zig/std/rand.zig

const std = @import("index.zig");
const builtin = @import("builtin");
const assert = std.debug.assert;
const rand_test = @import("rand_test.zig");
const mem = std.mem;
const math = std.math;

pub const MT19937_32 = MersenneTwister(
    u32, 624, 397, 31,
    0x9908B0DF,
    11, 0xFFFFFFFF,
    7, 0x9D2C5680,
    15, 0xEFC60000,
    18, 1812433253);

pub const MT19937_64 = MersenneTwister(
    u64, 312, 156, 31,
    0xB5026F5AA96619E9,
    29, 0x5555555555555555,
    17, 0x71D67FFFEDA60000,
    37, 0xFFF7EEE000000000,
    43, 6364136223846793005);

/// Use `init` to initialize this state.
pub const Rand = struct {
    const Rng = if (@sizeOf(usize) >= 8) MT19937_64 else MT19937_32;

    rng: Rng,

    /// Initialize random state with the given seed.
    pub fn init(seed: usize) -> Rand {
        return Rand {
            .rng = Rng.init(seed),
        };
    }

    /// Get an integer or boolean with random bits.
    pub fn scalar(r: &Rand, comptime T: type) -> T {
        if (T == usize) {
            return r.rng.get();
        } else if (T == bool) {
            return (r.rng.get() & 0b1) == 0;
        } else {
            var result: [@sizeOf(T)]u8 = undefined;
            r.fillBytes(result[0..]);
            return mem.readInt(result, T, builtin.Endian.Little);
        }
    }

    /// Fill `buf` with randomness.
    pub fn fillBytes(r: &Rand, buf: []u8) {
        var bytes_left = buf.len;
        while (bytes_left >= @sizeOf(usize)) {
            mem.writeInt(buf[buf.len - bytes_left..], r.rng.get(), builtin.Endian.Little);
            bytes_left -= @sizeOf(usize);
        }
        if (bytes_left > 0) {
            var rand_val_array: [@sizeOf(usize)]u8 = undefined;
            mem.writeInt(rand_val_array[0..], r.rng.get(), builtin.Endian.Little);
            while (bytes_left > 0) {
                buf[buf.len - bytes_left] = rand_val_array[@sizeOf(usize) - bytes_left];
                bytes_left -= 1;
            }
        }
    }

    /// Get a random unsigned integer with even distribution between `start`
    /// inclusive and `end` exclusive.
    pub fn range(r: &Rand, comptime T: type, start: T, end: T) -> T {
        assert(start <= end);
        if (T.is_signed) {
            const uint = @IntType(false, T.bit_count);
            if (start >= 0 and end >= 0) {
                return T(r.range(uint, uint(start), uint(end)));
            } else if (start < 0 and end < 0) {
                // Can't overflow because the range is over signed ints
                return math.negateCast(r.range(uint, math.absCast(end), math.absCast(start)) + 1) catch unreachable;
            } else if (start < 0 and end >= 0) {
                const end_uint = uint(end);
                const total_range = math.absCast(start) + end_uint;
                const value = r.range(uint, 0, total_range);
                const result = if (value < end_uint) x: {
                    break :x T(value);
                } else if (value == end_uint) x: {
                    break :x start;
                } else x: {
                    // Can't overflow because the range is over signed ints
                    break :x math.negateCast(value - end_uint) catch unreachable;
                };
                return result;
            } else {
                unreachable;
            }
        } else {
            const total_range = end - start;
            const leftover = @maxValue(T) % total_range;
            const upper_bound = @maxValue(T) - leftover;
            var rand_val_array: [@sizeOf(T)]u8 = undefined;

            while (true) {
                r.fillBytes(rand_val_array[0..]);
                const rand_val = mem.readInt(rand_val_array, T, builtin.Endian.Little);
                if (rand_val < upper_bound) {
                    return start + (rand_val % total_range);
                }
            }
        }
    }

    /// Get a floating point value in the range 0.0..1.0.
    pub fn float(r: &Rand, comptime T: type) -> T {
        // TODO Implement this way instead:
        // const int = @int_type(false, @sizeOf(T) * 8);
        // const mask = ((1 << @float_mantissa_bit_count(T)) - 1);
        // const rand_bits = r.rng.scalar(int) & mask;
        // return @float_compose(T, false, 0, rand_bits) - 1.0
        const int_type = @IntType(false, @sizeOf(T) * 8);
        const precision = if (T == f32)
            16777216
        else if (T == f64)
            9007199254740992
        else
            @compileError("unknown floating point type")
        ;
        return T(r.range(int_type, 0, precision)) / T(precision);
    }
};

fn MersenneTwister(
    comptime int: type, comptime n: usize, comptime m: usize, comptime r: int,
    comptime a: int,
    comptime u: math.Log2Int(int), comptime d: int,
    comptime s: math.Log2Int(int), comptime b: int,
    comptime t: math.Log2Int(int), comptime c: int,
    comptime l: math.Log2Int(int), comptime f: int) -> type
{
    return struct {
        const Self = this;

        array: [n]int,
        index: usize,

        pub fn init(seed: int) -> Self {
            var mt = Self {
                .array = undefined,
                .index = n,
            };

            var prev_value = seed;
            mt.array[0] = prev_value;
            var i: usize = 1;
            while (i < n) : (i += 1) {
                prev_value = int(i) +% f *% (prev_value ^ (prev_value >> (int.bit_count - 2)));
                mt.array[i] = prev_value;
            }
            return mt;
        }

        pub fn get(mt: &Self) -> int {
            const mag01 = []int{0, a};
            const LM: int = (1 << r) - 1;
            const UM = ~LM;

            if (mt.index >= n) {
                var i: usize = 0;

                while (i < n - m) : (i += 1) {
                    const x = (mt.array[i] & UM) | (mt.array[i + 1] & LM);
                    mt.array[i] = mt.array[i + m] ^ (x >> 1) ^ mag01[usize(x & 0x1)];
                }

                while (i < n - 1) : (i += 1) {
                    const x = (mt.array[i] & UM) | (mt.array[i + 1] & LM);
                    mt.array[i] = mt.array[i + m - n] ^ (x >> 1) ^ mag01[usize(x & 0x1)];

                }
                const x = (mt.array[i] & UM) | (mt.array[0] & LM);
                mt.array[i] = mt.array[m - 1] ^ (x >> 1) ^ mag01[usize(x & 0x1)];

                mt.index = 0;
            }

            var x = mt.array[mt.index];
            mt.index += 1;

            x ^= ((x >> u) & d);
            x ^= ((x << s) & b);
            x ^= ((x << t) & c);
            x ^= (x >> l);

            return x;
        }
    };
}

test "rand float 32" {
    var r = Rand.init(42);
    var i: usize = 0;
    while (i < 1000) : (i += 1) {
        const val = r.float(f32);
        assert(val >= 0.0);
        assert(val < 1.0);
    }
}

test "rand.MT19937_64" {
    var rng = MT19937_64.init(rand_test.mt64_seed);
    for (rand_test.mt64_data) |value| {
        assert(value == rng.get());
    }
}

test "rand.MT19937_32" {
    var rng = MT19937_32.init(rand_test.mt32_seed);
    for (rand_test.mt32_data) |value| {
        assert(value == rng.get());
    }
}

test "rand.Rand.range" {
    var r = Rand.init(42);
    testRange(&r, -4, 3);
    testRange(&r, -4, -1);
    testRange(&r, 10, 14);
}

fn testRange(r: &Rand, start: i32, end: i32) {
    const count = usize(end - start);
    var values_buffer = []bool{false} ** 20;
    const values = values_buffer[0..count];
    var i: usize = 0;
    while (i < count) {
        const value = r.range(i32, start, end);
        const index = usize(value - start);
        if (!values[index]) {
            i += 1;
            values[index] = true;
        }
    }
}