michael@0: /*
michael@0:  *  Copyright (c) 2010 The WebM project authors. All Rights Reserved.
michael@0:  *
michael@0:  *  Use of this source code is governed by a BSD-style license
michael@0:  *  that can be found in the LICENSE file in the root of the source
michael@0:  *  tree. An additional intellectual property rights grant can be found
michael@0:  *  in the file PATENTS.  All contributing project authors may
michael@0:  *  be found in the AUTHORS file in the root of the source tree.
michael@0:  */
michael@0: 
michael@0: #include "vpx_ports/mem.h"
michael@0: #include "vpx_mem/vpx_mem.h"
michael@0: 
michael@0: #include "vp9/decoder/vp9_dboolhuff.h"
michael@0: 
michael@0: // This is meant to be a large, positive constant that can still be efficiently
michael@0: // loaded as an immediate (on platforms like ARM, for example).
michael@0: // Even relatively modest values like 100 would work fine.
michael@0: #define LOTS_OF_BITS 0x40000000
michael@0: 
michael@0: 
michael@0: int vp9_reader_init(vp9_reader *r, const uint8_t *buffer, size_t size) {
michael@0:   int marker_bit;
michael@0: 
michael@0:   r->buffer_end = buffer + size;
michael@0:   r->buffer = buffer;
michael@0:   r->value = 0;
michael@0:   r->count = -8;
michael@0:   r->range = 255;
michael@0: 
michael@0:   if (size && !buffer)
michael@0:     return 1;
michael@0: 
michael@0:   vp9_reader_fill(r);
michael@0:   marker_bit = vp9_read_bit(r);
michael@0:   return marker_bit != 0;
michael@0: }
michael@0: 
michael@0: void vp9_reader_fill(vp9_reader *r) {
michael@0:   const uint8_t *const buffer_end = r->buffer_end;
michael@0:   const uint8_t *buffer = r->buffer;
michael@0:   VP9_BD_VALUE value = r->value;
michael@0:   int count = r->count;
michael@0:   int shift = BD_VALUE_SIZE - 8 - (count + 8);
michael@0:   int loop_end = 0;
michael@0:   const int bits_left = (int)((buffer_end - buffer)*CHAR_BIT);
michael@0:   const int x = shift + CHAR_BIT - bits_left;
michael@0: 
michael@0:   if (x >= 0) {
michael@0:     count += LOTS_OF_BITS;
michael@0:     loop_end = x;
michael@0:   }
michael@0: 
michael@0:   if (x < 0 || bits_left) {
michael@0:     while (shift >= loop_end) {
michael@0:       count += CHAR_BIT;
michael@0:       value |= (VP9_BD_VALUE)*buffer++ << shift;
michael@0:       shift -= CHAR_BIT;
michael@0:     }
michael@0:   }
michael@0: 
michael@0:   r->buffer = buffer;
michael@0:   r->value = value;
michael@0:   r->count = count;
michael@0: }
michael@0: 
michael@0: const uint8_t *vp9_reader_find_end(vp9_reader *r) {
michael@0:   // Find the end of the coded buffer
michael@0:   while (r->count > CHAR_BIT && r->count < BD_VALUE_SIZE) {
michael@0:     r->count -= CHAR_BIT;
michael@0:     r->buffer--;
michael@0:   }
michael@0:   return r->buffer;
michael@0: }
michael@0: 
michael@0: int vp9_reader_has_error(vp9_reader *r) {
michael@0:   // Check if we have reached the end of the buffer.
michael@0:   //
michael@0:   // Variable 'count' stores the number of bits in the 'value' buffer, minus
michael@0:   // 8. The top byte is part of the algorithm, and the remainder is buffered
michael@0:   // to be shifted into it. So if count == 8, the top 16 bits of 'value' are
michael@0:   // occupied, 8 for the algorithm and 8 in the buffer.
michael@0:   //
michael@0:   // When reading a byte from the user's buffer, count is filled with 8 and
michael@0:   // one byte is filled into the value buffer. When we reach the end of the
michael@0:   // data, count is additionally filled with LOTS_OF_BITS. So when
michael@0:   // count == LOTS_OF_BITS - 1, the user's data has been exhausted.
michael@0:   //
michael@0:   // 1 if we have tried to decode bits after the end of stream was encountered.
michael@0:   // 0 No error.
michael@0:   return r->count > BD_VALUE_SIZE && r->count < LOTS_OF_BITS;
michael@0: }