thi-ng/umbrella

import type { GridIterOpts2D } from "./api.js";
import { __opts } from "./utils.js";

/**
 * Similar to {@link diagonal2d}, but yields 2D grid coordinates in zigzag
 * diagonal order starting at [0,0] and using given `cols` and `rows`.
 *
 * @param opts -
 */
export function* zigzagDiagonal2d(opts: GridIterOpts2D) {
    const { cols, rows, tx } = __opts(opts);
    const num = cols * rows - 1;
    for (
        let x = 0, y = 0, ny = 0, dx = -1, dy = 1, d = 0, down = true, i = 0;
        i <= num;
        i++
    ) {
        yield tx(x, y);
        if (i !== num) {
            do {
                if (y === ny) {
                    if (down) {
                        y++;
                        d++;
                        ny = 0;
                    } else {
                        x++;
                        ny = ++d;
                    }
                    down = !down;
                    dx *= -1;
                    dy *= -1;
                } else {
                    x += dx;
                    y += dy;
                }
            } while (x >= cols || y >= rows);
        }
    }
}