1 files changed, 175 insertions, 0 deletions
diff --git a/c/enc/literal_cost.c b/c/enc/literal_cost.c
new file mode 100644
index 0000000..91c691c
--- /dev/null
+++ b/c/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 <brotli/types.h>
+#include "./fast_log.h"
+#include "./port.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