Added header parser facility and tests to make sure it works properly

c/go/brotli/header.go

@@ -0,0 +1,63 @@
+package brotli
+
+import (
+	"encoding/hex"
+	"errors"
+)
+
+func BrotliParseHeader(data []byte) (returnedVersion byte, returnedSize uint64, err error) {
+	if len(data) < 4 {
+		return 0, 0, errors.New("Insufficient data provided in string: " + hex.EncodeToString(data))
+	}
+	hdr := uint64(data[0]) + uint64(data[1])*256 + uint64(data[2])*65536 + uint64(data[3])*65536*256
+	var bits uint
+	if hdr&1 == 0 {
+		bits = 1
+	} else if (hdr & 15) != 1 {
+		bits = 4
+	} else if (hdr & 127) != 0x11 {
+		bits = 7
+	} else {
+		bits = 14
+	}
+	hdr >>= bits
+	if (hdr & 1) != 0 {
+		return 0, 0, errors.New("Header incorrectly marked as last block")
+	}
+	hdr >>= 1
+	bits += 1
+	if (hdr & 3) != 3 {
+		return 0, 0, errors.New("Header incorrectly contains file data")
+	}
+	hdr >>= 2
+	bits += 2
+	if (hdr & 1) != 0 {
+		return 0, 0, errors.New("Reserved 0 metadata bit is set to nonzero value")
+	}
+	hdr >>= 1
+	bits += 1
+	if (hdr & 3) != 1 {
+		return 0, 0, errors.New("Header should only need 1 byte of length data")
+	}
+	hdr >>= 2
+	bits += 2
+	num_raw_header_bytes := 1 + (hdr & 0xff)
+	bits += 8
+	byte_offset := ((uint64(bits) + 7) / 8)
+	if uint64(len(data)) < byte_offset+num_raw_header_bytes {
+		return 0, 0, errors.New("Insufficient data to accomodate number of raw header bytes " + hex.EncodeToString(data))
+	}
+	if data[byte_offset] != 0xe1 || data[byte_offset+1] != 0x97 || (data[byte_offset+2]&0xf0) != 0x80 || (data[byte_offset+2]&0xf) > 2 {
+		return 0, 0, errors.New("Header does not start with E1978X: " + hex.EncodeToString(data[byte_offset:byte_offset+3]))
+	}
+	version := data[byte_offset+3]
+	size_le := data[byte_offset+4 : byte_offset+num_raw_header_bytes]
+	total_size := uint64(0)
+	for index := uint(0); index < uint(len(size_le)); index += 1 {
+		total_size += uint64(size_le[index]&0x7f) << (7 * index)
+		if 0 == (size_le[index] & 0x80) {
+			break
+		}
+	}
+	return version, total_size, nil
+}

c/go/interface_test.go

@@ -46,7 +46,18 @@ func TestCompressWriter(*testing.T) {
 	if len(outBuffer.Bytes()) > 800000 {
 		panic(fmt.Sprintf("Buffer too large: %d", len(outBuffer.Bytes())))
 	}
+	version, size, err := brotli.BrotliParseHeader(outBuffer.Bytes())
+	if err != nil {
+		panic(err)
+	}
+	if version != byte(brotli.BrotliEncoderVersion()&0xff) {
+		panic(version)
+	}
+	if size != uint64(len(data)) {
+		panic(size)
+	}
 }
+
 func TestCompressRoundtrip(*testing.T) {
 	tmp := testData()
 	data := tmp[:len(tmp)-17]
@@ -211,7 +222,6 @@ func TestCompressReader(*testing.T) {
 	var options = brotli.CompressionOptions{
 		NumThreads: 1,
 		Quality:    4,
-		Catable:    true,
 		Appendable: true,
 		Magic:      true,
 	}
@@ -229,6 +239,16 @@ func TestCompressReader(*testing.T) {
 	if len(outBuffer.Bytes()) > 800000 {
 		panic(fmt.Sprintf("Buffer too large: %d", len(outBuffer.Bytes())))
 	}
+	version, size, err := brotli.BrotliParseHeader(outBuffer.Bytes())
+	if err != nil {
+		panic(err)
+	}
+	if version != byte(brotli.BrotliEncoderVersion()&0xff) {
+		panic(version)
+	}
+	if size != uint64(len(data)) {
+		panic(size)
+	}
 }
 func TestCompressReaderClose(*testing.T) {
 	data := testData()
@@ -259,6 +279,16 @@ func TestCompressReaderClose(*testing.T) {
 	if err != nil {
 		panic(err)
 	}
+	version, size, err := brotli.BrotliParseHeader(outBuffer.Bytes())
+	if err != nil {
+		panic(err)
+	}
+	if version != byte(brotli.BrotliEncoderVersion()&0xff) {
+		panic(version)
+	}
+	if size != uint64(len(data)) {
+		panic(size)
+	}
 }
 
 func TestCompressReaderEarlyClose(*testing.T) {