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gitea/vendor/github.com/dsnet/compress/internal/prefix/encoder.go

66 lines
1.8 KiB

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