zig/std/rand.zig

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// Mersenne Twister
const ARRAY_SIZE : u16 = 624;
/// Use `rand_init` to initialize this state.
pub struct Rand {
array: [ARRAY_SIZE]u32,
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index: @typeof(ARRAY_SIZE),
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/// Initialize random state with the given seed.
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pub fn init(r: &Rand, seed: u32) => {
r.index = 0;
r.array[0] = seed;
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var i : @typeof(ARRAY_SIZE) = 1;
while (i < ARRAY_SIZE) {
const prev_value : u64 = r.array[i - 1];
r.array[i] = ((prev_value ^ (prev_value << 30)) * 0x6c078965 + i) as u32;
i += 1;
}
}
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/// Get 32 bits of randomness.
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pub fn get_u32(r: &Rand) u32 => {
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if (r.index == 0) {
r.generate_numbers();
}
// temper the number
var y : u32 = r.array[r.index];
y ^= y >> 11;
y ^= (y >> 7) & 0x9d2c5680;
y ^= (y >> 15) & 0xefc60000;
y ^= y >> 18;
r.index = (r.index + 1) % ARRAY_SIZE;
return y;
}
/// Fill `buf` with randomness.
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pub fn get_bytes(r: &Rand, buf: []u8) => {
var bytes_left = r.get_bytes_aligned(buf);
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if (bytes_left > 0) {
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var rand_val_array : [@sizeof(u32)]u8;
*(rand_val_array.ptr as (&u32)) = r.get_u32();
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while (bytes_left > 0) {
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buf[buf.len - bytes_left] = rand_val_array[@sizeof(u32) - bytes_left];
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bytes_left -= 1;
}
}
}
/// Get a random unsigned integer with even distribution between `start`
/// inclusive and `end` exclusive.
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pub fn range_u64(r: &Rand, start: u64, end: u64) u64 => {
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const range = end - start;
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const leftover = @max_value(u64) % range;
const upper_bound = @max_value(u64) - leftover;
var rand_val_array : [@sizeof(u64)]u8;
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while (true) {
r.get_bytes_aligned(rand_val_array);
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const rand_val = *(rand_val_array.ptr as (&u64));
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if (rand_val < upper_bound) {
return start + (rand_val % range);
}
}
}
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fn generate_numbers(r: &Rand) => {
var i : @typeof(ARRAY_SIZE) = 0;
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while (i < ARRAY_SIZE) {
const y : u32 = (r.array[i] & 0x80000000) + (r.array[(i + 1) % ARRAY_SIZE] & 0x7fffffff);
const untempered : u32 = r.array[(i + 397) % ARRAY_SIZE] ^ (y >> 1);
r.array[i] = if ((y % 2) == 0) {
untempered
} else {
// y is odd
untempered ^ 0x9908b0df
};
i += 1;
}
}
// does not populate the remaining (buf.len % 4) bytes
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fn get_bytes_aligned(r: &Rand, buf: []u8) usize => {
var bytes_left = buf.len;
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while (bytes_left >= 4) {
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*(&buf[buf.len - bytes_left] as (&u32)) = r.get_u32();
bytes_left -= @sizeof(u32);
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}
return bytes_left;
}
}