tests/rand.mjs: Add MersenneTwister random number generator

The `tests/rand.c` is also added: a simplified adaptation of the
original algorithm implementation in C.  A 10k numbers test case shows
that the JavaScript version behaves the same that the C one does.

1 files changed, 55 insertions, 0 deletions

diff --git a/tests/rand.mjs b/tests/rand.mjs
new file mode 100644
index 0000000..ba4e3b1
--- /dev/null
+++ b/tests/rand.mjs
@@ -0,0 +1,55 @@
+const N          = 624;
+const M          = 397;
+const MATRIX_A   = 0x9908b0df;
+const UPPER_MASK = 0x80000000;
+const LOWER_MASK = 0x7fffffff;
+const mag01      = [ 0x0, MATRIX_A ];
+const INIT_MUL   = 1812433253;
+
+/// The given function generates random numbers on the [ 0, 0xffffffff ]-interval.
+export const makeRandom = (seed = (new Date()).getTime()) => {
+	const mt = new Array(N);
+	let mti  = null;
+
+	mt[0] = seed >>> 0;
+	for (mti = 1; mti < N; mti++) {
+		const s0 = mt[mti - 1] ^ (mt[mti - 1] >>> 30);
+		mt[mti] =
+			(((((s0 & 0xffff0000) >>> 16) * INIT_MUL) << 16) +
+			((   s0 & 0x0000ffff) * INIT_MUL) +
+			mti) >>> 0;
+	}
+
+	return () => {
+		if (mti >= N) {
+			let kk = null;
+
+			for (kk = 0; kk < N - M; kk++) {
+				const y0 =
+					(mt[kk]     & UPPER_MASK) |
+					(mt[kk + 1] & LOWER_MASK);
+				mt[kk] = mt[kk + M] ^ (y0 >>> 1) ^ mag01[y0 & 0x1];
+			}
+
+			for (; kk < N - 1; kk++) {
+				const y1 =
+					(mt[kk]     & UPPER_MASK) |
+					(mt[kk + 1] & LOWER_MASK);
+				mt[kk] = mt[kk + (M - N)] ^ (y1 >>> 1) ^ mag01[y1 & 0x1];
+			}
+
+			const y2 =
+				(mt[N - 1] & UPPER_MASK) |
+				(mt[0]     & LOWER_MASK);
+			mt[N - 1] = mt[M - 1] ^ (y2 >>> 1) ^ mag01[y2 & 0x1];
+			mti = 0;
+		}
+
+		let y = mt[mti++];
+		y ^= (y >>> 11);
+		y ^= (y <<  7)  & 0x9d2c5680;
+		y ^= (y <<  15) & 0xefc60000;
+		y ^= (y >>> 18);
+		return y >>> 0;
+	};
+};