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Diffstat (limited to 'modules/brotli/enc/literal_cost.c')
| -rw-r--r-- | modules/brotli/enc/literal_cost.c | 175 |
1 files changed, 175 insertions, 0 deletions
diff --git a/modules/brotli/enc/literal_cost.c b/modules/brotli/enc/literal_cost.c new file mode 100644 index 0000000000..c231100e34 --- /dev/null +++ b/modules/brotli/enc/literal_cost.c @@ -0,0 +1,175 @@ +/* Copyright 2013 Google Inc. All Rights Reserved. + + Distributed under MIT license. + See file LICENSE for detail or copy at https://opensource.org/licenses/MIT +*/ + +/* Literal cost model to allow backward reference replacement to be efficient. +*/ + +#include "./literal_cost.h" + +#include "../common/platform.h" +#include <brotli/types.h> +#include "./fast_log.h" +#include "./utf8_util.h" + +#if defined(__cplusplus) || defined(c_plusplus) +extern "C" { +#endif + +static size_t UTF8Position(size_t last, size_t c, size_t clamp) { + if (c < 128) { + return 0; /* Next one is the 'Byte 1' again. */ + } else if (c >= 192) { /* Next one is the 'Byte 2' of utf-8 encoding. */ + return BROTLI_MIN(size_t, 1, clamp); + } else { + /* Let's decide over the last byte if this ends the sequence. */ + if (last < 0xE0) { + return 0; /* Completed two or three byte coding. */ + } else { /* Next one is the 'Byte 3' of utf-8 encoding. */ + return BROTLI_MIN(size_t, 2, clamp); + } + } +} + +static size_t DecideMultiByteStatsLevel(size_t pos, size_t len, size_t mask, + const uint8_t* data) { + size_t counts[3] = { 0 }; + size_t max_utf8 = 1; /* should be 2, but 1 compresses better. */ + size_t last_c = 0; + size_t i; + for (i = 0; i < len; ++i) { + size_t c = data[(pos + i) & mask]; + ++counts[UTF8Position(last_c, c, 2)]; + last_c = c; + } + if (counts[2] < 500) { + max_utf8 = 1; + } + if (counts[1] + counts[2] < 25) { + max_utf8 = 0; + } + return max_utf8; +} + +static void EstimateBitCostsForLiteralsUTF8(size_t pos, size_t len, size_t mask, + const uint8_t* data, float* cost) { + /* max_utf8 is 0 (normal ASCII single byte modeling), + 1 (for 2-byte UTF-8 modeling), or 2 (for 3-byte UTF-8 modeling). */ + const size_t max_utf8 = DecideMultiByteStatsLevel(pos, len, mask, data); + size_t histogram[3][256] = { { 0 } }; + size_t window_half = 495; + size_t in_window = BROTLI_MIN(size_t, window_half, len); + size_t in_window_utf8[3] = { 0 }; + + size_t i; + { /* Bootstrap histograms. */ + size_t last_c = 0; + size_t utf8_pos = 0; + for (i = 0; i < in_window; ++i) { + size_t c = data[(pos + i) & mask]; + ++histogram[utf8_pos][c]; + ++in_window_utf8[utf8_pos]; + utf8_pos = UTF8Position(last_c, c, max_utf8); + last_c = c; + } + } + + /* Compute bit costs with sliding window. */ + for (i = 0; i < len; ++i) { + if (i >= window_half) { + /* Remove a byte in the past. */ + size_t c = + i < window_half + 1 ? 0 : data[(pos + i - window_half - 1) & mask]; + size_t last_c = + i < window_half + 2 ? 0 : data[(pos + i - window_half - 2) & mask]; + size_t utf8_pos2 = UTF8Position(last_c, c, max_utf8); + --histogram[utf8_pos2][data[(pos + i - window_half) & mask]]; + --in_window_utf8[utf8_pos2]; + } + if (i + window_half < len) { + /* Add a byte in the future. */ + size_t c = data[(pos + i + window_half - 1) & mask]; + size_t last_c = data[(pos + i + window_half - 2) & mask]; + size_t utf8_pos2 = UTF8Position(last_c, c, max_utf8); + ++histogram[utf8_pos2][data[(pos + i + window_half) & mask]]; + ++in_window_utf8[utf8_pos2]; + } + { + size_t c = i < 1 ? 0 : data[(pos + i - 1) & mask]; + size_t last_c = i < 2 ? 0 : data[(pos + i - 2) & mask]; + size_t utf8_pos = UTF8Position(last_c, c, max_utf8); + size_t masked_pos = (pos + i) & mask; + size_t histo = histogram[utf8_pos][data[masked_pos]]; + double lit_cost; + if (histo == 0) { + histo = 1; + } + lit_cost = FastLog2(in_window_utf8[utf8_pos]) - FastLog2(histo); + lit_cost += 0.02905; + if (lit_cost < 1.0) { + lit_cost *= 0.5; + lit_cost += 0.5; + } + /* Make the first bytes more expensive -- seems to help, not sure why. + Perhaps because the entropy source is changing its properties + rapidly in the beginning of the file, perhaps because the beginning + of the data is a statistical "anomaly". */ + if (i < 2000) { + lit_cost += 0.7 - ((double)(2000 - i) / 2000.0 * 0.35); + } + cost[i] = (float)lit_cost; + } + } +} + +void BrotliEstimateBitCostsForLiterals(size_t pos, size_t len, size_t mask, + const uint8_t* data, float* cost) { + if (BrotliIsMostlyUTF8(data, pos, mask, len, kMinUTF8Ratio)) { + EstimateBitCostsForLiteralsUTF8(pos, len, mask, data, cost); + return; + } else { + size_t histogram[256] = { 0 }; + size_t window_half = 2000; + size_t in_window = BROTLI_MIN(size_t, window_half, len); + + /* Bootstrap histogram. */ + size_t i; + for (i = 0; i < in_window; ++i) { + ++histogram[data[(pos + i) & mask]]; + } + + /* Compute bit costs with sliding window. */ + for (i = 0; i < len; ++i) { + size_t histo; + if (i >= window_half) { + /* Remove a byte in the past. */ + --histogram[data[(pos + i - window_half) & mask]]; + --in_window; + } + if (i + window_half < len) { + /* Add a byte in the future. */ + ++histogram[data[(pos + i + window_half) & mask]]; + ++in_window; + } + histo = histogram[data[(pos + i) & mask]]; + if (histo == 0) { + histo = 1; + } + { + double lit_cost = FastLog2(in_window) - FastLog2(histo); + lit_cost += 0.029; + if (lit_cost < 1.0) { + lit_cost *= 0.5; + lit_cost += 0.5; + } + cost[i] = (float)lit_cost; + } + } + } +} + +#if defined(__cplusplus) || defined(c_plusplus) +} /* extern "C" */ +#endif |