fst: implement app id parser

Signed-off-by: Ophestra Umiker <cat@ophivana.moe>

fst/id.go

@@ -0,0 +1,48 @@
+package fst
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+	"errors"
+	"fmt"
+)
+
+type ID [16]byte
+
+var (
+	ErrInvalidLength = errors.New("string representation must have a length of 32")
+)
+
+func (a *ID) String() string {
+	return hex.EncodeToString(a[:])
+}
+
+func NewAppID(id *ID) error {
+	_, err := rand.Read(id[:])
+	return err
+}
+
+func ParseAppID(id *ID, s string) error {
+	if len(s) != 32 {
+		return ErrInvalidLength
+	}
+
+	for i, b := range s {
+		if b < '0' || b > 'f' {
+			return fmt.Errorf("invalid char %q at byte %d", b, i)
+		}
+
+		v := uint8(b)
+		if v > '9' {
+			v = 10 + v - 'a'
+		} else {
+			v -= '0'
+		}
+		if i%2 == 0 {
+			v <<= 4
+		}
+		id[i/2] += v
+	}
+
+	return nil
+}