git.sesse.net Git - fjl/blob - bitsource.c

   1 #include <stdio.h>
   2 #include <stdlib.h>
   3 #include <stdbool.h>
   4 #include <string.h>
   5
   6 #include "bitsource.h"
   7
   8 #define MARKER_CHAR 0xff
   9 #define STUFF_MARKER 0x00
  10
  11 void init_bit_source(struct bit_source* source, input_func_t* input_func,
  12                      unsigned padding_bytes, void* userdata)
  13 {
  14         memset(source, 0, sizeof(*source));
  15         source->bytes = (uint8_t*)malloc(BYTERESERVOIR_SIZE);
  16         source->byte_read_ptr = source->bytes;
  17         source->input_func = input_func;
  18         source->padding_bytes_available = padding_bytes;
  19         source->userdata = userdata;
  20 }
  21
  22 void possibly_refill_slow_path(struct bit_source* source, unsigned num_bits)
  23 {
  24         // First, move out the data we already read.
  25         assert(source->bytes_available <= BYTERESERVOIR_SIZE);
  26         assert(source->byte_read_ptr >= source->bytes);
  27         memmove(source->bytes, source->byte_read_ptr, source->bytes_available);
  28         source->byte_read_ptr = source->bytes;
  29
  30         // Then, make sure there's stuff in the byte reservoir if we can.
  31         // Read data from the source until we have enough to satisfy the request.
  32         while (source->bits_available + 8 * source->bytes_available < num_bits) {
  33                 const size_t bytes_to_read = BYTERESERVOIR_SIZE - source->bytes_available;
  34                 const ssize_t bytes_read =
  35                         (*source->input_func)(source->userdata,
  36                                               source->bytes + source->bytes_available,
  37                                               bytes_to_read);
  38                 assert(bytes_read <= (ssize_t)bytes_to_read);
  39                 assert(bytes_read >= (ssize_t)-1);
  40
  41                 // TODO: We need better error handling here. setjmp()/longjmp()
  42                 // should hopefully do the trick, but we need to take care for
  43                 // suspension.
  44                 if (bytes_read == (ssize_t)-1) {
  45                         fprintf(stderr, "Input function returned error\n");
  46                         exit(1);
  47                 }
  48                 if (bytes_read == 0) {
  49                         source->source_eof = true;
  50                         if (source->padding_bytes_available > 0) {
  51                                 unsigned padding_to_add = source->padding_bytes_available;
  52                                 if (padding_to_add > bytes_to_read) {
  53                                         padding_to_add = bytes_to_read;
  54                                 }
  55                                 memset(source->bytes + source->bytes_available, 0, padding_to_add);
  56                                 source->padding_bytes_available -= padding_to_add;
  57                                 source->bytes_available += padding_to_add;
  58                         } else {
  59                                 fprintf(stderr, "Premature EOF\n");
  60                                 exit(1);
  61                         }
  62                 } else {
  63                         source->bytes_available += bytes_read;
  64                 }
  65         }
  66
  67         // Fill the bit reservoir one by one byte until we have enough.
  68         while (source->bits_available < num_bits) {
  69                 assert(source->bytes_available > 0);
  70                 assert(source->bits_available + 8 <= BITRESERVOIR_SIZE);
  71                 uint8_t byte = *(source->byte_read_ptr);
  72                 ++source->byte_read_ptr;
  73                 --source->bytes_available;
  74                 source->bits |= ((bitreservoir_t)byte << (BITRESERVOIR_SIZE - source->bits_available - 8));
  75                 source->bits_available += 8;
  76         }
  77 }