/**
 * earth-now.co
 * Author:  Simon-Pierre Boucher
 * Contact: contact@spboucher.ai
 * File:    apps/web/lib/tiers.ts
 * Purpose: Pure display-tier classification — metrics whose last visible digit ticks fast lead the page; slow ones go to the reference strip
 */

import { rateAt, type CounterModel, type DisplayHints } from "@earth-now/counter";
import type { MetricSummary } from "./api";

export type DisplayTier = "hero" | "live" | "country" | "realtime" | "reference";

export const HERO_METRIC_ID = "world_population";
export const COUNTRY_PREFIX = "country_population_";
export const CONTINENT_PREFIX = "continent_population_";

/**
 * Seconds between visible changes of the LAST displayed digit:
 * (10^-decimals) / (|rate| × displayScale). Infinity when the value doesn't move.
 * Pure — time is a parameter.
 */
export function secondsPerVisibleTick(
  model: CounterModel,
  hints: Pick<DisplayHints, "decimals" | "scale">,
  tMs: number,
): number {
  const rate = Math.abs(rateAt(model, tMs)) * (hints.scale ?? 1);
  if (rate === 0 || !Number.isFinite(rate)) return Infinity;
  return 10 ** -hints.decimals / rate;
}

/** A counter reads as "live" when its last digit changes at least every ~2.5 s. */
export const LIVE_TICK_THRESHOLD_S = 2.5;

export function tierFor(
  metric: MetricSummary,
  model: CounterModel | undefined,
  tMs: number,
): DisplayTier {
  if (metric.id === HERO_METRIC_ID) return "hero";
  if (metric.id.startsWith(COUNTRY_PREFIX) || metric.id.startsWith(CONTINENT_PREFIX)) return "country";
  if (metric.level === "rt") return "realtime";
  if (model === undefined) return "reference";
  // rate-of / depletion displays are static text (the rate itself moves slowly).
  if (metric.derived?.op === "rate-of" || metric.derived?.op === "depletion-countdown") {
    return "reference";
  }
  const spv = secondsPerVisibleTick(
    model,
    { decimals: metric.display.decimals, ...(metric.display.scale !== undefined ? { scale: metric.display.scale } : {}) },
    tMs,
  );
  return spv <= LIVE_TICK_THRESHOLD_S ? "live" : "reference";
}

/** Sort key for the live grid: fastest visible tick first. */
export function liveSortKey(
  metric: MetricSummary,
  model: CounterModel | undefined,
  tMs: number,
): number {
  if (model === undefined) return Infinity;
  return secondsPerVisibleTick(
    model,
    { decimals: metric.display.decimals, ...(metric.display.scale !== undefined ? { scale: metric.display.scale } : {}) },
    tMs,
  );
}