import { clamp } from './mood'
import type { Affinity } from './types'

const DAY = 86_400_000

export const GAIN = 0.01
export const DAILY_CAP = 0.05
export const LOSS = 0.03
const IDLE_GRACE_DAYS = 7
const IDLE_HALF_LIFE_DAYS = 30

// 升階門檻；降階門檻 = 升階門檻 − 0.05，形成遲滯
export const UP = [0.3, 0.55, 0.8]
export const HYSTERESIS = 0.05

export const dayKey = (ms: number) => new Date(ms).toISOString().slice(0, 10)

export const initialAffinity = (now: number): Affinity => ({ value: 0, stage: 0, dayKey: dayKey(now), dayGain: 0, lastSeen: now })

export function stageFor(value: number, current: number): number {
  let s = current
  while (s < UP.length && value >= UP[s]!) s++
  while (s > 0 && value < UP[s - 1]! - HYSTERESIS) s--
  return s
}

// 離開超過 7 天後，以 30 天半衰期回到 0
export function idleDecay(a: Affinity, now: number): Affinity {
  const idleDays = (now - a.lastSeen) / DAY
  if (idleDays <= IDLE_GRACE_DAYS) return a
  const value = a.value * Math.pow(0.5, (idleDays - IDLE_GRACE_DAYS) / IDLE_HALF_LIFE_DAYS)
  return { ...a, value, stage: stageFor(value, a.stage) }
}

export function interact(prev: Affinity, kind: 'meaningful' | 'negative', now: number): Affinity {
  let a = idleDecay(prev, now)
  const key = dayKey(now)
  if (a.dayKey !== key) a = { ...a, dayKey: key, dayGain: 0 }
  let { value, dayGain } = a
  if (kind === 'meaningful') {
    const gain = Math.max(0, Math.min(GAIN, DAILY_CAP - dayGain))
    value += gain
    dayGain += gain
  } else {
    value -= LOSS
  }
  value = clamp(value)
  return { value, dayGain, dayKey: key, lastSeen: now, stage: stageFor(value, a.stage) }
}