// carousel.js — pure carousel-swipe and crop-nudge logic (plan Step 5 +
// red-team D4). The simulator component wires pointer events through these
// so the swipe threshold, clamping, and keyboard nudge are unit-testable.

import { clampCropRect } from './cropViewport';

/**
 * Clamp a slide index into [0, n-1].
 */
export function clampIndex(index, n) {
  if (!Number.isInteger(n) || n < 1) return 0;

  return Math.min(Math.max(index, 0), n - 1);
}

/**
 * Resolve the slide index after a horizontal drag.
 *
 * @param {number} index current slide index
 * @param {number} dragDeltaX total pointer delta in px (positive = dragged
 *   right / finger moved right)
 * @param {number} threshold px distance that triggers a slide change
 * @param {number} n total slides
 * @returns {number} new clamped index
 *
 * Dragging LEFT (deltaX < 0) reveals the NEXT slide (index+1); dragging
 * RIGHT (deltaX > 0) goes back (index-1). Sub-threshold drags stay put.
 */
export function resolveIndex(index, dragDeltaX, threshold, n) {
  const base = clampIndex(index, n);

  if (dragDeltaX <= -threshold) return clampIndex(base + 1, n);

  if (dragDeltaX >= threshold) return clampIndex(base - 1, n);

  return base;
}

/**
 * Nudge a crop rect by whole pixels (red-team D4 keyboard positioning).
 * Delegates clamping to clampCropRect so the crop can never leave the
 * source. step lets Shift-arrow move in larger increments.
 */
export function nudgeCrop(crop, dx, dy, imgW, imgH, step = 1) {
  return clampCropRect(
    {
      ...crop,
      x: crop.x + dx * step,
      y: crop.y + dy * step,
    },
    imgW,
    imgH
  );
}