HealthLog/src/lib/format-locale.ts

/**
 * Locale-aware formatters.
 *
 * HealthLog's UI language is user-configurable (DE/EN). Numbers, dates, and
 * times must render in the active locale — not in the hard-coded de-DE that
 * lingered from the German-first era.
 *
 * Call via the `useFormatters()` client hook. For server-only code paths
 * (jobs, API routes, PDF generation) pass the locale explicitly through
 * `makeFormatters(locale)`. The optional second argument carries the
 * per-user timezone (v1.4.25 W7) — when omitted the formatter falls back
 * to `DISPLAY_TIMEZONE` so legacy callers keep rendering in Europe/Berlin.
 *
 * UTC is always preserved in the database.
 */

import type { Locale } from "./i18n/config";

/**
 * Fallback timezone for surfaces that are not yet user-scoped (admin
 * tables, audit log viewer, anything without a resolved user). User-
 * scoped surfaces should call `makeFormatters(locale, userTz)` with
 * the value from `resolveUserTimezone()`.
 */
export const DISPLAY_TIMEZONE = "Europe/Berlin";

/**
 * Map our short locale ("de" / "en") to a full BCP-47 tag suitable for
 * `Intl.*`. When we add locales, extend this map — do not rely on the short
 * tag directly since browsers differ in how they handle "en" vs "en-US".
 */
export const INTL_LOCALE_MAP: Record<Locale, string> = {
  de: "de-DE",
  en: "en-US",
};

export function resolveIntlLocale(locale: Locale): string {
  return INTL_LOCALE_MAP[locale] ?? "en-US";
}

type DateInput = Date | string | number;

function asDate(value: DateInput): Date {
  return value instanceof Date ? value : new Date(value);
}

export interface Formatters {
  /** Integer or decimal with the active locale's decimal separator. */
  number: (value: number, fractionDigits?: number) => string;
  /** Same as `number` but thousands-grouped. */
  integer: (value: number) => string;
  /** Percentage, e.g. 0.835 → "83,5 %" in DE / "83.5%" in EN. */
  percent: (value: number, fractionDigits?: number) => string;
  /** Short date, e.g. "19.02.2026" / "02/19/2026". */
  date: (value: DateInput) => string;
  /** Short date without year, e.g. "19.02." / "Feb 19". */
  dateShort: (value: DateInput) => string;
  /** Full date + time in 24h. */
  dateTime: (value: DateInput) => string;
  /** Time only in 24h, e.g. "14:30". */
  time: (value: DateInput) => string;
  /** Short weekday + date, e.g. "Do., 19.02." / "Thu, Feb 19". */
  dateWithWeekday: (value: DateInput) => string;
  /** For axis labels: 3-letter month abbreviation. */
  monthShort: (value: DateInput) => string;
}

export function makeFormatters(locale: Locale, userTz?: string): Formatters {
  const intlLocale = resolveIntlLocale(locale);
  const tz = userTz && userTz.length > 0 ? userTz : DISPLAY_TIMEZONE;

  return {
    number: (value, fractionDigits) =>
      new Intl.NumberFormat(intlLocale, {
        minimumFractionDigits: fractionDigits,
        maximumFractionDigits: fractionDigits,
      }).format(value),

    integer: (value) =>
      new Intl.NumberFormat(intlLocale, { maximumFractionDigits: 0 }).format(
        value,
      ),

    percent: (value, fractionDigits = 0) =>
      new Intl.NumberFormat(intlLocale, {
        style: "percent",
        minimumFractionDigits: fractionDigits,
        maximumFractionDigits: fractionDigits,
      }).format(value),

    date: (value) =>
      asDate(value).toLocaleDateString(intlLocale, {
        timeZone: tz,
        day: "2-digit",
        month: "2-digit",
        year: "numeric",
      }),

    dateShort: (value) =>
      asDate(value).toLocaleDateString(intlLocale, {
        timeZone: tz,
        day: "2-digit",
        month: "2-digit",
      }),

    dateTime: (value) =>
      asDate(value).toLocaleString(intlLocale, {
        timeZone: tz,
        day: "2-digit",
        month: "2-digit",
        year: "numeric",
        hour: "2-digit",
        minute: "2-digit",
        hour12: false,
      }),

    time: (value) =>
      asDate(value).toLocaleTimeString(intlLocale, {
        timeZone: tz,
        hour: "2-digit",
        minute: "2-digit",
        hour12: false,
      }),

    dateWithWeekday: (value) =>
      asDate(value).toLocaleDateString(intlLocale, {
        timeZone: tz,
        weekday: "short",
        day: "2-digit",
        month: "2-digit",
      }),

    monthShort: (value) =>
      asDate(value).toLocaleDateString(intlLocale, {
        timeZone: tz,
        month: "short",
      }),
  };
}

/**
 * Parse locale from a `Accept-Language` header or `healthlog-locale` cookie.
 * Server-side helper for jobs, API routes, and PDF generation where there is
 * no React context.
 */
export function parseLocaleFromAcceptLanguage(header: string | null): Locale {
  if (!header) return "en";
  const primary = header.split(",")[0]?.trim().toLowerCase() ?? "";
  if (primary.startsWith("de")) return "de";
  return "en";
}