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created separate function for offset computation

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This commit is contained in:
Paul Cruz 2017-08-14 14:20:12 -07:00
parent b6d6be58c9
commit d3e57db0bd
1 changed files with 51 additions and 45 deletions
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96
doc/educational_decoder/zstd_decompress.c
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@ -356,6 +356,8 @@ static void execute_sequences(frame_context_t *const ctx, ostream_t *const out,
static u32 copy_literals(sequence_command_t seq, istream_t *litstream,
ostream_t *const out);
static size_t compute_offset(sequence_command_t seq, u64 *const offset_hist);
/******* END ZSTD HELPER STRUCTS AND PROTOTYPES *******************************/
size_t ZSTD_decompress(void *const dst, const size_t dst_len,
@ -1263,51 +1265,7 @@ static void execute_sequences(frame_context_t *const ctx, ostream_t *const out,
const u32 literals_size = copy_literals(seq, &litstream, out);
total_output += literals_size;
}
size_t offset;
// Offsets are special, we need to handle the repeat offsets
if (seq.offset <= 3) {
// "The first 3 values define a repeated offset and we will call
// them Repeated_Offset1, Repeated_Offset2, and Repeated_Offset3.
// They are sorted in recency order, with Repeated_Offset1 meaning
// 'most recent one'".
// Use 0 indexing for the array
u32 idx = seq.offset - 1;
if (seq.literal_length == 0) {
// "There is an exception though, when current sequence's
// literals length is 0. In this case, repeated offsets are
// shifted by one, so Repeated_Offset1 becomes Repeated_Offset2,
// Repeated_Offset2 becomes Repeated_Offset3, and
// Repeated_Offset3 becomes Repeated_Offset1 - 1_byte."
idx++;
}
if (idx == 0) {
offset = offset_hist[0];
} else {
// If idx == 3 then literal length was 0 and the offset was 3,
// as per the exception listed above
offset = idx < 3 ? offset_hist[idx] : offset_hist[0] - 1;
// If idx == 1 we don't need to modify offset_hist[2], since
// we're using the second-most recent code
if (idx > 1) {
offset_hist[2] = offset_hist[1];
}
offset_hist[1] = offset_hist[0];
offset_hist[0] = offset;
}
} else {
// When it's not a repeat offset:
// "if (Offset_Value > 3) offset = Offset_Value - 3;"
offset = seq.offset - 3;
// Shift back history
offset_hist[2] = offset_hist[1];
offset_hist[1] = offset_hist[0];
offset_hist[0] = offset;
}
size_t offset = compute_offset(seq, offset_hist);
size_t match_length = seq.match_length;
@ -1377,6 +1335,54 @@ static u32 copy_literals(const sequence_command_t seq, istream_t *litstream,
return seq.literal_length;
}
static size_t compute_offset(sequence_command_t seq, u64 *const offset_hist) {
size_t offset;
// Offsets are special, we need to handle the repeat offsets
if (seq.offset <= 3) {
// "The first 3 values define a repeated offset and we will call
// them Repeated_Offset1, Repeated_Offset2, and Repeated_Offset3.
// They are sorted in recency order, with Repeated_Offset1 meaning
// 'most recent one'".
// Use 0 indexing for the array
u32 idx = seq.offset - 1;
if (seq.literal_length == 0) {
// "There is an exception though, when current sequence's
// literals length is 0. In this case, repeated offsets are
// shifted by one, so Repeated_Offset1 becomes Repeated_Offset2,
// Repeated_Offset2 becomes Repeated_Offset3, and
// Repeated_Offset3 becomes Repeated_Offset1 - 1_byte."
idx++;
}
if (idx == 0) {
offset = offset_hist[0];
} else {
// If idx == 3 then literal length was 0 and the offset was 3,
// as per the exception listed above
offset = idx < 3 ? offset_hist[idx] : offset_hist[0] - 1;
// If idx == 1 we don't need to modify offset_hist[2], since
// we're using the second-most recent code
if (idx > 1) {
offset_hist[2] = offset_hist[1];
}
offset_hist[1] = offset_hist[0];
offset_hist[0] = offset;
}
} else {
// When it's not a repeat offset:
// "if (Offset_Value > 3) offset = Offset_Value - 3;"
offset = seq.offset - 3;
// Shift back history
offset_hist[2] = offset_hist[1];
offset_hist[1] = offset_hist[0];
offset_hist[0] = offset;
}
return offset;
}
/******* END SEQUENCE EXECUTION ***********************************************/
/******* OUTPUT SIZE COUNTING *************************************************/