js: implement more functions for JS backend (#12167)

vlib/rand/rand.js.v

@@ -1,5 +1,7 @@
 module rand
 
+import time
+
 // init initializes the default RNG.
 fn init() {
 	default_rng = new_default()
@@ -15,3 +17,51 @@ pub fn string(len int) string {
 
 	return result
 }
+
+const (
+	ulid_encoding = '0123456789ABCDEFGHJKMNPQRSTVWXYZ'
+)
+
+// ulid generates an Unique Lexicographically sortable IDentifier.
+// See https://github.com/ulid/spec .
+// NB: ULIDs can leak timing information, if you make them public, because
+// you can infer the rate at which some resource is being created, like
+// users or business transactions.
+// (https://news.ycombinator.com/item?id=14526173)
+pub fn ulid() string {
+	return ulid_at_millisecond(u64(time.utc().unix_time_milli()))
+}
+
+// ulid_at_millisecond does the same as `ulid` but takes a custom Unix millisecond timestamp via `unix_time_milli`.
+pub fn ulid_at_millisecond(unix_time_milli u64) string {
+	mut buf := []byte{cap: 27}
+	mut t := unix_time_milli
+	mut i := 9
+	for i >= 0 {
+		buf[i] = rand.ulid_encoding[int(t & 0x1f)]
+		t = t >> 5
+		i--
+	}
+
+	mut x := default_rng.u64()
+	i = 10
+	for i < 19 {
+		buf[i] = rand.ulid_encoding[int(x & 0x1f)]
+
+		x = x >> 5
+		i++
+	}
+
+	x = default_rng.u64()
+	for i < 26 {
+		buf[i] = rand.ulid_encoding[int(x & 0x1f)]
+		x = x >> 5
+		i++
+	}
+
+	res := ''
+	println(buf)
+	#res.str = buf.arr.arr.map(String.fromCharCode).join('')
+
+	return res
+}