnMut(&CalcNode<L>) -> Result<Option<CalcNode<L>>, ()>,
   {
       if let Some(node) = mapping_fn(self)? {
           *self = node;
           // Assume that any sub-nodes don't need to be mutated.
           return Ok(());
       }
       match self {
           Self::Leaf(_) | Self::Anchor(_) | Self::AnchorSize(_) => (),
           Self::Negate(child) | Self::Invert(child) | Self::Abs(child) | Self::Sign(child) => {
               child.map_node_internal(mapping_fn)?;
           },
           Self::Sum(children)
           | Self::Product(children)
           | Self::Hypot(children)
           | Self::MinMax(children, _) => {
               for child in children.iter_mut() {
                   child.map_node_internal(mapping_fn)?;
               }
           },
           Self::Clamp { min, center, max } => {
               min.map_node_internal(mapping_fn)?;
               center.map_node_internal(mapping_fn)?;
               max.map_node_internal(mapping_fn)?;
           },
           Self::Round { value, step, .. } => {
               value.map_node_internal(mapping_fn)?;
               step.map_node_internal(mapping_fn)?;
           },
           Self::ModRem {
               dividend, divisor, ..
           } => {
               dividend.map_node_internal(mapping_fn)?;
               divisor.map_node_internal(mapping_fn)?;
           },
       };
       Ok(())
   }

   fn is_negative_leaf(&self) -> Result<bool, ()> {
       Ok(match *self {
           Self::Leaf(ref l) => l.is_negative()?,
           _ => false,
       })
   }

   fn is_zero_leaf(&self) -> Result<bool, ()> {
       Ok(match *self {
           Self::Leaf(ref l) => l.is_zero()?,
           _ => false,
       })
   }

   fn is_infinite_leaf(&self) -> Result<bool, ()> {
       Ok(match *self {
           Self::Leaf(ref l) => l.is_infinite()?,
           _ => false,
       })
   }

   fn is_nan_leaf(&self) -> Result<bool, ()> {
       Ok(match *self {
           Self::Leaf(ref l) => l.is_nan()?,
           _ => false,
       })
   }

   /// Visits all the nodes in this calculation tree recursively, starting by
   /// the leaves and bubbling all the way up.
   ///
   /// This is useful for simplification, but can also be used for validation
   /// and such.
   pub fn visit_depth_first(&mut self, mut f: impl FnMut(&mut Self)) {
       self.visit_depth_first_internal(&mut f)
   }

   fn visit_depth_first_internal(&mut self, f: &mut impl FnMut(&mut Self)) {
       match *self {
           Self::Clamp {
               ref mut min,
               ref mut center,
               ref mut max,
           } => {
               min.visit_depth_first_internal(f);
               center.visit_depth_first_internal(f);
               max.visit_depth_first_internal(f);
           },
           Self::Round {
               ref mut value,
               ref mut step,
               ..
           } => {
               value.visit_depth_first_internal(f);
               step.visit_depth_first_internal(f);
           },
           Self::ModRem {
               ref mut dividend,
               ref mut divisor,
               ..
           } => {
               dividend.visit_depth_first_internal(f);
               divisor.visit_depth_first_internal(f);
           },
           Self::Sum(ref mut children)
           | Self::Product(ref mut children)
           | Self::MinMax(ref mut children, _)
           | Self::Hypot(ref mut children) => {
               for child in &mut **children {
                   child.visit_depth_first_internal(f);
               }
           },
           Self::Negate(ref mut value) | Self::Invert(ref mut value) => {
               value.visit_depth_first_internal(f);
           },
           Self::Abs(ref mut value) | Self::Sign(ref mut value) => {
               value.visit_depth_first_internal(f);
           },
           Self::Leaf(..) | Self::Anchor(..) | Self::AnchorSize(..) => {},
       }
       f(self);
   }

   /// This function simplifies and sorts the calculation of the specified node. It simplifies
   /// directly nested nodes while assuming that all nodes below it have already been simplified.
   /// It is recommended to use this function in combination with `visit_depth_first()`.
   ///
   /// This function is necessary only if the node needs to be preserved after parsing,
   /// specifically for `<length-percentage>` cases where the calculation contains percentages or
   /// relative units. Otherwise, the node can be evaluated using `resolve()`, which will
   /// automatically provide a simplified value.
   ///
   /// <https://drafts.csswg.org/css-values-4/#calc-simplification>
   pub fn simplify_and_sort_direct_children(&mut self) {
       macro_rules! replace_self_with {
           ($slot:expr) => {{
               let result = mem::replace($slot, Self::dummy());
               *self = result;
           }};
       }

       macro_rules! value_or_stop {
           ($op:expr) => {{
               match $op {
                   Ok(value) => value,
                   Err(_) => return,
               }
           }};
       }

       match *self {
           Self::Clamp {
               ref mut min,
               ref mut center,
               ref mut max,
           } => {
               // NOTE: clamp() is max(min, min(center, max))
               let min_cmp_center = match min.compare(&center, PositivePercentageBasis::Unknown) {
                   Some(o) => o,
                   None => return,
               };

               // So if we can prove that min is more than center, then we won,
               // as that's what we should always return.
               if matches!(min_cmp_center, cmp::Ordering::Greater) {
                   replace_self_with!(&mut **min);
                   return;
               }

               // Otherwise try with max.
               let max_cmp_center = match max.compare(&center, PositivePercentageBasis::Unknown) {
                   Some(o) => o,
                   None => return,
               };

               if matches!(max_cmp_center, cmp::Ordering::Less) {
                   // max is less than center, so we need to return effectively
                   // `max(min, max)`.
                   let max_cmp_min = match max.compare(&min, PositivePercentageBasis::Unknown) {
                       Some(o) => o,
                       None => return,
                   };

                   if matches!(max_cmp_min, cmp::Ordering::Less) {
                       replace_self_with!(&mut **min);
                       return;
                   }

                   replace_self_with!(&mut **max);
                   return;
               }

               // Otherwise we're the center node.
               replace_self_with!(&mut **center);
           },
           Self::Round {
               strategy,
               ref mut value,
               ref mut step,
           } => {
               if value_or_stop!(step.is_zero_leaf()) {
                   value_or_stop!(value.coerce_to_value(f32::NAN));
                   replace_self_with!(&mut **value);
                   return;
               }

               if value_or_stop!(value.is_infinite_leaf())
                   && value_or_stop!(step.is_infinite_leaf())
               {
                   value_or_stop!(value.coerce_to_value(f32::NAN));
                   replace_self_with!(&mut **value);
                   return;
               }

               if value_or_stop!(value.is_infinite_leaf()) {
                   replace_self_with!(&mut **value);
                   return;
               }

               if value_or_stop!(step.is_infinite_leaf()) {
                   match strategy {
                       RoundingStrategy::Nearest | RoundingStrategy::ToZero => {
                           value_or_stop!(value.coerce_to_value(0.0));
                           replace_self_with!(&mut **value);
                           return;
                       },
                       RoundingStrategy::Up => {
                           if !value_or_stop!(value.is_negative_leaf())
                               && !value_or_stop!(value.is_zero_leaf())
                           {
                               value_or_stop!(value.coerce_to_value(f32::INFINITY));
                               replace_self_with!(&mut **value);
                               return;
                           } else if !value_or_stop!(value.is_negative_leaf())
                               && value_or_stop!(value.is_zero_leaf())
                           {
                               replace_self_with!(&mut **value);
                               return;
                           } else {
                               value_or_stop!(value.coerce_to_value(0.0));
                               replace_self_with!(&mut **value);
                               return;
                           }
                       },
                       RoundingStrategy::Down => {
                           if value_or_stop!(value.is_negative_leaf())
                               && !value_or_stop!(value.is_zero_leaf())
                           {
                               value_or_stop!(value.coerce_to_value(f32::INFINITY));
                               replace_self_with!(&mut **value);
                               return;
                           } else if value_or_stop!(value.is_negative_leaf())
                               && value_or_stop!(value.is_zero_leaf())
                           {
                               replace_self_with!(&mut **value);
                               return;
                           } else {
                               value_or_stop!(value.coerce_to_value(0.0));
                               replace_self_with!(&mut **value);
                               return;
                           }
                       },
                   }
               }

               if value_or_stop!(step.is_negative_leaf()) {
                   step.negate();
               }

               let remainder = value_or_stop!(value.try_op(step, Rem::rem));
               if value_or_stop!(remainder.is_zero_leaf()) {
                   replace_self_with!(&mut **value);
                   return;
               }

               let (mut lower_bound, mut upper_bound) = if value_or_stop!(value.is_negative_leaf())
               {
                   let upper_bound = value_or_stop!(value.try_op(&remainder, Sub::sub));
                   let lower_bound = value_or_stop!(upper_bound.try_op(&step, Sub::sub));

                   (lower_bound, upper_bound)
               } else {
                   let lower_bound = value_or_stop!(value.try_op(&remainder, Sub::sub));
                   let upper_bound = value_or_stop!(lower_bound.try_op(&step, Add::add));

                   (lower_bound, upper_bound)
               };

               match strategy {
                   RoundingStrategy::Nearest => {
                       let lower_diff = value_or_stop!(value.try_op(&lower_bound, Sub::sub));
                       let upper_diff = value_or_stop!(upper_bound.try_op(value, Sub::sub));
                       // In case of a tie, use the upper bound
                       if lower_diff.lt(&upper_diff, PositivePercentageBasis::Unknown) {
                           replace_self_with!(&mut lower_bound);
                       } else {
                           replace_self_with!(&mut upper_bound);
                       }
                   },
                   RoundingStrategy::Up => {
                       replace_self_with!(&mut upper_bound);
                   },
                   RoundingStrategy::Down => {
                       replace_self_with!(&mut lower_bound);
                   },
                   RoundingStrategy::ToZero => {
                       let mut lower_diff = lower_bound.clone();
                       let mut upper_diff = upper_bound.clone();

                       if value_or_stop!(lower_diff.is_negative_leaf()) {
                           lower_diff.negate();
                       }

                       if value_or_stop!(upper_diff.is_negative_leaf()) {
                           upper_diff.negate();
                       }

                       // In case of a tie, use the upper bound
                       if lower_diff.lt(&upper_diff, PositivePercentageBasis::Unknown) {
                           replace_self_with!(&mut lower_bound);
                       } else {
                           replace_self_with!(&mut upper_bound);
                       }
                   },
               };
           },
           Self::ModRem {
               ref dividend,
               ref divisor,
               op,
           } => {
               let mut result = value_or_stop!(dividend.try_op(divisor, |a, b| op.apply(a, b)));
               replace_self_with!(&mut result);
           },
           Self::MinMax(ref mut children, op) => {
               let winning_order = match op {
                   MinMaxOp::Min => cmp::Ordering::Less,
                   MinMaxOp::Max => cmp::Ordering::Greater,
               };

               if value_or_stop!(children[0].is_nan_leaf()) {
                   replace_self_with!(&mut children[0]);
                   return;
               }

               let mut result = 0;
               for i in 1..children.len() {
                   if value_or_stop!(children[i].is_nan_leaf()) {
                       replace_self_with!(&mut children[i]);
                       return;
                   }
                   let o = match children[i]
                       .compare(&children[result], PositivePercentageBasis::Unknown)
                   {
                       // We can't compare all the children, so we can't
                       // know which one will actually win. Bail out and
                       // keep ourselves as a min / max function.
                       //
                       // TODO: Maybe we could simplify compatible children,
                       // see https://github.com/w3c/csswg-drafts/issues/4756
                       None => return,
                       Some(o) => o,
                   };

                   if o == winning_order {
                       result = i;
                   }
               }

               replace_self_with!(&mut children[result]);
           },
           Self::Sum(ref mut children_slot) => {
               let mut sums_to_merge = SmallVec::<[_; 3]>::new();
               let mut extra_kids = 0;
               for (i, child) in children_slot.iter().enumerate() {
                   if let Self::Sum(ref children) = *child {
                       extra_kids += children.len();
                       sums_to_merge.push(i);
                   }
               }

               // If we only have one kid, we've already simplified it, and it
               // doesn't really matter whether it's a sum already or not, so
               // lift it up and continue.
               if children_slot.len() == 1 {
                   replace_self_with!(&mut children_slot[0]);
                   return;
               }

               let mut children = mem::take(children_slot).into_vec();

               if !sums_to_merge.is_empty() {
                   children.reserve(extra_kids - sums_to_merge.len());
                   // Merge all our nested sums, in reverse order so that the
                   // list indices are not invalidated.
                   for i in sums_to_merge.drain(..).rev() {
                       let kid_children = match children.swap_remove(i) {
                           Self::Sum(c) => c,
                           _ => unreachable!(),
                       };

                       // This would be nicer with
                       // https://github.com/rust-lang/rust/issues/59878 fixed.
                       children.extend(kid_children.into_vec());
                   }
               }

               debug_assert!(children.len() >= 2, "Should still have multiple kids!");

               // Sort by spec order.
               children.sort_unstable_by_key(|c| c.sort_key());

               // NOTE: if the function returns true, by the docs of dedup_by,
               // a is removed.
               children.dedup_by(|a, b| b.try_sum_in_place(a).is_ok());

               if children.len() == 1 {
                   // If only one children remains, lift it up, and carry on.
                   replace_self_with!(&mut children[0]);
               } else {
                   // Else put our simplified children back.
                   *children_slot = children.into_boxed_slice().into();
               }
           },
           Self::Product(ref mut children_slot) => {
               let mut products_to_merge = SmallVec::<[_; 3]>::new();
               let mut extra_kids = 0;
               for (i, child) in children_slot.iter().enumerate() {
                   if let Self::Product(ref children) = *child {
                       extra_kids += children.len();
                       products_to_merge.push(i);
                   }
               }

               // If we only have one kid, we've already simplified it, and it
               // doesn't really matter whether it's a product already or not,
               // so lift it up and continue.
               if children_slot.len() == 1 {
                   replace_self_with!(&mut children_slot[0]);
                   return;
               }

               let mut children = mem::take(children_slot).into_vec();

               if !products_to_merge.is_empty() {
                   children.reserve(extra_kids - products_to_merge.len());
                   // Merge all our nested sums, in reverse order so that the
                   // list indices are not invalidated.
                   for i in products_to_merge.drain(..).rev() {
                       let kid_children = match children.swap_remove(i) {
                           Self::Product(c) => c,
                           _ => unreachable!(),
                       };

                       // This would be nicer with
                       // https://github.com/rust-lang/rust/issues/59878 fixed.
                       children.extend(kid_children.into_vec());
                   }
               }

               debug_assert!(children.len() >= 2, "Should still have multiple kids!");

               // Sort by spec order.
               children.sort_unstable_by_key(|c| c.sort_key());

               // NOTE: if the function returns true, by the docs of dedup_by,
               // a is removed.
               children.dedup_by(|right, left| left.try_product_in_place(right));

               if children.len() == 1 {
                   // If only one children remains, lift it up, and carry on.
                   replace_self_with!(&mut children[0]);
               } else {
                   // Else put our simplified children back.
                   *children_slot = children.into_boxed_slice().into();
               }
           },
           Self::Hypot(ref children) => {
               let mut result = value_or_stop!(children[0].try_op(&children[0], Mul::mul));

               for child in children.iter().skip(1) {
                   let square = value_or_stop!(child.try_op(&child, Mul::mul));
                   result = value_or_stop!(result.try_op(&square, Add::add));
               }

               result = value_or_stop!(result.try_op(&result, |a, _| a.sqrt()));

               replace_self_with!(&mut result);
           },
           Self::Abs(ref mut child) => {
               if let CalcNode::Leaf(leaf) = child.as_mut() {
                   value_or_stop!(leaf.map(|v| v.abs()));
                   replace_self_with!(&mut **child);
               }
           },
           Self::Sign(ref mut child) => {
               if let CalcNode::Leaf(leaf) = child.as_mut() {
                   let mut result = Self::Leaf(value_or_stop!(L::sign_from(leaf)));
                   replace_self_with!(&mut result);
               }
           },
           Self::Negate(ref mut child) => {
               // Step 6.
               match &mut **child {
                   CalcNode::Leaf(_) => {
                       // 1. If root’s child is a numeric value, return an equivalent numeric value, but
                       // with the value negated (0 - value).
                       child.negate();
                       replace_self_with!(&mut **child);
                   },
                   CalcNode::Negate(value) => {
                       // 2. If root’s child is a Negate node, return the child’s child.
                       replace_self_with!(&mut **value);
                   },
                   _ => {
                       // 3. Return root.
                   },
               }
           },
           Self::Invert(ref mut child) => {
               // Step 7.
               match &mut **child {
                   CalcNode::Leaf(leaf) => {
                       // 1. If root’s child is a number (not a percentage or dimension) return the
                       // reciprocal of the child’s value.
                       if leaf.unit().is_empty() {
                           value_or_stop!(child.map(|v| 1.0 / v));
                           replace_self_with!(&mut **child);
                       }
                   },
                   CalcNode::Invert(value) => {
                       // 2. If root’s child is an Invert node, return the child’s child.
                       replace_self_with!(&mut **value);
                   },
                   _ => {
                       // 3. Return root.
                   },
               }
           },
           Self::Leaf(ref mut l) => {
               l.simplify();
           },
           Self::Anchor(ref mut f) => {
               if let GenericAnchorSide::Percentage(ref mut n) = f.side {
                   n.simplify_and_sort();
               }
               if let Some(fallback) = f.fallback.as_mut() {
                   fallback.simplify_and_sort();
               }
           },
           Self::AnchorSize(ref mut f) => {
               if let Some(fallback) = f.fallback.as_mut() {
                   fallback.simplify_and_sort();
               }
           },
       }
   }

   /// Simplifies and sorts the kids in the whole calculation subtree.
   pub fn simplify_and_sort(&mut self) {
       self.visit_depth_first(|node| node.simplify_and_sort_direct_children())
   }

   fn to_css_impl<W>(&self, dest: &mut CssWriter<W>, level: ArgumentLevel) -> fmt::Result
   where
       W: Write,
   {
       let write_closing_paren = match *self {
           Self::MinMax(_, op) => {
               dest.write_str(match op {
                   MinMaxOp::Max => "max(",
                   MinMaxOp::Min => "min(",
               })?;
               true
           },
           Self::Clamp { .. } => {
               dest.write_str("clamp(")?;
               true
           },
           Self::Round { strategy, .. } => {
               match strategy {
                   RoundingStrategy::Nearest => dest.write_str("round("),
                   RoundingStrategy::Up => dest.write_str("round(up, "),
                   RoundingStrategy::Down => dest.write_str("round(down, "),
                   RoundingStrategy::ToZero => dest.write_str("round(to-zero, "),
               }?;

               true
           },
           Self::ModRem { op, .. } => {
               dest.write_str(match op {
                   ModRemOp::Mod => "mod(",
                   ModRemOp::Rem => "rem(",
               })?;

               true
           },
           Self::Hypot(_) => {
               dest.write_str("hypot(")?;
               true
           },
           Self::Abs(_) => {
               dest.write_str("abs(")?;
               true
           },
           Self::Sign(_) => {
               dest.write_str("sign(")?;
               true
           },
           Self::Negate(_) => {
               // We never generate a [`Negate`] node as the root of a calculation, only inside
               // [`Sum`] nodes as a child. Because negate nodes are handled by the [`Sum`] node
               // directly (see below), this node will never be serialized.
               debug_assert!(
                   false,
                   "We never serialize Negate nodes as they are handled inside Sum nodes."
               );
               dest.write_str("(-1 * ")?;
               true
           },
           Self::Invert(_) => {
               if matches!(level, ArgumentLevel::CalculationRoot) {
                   dest.write_str("calc")?;
               }
               dest.write_str("(1 / ")?;
               true
           },
           Self::Sum(_) | Self::Product(_) => match level {
               ArgumentLevel::CalculationRoot => {
                   dest.write_str("calc(")?;
                   true
               },
               ArgumentLevel::ArgumentRoot => false,
               ArgumentLevel::Nested => {
                   dest.write_str("(")?;
                   true
               },
           },
           Self::Leaf(_) | Self::Anchor(_) | Self::AnchorSize(_) => match level {
               ArgumentLevel::CalculationRoot => {
                   dest.write_str("calc(")?;
                   true
               },
               ArgumentLevel::ArgumentRoot | ArgumentLevel::Nested => false,
           },
       };

       match *self {
           Self::MinMax(ref children, _) | Self::Hypot(ref children) => {
               let mut first = true;
               for child in &**children {
                   if !first {
                       dest.write_str(", ")?;
                   }
                   first = false;
                   child.to_css_impl(dest, ArgumentLevel::ArgumentRoot)?;
               }
           },
           Self::Negate(ref value) | Self::Invert(ref value) => {
               value.to_css_impl(dest, ArgumentLevel::Nested)?
           },
           Self::Sum(ref children) => {
               let mut first = true;
               for child in &**children {
                   if !first {
                       match child {
                           Self::Leaf(l) => {
                               if let Ok(true) = l.is_negative() {
                                   dest.write_str(" - ")?;
                                   let mut negated = l.clone();
                                   // We can unwrap here, because we already
                                   // checked if the value inside is negative.
                                   negated.map(std::ops::Neg::neg).unwrap();
                                   negated.to_css(dest)?;
                               } else {
                                   dest.write_str(" + ")?;
                                   l.to_css(dest)?;
                               }
                           },
                           Self::Negate(n) => {
                               dest.write_str(" - ")?;
                               n.to_css_impl(dest, ArgumentLevel::Nested)?;
                           },
                           _ => {
                               dest.write_str(" + ")?;
                               child.to_css_impl(dest, ArgumentLevel::Nested)?;
                           },
                       }
                   } else {
                       first = false;
                       child.to_css_impl(dest, ArgumentLevel::Nested)?;
                   }
               }
           },
           Self::Product(ref children) => {
               let mut first = true;
               for child in &**children {
                   if !first {
                       match child {
                           Self::Invert(n) => {
                               dest.write_str(" / ")?;
                               n.to_css_impl(dest, ArgumentLevel::Nested)?;
                           },
                           _ => {
                               dest.write_str(" * ")?;
                               child.to_css_impl(dest, ArgumentLevel::Nested)?;
                           },
                       }
                   } else {
                       first = false;
                       child.to_css_impl(dest, ArgumentLevel::Nested)?;
                   }
               }
           },
           Self::Clamp {
               ref min,
               ref center,
               ref max,
           } => {
               min.to_css_impl(dest, ArgumentLevel::ArgumentRoot)?;
               dest.write_str(", ")?;
               center.to_css_impl(dest, ArgumentLevel::ArgumentRoot)?;
               dest.write_str(", ")?;
               max.to_css_impl(dest, ArgumentLevel::ArgumentRoot)?;
           },
           Self::Round {
               ref value,
               ref step,
               ..
           } => {
               value.to_css_impl(dest, ArgumentLevel::ArgumentRoot)?;
               dest.write_str(", ")?;
               step.to_css_impl(dest, ArgumentLevel::ArgumentRoot)?;
           },
           Self::ModRem {
               ref dividend,
               ref divisor,
               ..
           } => {
               dividend.to_css_impl(dest, ArgumentLevel::ArgumentRoot)?;
               dest.write_str(", ")?;
               divisor.to_css_impl(dest, ArgumentLevel::ArgumentRoot)?;
           },
           Self::Abs(ref v) | Self::Sign(ref v) => {
               v.to_css_impl(dest, ArgumentLevel::ArgumentRoot)?
           },
           Self::Leaf(ref l) => l.to_css(dest)?,
           Self::Anchor(ref f) => f.to_css(dest)?,
           Self::AnchorSize(ref f) => f.to_css(dest)?,
       }

       if write_closing_paren {
           dest.write_char(')')?;
       }
       Ok(())
   }

   fn to_typed_impl(&self, level: ArgumentLevel) -> Option<TypedValue> {
       // XXX Only supporting Sum and Leaf for now
       match *self {
           Self::Sum(ref children) => {
               let mut values = ThinVec::new();
               for child in &**children {
                   if let Some(TypedValue::Numeric(inner)) =
                       child.to_typed_impl(ArgumentLevel::Nested)
                   {
                       values.push(inner);
                   }
               }
               Some(TypedValue::Numeric(NumericValue::Sum { values }))
           },
           Self::Leaf(ref l) => match l.to_typed() {
               Some(TypedValue::Numeric(inner)) => match level {
                   ArgumentLevel::CalculationRoot => {
                       Some(TypedValue::Numeric(NumericValue::Sum {
                           values: ThinVec::from([inner]),
                       }))
                   },
                   ArgumentLevel::ArgumentRoot | ArgumentLevel::Nested => {
                       Some(TypedValue::Numeric(inner))
                   },
               },
               _ => None,
           },
           _ => None,
       }
   }

   fn compare(
       &self,
       other: &Self,
       basis_positive: PositivePercentageBasis,
   ) -> Option<cmp::Ordering> {
       match (self, other) {
           (&CalcNode::Leaf(ref one), &CalcNode::Leaf(ref other)) => {
               one.compare(other, basis_positive)
           },
           _ => None,
       }
   }

   compare_helpers!();
}

impl<L: CalcNodeLeaf> ToCss for CalcNode<L> {
   /// <https://drafts.csswg.org/css-values/#calc-serialize>
   fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
   where
       W: Write,
   {
       self.to_css_impl(dest, ArgumentLevel::CalculationRoot)
   }
}

impl<L: CalcNodeLeaf> ToTyped for CalcNode<L> {
   fn to_typed(&self) -> Option<TypedValue> {
       self.to_typed_impl(ArgumentLevel::CalculationRoot)
   }
}

#[cfg(test)]
mod tests {
   use super::*;

   #[test]
   fn can_sum_with_checks() {
       assert!(CalcUnits::LENGTH.can_sum_with(CalcUnits::LENGTH));
       assert!(CalcUnits::LENGTH.can_sum_with(CalcUnits::PERCENTAGE));
       assert!(CalcUnits::LENGTH.can_sum_with(CalcUnits::LENGTH_PERCENTAGE));

       assert!(CalcUnits::PERCENTAGE.can_sum_with(CalcUnits::LENGTH));
       assert!(CalcUnits::PERCENTAGE.can_sum_with(CalcUnits::PERCENTAGE));
       assert!(CalcUnits::PERCENTAGE.can_sum_with(CalcUnits::LENGTH_PERCENTAGE));

       assert!(CalcUnits::LENGTH_PERCENTAGE.can_sum_with(CalcUnits::LENGTH));
       assert!(CalcUnits::LENGTH_PERCENTAGE.can_sum_with(CalcUnits::PERCENTAGE));
       assert!(CalcUnits::LENGTH_PERCENTAGE.can_sum_with(CalcUnits::LENGTH_PERCENTAGE));

       assert!(!CalcUnits::ANGLE.can_sum_with(CalcUnits::TIME));
       assert!(CalcUnits::ANGLE.can_sum_with(CalcUnits::ANGLE));

       assert!(!(CalcUnits::ANGLE | CalcUnits::TIME).can_sum_with(CalcUnits::ANGLE));
       assert!(!CalcUnits::ANGLE.can_sum_with(CalcUnits::ANGLE | CalcUnits::TIME));
       assert!(
           !(CalcUnits::ANGLE | CalcUnits::TIME).can_sum_with(CalcUnits::ANGLE | CalcUnits::TIME)
       );
   }
}