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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 mt19937
+
+import math.bits
+
+/*
+C++ functions for MT19937, with initialization improved 2002/2/10.
+   Coded by Takuji Nishimura and Makoto Matsumoto.
+   This is a faster version by taking Shawn Cokus's optimization,
+   Matthe Bellew's simplification, Isaku Wada's real version.
+
+   Copyright (C) 1997 - 2002, Makoto Matsumoto and Takuji Nishimura,
+   All rights reserved.
+
+   Redistribution and use in source and binary forms, with or without
+   modification, are permitted provided that the following conditions
+   are met:
+
+     1. Redistributions of source code must retain the above copyright
+        notice, this list of conditions and the following disclaimer.
+
+     2. Redistributions in binary form must reproduce the above copyright
+        notice, this list of conditions and the following disclaimer in the
+        documentation and/or other materials provided with the distribution.
+
+     3. The names of its contributors may not be used to endorse or promote
+        products derived from this software without specific prior written
+        permission.
+
+   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+   "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+   LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+   A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+   CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+   EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+   PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+   PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+   LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+   NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+   SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+
+   Any feedback is very welcome.
+   http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/emt.html
+   email: m-mat @ math.sci.hiroshima-u.ac.jp (remove space)
+*/
+const (
+	nn            = 312
+	mm            = 156
+	matrix_a      = 0xB5026F5AA96619E9
+	um            = 0xFFFFFFFF80000000
+	lm            = 0x7FFFFFFF
+	inv_f64_limit = 1.0 / 9007199254740992.0
+)
+
+// MT19937RNG is generator that uses the Mersenne Twister algorithm with period 2^19937.
+// **NOTE**: The RNG is not seeded when instantiated so remember to seed it before use.
+pub struct MT19937RNG {
+mut:
+	state    []u64 = []u64{len: mt19937.nn}
+	mti      int   = mt19937.nn
+	next_rnd u32
+	has_next bool
+}
+
+// calculate_state returns a random state array calculated from the `seed_data`.
+fn calculate_state(seed_data []u32, mut state []u64) []u64 {
+	lo := u64(seed_data[0])
+	hi := u64(seed_data[1])
+	state[0] = u64((hi << 32) | lo)
+	for j := 1; j < mt19937.nn; j++ {
+		state[j] = u64(6364136223846793005) * (state[j - 1] ^ (state[j - 1] >> 62)) + u64(j)
+	}
+	return *state
+}
+
+// seed sets the current random state based on `seed_data`.
+// seed expects `seed_data` to be only two `u32`s in little-endian format as [lower, higher].
+pub fn (mut rng MT19937RNG) seed(seed_data []u32) {
+	if seed_data.len != 2 {
+		eprintln('mt19937 needs only two 32bit integers as seed: [lower, higher]')
+		exit(1)
+	}
+	// calculate 2 times because MT19937RNG init didn't call calculate_state.
+	rng.state = calculate_state(seed_data, mut rng.state)
+	rng.state = calculate_state(seed_data, mut rng.state)
+	rng.mti = mt19937.nn
+	rng.next_rnd = 0
+	rng.has_next = false
+}
+
+// u32 returns a pseudorandom 32bit int in range `[0, 2³²)`.
+[inline]
+pub fn (mut rng MT19937RNG) u32() u32 {
+	if rng.has_next {
+		rng.has_next = false
+		return rng.next_rnd
+	}
+	ans := rng.u64()
+	rng.next_rnd = u32(ans >> 32)
+	rng.has_next = true
+	return u32(ans & 0xffffffff)
+}
+
+// u64 returns a pseudorandom 64bit int in range `[0, 2⁶⁴)`.
+[inline]
+pub fn (mut rng MT19937RNG) u64() u64 {
+	mag01 := [u64(0), u64(mt19937.matrix_a)]
+	mut x := u64(0)
+	mut i := int(0)
+	if rng.mti >= mt19937.nn {
+		for i = 0; i < mt19937.nn - mt19937.mm; i++ {
+			x = (rng.state[i] & mt19937.um) | (rng.state[i + 1] & mt19937.lm)
+			rng.state[i] = rng.state[i + mt19937.mm] ^ (x >> 1) ^ mag01[int(x & 1)]
+		}
+		for i < mt19937.nn - 1 {
+			x = (rng.state[i] & mt19937.um) | (rng.state[i + 1] & mt19937.lm)
+			rng.state[i] = rng.state[i + (mt19937.mm - mt19937.nn)] ^ (x >> 1) ^ mag01[int(x & 1)]
+			i++
+		}
+		x = (rng.state[mt19937.nn - 1] & mt19937.um) | (rng.state[0] & mt19937.lm)
+		rng.state[mt19937.nn - 1] = rng.state[mt19937.mm - 1] ^ (x >> 1) ^ mag01[int(x & 1)]
+		rng.mti = 0
+	}
+	x = rng.state[rng.mti]
+	rng.mti++
+	x ^= (x >> 29) & 0x5555555555555555
+	x ^= (x << 17) & 0x71D67FFFEDA60000
+	x ^= (x << 37) & 0xFFF7EEE000000000
+	x ^= (x >> 43)
+	return x
+}
+
+// int returns a 32-bit signed (possibly negative) `int`.
+[inline]
+pub fn (mut rng MT19937RNG) int() int {
+	return int(rng.u32())
+}
+
+// i64 returns a 64-bit signed (possibly negative) `i64`.
+[inline]
+pub fn (mut rng MT19937RNG) i64() i64 {
+	return i64(rng.u64())
+}
+
+// int31 returns a 31bit positive pseudorandom `int`.
+[inline]
+pub fn (mut rng MT19937RNG) int31() int {
+	return int(rng.u32() >> 1)
+}
+
+// int63 returns a 63bit positive pseudorandom `i64`.
+[inline]
+pub fn (mut rng MT19937RNG) int63() i64 {
+	return i64(rng.u64() >> 1)
+}
+
+// u32n returns a 32bit `u32` in range `[0, max)`.
+[inline]
+pub fn (mut rng MT19937RNG) u32n(max u32) u32 {
+	if max == 0 {
+		eprintln('max must be positive integer.')
+		exit(1)
+	}
+	// Check SysRNG in system_rng.c.v for explanation
+	bit_len := bits.len_32(max)
+	if bit_len == 32 {
+		for {
+			value := rng.u32()
+			if value < max {
+				return value
+			}
+		}
+	} else {
+		mask := (u32(1) << (bit_len + 1)) - 1
+		for {
+			value := rng.u32() & mask
+			if value < max {
+				return value
+			}
+		}
+	}
+	return u32(0)
+}
+
+// u64n returns a 64bit `u64` in range `[0, max)`.
+[inline]
+pub fn (mut rng MT19937RNG) u64n(max u64) u64 {
+	if max == 0 {
+		eprintln('max must be positive integer.')
+		exit(1)
+	}
+	bit_len := bits.len_64(max)
+	if bit_len == 64 {
+		for {
+			value := rng.u64()
+			if value < max {
+				return value
+			}
+		}
+	} else {
+		mask := (u64(1) << (bit_len + 1)) - 1
+		for {
+			value := rng.u64() & mask
+			if value < max {
+				return value
+			}
+		}
+	}
+	return u64(0)
+}
+
+// u32n returns a pseudorandom `u32` value that is guaranteed to be in range `[min, max)`.
+[inline]
+pub fn (mut rng MT19937RNG) u32_in_range(min u32, max u32) u32 {
+	if max <= min {
+		eprintln('max must be greater than min.')
+		exit(1)
+	}
+	return min + rng.u32n(max - min)
+}
+
+// u64n returns a pseudorandom `u64` value that is guaranteed to be in range `[min, max)`.
+[inline]
+pub fn (mut rng MT19937RNG) u64_in_range(min u64, max u64) u64 {
+	if max <= min {
+		eprintln('max must be greater than min.')
+		exit(1)
+	}
+	return min + rng.u64n(max - min)
+}
+
+// intn returns a 32bit positive `int` in range `[0, max)`.
+[inline]
+pub fn (mut rng MT19937RNG) intn(max int) int {
+	if max <= 0 {
+		eprintln('max has to be positive.')
+		exit(1)
+	}
+	return int(rng.u32n(u32(max)))
+}
+
+// i64n returns a 64bit positive `i64` in range `[0, max)`.
+[inline]
+pub fn (mut rng MT19937RNG) i64n(max i64) i64 {
+	if max <= 0 {
+		eprintln('max has to be positive.')
+		exit(1)
+	}
+	return i64(rng.u64n(u64(max)))
+}
+
+// int_in_range returns a 32bit positive `int` in range `[min, max)`.
+[inline]
+pub fn (mut rng MT19937RNG) int_in_range(min int, max int) int {
+	if max <= min {
+		eprintln('max must be greater than min.')
+		exit(1)
+	}
+	return min + rng.intn(max - min)
+}
+
+// i64_in_range returns a 64bit positive `i64` in range `[min, max)`.
+[inline]
+pub fn (mut rng MT19937RNG) i64_in_range(min i64, max i64) i64 {
+	if max <= min {
+		eprintln('max must be greater than min.')
+		exit(1)
+	}
+	return min + rng.i64n(max - min)
+}
+
+// f32 returns a 32bit real (`f32`) in range `[0, 1)`.
+[inline]
+pub fn (mut rng MT19937RNG) f32() f32 {
+	return f32(rng.f64())
+}
+
+// f64 returns 64bit real (`f64`) in range `[0, 1)`.
+[inline]
+pub fn (mut rng MT19937RNG) f64() f64 {
+	return f64(rng.u64() >> 11) * mt19937.inv_f64_limit
+}
+
+// f32n returns a 32bit real (`f32`) in range [0, max)`.
+[inline]
+pub fn (mut rng MT19937RNG) f32n(max f32) f32 {
+	if max <= 0 {
+		eprintln('max has to be positive.')
+		exit(1)
+	}
+	return rng.f32() * max
+}
+
+// f64n returns a 64bit real (`f64`) in range `[0, max)`.
+[inline]
+pub fn (mut rng MT19937RNG) f64n(max f64) f64 {
+	if max <= 0 {
+		eprintln('max has to be positive.')
+		exit(1)
+	}
+	return rng.f64() * max
+}
+
+// f32_in_range returns a pseudorandom `f32` that lies in range `[min, max)`.
+[inline]
+pub fn (mut rng MT19937RNG) f32_in_range(min f32, max f32) f32 {
+	if max <= min {
+		eprintln('max must be greater than min.')
+		exit(1)
+	}
+	return min + rng.f32n(max - min)
+}
+
+// i64_in_range returns a pseudorandom `i64` that lies in range `[min, max)`.
+[inline]
+pub fn (mut rng MT19937RNG) f64_in_range(min f64, max f64) f64 {
+	if max <= min {
+		eprintln('max must be greater than min.')
+		exit(1)
+	}
+	return min + rng.f64n(max - min)
+}