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// Copyright 2020 The LUCI Authors.
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
package spanutil
import (
var zstdHeader = []byte("ztd\n")
// Globally shared zstd encoder and decoder. We use only their EncodeAll and
// DecodeAll methods which are allowed to be used concurrently. Internally, both
// the encode and the decoder have worker pools (limited by GOMAXPROCS) and each
// concurrent EncodeAll/DecodeAll call temporary consumes one worker (so overall
// we do not run more jobs that we have cores for).
var (
zstdEncoder *zstd.Encoder
zstdDecoder *zstd.Decoder
func init() {
var err error
if zstdEncoder, err = zstd.NewWriter(nil); err != nil {
panic(err) // this is impossible
if zstdDecoder, err = zstd.NewReader(nil); err != nil {
panic(err) // this is impossible
// Compressed instructs ToSpanner and FromSpanner functions to compress the
// content with encoding.
type Compressed []byte
// ToSpanner implements Value.
func (c Compressed) ToSpanner() interface{} {
if len(c) == 0 {
// Do not store empty bytes.
return []byte(nil)
return Compress(c)
// SpannerPtr implements Ptr.
func (c *Compressed) SpannerPtr(b *Buffer) interface{} {
return &b.ByteSlice
// FromSpanner implements Ptr.
func (c *Compressed) FromSpanner(b *Buffer) error {
if len(b.ByteSlice) == 0 {
// do not set to nil; otherwise we lose the buffer.
*c = (*c)[:0]
} else {
// *c might be pointing to an existing memory buffer.
// Try to reuse it for decoding.
var err error
if *c, err = Decompress(b.ByteSlice, *c); err != nil {
return err
return nil
// Compress compresses data using zstd.
func Compress(data []byte) []byte {
out := make([]byte, 0, len(data)/2+len(zstdHeader)) // hope for at least 2x compression
out = append(out, zstdHeader...)
return zstdEncoder.EncodeAll(data, out)
// Decompress decompresses the src compressed with Compress to dest.
// dest is the buffer for decompressed content, it will be reset to 0 length
// before taking the content.
func Decompress(src, dest []byte) ([]byte, error) {
if !bytes.HasPrefix(src, zstdHeader) {
return nil, errors.Reason("expected ztd header").Err()
dest, err := zstdDecoder.DecodeAll(src[len(zstdHeader):], dest[:0])
if err != nil {
return nil, errors.Annotate(err, "failed to decode from zstd").Err()
return dest, nil