Update brotli to v1.0.0-snapshot.

third_party/brotli/enc/literal_cost.cc

-/* 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 <math.h>
-#include <algorithm>
-
-#include "./fast_log.h"
-#include "./types.h"
-#include "./utf8_util.h"
-
-namespace brotli {
-
-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 std::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 std::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 utf8_pos = 0;
-  for (size_t i = 0; i < len; ++i) {
-    size_t c = data[(pos + i) & mask];
-    utf8_pos = UTF8Position(last_c, c, 2);
-    ++counts[utf8_pos];
-    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 = std::min(window_half, len);
-  size_t in_window_utf8[3] = { 0 };
-
-  // Bootstrap histograms.
-  size_t last_c = 0;
-  size_t utf8_pos = 0;
-  for (size_t 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 (size_t 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]];
-    if (histo == 0) {
-      histo = 1;
-    }
-    double 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 - (static_cast<double>(2000 - i) / 2000.0 * 0.35);
-    }
-    cost[i] = static_cast<float>(lit_cost);
-  }
-}
-
-void EstimateBitCostsForLiterals(size_t pos, size_t len, size_t mask,
-                                 const uint8_t *data, float *cost) {
-  if (IsMostlyUTF8(data, pos, mask, len, kMinUTF8Ratio)) {
-    EstimateBitCostsForLiteralsUTF8(pos, len, mask, data, cost);
-    return;
-  }
-  size_t histogram[256] = { 0 };
-  size_t window_half = 2000;
-  size_t in_window = std::min(window_half, len);
-
-  // Bootstrap histogram.
-  for (size_t i = 0; i < in_window; ++i) {
-    ++histogram[data[(pos + i) & mask]];
-  }
-
-  // Compute bit costs with sliding window.
-  for (size_t i = 0; i < len; ++i) {
-    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;
-    }
-    size_t 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] = static_cast<float>(lit_cost);
-  }
-}
-
-
-}  // namespace brotli