/**
 * Startet den Kamerastrom in einem <video>-Element.
 * Bevorzugt die Ruecckamera und eine hohe Aufloesung, damit die
 * kleine Etikettenschrift lesbar bleibt.
 */
export async function startCamera(videoElement) {
  const stream = await navigator.mediaDevices.getUserMedia({
    video: {
      facingMode: { ideal: 'environment' },
      width: { ideal: 1920 },
      height: { ideal: 1080 },
    },
    audio: false,
  });

  videoElement.srcObject = stream;
  videoElement.setAttribute('playsinline', '');

  try {
    await videoElement.play();
  } catch (error) {
    for (const track of stream.getTracks()) track.stop();
    videoElement.srcObject = null;
    throw error;
  }

  return {
    stop() {
      for (const track of stream.getTracks()) track.stop();
      videoElement.srcObject = null;
    },
  };
}

function drawScaled(source, sourceWidth, sourceHeight, maxEdge) {
  const scale = Math.min(1, maxEdge / Math.max(sourceWidth, sourceHeight));
  const width = Math.round(sourceWidth * scale);
  const height = Math.round(sourceHeight * scale);

  const canvas = document.createElement('canvas');
  canvas.width = width;
  canvas.height = height;
  const context = canvas.getContext('2d', { willReadFrequently: true });
  context.drawImage(source, 0, 0, width, height);
  return context.getImageData(0, 0, width, height);
}

function assertHasSize(width, height) {
  if (!(width > 0) || !(height > 0)) {
    throw new Error(
      'Das Kamerabild ist noch nicht bereit. Bitte gleich noch einmal versuchen.',
    );
  }
}

/** Einzelbild aus dem laufenden Kamerastrom. */
export function grabFrame(videoElement, maxEdge = 1280) {
  assertHasSize(videoElement.videoWidth, videoElement.videoHeight);
  return drawScaled(
    videoElement,
    videoElement.videoWidth,
    videoElement.videoHeight,
    maxEdge,
  );
}

/** Ersatzweg ohne Kamera: Bilddatei auswaehlen (z. B. am Rechner). */
export async function imageDataFromFile(file, maxEdge = 1280) {
  const bitmap = await createImageBitmap(file);
  try {
    assertHasSize(bitmap.width, bitmap.height);
    const data = drawScaled(bitmap, bitmap.width, bitmap.height, maxEdge);
    return data;
  } finally {
    bitmap.close();
  }
}