Add brotli compressor

This commit is for the encoder for brotli compression format.
Brotli is a generic byte-level compression algorithm.

brotli/enc/literal_cost.cc

@@ -0,0 +1,60 @@
+// Copyright 2013 Google Inc. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// Literal cost model to allow backward reference replacement to be efficient.
+
+#include "./literal_cost.h"
+
+#include <math.h>
+#include <stdint.h>
+#include <algorithm>
+
+namespace brotli {
+
+void EstimateBitCostsForLiterals(size_t len, const uint8_t *data, float *cost) {
+  int histogram[256] = { 0 };
+  int window_half = 2000;
+  int in_window = std::min(static_cast<size_t>(window_half), len);
+
+  // Bootstrap histogram.
+  for (int i = 0; i < in_window; ++i) {
+    ++histogram[data[i]];
+  }
+
+  // Compute bit costs with sliding window.
+  for (int i = 0; i < len; ++i) {
+    if (i - window_half >= 0) {
+      // Remove a byte in the past.
+      --histogram[data[i - window_half]];
+      --in_window;
+    }
+    if (i + window_half < len) {
+      // Add a byte in the future.
+      ++histogram[data[i + window_half]];
+      ++in_window;
+    }
+    int histo = histogram[data[i]];
+    if (histo == 0) {
+      histo = 1;
+    }
+    cost[i] = log2(static_cast<double>(in_window) / histo);
+    cost[i] += 0.03;
+    if (cost[i] < 1.0) {
+      cost[i] *= 0.5;
+      cost[i] += 0.5;
+    }
+  }
+}
+
+}  // namespace brotli