crypto.rand: add PRNG function read_u64

vlib/crypto/rand/rand_test.v

@@ -4,7 +4,7 @@
 
 import crypto.rand
 
-fn test_crypto_rand() {
+fn test_crypto_rand_read() {
 	no_bytes := 100
 	max_percentage_diff := 20
 
@@ -28,3 +28,16 @@ fn test_crypto_rand() {
 
 	assert diff_percentage <= max_percentage_diff
 }
+
+fn test_crypto_rand_read_u64() {
+	max := u64(200)
+	r1 := rand.read_u64(max) or {
+		assert false
+		return
+	}
+	r2 := rand.read_u64(max) or {
+		assert false
+		return
+	}
+	assert r1 > u64(0) && r2 > u64(0) && r1 < max && r2 < max
+}

vlib/crypto/rand/utils.v

@@ -0,0 +1,63 @@
+// Copyright (c) 2019 Alexander Medvednikov. All rights reserved.
+// Use of this source code is governed by an MIT license
+// that can be found in the LICENSE file.
+
+module rand
+
+import(
+	math
+	encoding.binary
+)
+
+pub fn read_u64(max u64) u64? {
+	if max <= u64(0) {
+		return error('crypto.rand: argument to read_u64 is <= 0')
+	}
+	// bitlen := int(math.floor(math.log2(f64(max))+1))
+	bitlen := int(math.floor(math.log(f64(max))/math.log(2)) + 1)
+	if bitlen == 0 {
+		return u64(0)
+	}
+	k := (bitlen + 7) / 8
+	mut b := u64(bitlen % 8)
+	if b == u64(0) {
+		b = u64(8)
+	}
+	mut n := u64(0)
+	for {
+		mut bytes := read(k) or {
+			return error(err)
+		}
+		bytes[0] &= byte(int(u64(1)<<b) - 1)
+		x := bytes_to_u64(bytes)
+		n = x[0]
+		if x.len > 1 {
+			n = u64(u32(x[1])<<u32(32)) | n
+		}
+		if int(n) < 0 {
+			n = -n
+		}
+		if n < max {
+			return n
+		}
+	}
+	return n
+}
+
+fn bytes_to_u64(b []byte) []u64 {   
+	mut z := [u64(0)].repeat((b.len + 8 - 1) / 8)
+	mut i := b.len
+	for k := 0; i >= 8; k++ {
+		z[k] = binary.big_endian_u64(b.slice(i-8, i))
+		i -= 8
+	}
+	if i > 0 {
+		mut d := u64(0)
+		for s := u64(0); i > 0; s += u64(8) {
+			d |= u64(u64(b[i-1]) << s)
+			i--
+		}
+		z[z.len-1] = d
+	}
+	return z
+}