internal/compress: Import flate and such from klauspost/compress

1 files changed, 71 insertions, 0 deletions

diff --git a/internal/compress/zlib/writer_header.go b/internal/compress/zlib/writer_header.go
new file mode 100644
index 00000000..43d3bdf5
--- /dev/null
+++ b/internal/compress/zlib/writer_header.go
@@ -0,0 +1,71 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package zlib
+
+import (
+	"encoding/binary"
+
+	"codeberg.org/lindenii/furgit/internal/adler32"
+	"codeberg.org/lindenii/furgit/internal/compress/flate"
+)
+
+// writeHeader writes the ZLIB header.
+func (z *Writer) writeHeader() (err error) {
+	z.wroteHeader = true
+	// ZLIB has a two-byte header (as documented in RFC 1950).
+	// The first four bits is the CINFO (compression info), which is 7 for the default deflate window size.
+	// The next four bits is the CM (compression method), which is 8 for deflate.
+	z.scratch[0] = 0x78
+	// The next two bits is the FLEVEL (compression level). The four values are:
+	// 0=fastest, 1=fast, 2=default, 3=best.
+	// The next bit, FDICT, is set if a dictionary is given.
+	// The final five FCHECK bits form a mod-31 checksum.
+	switch z.level {
+	case -2, 0, 1:
+		z.scratch[1] = 0 << 6
+	case 2, 3, 4, 5:
+		z.scratch[1] = 1 << 6
+	case 6, -1:
+		z.scratch[1] = 2 << 6
+	case 7, 8, 9:
+		z.scratch[1] = 3 << 6
+	default:
+		panic("unreachable")
+	}
+
+	if z.dict != nil {
+		z.scratch[1] |= 1 << 5
+	}
+
+	z.scratch[1] += uint8(31 - binary.BigEndian.Uint16(z.scratch[:2])%31) //#nosec G115
+
+	_, err = z.w.Write(z.scratch[0:2])
+	if err != nil {
+		return err
+	}
+
+	if z.dict != nil {
+		// The next four bytes are the Adler-32 checksum of the dictionary.
+		binary.BigEndian.PutUint32(z.scratch[:], adler32.Checksum(z.dict))
+
+		_, err = z.w.Write(z.scratch[0:4])
+		if err != nil {
+			return err
+		}
+	}
+
+	if z.compressor == nil {
+		// Initialize deflater unless the Writer is being reused
+		// after a Reset call.
+		z.compressor, err = flate.NewWriterDict(z.w, z.level, z.dict)
+		if err != nil {
+			return err
+		}
+
+		z.digest = adler32.New()
+	}
+
+	return nil
+}