compress/bzip2: fix bugs

(Once again, proving that a fuzz tester is the first thing that I
should try.)

R=bradfitzgo
CC=golang-dev
https://golang.org/cl/4227042

src/pkg/compress/bzip2/bzip2.go

@@ -141,6 +141,8 @@ func (bz2 *reader) read(buf []byte) (n int, err os.Error) {
 
 		if bz2.lastByte == int(b) {
 			bz2.byteRepeats++
+		} else {
+			bz2.byteRepeats = 0
 		}
 		bz2.lastByte = int(b)
 
@@ -229,6 +231,18 @@ func (bz2 *reader) readBlock() (err os.Error) {
 		treeIndexes[i] = uint8(mtfTreeDecoder.Decode(c))
 	}
 
+	// The list of symbols for the move-to-front transform is taken from
+	// the previously decoded symbol bitmap.
+	symbols := make([]byte, numSymbols)
+	nextSymbol := 0
+	for i := 0; i < 256; i++ {
+		if symbolPresent[i] {
+			symbols[nextSymbol] = byte(i)
+			nextSymbol++
+		}
+	}
+	mtf := newMTFDecoder(symbols)
+
 	numSymbols += 2 // to account for RUNA and RUNB symbols
 	huffmanTrees := make([]huffmanTree, numHuffmanTrees)
 
@@ -259,18 +273,6 @@ func (bz2 *reader) readBlock() (err os.Error) {
 		}
 	}
 
-	// The list of symbols for the move-to-front transform is taken from
-	// the previously decoded symbol bitmap.
-	symbols := make([]byte, numSymbols)
-	nextSymbol := 0
-	for i := 0; i < 256; i++ {
-		if symbolPresent[i] {
-			symbols[nextSymbol] = byte(i)
-			nextSymbol++
-		}
-	}
-	mtf := newMTFDecoder(symbols)
-
 	selectorIndex := 1 // the next tree index to use
 	currentHuffmanTree := huffmanTrees[treeIndexes[0]]
 	bufIndex := 0 // indexes bz2.buf, the output buffer.

src/pkg/compress/bzip2/move_to_front.go

@@ -13,7 +13,7 @@ package bzip2
 // as the symbol will be at the front of the list after the first access.
 type moveToFrontDecoder struct {
 	// Rather than actually keep the list in memory, the symbols are stored
-	// as a circular, double linked list which the symbol indexed by head
+	// as a circular, double linked list with the symbol indexed by head
 	// at the front of the list.
 	symbols []byte
 	next    []uint8
@@ -24,6 +24,10 @@ type moveToFrontDecoder struct {
 // newMTFDecoder creates a move-to-front decoder with an explicit initial list
 // of symbols.
 func newMTFDecoder(symbols []byte) *moveToFrontDecoder {
+	if len(symbols) > 256 {
+		panic("too many symbols")
+	}
+
 	m := &moveToFrontDecoder{
 		symbols: symbols,
 		next:    make([]uint8, len(symbols)),