/**
 * earth-now.co
 * Author:  Simon-Pierre Boucher
 * Contact: contact@spboucher.ai
 * File:    packages/counter/src/format.ts
 * Purpose: All UI number formatting — sigFigs honesty cap, Intl.NumberFormat, rates and uncertainty; never inline toFixed elsewhere
 */

import type { DisplayHints } from "./counter-model.js";

/** Round to n significant digits (the honesty cap: never display precision the model can't justify). */
export function roundToSigFigs(value: number, sigFigs: number): number {
  if (value === 0 || !Number.isFinite(value)) return value;
  const magnitude = Math.floor(Math.log10(Math.abs(value)));
  const factor = 10 ** (sigFigs - 1 - magnitude);
  return Math.round(value * factor) / factor;
}

export interface FormatOptions {
  locale?: string;
  /**
   * Animated tickers keep the trailing digits moving (the units digit animates)
   * while static contexts (badge alt text, tooltips, share cards) apply the
   * sigFigs cap strictly. Default: true (strict).
   */
  applySigFigs?: boolean;
}

/** Format a counter value according to its display hints. Fallback text on non-finite input. */
export function formatValue(
  value: number,
  hints: DisplayHints,
  options: FormatOptions = {},
): string {
  if (!Number.isFinite(value)) return "—";
  const { locale = "en", applySigFigs = true } = options;
  const scaled = value * (hints.scale ?? 1);
  const v =
    applySigFigs && hints.sigFigs !== undefined ? roundToSigFigs(scaled, hints.sigFigs) : scaled;
  return new Intl.NumberFormat(locale, {
    minimumFractionDigits: hints.decimals,
    maximumFractionDigits: hints.decimals,
  }).format(v);
}

/** Format an instantaneous rate at a human-friendly cadence (/s, /min, /h). */
export function formatRate(
  perSecond: number,
  hints: DisplayHints,
  options: FormatOptions = {},
): string {
  if (!Number.isFinite(perSecond)) return "—";
  const { locale = "en" } = options;
  const perSecondScaled = perSecond * (hints.scale ?? 1);
  const abs = Math.abs(perSecondScaled);
  let scaled = perSecondScaled;
  let suffix = "/s";
  if (abs < 1 / 60) {
    scaled = perSecondScaled * 3600;
    suffix = "/h";
  } else if (abs < 1) {
    scaled = perSecondScaled * 60;
    suffix = "/min";
  }
  const formatted = new Intl.NumberFormat(locale, {
    maximumSignificantDigits: Math.min(hints.sigFigs ?? 3, 3),
  }).format(scaled);
  return `${formatted}${suffix}`;
}

/**
 * Compact display for chart axis ticks (8.31 Md / 8.31B) — never used for the
 * ticking value itself. Honors the display scale and caps significant digits.
 */
export function formatCompact(
  value: number,
  hints: DisplayHints,
  options: FormatOptions = {},
): string {
  if (!Number.isFinite(value)) return "—";
  const { locale = "en" } = options;
  const scaled = value * (hints.scale ?? 1);
  return new Intl.NumberFormat(locale, {
    notation: "compact",
    maximumSignificantDigits: Math.min(hints.sigFigs ?? 3, 4),
  }).format(scaled);
}

/** Format a 90 % CI as "low – high" with the same hints (for tooltips and the methodology page). */
export function formatUncertainty(
  low: number,
  high: number,
  hints: DisplayHints,
  options: FormatOptions = {},
): string {
  return `${formatValue(low, hints, options)} – ${formatValue(high, hints, options)}`;
}