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lib/deflate_compress: select min_size_to_compress based on level #92

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Oct 19, 2020
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lib/deflate_compress: select min_size_to_compress based on level #92

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25 changes: 17 additions & 8 deletions lib/deflate_compress.c
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Original file line number Diff line number Diff line change
Expand Up @@ -366,6 +366,9 @@ struct libdeflate_compressor {
/* The compression level with which this compressor was created. */
unsigned compression_level;

/* Anything smaller than this we won't bother trying to compress. */
unsigned min_size_to_compress;

/* Temporary space for Huffman code output */
u32 precode_freqs[DEFLATE_NUM_PRECODE_SYMS];
u8 precode_lens[DEFLATE_NUM_PRECODE_SYMS];
Expand Down Expand Up @@ -2688,6 +2691,9 @@ libdeflate_alloc_compressor(int compression_level)
struct libdeflate_compressor *c;
size_t size = offsetof(struct libdeflate_compressor, p);

if (compression_level < 0 || compression_level > 12)
return NULL;

#if SUPPORT_NEAR_OPTIMAL_PARSING
if (compression_level >= 8)
size += sizeof(c->p.n);
Expand All @@ -2702,6 +2708,14 @@ libdeflate_alloc_compressor(int compression_level)
if (!c)
return NULL;

c->compression_level = compression_level;

/*
* The higher the compression level, the more we should bother trying to
* compress very small inputs.
*/
c->min_size_to_compress = 56 - (compression_level * 4);

switch (compression_level) {
case 0:
c->impl = deflate_compress_none;
Expand Down Expand Up @@ -2766,7 +2780,7 @@ libdeflate_alloc_compressor(int compression_level)
c->nice_match_length = 80;
c->p.n.num_optim_passes = 3;
break;
case 12:
default:
c->impl = deflate_compress_near_optimal;
c->max_search_depth = 100;
c->nice_match_length = 133;
Expand All @@ -2778,19 +2792,14 @@ libdeflate_alloc_compressor(int compression_level)
c->max_search_depth = 150;
c->nice_match_length = 200;
break;
case 9:
default:
c->impl = deflate_compress_lazy;
c->max_search_depth = 200;
c->nice_match_length = DEFLATE_MAX_MATCH_LEN;
break;
#endif
default:
libdeflate_aligned_free(c);
return NULL;
}

c->compression_level = compression_level;

deflate_init_offset_slot_fast(c);
deflate_init_static_codes(c);

Expand All @@ -2806,7 +2815,7 @@ libdeflate_deflate_compress(struct libdeflate_compressor *c,
return 0;

/* For extremely small inputs just use a single uncompressed block. */
if (unlikely(in_nbytes < 16)) {
if (unlikely(in_nbytes < c->min_size_to_compress)) {
struct deflate_output_bitstream os;
deflate_init_output(&os, out, out_nbytes_avail);
if (in_nbytes == 0)
Expand Down