internal/pkg: decompress zstd streams

This uses internal/zstd until compress/zstd becomes available.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/zstd/bits.go

@@ -0,0 +1,130 @@
+// Copyright 2023 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 zstd
+
+import (
+	"math/bits"
+)
+
+// block is the data for a single compressed block.
+// The data starts immediately after the 3 byte block header,
+// and is Block_Size bytes long.
+type block []byte
+
+// bitReader reads a bit stream going forward.
+type bitReader struct {
+	r    *Reader // for error reporting
+	data block   // the bits to read
+	off  uint32  // current offset into data
+	bits uint32  // bits ready to be returned
+	cnt  uint32  // number of valid bits in the bits field
+}
+
+// makeBitReader makes a bit reader starting at off.
+func (r *Reader) makeBitReader(data block, off int) bitReader {
+	return bitReader{
+		r:    r,
+		data: data,
+		off:  uint32(off),
+	}
+}
+
+// moreBits is called to read more bits.
+// This ensures that at least 16 bits are available.
+func (br *bitReader) moreBits() error {
+	for br.cnt < 16 {
+		if br.off >= uint32(len(br.data)) {
+			return br.r.makeEOFError(int(br.off))
+		}
+		c := br.data[br.off]
+		br.off++
+		br.bits |= uint32(c) << br.cnt
+		br.cnt += 8
+	}
+	return nil
+}
+
+// val is called to fetch a value of b bits.
+func (br *bitReader) val(b uint8) uint32 {
+	r := br.bits & ((1 << b) - 1)
+	br.bits >>= b
+	br.cnt -= uint32(b)
+	return r
+}
+
+// backup steps back to the last byte we used.
+func (br *bitReader) backup() {
+	for br.cnt >= 8 {
+		br.off--
+		br.cnt -= 8
+	}
+}
+
+// makeError returns an error at the current offset wrapping a string.
+func (br *bitReader) makeError(msg string) error {
+	return br.r.makeError(int(br.off), msg)
+}
+
+// reverseBitReader reads a bit stream in reverse.
+type reverseBitReader struct {
+	r     *Reader // for error reporting
+	data  block   // the bits to read
+	off   uint32  // current offset into data
+	start uint32  // start in data; we read backward to start
+	bits  uint32  // bits ready to be returned
+	cnt   uint32  // number of valid bits in bits field
+}
+
+// makeReverseBitReader makes a reverseBitReader reading backward
+// from off to start. The bitstream starts with a 1 bit in the last
+// byte, at off.
+func (r *Reader) makeReverseBitReader(data block, off, start int) (reverseBitReader, error) {
+	streamStart := data[off]
+	if streamStart == 0 {
+		return reverseBitReader{}, r.makeError(off, "zero byte at reverse bit stream start")
+	}
+	rbr := reverseBitReader{
+		r:     r,
+		data:  data,
+		off:   uint32(off),
+		start: uint32(start),
+		bits:  uint32(streamStart),
+		cnt:   uint32(7 - bits.LeadingZeros8(streamStart)),
+	}
+	return rbr, nil
+}
+
+// val is called to fetch a value of b bits.
+func (rbr *reverseBitReader) val(b uint8) (uint32, error) {
+	if !rbr.fetch(b) {
+		return 0, rbr.r.makeEOFError(int(rbr.off))
+	}
+
+	rbr.cnt -= uint32(b)
+	v := (rbr.bits >> rbr.cnt) & ((1 << b) - 1)
+	return v, nil
+}
+
+// fetch is called to ensure that at least b bits are available.
+// It reports false if this can't be done,
+// in which case only rbr.cnt bits are available.
+func (rbr *reverseBitReader) fetch(b uint8) bool {
+	for rbr.cnt < uint32(b) {
+		if rbr.off <= rbr.start {
+			return false
+		}
+		rbr.off--
+		c := rbr.data[rbr.off]
+		rbr.bits <<= 8
+		rbr.bits |= uint32(c)
+		rbr.cnt += 8
+	}
+	return true
+}
+
+// makeError returns an error at the current offset wrapping a string.
+func (rbr *reverseBitReader) makeError(msg string) error {
+	return rbr.r.makeError(int(rbr.off), msg)
+}