inflate: Generate code for byte readers

flate/gen_inflate.go

@@ -0,0 +1,274 @@
+// +build generate
+
+//go:generate go run $GOFILE && gofmt -w 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"}
+	names := []string{"BytesBuffer", "BytesReader", "BufioReader", "StringsReader"}
+	imports := []string{"bytes", "bufio", "io", "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 := `
+
+// Decode a single Huffman block from f.
+// 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.($TYPE$)
+	moreBits := func() error {
+		c, err := fr.ReadByte()
+		if err != nil {
+			return noEOF(err)
+		}
+		f.roffset++
+		f.b |= uint32(c) << f.nb
+		f.nb += 8
+		return nil
+	}
+
+	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)
+			// 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,
+			// inline call to moreBits and reassign b,nb back to f on return.
+			nb, b := f.nb, f.b
+			for {
+				for nb < n {
+					c, err := fr.ReadByte()
+					if err != nil {
+						f.b = b
+						f.nb = nb
+						f.err = noEOF(err)
+						return
+					}
+					f.roffset++
+					b |= uint32(c) << (nb & 31)
+					nb += 8
+				}
+				chunk := f.hl.chunks[b&(huffmanNumChunks-1)]
+				n = uint(chunk & huffmanCountMask)
+				if n > huffmanChunkBits {
+					chunk = f.hl.links[chunk>>huffmanValueShift][(b>>huffmanChunkBits)&f.hl.linkMask]
+					n = uint(chunk & huffmanCountMask)
+				}
+				if n <= nb {
+					if n == 0 {
+						f.b = b
+						f.nb = nb
+						if debugDecode {
+							fmt.Println("huffsym: n==0")
+						}
+						f.err = CorruptInputError(f.roffset)
+						return
+					}
+					f.b = b >> (n & 31)
+					f.nb = nb - n
+					v = int(chunk >> huffmanValueShift)
+					break
+				}
+			}
+		}
+
+		var n uint // number of bits extra
+		var length int
+		var err error
+		switch {
+		case v < 256:
+			f.dict.writeByte(byte(v))
+			if f.dict.availWrite() == 0 {
+				f.toRead = f.dict.readFlush()
+				f.step = (*decompressor).$FUNCNAME$
+				f.stepState = stateInit
+				return
+			}
+			goto readLiteral
+		case v == 256:
+			f.finishBlock()
+			return
+		// otherwise, reference to older data
+		case v < 265:
+			length = v - (257 - 3)
+			n = 0
+		case v < 269:
+			length = v*2 - (265*2 - 11)
+			n = 1
+		case v < 273:
+			length = v*4 - (269*4 - 19)
+			n = 2
+		case v < 277:
+			length = v*8 - (273*8 - 35)
+			n = 3
+		case v < 281:
+			length = v*16 - (277*16 - 67)
+			n = 4
+		case v < 285:
+			length = v*32 - (281*32 - 131)
+			n = 5
+		case v < maxNumLit:
+			length = 258
+			n = 0
+		default:
+			if debugDecode {
+				fmt.Println(v, ">= maxNumLit")
+			}
+			f.err = CorruptInputError(f.roffset)
+			return
+		}
+		if n > 0 {
+			for f.nb < n {
+				if err = moreBits(); err != nil {
+					if debugDecode {
+						fmt.Println("morebits n>0:", err)
+					}
+					f.err = err
+					return
+				}
+			}
+			length += int(f.b & uint32(1<<n-1))
+			f.b >>= n
+			f.nb -= n
+		}
+
+		var dist int
+		if f.hd == nil {
+			for f.nb < 5 {
+				if err = moreBits(); err != nil {
+					if debugDecode {
+						fmt.Println("morebits f.nb<5:", err)
+					}
+					f.err = err
+					return
+				}
+			}
+			dist = int(bits.Reverse8(uint8(f.b & 0x1F << 3)))
+			f.b >>= 5
+			f.nb -= 5
+		} else {
+			if dist, err = f.huffSym(f.hd); err != nil {
+				if debugDecode {
+					fmt.Println("huffsym:", err)
+				}
+				f.err = err
+				return
+			}
+		}
+
+		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
+			for f.nb < nb {
+				if err = moreBits(); err != nil {
+					if debugDecode {
+						fmt.Println("morebits f.nb<nb:", err)
+					}
+					f.err = err
+					return
+				}
+			}
+			extra |= int(f.b & uint32(1<<nb-1))
+			f.b >>= nb
+			f.nb -= nb
+			dist = 1<<(nb+1) + 1 + extra
+		default:
+			if debugDecode {
+				fmt.Println("dist too big:", dist, maxNumDist)
+			}
+			f.err = CorruptInputError(f.roffset)
+			return
+		}
+
+		// No check on length; encoding can be prescient.
+		if dist > f.dict.histSize() {
+			if debugDecode {
+				fmt.Println("dist > f.dict.histSize():", dist, f.dict.histSize())
+			}
+			f.err = CorruptInputError(f.roffset)
+			return
+		}
+
+		f.copyLen, f.copyDist = length, dist
+		goto copyHistory
+	}
+
+copyHistory:
+	// Perform a backwards copy according to RFC section 3.2.3.
+	{
+		cnt := f.dict.tryWriteCopy(f.copyDist, f.copyLen)
+		if cnt == 0 {
+			cnt = f.dict.writeCopy(f.copyDist, f.copyLen)
+		}
+		f.copyLen -= cnt
+
+		if f.dict.availWrite() == 0 || f.copyLen > 0 {
+			f.toRead = f.dict.readFlush()
+			f.step = (*decompressor).$FUNCNAME$ // We need to continue this work
+			f.stepState = stateDict
+			return
+		}
+		goto readLiteral
+	}
+}
+
+`
+	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.huffmanBlockGeneric")
+	f.WriteString("\t}\n}\n")
+}

flate/inflate.go

@@ -342,15 +342,15 @@ func (f *decompressor) nextBlock() {
 		// compressed, fixed Huffman tables
 		f.hl = &fixedHuffmanDecoder
 		f.hd = nil
-		f.huffmanBlock()
+		f.huffmanBlockDecoder()()
 	case 2:
 		// compressed, dynamic Huffman tables
 		if f.err = f.readHuffman(); f.err != nil {
 			break
 		}
 		f.hl = &f.h1
 		f.hd = &f.h2
-		f.huffmanBlock()
+		f.huffmanBlockDecoder()()
 	default:
 		// 3 is reserved.
 		if debugDecode {
@@ -564,7 +564,7 @@ func (f *decompressor) readHuffman() error {
 // 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) huffmanBlock() {
+func (f *decompressor) huffmanBlockGeneric() {
 	const (
 		stateInit = iota // Zero value must be stateInit
 		stateDict
@@ -637,7 +637,7 @@ readLiteral:
 			f.dict.writeByte(byte(v))
 			if f.dict.availWrite() == 0 {
 				f.toRead = f.dict.readFlush()
-				f.step = (*decompressor).huffmanBlock
+				f.step = (*decompressor).huffmanBlockGeneric
 				f.stepState = stateInit
 				return
 			}
@@ -765,7 +765,7 @@ copyHistory:
 
 		if f.dict.availWrite() == 0 || f.copyLen > 0 {
 			f.toRead = f.dict.readFlush()
-			f.step = (*decompressor).huffmanBlock // We need to continue this work
+			f.step = (*decompressor).huffmanBlockGeneric // We need to continue this work
 			f.stepState = stateDict
 			return
 		}