Add Decode (#4)

Author: cristaloleg

example_test.go

@@ -7,11 +7,18 @@ import (
 )
 
 func Example() {
-	s := miniid.Encode(1234)
-	fmt.Printf("num: %d -> %s\n", 1234, s)
+	const num = uint64(1234)
+	const long = uint64(405160095143)
+	const str1 = "Ju"
+	const str2 = "078FWT6F"
 
-	f := miniid.EncodeFixed(1234, 8)
-	fmt.Printf("fix: %d -> %s\n", 1234, f)
+	s := miniid.Encode(num)
+	d1 := miniid.Decode(str1)
+	fmt.Printf("num: %d -> %s -> %d\n", num, s, d1)
+
+	f := miniid.EncodeFixed(long, 8)
+	d2 := miniid.Decode(str2)
+	fmt.Printf("fix: %d -> %s -> %d\n", long, f, d2)
 
 	gen := miniid.New(1200)
 
@@ -21,7 +28,7 @@ func Example() {
 	fmt.Println()
 
 	// Output:
-	// num: 1234 -> Ju
-	// fix: 1234 -> 000000Ju
+	// num: 1234 -> Ju -> 1234
+	// fix: 405160095143 -> 078FWT6F -> 405160095143
 	// JN JO JP JQ JR JS JT JU JV JW
 }

miniid.go

@@ -2,6 +2,22 @@ package miniid
 
 import "sync/atomic"
 
+const (
+	Alphabet = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
+	base     = uint64(len(Alphabet))
+)
+
+var index = func() [256]int {
+	var index [256]int
+	for i := range index {
+		index[i] = -1
+	}
+	for i, c := range Alphabet {
+		index[c] = i
+	}
+	return index
+}()
+
 // Encode number in base62.
 func Encode(n uint64) string {
 	return encodeBase62(n)
@@ -12,6 +28,20 @@ func EncodeFixed(n uint64, length int) string {
 	return encodeFixedBase62(n, length)
 }
 
+// Decode converts a base62-encoded string back into a uint64 number.
+// It panics if the input contains characters not in the base62 alphabet.
+func Decode(s string) uint64 {
+	var n uint64
+	for i := 0; i < len(s); i++ {
+		val := index[s[i]]
+		if val < 0 {
+			panic("invalid base62 character")
+		}
+		n = n*base + uint64(val)
+	}
+	return n
+}
+
 // Generator can encode integers into base62 strings.
 type Generator struct {
 	currID atomic.Int64
@@ -31,11 +61,8 @@ func (g *Generator) Next() string {
 }
 
 func encodeBase62(num uint64) string {
-	const alphabet = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
-	const base = uint64(len(alphabet))
-
 	if num == 0 {
-		return string(alphabet[0])
+		return string(Alphabet[0])
 	}
 
 	const maxLen = 16
@@ -44,19 +71,16 @@ func encodeBase62(num uint64) string {
 
 	for ; num > 0; i-- {
 		rem := num % base
-		result[i] = alphabet[rem]
+		result[i] = Alphabet[rem]
 		num /= base
 	}
 	return string(result[i+1:])
 }
 
 func encodeFixedBase62(num uint64, length int) string {
-	const alphabet = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
-	const base = uint64(len(alphabet))
-
 	buf := make([]byte, length)
 	for i := length - 1; i >= 0; i-- {
-		buf[i] = alphabet[num%base]
+		buf[i] = Alphabet[num%base]
 		num /= base
 	}
 	return string(buf)

miniid_test.go

@@ -62,6 +62,20 @@ func TestEncodeFixed(t *testing.T) {
 	}
 }
 
+func TestEncodeDecode(t *testing.T) {
+	tests := []uint64{
+		0, 1, 10, 61, 62, 12345, 999999, 405160095143, 1<<63 - 1,
+	}
+
+	for _, n := range tests {
+		enc := Encode(n)
+		dec := Decode(enc)
+		if dec != n {
+			t.Fatalf("Encode/Decode failed for %d: got %d (encoded: %s)", n, dec, enc)
+		}
+	}
+}
+
 func TestGenerator(t *testing.T) {
 	testCases := []struct {
 		num  uint64