56 lines
1.1 KiB
V
56 lines
1.1 KiB
V
// Copyright (c) 2019-2021 Alexander Medvednikov. All rights reserved.
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// Use of this source code is governed by an MIT license
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// that can be found in the LICENSE file.
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module rand
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import math.bits
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import encoding.binary
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// int_u64 returns a random unsigned 64-bit integer `u64` read from a real OS source of entropy.
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pub fn int_u64(max u64) ?u64 {
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bitlen := bits.len_64(max)
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if bitlen == 0 {
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return u64(0)
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}
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k := (bitlen + 7) / 8
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mut b := u64(bitlen % 8)
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if b == u64(0) {
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b = u64(8)
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}
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mut n := u64(0)
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for {
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mut bytes := read(k) ?
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bytes[0] &= byte(int(u64(1) << b) - 1)
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x := bytes_to_u64(bytes)
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n = x[0]
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// NOTE: maybe until we have bigint could do it another way?
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// if x.len > 1 {
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// n = u64(u32(x[1])<<u32(32)) | n
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// }
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if n < max {
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return n
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}
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}
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return n
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}
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fn bytes_to_u64(b []byte) []u64 {
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ws := 64 / 8
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mut z := []u64{len: ((b.len + ws - 1) / ws)}
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mut i := b.len
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for k := 0; i >= ws; k++ {
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z[k] = binary.big_endian_u64(b[i - ws..i])
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i -= ws
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}
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if i > 0 {
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mut d := u64(0)
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for s := u64(0); i > 0; s += u64(8) {
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d |= u64(b[i - 1]) << s
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i--
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}
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z[z.len - 1] = d
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}
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return z
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}
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