lib: Add file name compression

Allows to compress short arbitrary strings and returns a string using base64 url encoding.

Generator for tables included and a few samples has been added. Add more to init.go

Tested with fuzzing for crash resistance and symmetry, see fuzz.go

lib/encoder/filename/init.go

@@ -0,0 +1,89 @@
+package filename
+
+import (
+	"encoding/base64"
+	"sync"
+
+	"github.com/klauspost/compress/huff0"
+)
+
+// encodeURL is base64 url encoding values.
+const encodeURL = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_"
+
+// decodeMap will return x = decodeMap[encodeURL[byte(x)]] - 1 if x >= 0 and x < 64, otherwise -1 is returned.
+var decodeMap [256]byte
+
+// maxLength is the maximum length that will be attempted to be compressed.
+const maxLength = 256
+
+var (
+	initOnce sync.Once // Used to control init of tables.
+
+	encTables     [64]*huff0.Scratch // Encoders.
+	encTableLocks [64]sync.Mutex     // Temporary locks for encoders since they are stateful.
+	decTables     [64]*huff0.Decoder // Stateless decoders.
+)
+
+const (
+	tableUncompressed = 0
+	tableRLE          = 61
+	tableCustom       = 62
+	tableReserved     = 63
+)
+
+// predefined tables as base64 URL encoded string.
+var tablesData = [64]string{
+	// Uncompressed
+	tableUncompressed: "",
+	// ncw home directory
+	1: "MRDIEtAAMAzDMAzDSjX_ybu0w97bb-L3b2mR-rUl5LXW3lZII43kIDMzM1NXu3okgQs=",
+	// ncw images
+	2: "IhDIAEAA______-Pou_4Sf5z-uS-39MVWjullFLKM7EBECs=",
+	// ncw Google Drive:
+	3: "JxDQAIIBMDMzMwOzbv7nJJCyd_m_9D2llCarnQX33nvvlFKEhUxAAQ==",
+	// Hex
+	4: "ExDoSTD___-tfXfhJ0hKSkryTxU=",
+	// Base64
+	5: "JRDIcQf_______8PgIiIiIgINkggARHlkQwSSCCBxHFYINHdfXI=",
+
+	// Special tables:
+	// Compressed data has its own table.
+	tableCustom: "",
+	// Reserved for extension.
+	tableReserved: "",
+}
+
+func initCoders() {
+	initOnce.Do(func() {
+		// Init base 64 decoder.
+		for i, v := range encodeURL {
+			decodeMap[v] = byte(i) + 1
+		}
+
+		// Initialize encoders and decoders.
+		for i, dataString := range tablesData {
+			if len(dataString) == 0 {
+				continue
+			}
+			data, err := base64.URLEncoding.DecodeString(dataString)
+			if err != nil {
+				panic(err)
+			}
+			s, _, err := huff0.ReadTable(data, nil)
+			if err != nil {
+				panic(err)
+			}
+
+			// We want to save at least len(in) >> 5
+			s.WantLogLess = 5
+			s.Reuse = huff0.ReusePolicyMust
+			encTables[i] = s
+			decTables[i] = s.Decoder()
+		}
+		// Add custom table type.
+		var s huff0.Scratch
+		s.Reuse = huff0.ReusePolicyNone
+		encTables[tableCustom] = &s
+		decTables[tableCustom] = nil
+	})
+}