Probably should name the custom packages specially

1 files changed, 69 insertions, 0 deletions

diff --git a/internal/zlibx/decompress.go b/internal/zlibx/decompress.go
new file mode 100644
index 00000000..34e62c6f
--- /dev/null
+++ b/internal/zlibx/decompress.go
@@ -0,0 +1,69 @@
+package zlibx
+
+import (
+	"encoding/binary"
+	"io"
+
+	"git.sr.ht/~runxiyu/furgit/internal/adler32"
+	"git.sr.ht/~runxiyu/furgit/internal/bufpool"
+	"git.sr.ht/~runxiyu/furgit/internal/flatex"
+)
+
+// Decompress inflates the provided zlib wrapped stream and returns the
+// uncompressed data inside a pooled bufpool.Buffer.
+func Decompress(src []byte) (bufpool.Buffer, error) {
+	return DecompressDict(src, nil)
+}
+
+// DecompressDict is like Decompress but accepts a preset dictionary. The
+// dictionary must match the checksum embedded in the stream if the dictionary
+// flag is present.
+func DecompressDict(src []byte, dict []byte) (bufpool.Buffer, error) {
+	if len(src) < 6 {
+		return bufpool.Buffer{}, io.ErrUnexpectedEOF
+	}
+
+	cmf := src[0]
+	flg := src[1]
+	if (cmf&0x0f != zlibDeflate) || (cmf>>4 > zlibMaxWindow) || (binary.BigEndian.Uint16(src[:2])%31 != 0) {
+		return bufpool.Buffer{}, ErrHeader
+	}
+
+	offset := 2
+	haveDict := flg&0x20 != 0
+	if haveDict {
+		if len(src) < offset+4 {
+			return bufpool.Buffer{}, io.ErrUnexpectedEOF
+		}
+		if dict == nil {
+			return bufpool.Buffer{}, ErrDictionary
+		}
+		checksum := binary.BigEndian.Uint32(src[offset : offset+4])
+		if checksum != adler32.Checksum(dict) {
+			return bufpool.Buffer{}, ErrDictionary
+		}
+		offset += 4
+	}
+
+	if len(src[offset:]) < 4 {
+		return bufpool.Buffer{}, io.ErrUnexpectedEOF
+	}
+
+	deflateData := src[offset:]
+	out, consumed, err := flatex.DecompressDict(deflateData, dict)
+	if err != nil {
+		return bufpool.Buffer{}, err
+	}
+
+	checksumPos := offset + consumed
+	if checksumPos+4 > len(src) {
+		out.Release()
+		return bufpool.Buffer{}, io.ErrUnexpectedEOF
+	}
+	expected := binary.BigEndian.Uint32(src[checksumPos : checksumPos+4])
+	if expected != adler32.Checksum(out.Bytes()) {
+		out.Release()
+		return bufpool.Buffer{}, ErrChecksum
+	}
+	return out, nil
+}