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reduce educational decoder to single frame decompression

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This commit is contained in:
Paul Cruz
2017-08-11 14:34:49 -07:00
parent cae4024889
commit 736a28d835
1 changed files with 13 additions and 89 deletions
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102
doc/educational_decoder/zstd_decompress.c
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@ -28,6 +28,7 @@ size_t ZSTD_decompress_with_dict(void *const dst, const size_t dst_len,
/// Get the decompressed size of an input stream so memory can be allocated in
/// advance
/// Returns -1 if the size can't be determined
/// Assumes decompression of a single frame
size_t ZSTD_get_decompressed_size(const void *const src, const size_t src_len);
/******* UTILITY MACROS AND TYPES *********************************************/
@ -396,9 +397,9 @@ size_t ZSTD_decompress_with_dict(void *const dst, const size_t dst_len,
// Multiple frames can be appended into a single file or stream. A frame is
// totally independent, has a defined beginning and end, and a set of
// parameters which tells the decoder how to decompress it."
while (IO_istream_len(&in) > 0) {
decode_frame(&out, &in, &parsed_dict);
}
/* this decoder assumes decompression of a single frame */
decode_frame(&out, &in, &parsed_dict);
free_dictionary(&parsed_dict);
@ -424,30 +425,6 @@ static void decompress_data(frame_context_t *const ctx, ostream_t *const out,
static void decode_frame(ostream_t *const out, istream_t *const in,
const dictionary_t *const dict) {
const u32 magic_number = IO_read_bits(in, 32);
// Skippable frame
//
// "Magic_Number
//
// 4 Bytes, little-endian format. Value : 0x184D2A5?, which means any value
// from 0x184D2A50 to 0x184D2A5F. All 16 values are valid to identify a
// skippable frame."
if ((magic_number & ~0xFU) == 0x184D2A50U) {
// "Skippable frames allow the insertion of user-defined data into a
// flow of concatenated frames. Its design is pretty straightforward,
// with the sole objective to allow the decoder to quickly skip over
// user-defined data and continue decoding.
//
// Skippable frames defined in this specification are compatible with
// LZ4 ones."
const size_t frame_size = IO_read_bits(in, 32);
// skip over frame
IO_advance_input(in, frame_size);
return;
}
// Zstandard frame
//
// "Magic_Number
@ -460,8 +437,8 @@ static void decode_frame(ostream_t *const out, istream_t *const in,
return;
}
// not a real frame
ERROR("Invalid magic number");
// not a real frame or a skippable frame
ERROR("Tried to decode non-ZSTD frame");
}
/// Decode a frame that contains compressed data. Not all frames do as there
@ -1420,28 +1397,17 @@ static void execute_sequences(frame_context_t *const ctx, ostream_t *const out,
/******* END SEQUENCE EXECUTION ***********************************************/
/******* OUTPUT SIZE COUNTING *************************************************/
static void traverse_frame(const frame_header_t *const header, istream_t *const in);
/// Get the decompressed size of an input stream so memory can be allocated in
/// advance.
/// This is more complex than the implementation in the reference
/// implementation, as this API allows for the decompression of multiple
/// concatenated frames.
/// This implementation assumes `src` points to a single ZSTD-compressed frame
size_t ZSTD_get_decompressed_size(const void *src, const size_t src_len) {
istream_t in = IO_make_istream(src, src_len);
size_t dst_size = 0;
// Each frame header only gives us the size of its frame, so iterate over
// all
// frames
while (IO_istream_len(&in) > 0) {
// get decompressed size from ZSTD frame header
{
const u32 magic_number = IO_read_bits(&in, 32);
if ((magic_number & ~0xFU) == 0x184D2A50U) {
// skippable frame, this has no impact on output size
const size_t frame_size = IO_read_bits(&in, 32);
IO_advance_input(&in, frame_size);
} else if (magic_number == 0xFD2FB528U) {
if (magic_number == 0xFD2FB528U) {
// ZSTD frame
frame_header_t header;
parse_frame_header(&header, &in);
@ -1451,54 +1417,13 @@ size_t ZSTD_get_decompressed_size(const void *src, const size_t src_len) {
return -1;
}
dst_size += header.frame_content_size;
// Consume the input from the frame to reach the start of the next
traverse_frame(&header, &in);
return header.frame_content_size;
} else {
// not a real frame
ERROR("Invalid magic number");
// not a real frame or skippable frame
ERROR("ZSTD frame magic number did not match");
}
}
return dst_size;
}
/// Iterate over each block in a frame to find the end of it, to get to the
/// start of the next frame
static void traverse_frame(const frame_header_t *const header, istream_t *const in) {
int last_block = 0;
do {
// Parse the block header
last_block = IO_read_bits(in, 1);
const int block_type = IO_read_bits(in, 2);
const size_t block_len = IO_read_bits(in, 21);
switch (block_type) {
case 0: // Raw block, block_len bytes
IO_advance_input(in, block_len);
break;
case 1: // RLE block, 1 byte
IO_advance_input(in, 1);
break;
case 2: // Compressed block, compressed size is block_len
IO_advance_input(in, block_len);
break;
case 3:
// Reserved block type
CORRUPTION();
break;
default:
IMPOSSIBLE();
}
} while (!last_block);
if (header->content_checksum_flag) {
IO_advance_input(in, 4);
}
}
/******* END OUTPUT SIZE COUNTING *********************************************/
/******* DICTIONARY PARSING ***************************************************/
@ -2355,4 +2280,3 @@ static void FSE_copy_dtable(FSE_dtable *const dst, const FSE_dtable *const src)
memcpy(dst->new_state_base, src->new_state_base, size * sizeof(u16));
}
/******* END FSE PRIMITIVES ***************************************************/