/*
 * theme.ts — PX-02: the Telegram theme signal → data-tg-theme slot switch.
 * ---------------------------------------------------------------------
 * Light/dark is a SLOT SWITCH, not a color import: Telegram's signals
 * (`colorScheme`, or the luminance of `themeParams.header_bg_color` as
 * the old-client fallback) select between two Rams token pairs defined
 * in theme.css (`--shell-bg` / `--shell-on-surface`). Telegram's own hex
 * values are read ONLY as a 0–1 luminance signal (T-03-11) — no
 * Telegram color ever reaches a CSS custom property.
 *
 * Pitfall 7 (D-10): applyTheme's ONLY statement is the data-attribute
 * set. MapLibre is deliberately absent — the map style is never re-set
 * on themeChanged; CSS custom properties re-slot and the map stays
 * Rams-constant. The invariant is greppable (theme.test.mjs asserts no
 * setStyle call exists in this source).
 */
import type { TelegramWebAppLike } from "./webapp.ts";

type ThemeBranch = "light" | "dark";

export function resolveBranch(wa: TelegramWebAppLike): ThemeBranch {
  // Primary signal — present on all clients this phase targets.
  if (wa.colorScheme === "dark" || wa.colorScheme === "light") return wa.colorScheme;

  // Fallback (UI-SPEC §Theme mapping): luminance of header_bg_color ≥ 0.5 → light.
  const hex = wa.themeParams?.header_bg_color;
  if (typeof hex === "string" && /^#[0-9a-f]{6}$/i.test(hex)) {
    const r = parseInt(hex.slice(1, 3), 16);
    const g = parseInt(hex.slice(3, 5), 16);
    const b = parseInt(hex.slice(5, 7), 16);
    const lum = (0.2126 * r + 0.7152 * g + 0.0722 * b) / 255;
    return lum >= 0.5 ? "light" : "dark";
  }
  return "light";
}

export function applyTheme(wa: TelegramWebAppLike): void {
  // The ONLY statement a theme change is allowed to touch. MapLibre is
  // deliberately absent from this function (D-10 / RESEARCH Pitfall 7).
  document.documentElement.dataset.tgTheme = resolveBranch(wa);
}

export function watchTheme(wa: TelegramWebAppLike): void {
  applyTheme(wa);
  wa.onEvent?.("themeChanged", () => applyTheme(wa));
}