export class Rng {
    genU32() { throw new Error('not implemented'); }

    // precisely mirrors Rust's `rand::Rng::gen_range()` for `Range<u32>`
    genRange(min, max) {
        let range = max - min;
        let zone = (range << Math.clz32(range)) - 1 >>> 0;
        let v, lo, hi;
        do {
            v = this.genU32();
            lo = Math.imul(v, range) >>> 0; // (u32) (v * range)
        } while (lo > zone);
        hi = BigInt(v) * BigInt(range) >> 32n;
        return min + Number(hi);
    }
}

export class SplitMix64 extends Rng {
    constructor(seed) {
        super();
        this.seed = BigInt(seed);
    }

    clone() {
        return new this.constructor(this.seed);
    }

    genU32() {
        this.seed = BigInt.asUintN(64, this.seed + 0x9E3779B97F4A7C15n);
        let z = this.seed;
        z ^= BigInt.asUintN(64, z >> 33n);
        z  = BigInt.asUintN(64, z * 0x62A9D9ED799705F5n);
        z ^= BigInt.asUintN(64, z >> 28n);
        z  = BigInt.asUintN(64, z * 0xCB24D0A5C88C35B3n);
        return Number(z >> 32n);
    }
}