tor-browser

getbits.c (4988B)

---

/*
* Copyright © 2018, VideoLAN and dav1d authors
* Copyright © 2018, Two Orioles, LLC
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* 1. Redistributions of source code must retain the above copyright notice, this
*    list of conditions and the following disclaimer.
*
* 2. Redistributions in binary form must reproduce the above copyright notice,
*    this list of conditions and the following disclaimer in the documentation
*    and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
* WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
* DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
* ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
* (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
* LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
* ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
* SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/

#include "config.h"

#include <limits.h>

#include "common/intops.h"

#include "src/getbits.h"

void dav1d_init_get_bits(GetBits *const c, const uint8_t *const data,
                        const size_t sz)
{
   assert(sz);
   c->ptr = c->ptr_start = data;
   c->ptr_end = &c->ptr_start[sz];
   c->state = 0;
   c->bits_left = 0;
   c->error = 0;
}

unsigned dav1d_get_bit(GetBits *const c) {
   if (!c->bits_left) {
       if (c->ptr >= c->ptr_end) {
           c->error = 1;
       } else {
           const unsigned state = *c->ptr++;
           c->bits_left = 7;
           c->state = (uint64_t) state << 57;
           return state >> 7;
       }
   }

   const uint64_t state = c->state;
   c->bits_left--;
   c->state = state << 1;
   return (unsigned) (state >> 63);
}

static inline void refill(GetBits *const c, const int n) {
   assert(c->bits_left >= 0 && c->bits_left < 32);
   unsigned state = 0;
   do {
       if (c->ptr >= c->ptr_end) {
           c->error = 1;
           if (state) break;
           return;
       }
       state = (state << 8) | *c->ptr++;
       c->bits_left += 8;
   } while (n > c->bits_left);
   c->state |= (uint64_t) state << (64 - c->bits_left);
}

#define GET_BITS(name, type, type64)            \
type name(GetBits *const c, const int n) {      \
   assert(n > 0 && n <= 32);                   \
   /* Unsigned cast avoids refill after eob */ \
   if ((unsigned) n > (unsigned) c->bits_left) \
       refill(c, n);                           \
   const uint64_t state = c->state;            \
   c->bits_left -= n;                          \
   c->state = state << n;                      \
   return (type) ((type64) state >> (64 - n)); \
}

GET_BITS(dav1d_get_bits,  unsigned, uint64_t)
GET_BITS(dav1d_get_sbits, int,      int64_t)

unsigned dav1d_get_uleb128(GetBits *const c) {
   uint64_t val = 0;
   unsigned i = 0, more;

   do {
       const int v = dav1d_get_bits(c, 8);
       more = v & 0x80;
       val |= ((uint64_t) (v & 0x7F)) << i;
       i += 7;
   } while (more && i < 56);

   if (val > UINT32_MAX || more) {
       c->error = 1;
       return 0;
   }

   return (unsigned) val;
}

unsigned dav1d_get_uniform(GetBits *const c, const unsigned max) {
   // Output in range [0..max-1]
   // max must be > 1, or else nothing is read from the bitstream
   assert(max > 1);
   const int l = ulog2(max) + 1;
   assert(l > 1);
   const unsigned m = (1U << l) - max;
   const unsigned v = dav1d_get_bits(c, l - 1);
   return v < m ? v : (v << 1) - m + dav1d_get_bit(c);
}

unsigned dav1d_get_vlc(GetBits *const c) {
   if (dav1d_get_bit(c))
       return 0;

   int n_bits = 0;
   do {
       if (++n_bits == 32)
           return UINT32_MAX;
   } while (!dav1d_get_bit(c));

   return ((1U << n_bits) - 1) + dav1d_get_bits(c, n_bits);
}

static unsigned get_bits_subexp_u(GetBits *const c, const unsigned ref,
                                 const unsigned n)
{
   unsigned v = 0;

   for (int i = 0;; i++) {
       const int b = i ? 3 + i - 1 : 3;

       if (n < v + 3 * (1 << b)) {
           v += dav1d_get_uniform(c, n - v + 1);
           break;
       }

       if (!dav1d_get_bit(c)) {
           v += dav1d_get_bits(c, b);
           break;
       }

       v += 1 << b;
   }

   return ref * 2 <= n ? inv_recenter(ref, v) : n - inv_recenter(n - ref, v);
}

int dav1d_get_bits_subexp(GetBits *const c, const int ref, const unsigned n) {
   return (int) get_bits_subexp_u(c, ref + (1 << n), 2 << n) - (1 << n);
}