void input_reader(png_structp png_ptr, png_bytep data, png_size_t length) {
InputStream *istr = (InputStream*)png_get_io_ptr(png_ptr);
int ct = istr->ReadChars(length, data);
if(ct != length)
png_error(png_ptr, "unexpected EOF");
}
// Gif helper function - loads the given number of bits from the file. File format: byte indicating
// block length, followed by a block of that length. All blocks are logically concatenated until
// a block of length 0 is reached.
static int LoadGifBits(InputStream input, unsigned char *buffer, int num_bits,
int *buffer_pos, int *buffer_end) {
if (*buffer_pos + num_bits >= *buffer_end) {
if (*buffer_end >= 16)
buffer[0] = buffer[*buffer_end/8 - 2];
if (*buffer_end >= 8)
buffer[1] = buffer[*buffer_end/8 - 1];
unsigned char block_size;
if (!input.ReadChars(1, &block_size) || block_size == 0)
return -1;
if (input.ReadChars(block_size, buffer + 2) < block_size)
return -1;
*buffer_pos = 16 - (*buffer_end - *buffer_pos);
*buffer_end = 8 * (int(block_size) + 2);
}
unsigned int value = 0;
for (int i = *buffer_pos, j = 0; j < num_bits; i++, j++)
value |= (((buffer[i / 8] & (1 << (i % 8))) > 0) << j);
*buffer_pos += num_bits;
return value;
}