Implement zstd compression

Zstd compression can be enabled by providing `-zstd` or `-zstd-level <level>`
to `init`, `add`, or `backup`. With `-zstd` the compression level will be
`default`, and with `-zstd-level` the level can be any of `fastest`, `default`,
`better`, or `best`.

src/duplicacy_backupmanager.go

@@ -47,6 +47,10 @@ func (manager *BackupManager) SetDryRun(dryRun bool) {
 	manager.config.dryRun = dryRun
 }
 
+func (manager *BackupManager) SetCompressionLevel(level int) {
+	manager.config.CompressionLevel = level
+}
+
 // CreateBackupManager creates a backup manager using the specified 'storage'.  'snapshotID' is a unique id to
 // identify snapshots created for this repository.  'top' is the top directory of the repository.  'password' is the
 // master key which can be nil if encryption is not enabled.
@@ -138,6 +142,8 @@ func (manager *BackupManager) Backup(top string, quickMode bool, threads int, ta
 
 	LOG_DEBUG("BACKUP_PARAMETERS", "top: %s, quick: %t, tag: %s", top, quickMode, tag)
 
+	manager.config.PrintCompressionLevel()
+
 	if manager.config.DataShards != 0 && manager.config.ParityShards != 0 {
 		LOG_INFO("BACKUP_ERASURECODING", "Erasure coding is enabled with %d data shards and %d parity shards",
 		         manager.config.DataShards, manager.config.ParityShards)

src/duplicacy_chunk.go

@@ -24,6 +24,7 @@ import (
 	"github.com/bkaradzic/go-lz4"
 	"github.com/minio/highwayhash"
 	"github.com/klauspost/reedsolomon"
+	"github.com/klauspost/compress/zstd"
 
 	// This is a fork of github.com/minio/highwayhash at 1.0.1 that computes incorrect hash on
 	// arm64 machines.  We need this fork to be able to read the chunks created by Duplicacy
@@ -267,6 +268,38 @@ func (chunk *Chunk) Encrypt(encryptionKey []byte, derivationKey string, isMetada
 		deflater, _ := zlib.NewWriterLevel(encryptedBuffer, chunk.config.CompressionLevel)
 		deflater.Write(chunk.buffer.Bytes())
 		deflater.Close()
+	} else if chunk.config.CompressionLevel >= ZSTD_COMPRESSION_LEVEL_FASTEST && chunk.config.CompressionLevel <= ZSTD_COMPRESSION_LEVEL_BEST  {
+		encryptedBuffer.Write([]byte("ZSTD"))
+
+		compressionLevel := zstd.SpeedDefault
+		if chunk.config.CompressionLevel == ZSTD_COMPRESSION_LEVEL_FASTEST {
+			compressionLevel = zstd.SpeedFastest
+		} else if chunk.config.CompressionLevel == ZSTD_COMPRESSION_LEVEL_BETTER {
+			compressionLevel = zstd.SpeedBetterCompression
+		} else if chunk.config.CompressionLevel == ZSTD_COMPRESSION_LEVEL_BEST {
+			compressionLevel = zstd.SpeedBestCompression
+		}
+
+		deflater, err := zstd.NewWriter(encryptedBuffer, zstd.WithEncoderLevel(compressionLevel))
+		if err != nil {
+			return err
+		}
+
+		// Make sure we have enough space in encryptedBuffer
+		availableLength := encryptedBuffer.Cap() - len(encryptedBuffer.Bytes())
+		maximumLength := deflater.MaxEncodedSize(chunk.buffer.Len())
+		if availableLength < maximumLength {
+			encryptedBuffer.Grow(maximumLength - availableLength)
+		}
+		_, err = deflater.Write(chunk.buffer.Bytes())
+		if err != nil {
+			return fmt.Errorf("ZSTD compression error: %v", err)
+		}
+
+		err = deflater.Close()
+		if err != nil {
+			return fmt.Errorf("ZSTD compression error: %v", err)
+		}
 	} else if chunk.config.CompressionLevel == DEFAULT_COMPRESSION_LEVEL {
 		encryptedBuffer.Write([]byte("LZ4 "))
 		// Make sure we have enough space in encryptedBuffer
@@ -361,7 +394,6 @@ func (chunk *Chunk) Encrypt(encryptionKey []byte, derivationKey string, isMetada
 	chunk.buffer.Write(header)
 
 	return nil
-
 }
 
 // This is to ensure compatibility with Vertical Backup, which still uses HMAC-SHA256 (instead of HMAC-BLAKE2) to
@@ -633,6 +665,24 @@ func (chunk *Chunk) Decrypt(encryptionKey []byte, derivationKey string) (err err
 		chunk.hash = nil
 		return nil, rewriteNeeded
 	}
+
+	if len(compressed) > 4 && string(compressed[:4]) == "ZSTD" {
+		chunk.buffer.Reset()
+		chunk.hasher = chunk.config.NewKeyedHasher(chunk.config.HashKey)
+		chunk.hash = nil
+
+		encryptedBuffer.Read(encryptedBuffer.Bytes()[:4])
+		inflater, err := zstd.NewReader(encryptedBuffer)
+		if err != nil {
+			return err, false
+		}
+		defer inflater.Close()
+		if _, err = io.Copy(chunk, inflater); err != nil {
+			return err, false
+		}
+		return nil, rewriteNeeded
+	}
+
 	inflater, err := zlib.NewReader(encryptedBuffer)
 	if err != nil {
 		return err, false

src/duplicacy_config.go

@@ -35,6 +35,19 @@ var DEFAULT_KEY = []byte("duplicacy")
 // standard zlib levels of -1 to 9.
 var DEFAULT_COMPRESSION_LEVEL = 100
 
+// zstd compression levels starting from 200
+var ZSTD_COMPRESSION_LEVEL_FASTEST = 200
+var ZSTD_COMPRESSION_LEVEL_DEFAULT = 201
+var ZSTD_COMPRESSION_LEVEL_BETTER = 202
+var ZSTD_COMPRESSION_LEVEL_BEST = 203
+
+var ZSTD_COMPRESSION_LEVELS = map[string]int {
+    "fastest": ZSTD_COMPRESSION_LEVEL_FASTEST,
+	"default": ZSTD_COMPRESSION_LEVEL_DEFAULT,
+	"better":  ZSTD_COMPRESSION_LEVEL_BETTER,
+	"best":    ZSTD_COMPRESSION_LEVEL_BEST,
+}
+
 // The new banner of the config file (to differentiate from the old format where the salt and iterations are fixed)
 var CONFIG_BANNER = "duplicacy\001"
 
@@ -202,6 +215,14 @@ func (config *Config) Print() {
 
 }
 
+func (config *Config) PrintCompressionLevel() {
+	for name, level := range ZSTD_COMPRESSION_LEVELS {
+		if level == config.CompressionLevel {
+			LOG_INFO("COMPRESSION_LEVEL", "Zstd compression is enabled (level: %s)", name)
+		}
+	}
+}
+
 func CreateConfigFromParameters(compressionLevel int, averageChunkSize int, maximumChunkSize int, mininumChunkSize int,
 	isEncrypted bool, copyFrom *Config, bitCopy bool) (config *Config) {
 
@@ -294,7 +315,10 @@ func (config *Config) PutChunk(chunk *Chunk) {
 }
 
 func (config *Config) NewKeyedHasher(key []byte) hash.Hash {
-	if config.CompressionLevel == DEFAULT_COMPRESSION_LEVEL {
+	// Early versions of Duplicacy used SHA256 as the hash function for chunk IDs at the time when
+	// only zlib compression was supported.  Later SHA256 was replaced by Blake2b and LZ4 was used
+	// for compression (with compression level set to 100).
+	if config.CompressionLevel >= DEFAULT_COMPRESSION_LEVEL {
 		hasher, err := blake2.New(&blake2.Config{Size: 32, Key: key})
 		if err != nil {
 			LOG_ERROR("HASH_KEY", "Invalid hash key: %x", key)
@@ -339,7 +363,7 @@ func (hasher *DummyHasher) BlockSize() int {
 func (config *Config) NewFileHasher() hash.Hash {
 	if SkipFileHash {
 		return &DummyHasher{}
-	} else if config.CompressionLevel == DEFAULT_COMPRESSION_LEVEL {
+	} else if config.CompressionLevel >= DEFAULT_COMPRESSION_LEVEL {
 		hasher, _ := blake2.New(&blake2.Config{Size: 32})
 		return hasher
 	} else {