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// Copyright (c) 2019-2021 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.bits
import encoding.binary
// int_u64 returns a random unsigned 64-bit integer `u64` read from a real OS source of entropy.
pub fn int_u64(max u64) ?u64 {
bitlen := bits.len_64(max)
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) ?
bytes[0] &= byte(int(u64(1) << b) - 1)
x := bytes_to_u64(bytes)
n = x[0]
// NOTE: maybe until we have bigint could do it another way?
// if x.len > 1 {
// n = u64(u32(x[1])<<u32(32)) | n
// }
if n < max {
return n
}
}
return n
}
fn bytes_to_u64(b []byte) []u64 {
ws := 64 / 8
mut z := []u64{len: ((b.len + ws - 1) / ws)}
mut i := b.len
for k := 0; i >= ws; k++ {
z[k] = binary.big_endian_u64(b[i - ws..i])
i -= ws
}
if i > 0 {
mut d := u64(0)
for s := u64(0); i > 0; s += u64(8) {
d |= u64(b[i - 1]) << s
i--
}
z[z.len - 1] = d
}
return z
}
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