/**
 * Reads the CSS colors the server meets: a dumped page's `theme-color`, which
 * the generic link preview keeps as its tint (`normalizeCssColor`), and the
 * theme stylesheets' tokens, which the emails resolve (`theme-tokens.ts`).
 *
 * @module
 */

/** An sRGB color with its alpha, each channel 0–255 and alpha 0–1. */
export interface Rgba {
  r: number;
  g: number;
  b: number;
  a: number;
}

/**
 * The named colors sites actually put in `theme-color`, and `transparent`,
 * which the themes write; the rest of CSS's names aren't worth carrying.
 */
const NAMED: Record<string, string> = {
  transparent: "#00000000",
  black: "#000000",
  white: "#ffffff",
  red: "#ff0000",
  green: "#008000",
  blue: "#0000ff",
  yellow: "#ffff00",
  orange: "#ffa500",
  purple: "#800080",
  gray: "#808080",
  grey: "#808080",
  silver: "#c0c0c0",
  maroon: "#800000",
  navy: "#000080",
  teal: "#008080",
  olive: "#808000",
  lime: "#00ff00",
  aqua: "#00ffff",
  cyan: "#00ffff",
  fuchsia: "#ff00ff",
  magenta: "#ff00ff",
};

/** Reads a channel of `rgb()`, a number or a percentage of 255. */
function channel(text: string): number {
  const n = parseFloat(text);
  const value = text.endsWith("%") ? (n / 100) * 255 : n;
  return Math.max(0, Math.min(255, value));
}

/** Reads the alpha of `rgba()`, a number or a percentage of 1. */
function alpha(text: string): number {
  const n = parseFloat(text);
  return Math.max(0, Math.min(1, text.endsWith("%") ? n / 100 : n));
}

/**
 * Returns the color `text` writes as a hex (3, 4, 6 or 8 digits), an `rgb()`
 * or `rgba()` of numbers or percentages, or one of the common names; undefined
 * for any other form. Channels out of range are clamped.
 */
export function parseCssColor(text: string): Rgba | undefined {
  const value = text.trim().toLowerCase();
  const hex = /^#([0-9a-f]{3,8})$/.exec(NAMED[value] ?? value)?.[1];
  if (hex) {
    if (hex.length === 5 || hex.length === 7) return undefined;
    const full = hex.length <= 4 ? [...hex].map((c) => c + c).join("") : hex;
    const byte = (i: number) => parseInt(full.slice(i * 2, i * 2 + 2), 16);
    return {
      r: byte(0),
      g: byte(1),
      b: byte(2),
      a: full.length === 8 ? byte(3) / 255 : 1,
    };
  }
  const args = /^rgba?\(([^)]+)\)$/.exec(value)?.[1];
  if (!args) return undefined;
  const parts = args.split(/[\s,/]+/).filter(Boolean);
  if (parts.length !== 3 && parts.length !== 4) return undefined;
  const [r, g, b] = parts.slice(0, 3).map(channel);
  const a = parts[3] === undefined ? 1 : alpha(parts[3]);
  return [r, g, b, a].some(Number.isNaN) ? undefined : { r, g, b, a };
}

/** Writes `color` as a lowercase `#rrggbb`, its alpha dropped. */
export function hexOf({ r, g, b }: Rgba): string {
  return `#${
    [r, g, b].map((c) => Math.round(c).toString(16).padStart(2, "0")).join("")
  }`;
}