Added brotli enc/ and tools/ directories.

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