Rewritten block size constants with a formula.

Removed block size related maps.

lz4.go

@@ -10,6 +10,8 @@
 //
 package lz4
 
+import "math/bits"
+
 const (
 	// Extension is the LZ4 frame file name extension
 	Extension = ".lz4"
@@ -39,16 +41,23 @@ const (
 
 // map the block max size id with its value in bytes: 64Kb, 256Kb, 1Mb and 4Mb.
 const (
-	blockSize64K  = 64 << 10
-	blockSize256K = 256 << 10
-	blockSize1M   = 1 << 20
-	blockSize4M   = 4 << 20
+	blockSize64K = 1 << (16 + 2*iota)
+	blockSize256K
+	blockSize1M
+	blockSize4M
 )
 
-var (
-	bsMapID    = map[byte]int{4: blockSize64K, 5: blockSize256K, 6: blockSize1M, 7: blockSize4M}
-	bsMapValue = map[int]byte{blockSize64K: 4, blockSize256K: 5, blockSize1M: 6, blockSize4M: 7}
-)
+func blockSizeIndexToValue(i byte) int {
+	return 1 << (16 + 2*uint(i))
+}
+func isValidBlockSize(size int) bool {
+	const blockSizeMask = blockSize64K | blockSize256K | blockSize1M | blockSize4M
+
+	return size&blockSizeMask > 0 && bits.OnesCount(uint(size)) == 1
+}
+func blockSizeValueToIndex(size int) byte {
+	return 4 + byte(bits.TrailingZeros(uint(size)>>16)/2)
+}
 
 // Header describes the various flags that can be set on a Writer or obtained from a Reader.
 // The default values match those of the LZ4 frame format definition

reader.go

@@ -88,10 +88,10 @@ func (z *Reader) readHeader(first bool) error {
 	z.NoChecksum = b>>2&1 == 0
 
 	bmsID := buf[1] >> 4 & 0x7
-	bSize, ok := bsMapID[bmsID]
-	if !ok {
+	if bmsID < 4 || bmsID > 7 {
 		return fmt.Errorf("lz4: invalid block max size ID: %d", bmsID)
 	}
+	bSize := blockSizeIndexToValue(bmsID - 4)
 	z.BlockMaxSize = bSize
 
 	// Allocate the compressed/uncompressed buffers.

writer.go

@@ -36,14 +36,14 @@ func NewWriter(dst io.Writer) *Writer {
 func (z *Writer) writeHeader() error {
 	// Default to 4Mb if BlockMaxSize is not set.
 	if z.Header.BlockMaxSize == 0 {
-		z.Header.BlockMaxSize = bsMapID[7]
+		z.Header.BlockMaxSize = blockSize4M
 	}
 	// The only option that needs to be validated.
 	bSize := z.Header.BlockMaxSize
-	bSizeID, ok := bsMapValue[bSize]
-	if !ok {
+	if !isValidBlockSize(bSize) {
 		return fmt.Errorf("lz4: invalid block max size: %d", bSize)
 	}
+	bSizeID := blockSizeValueToIndex(bSize)
 	// Allocate the compressed/uncompressed buffers.
 	// The compressed buffer cannot exceed the uncompressed one.
 	if cap(z.zdata) < bSize {