f8d9fa9e80
This upgrades all of libgo other than the runtime package to the Go 1.4 release. In Go 1.4 much of the runtime was rewritten into Go. Merging that code will take more time and will not change the API, so I'm putting it off for now. There are a few runtime changes anyhow, to accomodate other packages that rely on minor modifications to the runtime support. The compiler changes slightly to add a one-bit flag to each type descriptor kind that is stored directly in an interface, which for gccgo is currently only pointer types. Another one-bit flag (gcprog) is reserved because it is used by the gc compiler, but gccgo does not currently use it. There is another error check in the compiler since I ran across it during testing. gotools/: * Makefile.am (go_cmd_go_files): Sort entries. Add generate.go. * Makefile.in: Rebuild. From-SVN: r219627
54 lines
1.8 KiB
Go
54 lines
1.8 KiB
Go
// Copyright 2011 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package bzip2
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// moveToFrontDecoder implements a move-to-front list. Such a list is an
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// efficient way to transform a string with repeating elements into one with
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// many small valued numbers, which is suitable for entropy encoding. It works
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// by starting with an initial list of symbols and references symbols by their
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// index into that list. When a symbol is referenced, it's moved to the front
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// of the list. Thus, a repeated symbol ends up being encoded with many zeros,
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// as the symbol will be at the front of the list after the first access.
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type moveToFrontDecoder []byte
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// newMTFDecoder creates a move-to-front decoder with an explicit initial list
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// of symbols.
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func newMTFDecoder(symbols []byte) moveToFrontDecoder {
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if len(symbols) > 256 {
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panic("too many symbols")
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}
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return moveToFrontDecoder(symbols)
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}
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// newMTFDecoderWithRange creates a move-to-front decoder with an initial
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// symbol list of 0...n-1.
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func newMTFDecoderWithRange(n int) moveToFrontDecoder {
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if n > 256 {
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panic("newMTFDecoderWithRange: cannot have > 256 symbols")
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}
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m := make([]byte, n)
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for i := 0; i < n; i++ {
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m[i] = byte(i)
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}
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return moveToFrontDecoder(m)
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}
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func (m moveToFrontDecoder) Decode(n int) (b byte) {
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// Implement move-to-front with a simple copy. This approach
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// beats more sophisticated approaches in benchmarking, probably
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// because it has high locality of reference inside of a
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// single cache line (most move-to-front operations have n < 64).
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b = m[n]
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copy(m[1:], m[:n])
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m[0] = b
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return
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}
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// First returns the symbol at the front of the list.
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func (m moveToFrontDecoder) First() byte {
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return m[0]
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}
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