compress/flate: improve decompression speed

src/compress/flate/gen_inflate.go

@@ -0,0 +1,277 @@
+// Copyright 2020 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 flate implements the DEFLATE compressed data format, described in
+// RFC 1951.  The gzip and zlib packages implement access to DEFLATE-based file
+// formats.
+
+//go:build generate
+// +build generate
+
+//go:generate go run $GOFILE
+//go:generate go fmt inflate_gen.go
+
+package main
+
+import (
+	"os"
+	"strings"
+)
+
+func main() {
+	f, err := os.Create("inflate_gen.go")
+	if err != nil {
+		panic(err)
+	}
+	defer f.Close()
+	types := []string{"*bytes.Buffer", "*bytes.Reader", "*bufio.Reader", "*strings.Reader", "Reader"}
+	names := []string{"BytesBuffer", "BytesReader", "BufioReader", "StringsReader", "GenericReader"}
+	imports := []string{"bytes", "bufio", "strings", "math/bits"}
+	f.WriteString(`// Code generated by go generate gen_inflate.go. DO NOT EDIT.
+
+package flate
+
+import (
+`)
+
+	for _, imp := range imports {
+		f.WriteString("\t\"" + imp + "\"\n")
+	}
+	f.WriteString(")\n\n")
+
+	template := `
+
+// $FUNCNAME$ decodes a single Huffman block from f.
+// f.r must be a $TYPE$.
+// hl and hd are the Huffman states for the lit/length values
+// and the distance values, respectively. If hd == nil, using the
+// fixed distance encoding associated with fixed Huffman blocks.
+func (f *decompressor) $FUNCNAME$() {
+	const (
+		stateInit = iota // Zero value must be stateInit
+		stateDict
+	)
+	fr := f.r.($TYPE$)
+
+	// Optimization. Compiler isn't smart enough to keep f.b, f.nb in registers,
+	// but is smart enough to keep local variables in registers, so use nb and b,
+	// dict reference and reassign b, nb back to f on return.
+	fnb, fb, dict := f.nb, f.b, &f.dict
+	switch f.stepState {
+	case stateInit:
+		goto readLiteral
+	case stateDict:
+		goto copyHistory
+	}
+
+readLiteral:
+	// Read literal and/or (length, distance) according to RFC section 3.2.3.
+	{
+		var v int
+		{
+			// Inlined v, err := f.huffSym(f.hl)
+			// Since a huffmanDecoder can be empty or be composed of a degenerate tree
+			// with single element, huffSym must error on these two edge cases. In both
+			// cases, the chunks slice will be 0 for the invalid sequence, leading it
+			// satisfy the n == 0 check below.
+			n := uint(f.hl.maxRead)
+			for {
+				for fnb < n {
+					c, err := fr.ReadByte()
+					if err != nil {
+						f.b, f.nb = fb, fnb
+						f.err = noEOF(err)
+						return
+					}
+					f.roffset++
+					fb |= uint32(c) << (fnb & 31)
+					fnb += 8
+				}
+				chunk := f.hl.chunks[fb&(huffmanNumChunks-1)]
+				n = uint(chunk & huffmanCountMask)
+				if n > huffmanChunkBits {
+					chunk = f.hl.links[chunk>>huffmanValueShift][(fb>>huffmanChunkBits)&f.hl.linkMask]
+					n = uint(chunk & huffmanCountMask)
+				}
+				if n <= fnb {
+					if n == 0 {
+						f.b, f.nb = fb, fnb
+						f.err = CorruptInputError(f.roffset)
+						return
+					}
+					fb = fb >> (n & 31)
+					fnb = fnb - n
+					v = int(chunk >> huffmanValueShift)
+					break
+				}
+			}
+		}
+		var length int
+		switch {
+		case v < 256:
+			dict.writeByte(byte(v))
+			if dict.availWrite() == 0 {
+				f.toRead = dict.readFlush()
+				f.step = (*decompressor).$FUNCNAME$
+				f.stepState = stateInit
+				f.b, f.nb = fb, fnb
+				return
+			}
+			goto readLiteral
+		case v == 256:
+			f.b, f.nb = fb, fnb
+			f.finishBlock()
+			return
+		// otherwise, reference to older data
+		case v < 265:
+			// No extra bits 
+			length = v - (257 - 3)
+		case v < maxNumLit:
+			val := decCodeToLen[(v - 257)]
+			length = int(val.length) + 3
+			n := uint(val.extra)
+			for fnb < n {
+				c, err := fr.ReadByte()
+				if err != nil {
+					f.b, f.nb = fb, fnb
+					f.err = err
+					return
+				}
+				f.roffset++
+				fb |= uint32(c) << (fnb&31)
+				fnb += 8	
+			}
+			length += int(fb & bitMask32[n])
+			fb >>= n & 31
+			fnb -= n
+		default:
+			f.err = CorruptInputError(f.roffset)
+			f.b, f.nb = fb, fnb
+			return
+		}
+		var dist uint32
+		if f.hd == nil {
+			for fnb < 5 {
+				c, err := fr.ReadByte()
+				if err != nil {
+					f.b, f.nb = fb, fnb
+					f.err = err
+					return
+				}
+				f.roffset++
+				fb |= uint32(c) << (fnb&31)
+				fnb += 8
+			}
+			dist = uint32(bits.Reverse8(uint8(fb & 0x1F << 3)))
+			fb >>= 5
+			fnb -= 5
+		} else {
+			// Since a huffmanDecoder can be empty or be composed of a degenerate tree
+			// with single element, huffSym must error on these two edge cases. In both
+			// cases, the chunks slice will be 0 for the invalid sequence, leading it
+			// satisfy the n == 0 check below.
+			n := uint(f.hd.maxRead)
+			for {
+				for fnb < n {
+					c, err := fr.ReadByte()
+					if err != nil {
+						f.b, f.nb = fb, fnb
+						f.err = noEOF(err)
+						return
+					}
+					f.roffset++
+					fb |= uint32(c) << (fnb & 31)
+					fnb += 8
+				}
+				chunk := f.hd.chunks[fb&(huffmanNumChunks-1)]
+				n = uint(chunk & huffmanCountMask)
+				if n > huffmanChunkBits {
+					chunk = f.hd.links[chunk>>huffmanValueShift][(fb>>huffmanChunkBits)&f.hd.linkMask]
+					n = uint(chunk & huffmanCountMask)
+				}
+				if n <= fnb {
+					if n == 0 {
+						f.b, f.nb = fb, fnb
+						f.err = CorruptInputError(f.roffset)
+						return
+					}
+					fb = fb >> (n & 31)
+					fnb = fnb - n
+					dist = uint32(chunk >> huffmanValueShift)
+					break
+				}
+			}
+		}
+		switch {
+		case dist < 4:
+			dist++
+		case dist < maxNumDist:
+			nb := uint(dist-2) >> 1
+			// have 1 bit in bottom of dist, need nb more.
+			extra := (dist & 1) << (nb & 31)
+			for fnb < nb {
+				c, err := fr.ReadByte()
+				if err != nil {
+					f.b, f.nb = fb, fnb
+					f.err = err
+					return
+				}
+				f.roffset++
+				fb |= uint32(c) << (fnb&31)
+				fnb += 8
+			}
+			extra |= fb & bitMask32[nb]
+			fb >>= nb & 31
+			fnb -= nb
+			dist = 1<<((nb+1)&31) + 1 + extra
+		default:
+			f.b, f.nb = fb, fnb
+			f.err = CorruptInputError(f.roffset)
+			return
+		}
+		// No check on length; encoding can be prescient.
+		if dist > uint32(dict.histSize()) {
+			f.b, f.nb = fb, fnb
+			f.err = CorruptInputError(f.roffset)
+			return
+		}
+		f.copyLen, f.copyDist = length, int(dist)
+		goto copyHistory
+	}
+copyHistory:
+	// Perform a backwards copy according to RFC section 3.2.3.
+	{
+		cnt := dict.tryWriteCopy(f.copyDist, f.copyLen)
+		if cnt == 0 {
+			cnt = dict.writeCopy(f.copyDist, f.copyLen)
+		}
+		f.copyLen -= cnt
+		if dict.availWrite() == 0 || f.copyLen > 0 {
+			f.toRead = dict.readFlush()
+			f.step = (*decompressor).$FUNCNAME$ // We need to continue this work
+			f.stepState = stateDict
+			f.b, f.nb = fb, fnb
+			return
+		}
+		goto readLiteral
+	}
+	// Not reached
+}
+
+`
+	for i, t := range types {
+		s := strings.Replace(template, "$FUNCNAME$", "huffman"+names[i], -1)
+		s = strings.Replace(s, "$TYPE$", t, -1)
+		f.WriteString(s)
+	}
+	f.WriteString("func (f *decompressor) huffmanBlockDecoder() func() {\n")
+	f.WriteString("\tswitch f.r.(type) {\n")
+	for i, t := range types {
+		f.WriteString("\t\tcase " + t + ":\n")
+		f.WriteString("\t\t\treturn f.huffman" + names[i] + "\n")
+	}
+	f.WriteString("\t\tdefault:\n")
+	f.WriteString("\t\t\treturn f.huffmanGenericReader")
+	f.WriteString("\t}\n}\n")
+}