func (zr *Reader) decodePrefix(numSyms int) (syms []uint16) {
numSyms += 2 // Remove 0 symbol, add RUNA, RUNB, and EOF symbols
if numSyms < 3 {
panic(ErrCorrupt) // Not possible to encode EOF marker
}
// Read information about the trees and tree selectors.
var mtf internal.MoveToFront
numTrees := int(zr.rd.ReadBitsBE64(3))
if numTrees < minNumTrees || numTrees > maxNumTrees {
panic(ErrCorrupt)
}
numSels := int(zr.rd.ReadBitsBE64(15))
treeSels := make([]uint8, numSels)
for i := range treeSels {
sym, ok := zr.rd.TryReadSymbol(&decSel)
if !ok {
sym = zr.rd.ReadSymbol(&decSel)
}
if int(sym) >= numTrees {
panic(ErrCorrupt)
}
treeSels[i] = uint8(sym)
}
mtf.Decode(treeSels)
// Initialize prefix codes.
var codes2D [maxNumTrees][maxNumSyms]prefix.PrefixCode
var codes1D [maxNumTrees]prefix.PrefixCodes
var trees1D [maxNumTrees]prefix.Decoder
for i := range codes2D[:numTrees] {
pc := codes2D[i][:numSyms]
for j := range pc {
pc[j].Sym = uint32(j)
}
codes1D[i] = pc
}
zr.rd.ReadPrefixCodes(codes1D[:numTrees], trees1D[:numTrees])
// Read prefix encoded symbols of compressed data.
var tree *prefix.Decoder
var blkLen, selIdx int
for {
if blkLen == 0 {
blkLen = numBlockSyms
if selIdx >= len(treeSels) {
panic(ErrCorrupt)
}
tree = &trees1D[treeSels[selIdx]]
selIdx++
}
blkLen--
sym, ok := zr.rd.TryReadSymbol(tree)
if !ok {
sym = zr.rd.ReadSymbol(tree)
}
if int(sym) == numSyms-1 {
break // EOF marker
}
if int(sym) >= numSyms {
panic(ErrCorrupt) // Invalid symbol used
}
if len(syms) >= zr.level*blockSize {
panic(ErrCorrupt) // Block is too large
}
syms = append(syms, uint16(sym))
}
return syms
}
