encoding: add a hex sub-module (#11193)

vlib/encoding/hex/hex.v

@@ -0,0 +1,38 @@
+module hex
+
+import strconv
+import strings
+
+// decode converts a hex string into an array of bytes. The expected
+// input format is 2 ASCII characters for each output byte. If the provided
+// string length is not a multiple of 2, an implicit `0` is prepended to it.
+pub fn decode(s string) ?[]byte {
+	if s.len == 0 {
+		return []byte{}
+	} else if s.len <= 2 {
+		return [byte(strconv.parse_uint(s, 16, 8) ?)]
+	}
+	// calculate the first byte depending on if s.len is odd
+	val := byte(strconv.parse_uint(s[..2 - (s.len & 1)], 16, 8) ?)
+	// set cap to s.len/2 rounded up
+	mut bytes := []byte{len: 1, cap: (s.len + 1) >> 1, init: val}
+	// iterate over every 2 bytes
+	// the start index depends on if s.len is odd
+	for i := 2 - (s.len & 1); i < s.len; i += 2 {
+		bytes << byte(strconv.parse_uint(s[i..i + 2], 16, 8) ?)
+	}
+	return bytes
+}
+
+// encode converts an array of bytes into a string of ASCII hex bytes. The
+// output will always be a string with length a multiple of 2.
+[manualfree]
+pub fn encode(bytes []byte) string {
+	mut sb := strings.new_builder(bytes.len << 1)
+	for b in bytes {
+		sb.write_string(b.hex())
+	}
+	res := sb.str()
+	unsafe { sb.free() }
+	return res
+}