1 files changed, 198 insertions, 0 deletions
diff --git a/internal/zlibx/reader.go b/internal/zlibx/reader.go
new file mode 100644
index 00000000..80bbbb58
--- /dev/null
+++ b/internal/zlibx/reader.go
@@ -0,0 +1,198 @@
+// 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 implements reading and writing of zlib format compressed data,
+as specified in RFC 1950.
+
+This package differs from the standard library's compress/zlib package
+in that it pools readers to reduce allocations. Writers are unchanged.
+
+Note that closing the reader causes it to be returned to a pool for
+reuse. Therefore, the caller must not retain references to the
+reader after closing it; in the standard library's compress/zlib package,
+it is legal to Reset a closed reader and continue using it; that is
+not allowed here, so there is simply no Resetter interface.
+
+The implementation provides filters that uncompress during reading
+and compress during writing.  For example, to write compressed data
+to a buffer:
+
+	var b bytes.Buffer
+	w := zlib.NewWriter(&b)
+	w.Write([]byte("hello, world\n"))
+	w.Close()
+
+and to read that data back:
+
+	r, err := zlib.NewReader(&b)
+	io.Copy(os.Stdout, r)
+	r.Close()
+*/
+package zlibx
+
+import (
+	"bufio"
+	"encoding/binary"
+	"errors"
+	"hash"
+	"io"
+	"sync"
+
+	"git.sr.ht/~runxiyu/furgit/internal/adler32"
+	"git.sr.ht/~runxiyu/furgit/internal/flatex"
+)
+
+const (
+	zlibDeflate   = 8
+	zlibMaxWindow = 7
+)
+
+var (
+	// ErrChecksum is returned when reading ZLIB data that has an invalid checksum.
+	ErrChecksum = errors.New("zlib: invalid checksum")
+	// ErrDictionary is returned when reading ZLIB data that has an invalid dictionary.
+	ErrDictionary = errors.New("zlib: invalid dictionary")
+	// ErrHeader is returned when reading ZLIB data that has an invalid header.
+	ErrHeader = errors.New("zlib: invalid header")
+)
+
+var pool = sync.Pool{
+	New: func() any {
+		r := new(reader)
+		return r
+	},
+}
+
+type reader struct {
+	r            flatex.Reader
+	decompressor io.ReadCloser
+	digest       hash.Hash32
+	err          error
+	scratch      [4]byte
+}
+
+// NewReader creates a new ReadCloser.
+// Reads from the returned ReadCloser read and decompress data from r.
+// If r does not implement [io.ByteReader], the decompressor may read more
+// data than necessary from r.
+// It is the caller's responsibility to call Close on the ReadCloser when done.
+func NewReader(r io.Reader) (io.ReadCloser, error) {
+	return NewReaderDict(r, nil)
+}
+
+// NewReaderDict is like [NewReader] but uses a preset dictionary.
+// NewReaderDict ignores the dictionary if the compressed data does not refer to it.
+// If the compressed data refers to a different dictionary, NewReaderDict returns [ErrDictionary].
+func NewReaderDict(r io.Reader, dict []byte) (io.ReadCloser, error) {
+	v := pool.Get()
+	z, ok := v.(*reader)
+	if !ok {
+		panic("zlib: pool returned unexpected type")
+	}
+	err := z.Reset(r, dict)
+	if err != nil {
+		return nil, err
+	}
+	return z, nil
+}
+
+func (z *reader) Read(p []byte) (int, error) {
+	if z.err != nil {
+		return 0, z.err
+	}
+
+	var n int
+	n, z.err = z.decompressor.Read(p)
+	z.digest.Write(p[0:n])
+	if z.err != io.EOF {
+		// In the normal case we return here.
+		return n, z.err
+	}
+
+	// Finished file; check checksum.
+	if _, err := io.ReadFull(z.r, z.scratch[0:4]); err != nil {
+		if err == io.EOF {
+			err = io.ErrUnexpectedEOF
+		}
+		z.err = err
+		return n, z.err
+	}
+	// ZLIB (RFC 1950) is big-endian, unlike GZIP (RFC 1952).
+	checksum := binary.BigEndian.Uint32(z.scratch[:4])
+	if checksum != z.digest.Sum32() {
+		z.err = ErrChecksum
+		return n, z.err
+	}
+	return n, io.EOF
+}
+
+// Calling Close does not close the wrapped [io.Reader] originally passed to [NewReader].
+// In order for the ZLIB checksum to be verified, the reader must be
+// fully consumed until the [io.EOF].
+func (z *reader) Close() error {
+	if z.err != nil && z.err != io.EOF {
+		return z.err
+	}
+	z.err = z.decompressor.Close()
+	if z.err != nil {
+		return z.err
+	}
+
+	pool.Put(z)
+	return nil
+}
+
+func (z *reader) Reset(r io.Reader, dict []byte) error {
+	*z = reader{decompressor: z.decompressor}
+	if fr, ok := r.(flatex.Reader); ok {
+		z.r = fr
+	} else {
+		z.r = bufio.NewReader(r)
+	}
+
+	// Read the header (RFC 1950 section 2.2.).
+	_, z.err = io.ReadFull(z.r, z.scratch[0:2])
+	if z.err != nil {
+		if z.err == io.EOF {
+			z.err = io.ErrUnexpectedEOF
+		}
+		return z.err
+	}
+	h := binary.BigEndian.Uint16(z.scratch[:2])
+	if (z.scratch[0]&0x0f != zlibDeflate) || (z.scratch[0]>>4 > zlibMaxWindow) || (h%31 != 0) {
+		z.err = ErrHeader
+		return z.err
+	}
+	haveDict := z.scratch[1]&0x20 != 0
+	if haveDict {
+		_, z.err = io.ReadFull(z.r, z.scratch[0:4])
+		if z.err != nil {
+			if z.err == io.EOF {
+				z.err = io.ErrUnexpectedEOF
+			}
+			return z.err
+		}
+		checksum := binary.BigEndian.Uint32(z.scratch[:4])
+		if checksum != adler32.Checksum(dict) {
+			z.err = ErrDictionary
+			return z.err
+		}
+	}
+
+	if z.decompressor == nil {
+		if haveDict {
+			z.decompressor = flatex.NewReaderDict(z.r, dict)
+		} else {
+			z.decompressor = flatex.NewReader(z.r)
+		}
+	} else {
+		z.err = z.decompressor.(flatex.Resetter).Reset(z.r, dict)
+		if z.err != nil {
+			return z.err
+		}
+	}
+	z.digest = adler32.New()
+	return nil
+}