feat: add url shortner

pkg/shorturl/code.go

@@ -0,0 +1,38 @@
+package shorturl
+
+import (
+	"crypto/rand"
+	"fmt"
+	"math/big"
+	"regexp"
+)
+
+// CodeLength is the length of generated short codes. 10 base36 characters
+// give 36^10 ≈ 3.6 × 10^15 combinations.
+const CodeLength = 10
+
+// codeAlphabet is the base36 alphabet (lowercase letters + digits).
+const codeAlphabet = "abcdefghijklmnopqrstuvwxyz0123456789"
+
+var codeAlphabetBig = big.NewInt(int64(len(codeAlphabet)))
+
+// RandomCode returns a cryptographically random base36 code of CodeLength chars.
+func RandomCode() (string, error) {
+	buf := make([]byte, CodeLength)
+	for i := range buf {
+		n, err := rand.Int(rand.Reader, codeAlphabetBig)
+		if err != nil {
+			return "", fmt.Errorf("random code: %w", err)
+		}
+		buf[i] = codeAlphabet[n.Int64()]
+	}
+	return string(buf), nil
+}
+
+var validCodeRe = regexp.MustCompile(`^[a-z0-9]{4,16}$`)
+
+// ValidCode returns true if s could plausibly be a short code we issued.
+// Lenient on length (4-16) to allow future tuning without breaking old links.
+func ValidCode(s string) bool {
+	return validCodeRe.MatchString(s)
+}