shared/random.ts

export class SeededRandom {
  private state: number

  constructor(seed: number) {
    this.state = seed >>> 0
    if (this.state === 0) this.state = 0x6d2b79f5
  }

  next(): number {
    this.state += 0x6d2b79f5
    let t = this.state
    t = Math.imul(t ^ (t >>> 15), t | 1)
    t ^= t + Math.imul(t ^ (t >>> 7), t | 61)
    return ((t ^ (t >>> 14)) >>> 0) / 4294967296
  }

  int(maxExclusive: number): number {
    return Math.floor(this.next() * maxExclusive)
  }

  pick<T>(items: readonly T[]): T {
    if (items.length === 0) throw new Error('Cannot pick from an empty list')
    return items[this.int(items.length)]
  }

  fork(offset: number): SeededRandom {
    return new SeededRandom((this.state + Math.imul(offset + 1, 0x9e3779b9)) >>> 0)
  }
}

export function weightedIndex(weights: readonly number[], rng: SeededRandom): number {
  const total = weights.reduce((sum, w) => sum + Math.max(0, w), 0)
  if (total <= 0) return rng.int(weights.length)

  let needle = rng.next() * total
  for (let i = 0; i < weights.length; i++) {
    needle -= Math.max(0, weights[i])
    if (needle <= 0) return i
  }
  return weights.length - 1
}