/**
 * @file Lays out the Changes panel's History as a commit graph (the JetBrains /
 * `git log --graph` shape): each commit gets a column, and each row lists the
 * line segments drawn through it — a lane passing by, the lanes converging on
 * the commit from above, and the lines leaving it for its parents below.
 * Expects the commits in an order where no parent precedes a child
 * (`git log --date-order`); a parent outside the list leaves its lane running
 * off the bottom.
 * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
 */

/** One line in a row, in column units; y runs 0 (top) → 0.5 (node) → 1 (bottom). */
export interface GraphSeg {
  x1: number;
  y1: number;
  x2: number;
  y2: number;
  color: number;
}

export interface GraphRow {
  col: number;
  color: number;
  segs: GraphSeg[];
}

export function commitGraph(commits: { sha: string; parents: string[] }[]): {
  rows: GraphRow[];
  width: number;
} {
  // lanes[j] = the sha the lane in column j is waiting for; colors[j] its hue.
  const lanes: (string | null)[] = [];
  const colors: number[] = [];
  const hue = (j: number) => colors[j] ?? 0;
  let nextColor = 0;
  let width = 1;
  const open = () => {
    const free = lanes.indexOf(null);
    const j = free < 0 ? lanes.length : free;
    colors[j] = nextColor++;
    return j;
  };

  const rows = commits.map((c) => {
    const segs: GraphSeg[] = [];
    let col = lanes.indexOf(c.sha);
    // A tip nothing above points at: a lane of its own, no line from the top.
    if (col < 0) {
      col = open();
      lanes[col] = null;
    }
    lanes.forEach((s, j) => {
      if (s === null) return;
      segs.push(
        s === c.sha
          ? { x1: j, y1: 0, x2: col, y2: 0.5, color: hue(j) }
          : { x1: j, y1: 0, x2: j, y2: 1, color: hue(j) }
      );
    });
    const color = hue(col);
    lanes.forEach((s, j) => {
      if (s === c.sha) lanes[j] = null;
    });

    c.parents.forEach((p, i) => {
      let j = i === 0 ? col : lanes.indexOf(p);
      if (j < 0) j = open();
      lanes[j] = p;
      segs.push({ x1: col, y1: 0.5, x2: j, y2: 1, color: hue(j) });
    });
    while (lanes.length && lanes[lanes.length - 1] === null) lanes.pop();
    width = Math.max(width, lanes.length, col + 1);
    return { col, color, segs };
  });
  return { rows, width };
}