1 files changed, 53 insertions, 0 deletions
diff --git a/src/compress/bzip2/move_to_front.go b/src/compress/bzip2/move_to_front.go
new file mode 100644
index 000000000..526dfb34c
--- /dev/null
+++ b/src/compress/bzip2/move_to_front.go
@@ -0,0 +1,53 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bzip2
+
+// moveToFrontDecoder implements a move-to-front list. Such a list is an
+// efficient way to transform a string with repeating elements into one with
+// many small valued numbers, which is suitable for entropy encoding. It works
+// by starting with an initial list of symbols and references symbols by their
+// index into that list. When a symbol is referenced, it's moved to the front
+// of the list. Thus, a repeated symbol ends up being encoded with many zeros,
+// as the symbol will be at the front of the list after the first access.
+type moveToFrontDecoder []byte
+
+// newMTFDecoder creates a move-to-front decoder with an explicit initial list
+// of symbols.
+func newMTFDecoder(symbols []byte) moveToFrontDecoder {
+	if len(symbols) > 256 {
+		panic("too many symbols")
+	}
+	return moveToFrontDecoder(symbols)
+}
+
+// newMTFDecoderWithRange creates a move-to-front decoder with an initial
+// symbol list of 0...n-1.
+func newMTFDecoderWithRange(n int) moveToFrontDecoder {
+	if n > 256 {
+		panic("newMTFDecoderWithRange: cannot have > 256 symbols")
+	}
+
+	m := make([]byte, n)
+	for i := 0; i < n; i++ {
+		m[i] = byte(i)
+	}
+	return moveToFrontDecoder(m)
+}
+
+func (m moveToFrontDecoder) Decode(n int) (b byte) {
+	// Implement move-to-front with a simple copy. This approach
+	// beats more sophisticated approaches in benchmarking, probably
+	// because it has high locality of reference inside of a
+	// single cache line (most move-to-front operations have n < 64).
+	b = m[n]
+	copy(m[1:], m[:n])
+	m[0] = b
+	return
+}
+
+// First returns the symbol at the front of the list.
+func (m moveToFrontDecoder) First() byte {
+	return m[0]
+}