import { execFileSync } from 'node:child_process'

/**
 * The font the desktop's own terminal would use.
 *
 * GNOME Terminal is configured with `use-system-font` by default, so what it
 * renders is whatever `org.gnome.desktop.interface monospace-font-name` says.
 * Reading that — rather than hardcoding a family — is what makes "the same font
 * as GNOME Terminal" keep being true after someone changes their system font.
 */
export interface TerminalFont {
  /** A CSS font-family value. */
  family: string
  /** In CSS pixels, converted from the point size the desktop reports. */
  sizePx: number
}

/**
 * What to use when the desktop will not say, or is not GNOME.
 *
 * `ui-monospace` is deliberately first: it asks the engine for the system
 * monospace face, which is the same intent as the GSettings lookup, just
 * resolved one layer down.
 */
export const FALLBACK_TERMINAL_FONT: TerminalFont = {
  family:
    'ui-monospace, "JetBrains Mono", "Cascadia Code", Menlo, Consolas, "Noto Sans Mono CJK SC", monospace',
  sizePx: 13,
}

const DEFAULT_SIZE_PX = 13
/** Pango reports points; CSS wants pixels. 1pt = 1/72in, 1px = 1/96in. */
const PT_TO_PX = 96 / 72

/**
 * Parse a Pango font description such as `'Ubuntu Mono 13'`.
 *
 * The size is the *trailing* number, not the first: families legitimately
 * contain digits (`Noto Sans Mono 12` is a family called "Noto Sans Mono",
 * while `Fira Code 12` vs `Fira Code 2 12` are different things). Splitting on
 * the last whitespace-delimited number is the only rule that survives both.
 */
export function parseMonospaceFont(raw: string): TerminalFont | null {
  // Either quote style, since the description's quoting is not part of what it
  // means and the caller should not have to care which one it was handed.
  const unquoted = raw.trim().replace(/^['"](.*)['"]$/, '$1').trim()
  if (!unquoted) return null

  const match = /^(.*\S)\s+(\d+(?:\.\d+)?)$/.exec(unquoted)

  if (!match) {
    // A bare family name with no size. Usable, sized by our default.
    return { family: quoteFamily(unquoted), sizePx: DEFAULT_SIZE_PX }
  }

  const family = match[1].trim()
  const points = Number(match[2])
  if (!family || !Number.isFinite(points) || points <= 0) return null

  return {
    family: quoteFamily(family),
    sizePx: Math.round(points * PT_TO_PX * 100) / 100,
  }
}

/**
 * Always quote, on both paths.
 *
 * A CSS `font-family` value is a comma-separated stack, so an unquoted
 * `Noto Sans Mono` is read as three families — "Noto", then "Sans", then
 * "Mono" — and the terminal silently falls back to something else entirely.
 * Quoting a single-word family costs nothing.
 */
function quoteFamily(family: string): string {
  return `"${family.replace(/"/g, '')}"`
}

/**
 * Ask the desktop for its monospace font.
 *
 * Runs `gsettings` once at startup and falls back silently. A desktop that is
 * not GNOME-based simply will not answer, and that is not an error worth
 * surfacing — the fallback stack is the same request made a different way.
 */
export function readTerminalFont(): TerminalFont {
  try {
    const raw = execFileSync(
      'gsettings',
      ['get', 'org.gnome.desktop.interface', 'monospace-font-name'],
      { encoding: 'utf8', timeout: 3_000, stdio: ['ignore', 'pipe', 'ignore'] },
    )

    return parseMonospaceFont(raw) ?? FALLBACK_TERMINAL_FONT
  } catch {
    return FALLBACK_TERMINAL_FONT
  }
}