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/encode.go

@@ -0,0 +1,60 @@
+package filename
+
+import (
+	"encoding/base64"
+	"encoding/binary"
+
+	"github.com/klauspost/compress/huff0"
+)
+
+// Encode will encode the string and return a base64 (url) compatible version of it.
+// Calling Decode with the returned string should always succeed.
+// It is not a requirement that the input string is valid utf-8.
+func Encode(s string) string {
+	initCoders()
+	bestSize := len(s)
+	bestTable := tableUncompressed
+	org := []byte(s)
+	bestOut := []byte(s)
+
+	// Try all tables and choose the best
+	for i, enc := range encTables[:] {
+		if len(org) <= 1 || len(org) > maxLength {
+			// Use the uncompressed
+			break
+		}
+		if enc == nil {
+			continue
+		}
+		// Try to encode using table.
+		err := func() error {
+			encTableLocks[i].Lock()
+			defer encTableLocks[i].Unlock()
+			out, _, err := huff0.Compress1X(org, enc)
+			if err != nil {
+				return err
+			}
+			if len(out) < bestSize {
+				bestOut = bestOut[:len(out)]
+				bestTable = i
+				bestSize = len(out)
+				copy(bestOut, out)
+			}
+			return nil
+		}()
+		// If input is a single byte repeated store as RLE or save uncompressed.
+		if err == huff0.ErrUseRLE {
+			if len(org) > 2 {
+				// Encode as one byte repeated since it will be smaller than uncompressed.
+				n := binary.PutUvarint(bestOut, uint64(len(org)))
+				bestOut = bestOut[:n+1]
+				bestOut[n] = org[0]
+				bestSize = n + 1
+				bestTable = tableRLE
+			}
+			break
+		}
+	}
+
+	return string(encodeURL[bestTable]) + base64.URLEncoding.EncodeToString(bestOut)
+}