/**
 * earth-now.co
 * Author:  Simon-Pierre Boucher
 * Contact: contact@spboucher.ai
 * File:    packages/counter/src/windows.ts
 * Purpose: Reset-window helpers (today UTC / this year UTC / session) computed from the same model — pure, time is a parameter
 */

import type { CounterModel } from "./counter-model.js";
import { counterValue } from "./value.js";

export type CounterWindow = "today" | "ytd" | "session" | "total";

/** Start of the UTC day containing t (ms). */
export function startOfUtcDay(tMs: number): number {
  const d = new Date(tMs);
  return Date.UTC(d.getUTCFullYear(), d.getUTCMonth(), d.getUTCDate());
}

/** Start of the UTC year containing t (ms). */
export function startOfUtcYear(tMs: number): number {
  return Date.UTC(new Date(tMs).getUTCFullYear(), 0, 1);
}

/**
 * Value of a cumulative counter over a display window, derived from the SAME
 * model (same function server/client/badge): v(t) − v(windowStart).
 * "session" needs the caller-provided session start (user arrival time).
 */
export function windowValue(
  model: CounterModel,
  tMs: number,
  window: CounterWindow,
  sessionStartMs?: number,
): number {
  switch (window) {
    case "total":
      return counterValue(model, tMs);
    case "today":
      return counterValue(model, tMs) - counterValue(model, startOfUtcDay(tMs));
    case "ytd":
      return counterValue(model, tMs) - counterValue(model, startOfUtcYear(tMs));
    case "session": {
      if (sessionStartMs === undefined)
        throw new Error("windowValue: 'session' window requires sessionStartMs");
      return counterValue(model, tMs) - counterValue(model, sessionStartMs);
    }
  }
}