/**
 * Tightening the letter-spacing of a wordmark, by image rather than by text.
 *
 * The logo arrives as artwork: its spacing is drawn in, not typeset, so no
 * property can adjust it. But between two letters every pixel column is blank,
 * and a blank column carries no shape — dropping some of them moves the
 * letters closer without altering a single letterform.
 *
 * Working on the full-resolution source matters: doing this after the
 * downscale would compress columns that are already an average of ink and
 * paper, and smear the hairlines this logo can least afford to lose.
 */

export type Greyscale = {
  data: Uint8Array | Buffer;
  width: number;
  height: number;
};

/** Below this a pixel counts as ink; above it, as paper. */
const INK = 128;

export function columnHasInk({ data, width, height }: Greyscale, x: number): boolean {
  for (let y = 0; y < height; y += 1) {
    if ((data[y * width + x] ?? 255) < INK) {
      return true;
    }
  }
  return false;
}

/**
 * The columns to keep so that no run of blank ones is longer than `maxGap`.
 *
 * Blank columns at the very edges are dropped entirely: they are margin, not
 * spacing, and the caller sets its own.
 */
export function keptColumns(image: Greyscale, maxGap: number): number[] {
  const ink: boolean[] = [];
  for (let x = 0; x < image.width; x += 1) {
    ink.push(columnHasInk(image, x));
  }

  const first = ink.indexOf(true);
  const last = ink.lastIndexOf(true);
  if (first === -1) {
    return [];
  }

  const kept: number[] = [];
  let gap = 0;

  for (let x = first; x <= last; x += 1) {
    if (ink[x]) {
      gap = 0;
      kept.push(x);
      continue;
    }
    gap += 1;
    if (gap <= maxGap) {
      kept.push(x);
    }
  }

  return kept;
}

/** Rebuilds the image from the kept columns. */
export function tighten(image: Greyscale, maxGap: number): Greyscale {
  const columns = keptColumns(image, maxGap);
  const width = columns.length;
  const data = Buffer.alloc(width * image.height, 255);

  for (let y = 0; y < image.height; y += 1) {
    for (let index = 0; index < width; index += 1) {
      data[y * width + index] = image.data[y * image.width + (columns[index] ?? 0)] ?? 255;
    }
  }

  return { data, width, height: image.height };
}