From 404aeae4545d2426c089a5f8d5e82dae56f5212b Mon Sep 17 00:00:00 2001
From: Rutger Broekhoff
Date: Fri, 29 Dec 2023 21:31:53 +0100
Subject: Make Nix builds work

---
 vendor/github.com/klauspost/compress/s2/reader.go | 1062 +++++++++++++++++++++
 1 file changed, 1062 insertions(+)
 create mode 100644 vendor/github.com/klauspost/compress/s2/reader.go

(limited to 'vendor/github.com/klauspost/compress/s2/reader.go')

diff --git a/vendor/github.com/klauspost/compress/s2/reader.go b/vendor/github.com/klauspost/compress/s2/reader.go
new file mode 100644
index 0000000..2f01a39
--- /dev/null
+++ b/vendor/github.com/klauspost/compress/s2/reader.go
@@ -0,0 +1,1062 @@
+// Copyright 2011 The Snappy-Go Authors. All rights reserved.
+// Copyright (c) 2019+ Klaus Post. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package s2
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"math"
+	"runtime"
+	"sync"
+)
+
+// ErrCantSeek is returned if the stream cannot be seeked.
+type ErrCantSeek struct {
+	Reason string
+}
+
+// Error returns the error as string.
+func (e ErrCantSeek) Error() string {
+	return fmt.Sprintf("s2: Can't seek because %s", e.Reason)
+}
+
+// NewReader returns a new Reader that decompresses from r, using the framing
+// format described at
+// https://github.com/google/snappy/blob/master/framing_format.txt with S2 changes.
+func NewReader(r io.Reader, opts ...ReaderOption) *Reader {
+	nr := Reader{
+		r:        r,
+		maxBlock: maxBlockSize,
+	}
+	for _, opt := range opts {
+		if err := opt(&nr); err != nil {
+			nr.err = err
+			return &nr
+		}
+	}
+	nr.maxBufSize = MaxEncodedLen(nr.maxBlock) + checksumSize
+	if nr.lazyBuf > 0 {
+		nr.buf = make([]byte, MaxEncodedLen(nr.lazyBuf)+checksumSize)
+	} else {
+		nr.buf = make([]byte, MaxEncodedLen(defaultBlockSize)+checksumSize)
+	}
+	nr.readHeader = nr.ignoreStreamID
+	nr.paramsOK = true
+	return &nr
+}
+
+// ReaderOption is an option for creating a decoder.
+type ReaderOption func(*Reader) error
+
+// ReaderMaxBlockSize allows to control allocations if the stream
+// has been compressed with a smaller WriterBlockSize, or with the default 1MB.
+// Blocks must be this size or smaller to decompress,
+// otherwise the decoder will return ErrUnsupported.
+//
+// For streams compressed with Snappy this can safely be set to 64KB (64 << 10).
+//
+// Default is the maximum limit of 4MB.
+func ReaderMaxBlockSize(blockSize int) ReaderOption {
+	return func(r *Reader) error {
+		if blockSize > maxBlockSize || blockSize <= 0 {
+			return errors.New("s2: block size too large. Must be <= 4MB and > 0")
+		}
+		if r.lazyBuf == 0 && blockSize < defaultBlockSize {
+			r.lazyBuf = blockSize
+		}
+		r.maxBlock = blockSize
+		return nil
+	}
+}
+
+// ReaderAllocBlock allows to control upfront stream allocations
+// and not allocate for frames bigger than this initially.
+// If frames bigger than this is seen a bigger buffer will be allocated.
+//
+// Default is 1MB, which is default output size.
+func ReaderAllocBlock(blockSize int) ReaderOption {
+	return func(r *Reader) error {
+		if blockSize > maxBlockSize || blockSize < 1024 {
+			return errors.New("s2: invalid ReaderAllocBlock. Must be <= 4MB and >= 1024")
+		}
+		r.lazyBuf = blockSize
+		return nil
+	}
+}
+
+// ReaderIgnoreStreamIdentifier will make the reader skip the expected
+// stream identifier at the beginning of the stream.
+// This can be used when serving a stream that has been forwarded to a specific point.
+func ReaderIgnoreStreamIdentifier() ReaderOption {
+	return func(r *Reader) error {
+		r.ignoreStreamID = true
+		return nil
+	}
+}
+
+// ReaderSkippableCB will register a callback for chuncks with the specified ID.
+// ID must be a Reserved skippable chunks ID, 0x80-0xfd (inclusive).
+// For each chunk with the ID, the callback is called with the content.
+// Any returned non-nil error will abort decompression.
+// Only one callback per ID is supported, latest sent will be used.
+func ReaderSkippableCB(id uint8, fn func(r io.Reader) error) ReaderOption {
+	return func(r *Reader) error {
+		if id < 0x80 || id > 0xfd {
+			return fmt.Errorf("ReaderSkippableCB: Invalid id provided, must be 0x80-0xfd (inclusive)")
+		}
+		r.skippableCB[id] = fn
+		return nil
+	}
+}
+
+// ReaderIgnoreCRC will make the reader skip CRC calculation and checks.
+func ReaderIgnoreCRC() ReaderOption {
+	return func(r *Reader) error {
+		r.ignoreCRC = true
+		return nil
+	}
+}
+
+// Reader is an io.Reader that can read Snappy-compressed bytes.
+type Reader struct {
+	r           io.Reader
+	err         error
+	decoded     []byte
+	buf         []byte
+	skippableCB [0x80]func(r io.Reader) error
+	blockStart  int64 // Uncompressed offset at start of current.
+	index       *Index
+
+	// decoded[i:j] contains decoded bytes that have not yet been passed on.
+	i, j int
+	// maximum block size allowed.
+	maxBlock int
+	// maximum expected buffer size.
+	maxBufSize int
+	// alloc a buffer this size if > 0.
+	lazyBuf        int
+	readHeader     bool
+	paramsOK       bool
+	snappyFrame    bool
+	ignoreStreamID bool
+	ignoreCRC      bool
+}
+
+// GetBufferCapacity returns the capacity of the internal buffer.
+// This might be useful to know when reusing the same reader in combination
+// with the lazy buffer option.
+func (r *Reader) GetBufferCapacity() int {
+	return cap(r.buf)
+}
+
+// ensureBufferSize will ensure that the buffer can take at least n bytes.
+// If false is returned the buffer exceeds maximum allowed size.
+func (r *Reader) ensureBufferSize(n int) bool {
+	if n > r.maxBufSize {
+		r.err = ErrCorrupt
+		return false
+	}
+	if cap(r.buf) >= n {
+		return true
+	}
+	// Realloc buffer.
+	r.buf = make([]byte, n)
+	return true
+}
+
+// Reset discards any buffered data, resets all state, and switches the Snappy
+// reader to read from r. This permits reusing a Reader rather than allocating
+// a new one.
+func (r *Reader) Reset(reader io.Reader) {
+	if !r.paramsOK {
+		return
+	}
+	r.index = nil
+	r.r = reader
+	r.err = nil
+	r.i = 0
+	r.j = 0
+	r.blockStart = 0
+	r.readHeader = r.ignoreStreamID
+}
+
+func (r *Reader) readFull(p []byte, allowEOF bool) (ok bool) {
+	if _, r.err = io.ReadFull(r.r, p); r.err != nil {
+		if r.err == io.ErrUnexpectedEOF || (r.err == io.EOF && !allowEOF) {
+			r.err = ErrCorrupt
+		}
+		return false
+	}
+	return true
+}
+
+// skippable will skip n bytes.
+// If the supplied reader supports seeking that is used.
+// tmp is used as a temporary buffer for reading.
+// The supplied slice does not need to be the size of the read.
+func (r *Reader) skippable(tmp []byte, n int, allowEOF bool, id uint8) (ok bool) {
+	if id < 0x80 {
+		r.err = fmt.Errorf("interbal error: skippable id < 0x80")
+		return false
+	}
+	if fn := r.skippableCB[id-0x80]; fn != nil {
+		rd := io.LimitReader(r.r, int64(n))
+		r.err = fn(rd)
+		if r.err != nil {
+			return false
+		}
+		_, r.err = io.CopyBuffer(ioutil.Discard, rd, tmp)
+		return r.err == nil
+	}
+	if rs, ok := r.r.(io.ReadSeeker); ok {
+		_, err := rs.Seek(int64(n), io.SeekCurrent)
+		if err == nil {
+			return true
+		}
+		if err == io.ErrUnexpectedEOF || (r.err == io.EOF && !allowEOF) {
+			r.err = ErrCorrupt
+			return false
+		}
+	}
+	for n > 0 {
+		if n < len(tmp) {
+			tmp = tmp[:n]
+		}
+		if _, r.err = io.ReadFull(r.r, tmp); r.err != nil {
+			if r.err == io.ErrUnexpectedEOF || (r.err == io.EOF && !allowEOF) {
+				r.err = ErrCorrupt
+			}
+			return false
+		}
+		n -= len(tmp)
+	}
+	return true
+}
+
+// Read satisfies the io.Reader interface.
+func (r *Reader) Read(p []byte) (int, error) {
+	if r.err != nil {
+		return 0, r.err
+	}
+	for {
+		if r.i < r.j {
+			n := copy(p, r.decoded[r.i:r.j])
+			r.i += n
+			return n, nil
+		}
+		if !r.readFull(r.buf[:4], true) {
+			return 0, r.err
+		}
+		chunkType := r.buf[0]
+		if !r.readHeader {
+			if chunkType != chunkTypeStreamIdentifier {
+				r.err = ErrCorrupt
+				return 0, r.err
+			}
+			r.readHeader = true
+		}
+		chunkLen := int(r.buf[1]) | int(r.buf[2])<<8 | int(r.buf[3])<<16
+
+		// The chunk types are specified at
+		// https://github.com/google/snappy/blob/master/framing_format.txt
+		switch chunkType {
+		case chunkTypeCompressedData:
+			r.blockStart += int64(r.j)
+			// Section 4.2. Compressed data (chunk type 0x00).
+			if chunkLen < checksumSize {
+				r.err = ErrCorrupt
+				return 0, r.err
+			}
+			if !r.ensureBufferSize(chunkLen) {
+				if r.err == nil {
+					r.err = ErrUnsupported
+				}
+				return 0, r.err
+			}
+			buf := r.buf[:chunkLen]
+			if !r.readFull(buf, false) {
+				return 0, r.err
+			}
+			checksum := uint32(buf[0]) | uint32(buf[1])<<8 | uint32(buf[2])<<16 | uint32(buf[3])<<24
+			buf = buf[checksumSize:]
+
+			n, err := DecodedLen(buf)
+			if err != nil {
+				r.err = err
+				return 0, r.err
+			}
+			if r.snappyFrame && n > maxSnappyBlockSize {
+				r.err = ErrCorrupt
+				return 0, r.err
+			}
+
+			if n > len(r.decoded) {
+				if n > r.maxBlock {
+					r.err = ErrCorrupt
+					return 0, r.err
+				}
+				r.decoded = make([]byte, n)
+			}
+			if _, err := Decode(r.decoded, buf); err != nil {
+				r.err = err
+				return 0, r.err
+			}
+			if !r.ignoreCRC && crc(r.decoded[:n]) != checksum {
+				r.err = ErrCRC
+				return 0, r.err
+			}
+			r.i, r.j = 0, n
+			continue
+
+		case chunkTypeUncompressedData:
+			r.blockStart += int64(r.j)
+			// Section 4.3. Uncompressed data (chunk type 0x01).
+			if chunkLen < checksumSize {
+				r.err = ErrCorrupt
+				return 0, r.err
+			}
+			if !r.ensureBufferSize(chunkLen) {
+				if r.err == nil {
+					r.err = ErrUnsupported
+				}
+				return 0, r.err
+			}
+			buf := r.buf[:checksumSize]
+			if !r.readFull(buf, false) {
+				return 0, r.err
+			}
+			checksum := uint32(buf[0]) | uint32(buf[1])<<8 | uint32(buf[2])<<16 | uint32(buf[3])<<24
+			// Read directly into r.decoded instead of via r.buf.
+			n := chunkLen - checksumSize
+			if r.snappyFrame && n > maxSnappyBlockSize {
+				r.err = ErrCorrupt
+				return 0, r.err
+			}
+			if n > len(r.decoded) {
+				if n > r.maxBlock {
+					r.err = ErrCorrupt
+					return 0, r.err
+				}
+				r.decoded = make([]byte, n)
+			}
+			if !r.readFull(r.decoded[:n], false) {
+				return 0, r.err
+			}
+			if !r.ignoreCRC && crc(r.decoded[:n]) != checksum {
+				r.err = ErrCRC
+				return 0, r.err
+			}
+			r.i, r.j = 0, n
+			continue
+
+		case chunkTypeStreamIdentifier:
+			// Section 4.1. Stream identifier (chunk type 0xff).
+			if chunkLen != len(magicBody) {
+				r.err = ErrCorrupt
+				return 0, r.err
+			}
+			if !r.readFull(r.buf[:len(magicBody)], false) {
+				return 0, r.err
+			}
+			if string(r.buf[:len(magicBody)]) != magicBody {
+				if string(r.buf[:len(magicBody)]) != magicBodySnappy {
+					r.err = ErrCorrupt
+					return 0, r.err
+				} else {
+					r.snappyFrame = true
+				}
+			} else {
+				r.snappyFrame = false
+			}
+			continue
+		}
+
+		if chunkType <= 0x7f {
+			// Section 4.5. Reserved unskippable chunks (chunk types 0x02-0x7f).
+			// fmt.Printf("ERR chunktype: 0x%x\n", chunkType)
+			r.err = ErrUnsupported
+			return 0, r.err
+		}
+		// Section 4.4 Padding (chunk type 0xfe).
+		// Section 4.6. Reserved skippable chunks (chunk types 0x80-0xfd).
+		if chunkLen > maxChunkSize {
+			// fmt.Printf("ERR chunkLen: 0x%x\n", chunkLen)
+			r.err = ErrUnsupported
+			return 0, r.err
+		}
+
+		// fmt.Printf("skippable: ID: 0x%x, len: 0x%x\n", chunkType, chunkLen)
+		if !r.skippable(r.buf, chunkLen, false, chunkType) {
+			return 0, r.err
+		}
+	}
+}
+
+// DecodeConcurrent will decode the full stream to w.
+// This function should not be combined with reading, seeking or other operations.
+// Up to 'concurrent' goroutines will be used.
+// If <= 0, runtime.NumCPU will be used.
+// On success the number of bytes decompressed nil and is returned.
+// This is mainly intended for bigger streams.
+func (r *Reader) DecodeConcurrent(w io.Writer, concurrent int) (written int64, err error) {
+	if r.i > 0 || r.j > 0 || r.blockStart > 0 {
+		return 0, errors.New("DecodeConcurrent called after ")
+	}
+	if concurrent <= 0 {
+		concurrent = runtime.NumCPU()
+	}
+
+	// Write to output
+	var errMu sync.Mutex
+	var aErr error
+	setErr := func(e error) (ok bool) {
+		errMu.Lock()
+		defer errMu.Unlock()
+		if e == nil {
+			return aErr == nil
+		}
+		if aErr == nil {
+			aErr = e
+		}
+		return false
+	}
+	hasErr := func() (ok bool) {
+		errMu.Lock()
+		v := aErr != nil
+		errMu.Unlock()
+		return v
+	}
+
+	var aWritten int64
+	toRead := make(chan []byte, concurrent)
+	writtenBlocks := make(chan []byte, concurrent)
+	queue := make(chan chan []byte, concurrent)
+	reUse := make(chan chan []byte, concurrent)
+	for i := 0; i < concurrent; i++ {
+		toRead <- make([]byte, 0, r.maxBufSize)
+		writtenBlocks <- make([]byte, 0, r.maxBufSize)
+		reUse <- make(chan []byte, 1)
+	}
+	// Writer
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		for toWrite := range queue {
+			entry := <-toWrite
+			reUse <- toWrite
+			if hasErr() {
+				writtenBlocks <- entry
+				continue
+			}
+			n, err := w.Write(entry)
+			want := len(entry)
+			writtenBlocks <- entry
+			if err != nil {
+				setErr(err)
+				continue
+			}
+			if n != want {
+				setErr(io.ErrShortWrite)
+				continue
+			}
+			aWritten += int64(n)
+		}
+	}()
+
+	// Reader
+	defer func() {
+		close(queue)
+		if r.err != nil {
+			err = r.err
+			setErr(r.err)
+		}
+		wg.Wait()
+		if err == nil {
+			err = aErr
+		}
+		written = aWritten
+	}()
+
+	for !hasErr() {
+		if !r.readFull(r.buf[:4], true) {
+			if r.err == io.EOF {
+				r.err = nil
+			}
+			return 0, r.err
+		}
+		chunkType := r.buf[0]
+		if !r.readHeader {
+			if chunkType != chunkTypeStreamIdentifier {
+				r.err = ErrCorrupt
+				return 0, r.err
+			}
+			r.readHeader = true
+		}
+		chunkLen := int(r.buf[1]) | int(r.buf[2])<<8 | int(r.buf[3])<<16
+
+		// The chunk types are specified at
+		// https://github.com/google/snappy/blob/master/framing_format.txt
+		switch chunkType {
+		case chunkTypeCompressedData:
+			r.blockStart += int64(r.j)
+			// Section 4.2. Compressed data (chunk type 0x00).
+			if chunkLen < checksumSize {
+				r.err = ErrCorrupt
+				return 0, r.err
+			}
+			if chunkLen > r.maxBufSize {
+				r.err = ErrCorrupt
+				return 0, r.err
+			}
+			orgBuf := <-toRead
+			buf := orgBuf[:chunkLen]
+
+			if !r.readFull(buf, false) {
+				return 0, r.err
+			}
+
+			checksum := uint32(buf[0]) | uint32(buf[1])<<8 | uint32(buf[2])<<16 | uint32(buf[3])<<24
+			buf = buf[checksumSize:]
+
+			n, err := DecodedLen(buf)
+			if err != nil {
+				r.err = err
+				return 0, r.err
+			}
+			if r.snappyFrame && n > maxSnappyBlockSize {
+				r.err = ErrCorrupt
+				return 0, r.err
+			}
+
+			if n > r.maxBlock {
+				r.err = ErrCorrupt
+				return 0, r.err
+			}
+			wg.Add(1)
+
+			decoded := <-writtenBlocks
+			entry := <-reUse
+			queue <- entry
+			go func() {
+				defer wg.Done()
+				decoded = decoded[:n]
+				_, err := Decode(decoded, buf)
+				toRead <- orgBuf
+				if err != nil {
+					writtenBlocks <- decoded
+					setErr(err)
+					return
+				}
+				if !r.ignoreCRC && crc(decoded) != checksum {
+					writtenBlocks <- decoded
+					setErr(ErrCRC)
+					return
+				}
+				entry <- decoded
+			}()
+			continue
+
+		case chunkTypeUncompressedData:
+
+			// Section 4.3. Uncompressed data (chunk type 0x01).
+			if chunkLen < checksumSize {
+				r.err = ErrCorrupt
+				return 0, r.err
+			}
+			if chunkLen > r.maxBufSize {
+				r.err = ErrCorrupt
+				return 0, r.err
+			}
+			// Grab write buffer
+			orgBuf := <-writtenBlocks
+			buf := orgBuf[:checksumSize]
+			if !r.readFull(buf, false) {
+				return 0, r.err
+			}
+			checksum := uint32(buf[0]) | uint32(buf[1])<<8 | uint32(buf[2])<<16 | uint32(buf[3])<<24
+			// Read content.
+			n := chunkLen - checksumSize
+
+			if r.snappyFrame && n > maxSnappyBlockSize {
+				r.err = ErrCorrupt
+				return 0, r.err
+			}
+			if n > r.maxBlock {
+				r.err = ErrCorrupt
+				return 0, r.err
+			}
+			// Read uncompressed
+			buf = orgBuf[:n]
+			if !r.readFull(buf, false) {
+				return 0, r.err
+			}
+
+			if !r.ignoreCRC && crc(buf) != checksum {
+				r.err = ErrCRC
+				return 0, r.err
+			}
+			entry := <-reUse
+			queue <- entry
+			entry <- buf
+			continue
+
+		case chunkTypeStreamIdentifier:
+			// Section 4.1. Stream identifier (chunk type 0xff).
+			if chunkLen != len(magicBody) {
+				r.err = ErrCorrupt
+				return 0, r.err
+			}
+			if !r.readFull(r.buf[:len(magicBody)], false) {
+				return 0, r.err
+			}
+			if string(r.buf[:len(magicBody)]) != magicBody {
+				if string(r.buf[:len(magicBody)]) != magicBodySnappy {
+					r.err = ErrCorrupt
+					return 0, r.err
+				} else {
+					r.snappyFrame = true
+				}
+			} else {
+				r.snappyFrame = false
+			}
+			continue
+		}
+
+		if chunkType <= 0x7f {
+			// Section 4.5. Reserved unskippable chunks (chunk types 0x02-0x7f).
+			// fmt.Printf("ERR chunktype: 0x%x\n", chunkType)
+			r.err = ErrUnsupported
+			return 0, r.err
+		}
+		// Section 4.4 Padding (chunk type 0xfe).
+		// Section 4.6. Reserved skippable chunks (chunk types 0x80-0xfd).
+		if chunkLen > maxChunkSize {
+			// fmt.Printf("ERR chunkLen: 0x%x\n", chunkLen)
+			r.err = ErrUnsupported
+			return 0, r.err
+		}
+
+		// fmt.Printf("skippable: ID: 0x%x, len: 0x%x\n", chunkType, chunkLen)
+		if !r.skippable(r.buf, chunkLen, false, chunkType) {
+			return 0, r.err
+		}
+	}
+	return 0, r.err
+}
+
+// Skip will skip n bytes forward in the decompressed output.
+// For larger skips this consumes less CPU and is faster than reading output and discarding it.
+// CRC is not checked on skipped blocks.
+// io.ErrUnexpectedEOF is returned if the stream ends before all bytes have been skipped.
+// If a decoding error is encountered subsequent calls to Read will also fail.
+func (r *Reader) Skip(n int64) error {
+	if n < 0 {
+		return errors.New("attempted negative skip")
+	}
+	if r.err != nil {
+		return r.err
+	}
+
+	for n > 0 {
+		if r.i < r.j {
+			// Skip in buffer.
+			// decoded[i:j] contains decoded bytes that have not yet been passed on.
+			left := int64(r.j - r.i)
+			if left >= n {
+				tmp := int64(r.i) + n
+				if tmp > math.MaxInt32 {
+					return errors.New("s2: internal overflow in skip")
+				}
+				r.i = int(tmp)
+				return nil
+			}
+			n -= int64(r.j - r.i)
+			r.i = r.j
+		}
+
+		// Buffer empty; read blocks until we have content.
+		if !r.readFull(r.buf[:4], true) {
+			if r.err == io.EOF {
+				r.err = io.ErrUnexpectedEOF
+			}
+			return r.err
+		}
+		chunkType := r.buf[0]
+		if !r.readHeader {
+			if chunkType != chunkTypeStreamIdentifier {
+				r.err = ErrCorrupt
+				return r.err
+			}
+			r.readHeader = true
+		}
+		chunkLen := int(r.buf[1]) | int(r.buf[2])<<8 | int(r.buf[3])<<16
+
+		// The chunk types are specified at
+		// https://github.com/google/snappy/blob/master/framing_format.txt
+		switch chunkType {
+		case chunkTypeCompressedData:
+			r.blockStart += int64(r.j)
+			// Section 4.2. Compressed data (chunk type 0x00).
+			if chunkLen < checksumSize {
+				r.err = ErrCorrupt
+				return r.err
+			}
+			if !r.ensureBufferSize(chunkLen) {
+				if r.err == nil {
+					r.err = ErrUnsupported
+				}
+				return r.err
+			}
+			buf := r.buf[:chunkLen]
+			if !r.readFull(buf, false) {
+				return r.err
+			}
+			checksum := uint32(buf[0]) | uint32(buf[1])<<8 | uint32(buf[2])<<16 | uint32(buf[3])<<24
+			buf = buf[checksumSize:]
+
+			dLen, err := DecodedLen(buf)
+			if err != nil {
+				r.err = err
+				return r.err
+			}
+			if dLen > r.maxBlock {
+				r.err = ErrCorrupt
+				return r.err
+			}
+			// Check if destination is within this block
+			if int64(dLen) > n {
+				if len(r.decoded) < dLen {
+					r.decoded = make([]byte, dLen)
+				}
+				if _, err := Decode(r.decoded, buf); err != nil {
+					r.err = err
+					return r.err
+				}
+				if crc(r.decoded[:dLen]) != checksum {
+					r.err = ErrCorrupt
+					return r.err
+				}
+			} else {
+				// Skip block completely
+				n -= int64(dLen)
+				r.blockStart += int64(dLen)
+				dLen = 0
+			}
+			r.i, r.j = 0, dLen
+			continue
+		case chunkTypeUncompressedData:
+			r.blockStart += int64(r.j)
+			// Section 4.3. Uncompressed data (chunk type 0x01).
+			if chunkLen < checksumSize {
+				r.err = ErrCorrupt
+				return r.err
+			}
+			if !r.ensureBufferSize(chunkLen) {
+				if r.err != nil {
+					r.err = ErrUnsupported
+				}
+				return r.err
+			}
+			buf := r.buf[:checksumSize]
+			if !r.readFull(buf, false) {
+				return r.err
+			}
+			checksum := uint32(buf[0]) | uint32(buf[1])<<8 | uint32(buf[2])<<16 | uint32(buf[3])<<24
+			// Read directly into r.decoded instead of via r.buf.
+			n2 := chunkLen - checksumSize
+			if n2 > len(r.decoded) {
+				if n2 > r.maxBlock {
+					r.err = ErrCorrupt
+					return r.err
+				}
+				r.decoded = make([]byte, n2)
+			}
+			if !r.readFull(r.decoded[:n2], false) {
+				return r.err
+			}
+			if int64(n2) < n {
+				if crc(r.decoded[:n2]) != checksum {
+					r.err = ErrCorrupt
+					return r.err
+				}
+			}
+			r.i, r.j = 0, n2
+			continue
+		case chunkTypeStreamIdentifier:
+			// Section 4.1. Stream identifier (chunk type 0xff).
+			if chunkLen != len(magicBody) {
+				r.err = ErrCorrupt
+				return r.err
+			}
+			if !r.readFull(r.buf[:len(magicBody)], false) {
+				return r.err
+			}
+			if string(r.buf[:len(magicBody)]) != magicBody {
+				if string(r.buf[:len(magicBody)]) != magicBodySnappy {
+					r.err = ErrCorrupt
+					return r.err
+				}
+			}
+
+			continue
+		}
+
+		if chunkType <= 0x7f {
+			// Section 4.5. Reserved unskippable chunks (chunk types 0x02-0x7f).
+			r.err = ErrUnsupported
+			return r.err
+		}
+		if chunkLen > maxChunkSize {
+			r.err = ErrUnsupported
+			return r.err
+		}
+		// Section 4.4 Padding (chunk type 0xfe).
+		// Section 4.6. Reserved skippable chunks (chunk types 0x80-0xfd).
+		if !r.skippable(r.buf, chunkLen, false, chunkType) {
+			return r.err
+		}
+	}
+	return nil
+}
+
+// ReadSeeker provides random or forward seeking in compressed content.
+// See Reader.ReadSeeker
+type ReadSeeker struct {
+	*Reader
+	readAtMu sync.Mutex
+}
+
+// ReadSeeker will return an io.ReadSeeker and io.ReaderAt
+// compatible version of the reader.
+// If 'random' is specified the returned io.Seeker can be used for
+// random seeking, otherwise only forward seeking is supported.
+// Enabling random seeking requires the original input to support
+// the io.Seeker interface.
+// A custom index can be specified which will be used if supplied.
+// When using a custom index, it will not be read from the input stream.
+// The ReadAt position will affect regular reads and the current position of Seek.
+// So using Read after ReadAt will continue from where the ReadAt stopped.
+// No functions should be used concurrently.
+// The returned ReadSeeker contains a shallow reference to the existing Reader,
+// meaning changes performed to one is reflected in the other.
+func (r *Reader) ReadSeeker(random bool, index []byte) (*ReadSeeker, error) {
+	// Read index if provided.
+	if len(index) != 0 {
+		if r.index == nil {
+			r.index = &Index{}
+		}
+		if _, err := r.index.Load(index); err != nil {
+			return nil, ErrCantSeek{Reason: "loading index returned: " + err.Error()}
+		}
+	}
+
+	// Check if input is seekable
+	rs, ok := r.r.(io.ReadSeeker)
+	if !ok {
+		if !random {
+			return &ReadSeeker{Reader: r}, nil
+		}
+		return nil, ErrCantSeek{Reason: "input stream isn't seekable"}
+	}
+
+	if r.index != nil {
+		// Seekable and index, ok...
+		return &ReadSeeker{Reader: r}, nil
+	}
+
+	// Load from stream.
+	r.index = &Index{}
+
+	// Read current position.
+	pos, err := rs.Seek(0, io.SeekCurrent)
+	if err != nil {
+		return nil, ErrCantSeek{Reason: "seeking input returned: " + err.Error()}
+	}
+	err = r.index.LoadStream(rs)
+	if err != nil {
+		if err == ErrUnsupported {
+			// If we don't require random seeking, reset input and return.
+			if !random {
+				_, err = rs.Seek(pos, io.SeekStart)
+				if err != nil {
+					return nil, ErrCantSeek{Reason: "resetting stream returned: " + err.Error()}
+				}
+				r.index = nil
+				return &ReadSeeker{Reader: r}, nil
+			}
+			return nil, ErrCantSeek{Reason: "input stream does not contain an index"}
+		}
+		return nil, ErrCantSeek{Reason: "reading index returned: " + err.Error()}
+	}
+
+	// reset position.
+	_, err = rs.Seek(pos, io.SeekStart)
+	if err != nil {
+		return nil, ErrCantSeek{Reason: "seeking input returned: " + err.Error()}
+	}
+	return &ReadSeeker{Reader: r}, nil
+}
+
+// Seek allows seeking in compressed data.
+func (r *ReadSeeker) Seek(offset int64, whence int) (int64, error) {
+	if r.err != nil {
+		if !errors.Is(r.err, io.EOF) {
+			return 0, r.err
+		}
+		// Reset on EOF
+		r.err = nil
+	}
+
+	// Calculate absolute offset.
+	absOffset := offset
+
+	switch whence {
+	case io.SeekStart:
+	case io.SeekCurrent:
+		absOffset = r.blockStart + int64(r.i) + offset
+	case io.SeekEnd:
+		if r.index == nil {
+			return 0, ErrUnsupported
+		}
+		absOffset = r.index.TotalUncompressed + offset
+	default:
+		r.err = ErrUnsupported
+		return 0, r.err
+	}
+
+	if absOffset < 0 {
+		return 0, errors.New("seek before start of file")
+	}
+
+	if !r.readHeader {
+		// Make sure we read the header.
+		_, r.err = r.Read([]byte{})
+		if r.err != nil {
+			return 0, r.err
+		}
+	}
+
+	// If we are inside current block no need to seek.
+	// This includes no offset changes.
+	if absOffset >= r.blockStart && absOffset < r.blockStart+int64(r.j) {
+		r.i = int(absOffset - r.blockStart)
+		return r.blockStart + int64(r.i), nil
+	}
+
+	rs, ok := r.r.(io.ReadSeeker)
+	if r.index == nil || !ok {
+		currOffset := r.blockStart + int64(r.i)
+		if absOffset >= currOffset {
+			err := r.Skip(absOffset - currOffset)
+			return r.blockStart + int64(r.i), err
+		}
+		return 0, ErrUnsupported
+	}
+
+	// We can seek and we have an index.
+	c, u, err := r.index.Find(absOffset)
+	if err != nil {
+		return r.blockStart + int64(r.i), err
+	}
+
+	// Seek to next block
+	_, err = rs.Seek(c, io.SeekStart)
+	if err != nil {
+		return 0, err
+	}
+
+	r.i = r.j                     // Remove rest of current block.
+	r.blockStart = u - int64(r.j) // Adjust current block start for accounting.
+	if u < absOffset {
+		// Forward inside block
+		return absOffset, r.Skip(absOffset - u)
+	}
+	if u > absOffset {
+		return 0, fmt.Errorf("s2 seek: (internal error) u (%d) > absOffset (%d)", u, absOffset)
+	}
+	return absOffset, nil
+}
+
+// ReadAt reads len(p) bytes into p starting at offset off in the
+// underlying input source. It returns the number of bytes
+// read (0 <= n <= len(p)) and any error encountered.
+//
+// When ReadAt returns n < len(p), it returns a non-nil error
+// explaining why more bytes were not returned. In this respect,
+// ReadAt is stricter than Read.
+//
+// Even if ReadAt returns n < len(p), it may use all of p as scratch
+// space during the call. If some data is available but not len(p) bytes,
+// ReadAt blocks until either all the data is available or an error occurs.
+// In this respect ReadAt is different from Read.
+//
+// If the n = len(p) bytes returned by ReadAt are at the end of the
+// input source, ReadAt may return either err == EOF or err == nil.
+//
+// If ReadAt is reading from an input source with a seek offset,
+// ReadAt should not affect nor be affected by the underlying
+// seek offset.
+//
+// Clients of ReadAt can execute parallel ReadAt calls on the
+// same input source. This is however not recommended.
+func (r *ReadSeeker) ReadAt(p []byte, offset int64) (int, error) {
+	r.readAtMu.Lock()
+	defer r.readAtMu.Unlock()
+	_, err := r.Seek(offset, io.SeekStart)
+	if err != nil {
+		return 0, err
+	}
+	n := 0
+	for n < len(p) {
+		n2, err := r.Read(p[n:])
+		if err != nil {
+			// This will include io.EOF
+			return n + n2, err
+		}
+		n += n2
+	}
+	return n, nil
+}
+
+// ReadByte satisfies the io.ByteReader interface.
+func (r *Reader) ReadByte() (byte, error) {
+	if r.err != nil {
+		return 0, r.err
+	}
+	if r.i < r.j {
+		c := r.decoded[r.i]
+		r.i++
+		return c, nil
+	}
+	var tmp [1]byte
+	for i := 0; i < 10; i++ {
+		n, err := r.Read(tmp[:])
+		if err != nil {
+			return 0, err
+		}
+		if n == 1 {
+			return tmp[0], nil
+		}
+	}
+	return 0, io.ErrNoProgress
+}
+
+// SkippableCB will register a callback for chunks with the specified ID.
+// ID must be a Reserved skippable chunks ID, 0x80-0xfe (inclusive).
+// For each chunk with the ID, the callback is called with the content.
+// Any returned non-nil error will abort decompression.
+// Only one callback per ID is supported, latest sent will be used.
+// Sending a nil function will disable previous callbacks.
+func (r *Reader) SkippableCB(id uint8, fn func(r io.Reader) error) error {
+	if id < 0x80 || id > chunkTypePadding {
+		return fmt.Errorf("ReaderSkippableCB: Invalid id provided, must be 0x80-0xfe (inclusive)")
+	}
+	r.skippableCB[id] = fn
+	return nil
+}
-- 
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