1 files changed, 205 insertions, 0 deletions
diff --git a/internal/compress/zlib/reader.go b/internal/compress/zlib/reader.go
new file mode 100644
index 00000000..2d009887
--- /dev/null
+++ b/internal/compress/zlib/reader.go
@@ -0,0 +1,205 @@
+// 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 and writers to reduce allocations.
+
+Note that closing a reader or writer causes it to be returned to a pool
+for reuse. Therefore, the caller must not retain references to a
+reader or writer after closing it; in the standard library's
+compress/zlib package, it is legal to Reset a closed reader or writer
+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 zlib
+
+import (
+	"encoding/binary"
+	"errors"
+	"hash"
+	"io"
+	"sync"
+
+	"codeberg.org/lindenii/furgit/internal/compress/flate"
+)
+
+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 readerPool = sync.Pool{
+	New: func() any {
+		r := new(Reader)
+
+		return r
+	},
+}
+
+// Reader reads and verifies one zlib stream.
+//
+// Reader implements io.ReadCloser.
+type Reader struct {
+	r            flate.Reader
+	decompressor io.ReadCloser
+	digest       hash.Hash32
+	counter      *countingFlateReader
+	err          error
+	scratch      [4]byte
+}
+
+// countingFlateReader wraps flate input and tracks consumed bytes.
+type countingFlateReader struct {
+	inner flate.Reader
+	read  uint64
+}
+
+// Read implements io.Reader.
+func (reader *countingFlateReader) Read(dst []byte) (int, error) {
+	n, err := reader.inner.Read(dst)
+	reader.read += uint64(n)
+
+	return n, err
+}
+
+// ReadByte implements io.ByteReader.
+func (reader *countingFlateReader) ReadByte() (byte, error) {
+	b, err := reader.inner.ReadByte()
+	if err == nil {
+		reader.read++
+	}
+
+	return b, err
+}
+
+// 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) (*Reader, 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) (*Reader, error) {
+	v := readerPool.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
+}
+
+// Read decompresses bytes from receiver into p.
+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)
+
+	_, err := z.digest.Write(p[0:n])
+	if err != nil {
+		z.err = err
+
+		return n, z.err
+	}
+
+	if !errors.Is(z.err, io.EOF) {
+		// In the normal case we return here.
+		return n, z.err
+	}
+
+	// Finished file; check checksum.
+	_, err = io.ReadFull(z.r, z.scratch[0:4])
+	if err != nil {
+		if errors.Is(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
+}
+
+// InputConsumed returns compressed bytes consumed from stream input.
+//
+// This count includes the zlib header, deflate payload, and zlib checksum
+// trailer bytes read by the reader.
+func (z *Reader) InputConsumed() uint64 {
+	if z.counter == nil {
+		return 0
+	}
+
+	return z.counter.read
+}
+
+// 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].
+// Close returns the instance to a global pool; you MUST NOT keep references after Close.
+func (z *Reader) Close() error {
+	if z.err != nil && !errors.Is(z.err, io.EOF) {
+		return z.err
+	}
+
+	z.err = z.decompressor.Close()
+	if z.err != nil {
+		return z.err
+	}
+
+	readerPool.Put(z)
+
+	return nil
+}