94 lines
2.8 KiB
Zig
94 lines
2.8 KiB
Zig
// Mersenne Twister
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const ARRAY_SIZE : i16 = 624;
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/// Use `rand_init` to initialize this state.
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pub struct Rand {
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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) => {
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r.index = 0;
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r.array[0] = seed;
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var i : @typeof(ARRAY_SIZE) = 1;
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var prev_value: u64 = seed;
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while (i < ARRAY_SIZE) {
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r.array[i] = u32((prev_value ^ (prev_value << 30)) * 0x6c078965 + u32(i));
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prev_value = r.array[i];
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i += 1;
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}
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}
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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) {
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r.generate_numbers();
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}
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// temper the number
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var y : u32 = r.array[r.index];
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y ^= y >> 11;
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y ^= (y >> 7) & 0x9d2c5680;
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y ^= (y >> 15) & 0xefc60000;
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y ^= y >> 18;
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r.index = (r.index + 1) % ARRAY_SIZE;
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return y;
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}
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/// Fill `buf` with randomness.
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pub fn get_bytes(r: &Rand, buf: []u8) => {
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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;
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*((&u32)(rand_val_array.ptr)) = 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;
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}
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}
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}
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/// Get a random unsigned integer with even distribution between `start`
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/// 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;
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const upper_bound = @max_value(u64) - leftover;
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var rand_val_array : [@sizeof(u64)]u8;
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while (true) {
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r.get_bytes_aligned(rand_val_array);
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const rand_val = *(&u64)(rand_val_array.ptr);
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if (rand_val < upper_bound) {
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return start + (rand_val % range);
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}
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}
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}
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fn generate_numbers(r: &Rand) => {
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for (item, r.array, i) {
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const y : u32 = (item & 0x80000000) + (r.array[(i + 1) % ARRAY_SIZE] & 0x7fffffff);
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const untempered : u32 = r.array[(i + 397) % ARRAY_SIZE] ^ (y >> 1);
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r.array[i] = if ((y % 2) == 0) {
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untempered
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} else {
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// y is odd
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untempered ^ 0x9908b0df
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};
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}
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}
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// does not populate the remaining (buf.len % 4) bytes
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fn get_bytes_aligned(r: &Rand, buf: []u8) isize => {
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var bytes_left = buf.len;
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while (bytes_left >= 4) {
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*((&u32)(&buf[buf.len - bytes_left])) = r.get_u32();
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bytes_left -= @sizeof(u32);
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}
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return bytes_left;
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}
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}
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