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67 lines
1.8 KiB
67 lines
1.8 KiB
// Copyright 2015, Joe Tsai. 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.md file.
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package prefix
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import (
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"sort"
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"github.com/dsnet/compress/internal"
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)
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type Encoder struct {
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chunks []uint32 // First-level lookup map
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chunkMask uint32 // Mask the length of the chunks table
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NumSyms uint32 // Number of symbols
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}
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// Init initializes Encoder according to the codes provided.
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func (pe *Encoder) Init(codes PrefixCodes) {
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// Handle special case trees.
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if len(codes) <= 1 {
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switch {
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case len(codes) == 0: // Empty tree (should error if used later)
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*pe = Encoder{chunks: pe.chunks[:0], NumSyms: 0}
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case len(codes) == 1 && codes[0].Len == 0: // Single code tree (bit-length of zero)
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pe.chunks = append(pe.chunks[:0], codes[0].Val<<countBits|0)
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*pe = Encoder{chunks: pe.chunks[:1], NumSyms: 1}
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default:
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panic("invalid codes")
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}
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return
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}
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if internal.Debug && !sort.IsSorted(prefixCodesBySymbol(codes)) {
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panic("input codes is not sorted")
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}
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if internal.Debug && !(codes.checkLengths() && codes.checkPrefixes()) {
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panic("detected incomplete or overlapping codes")
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}
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// Enough chunks to contain all the symbols.
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numChunks := 1
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for n := len(codes) - 1; n > 0; n >>= 1 {
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numChunks <<= 1
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}
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pe.NumSyms = uint32(len(codes))
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retry:
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// Allocate and reset chunks.
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pe.chunks = allocUint32s(pe.chunks, numChunks)
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pe.chunkMask = uint32(numChunks - 1)
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for i := range pe.chunks {
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pe.chunks[i] = 0 // Logic below relies on zero value as uninitialized
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}
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// Insert each symbol, checking that there are no conflicts.
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for _, c := range codes {
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if pe.chunks[c.Sym&pe.chunkMask] > 0 {
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// Collision found our "hash" table, so grow and try again.
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numChunks <<= 1
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goto retry
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}
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pe.chunks[c.Sym&pe.chunkMask] = c.Val<<countBits | c.Len
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}
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}
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