/**
 * Pure geometry for the A-Z jump strip (see AlphabetScrollBar.svelte).
 *
 * The strip stretches from wherever it sits down to a floor, and the floor is
 * the whole question: get it wrong and the tail of the alphabet renders past
 * the bottom of the scroller, under the mini player / bottom nav, where it
 * cannot be tapped.
 *
 * The floor is the scroller's own bottom edge — never the viewport's. The strip
 * used to size itself as `window.innerHeight` minus a hardcoded guess at the
 * bars' height (5rem/7rem/11rem by platform and mini-player visibility), which
 * dates from when those bars were `position: fixed` overlays. They are in-flow
 * flex siblings below the scroller now (see BottomUi.svelte), so the scroller's
 * bottom edge *is* the top of the mini player, measured exactly, every frame —
 * and the guess was short on every device with a navigation/gesture bar,
 * because `--safe-bottom` is padded inside BottomUi and the guess never knew
 * about it.
 *
 * Extracted from the component so the floor rule is unit-testable — the
 * component only supplies measurements.
 *
 * TRACES: UR-007 | DR-007, DR-262
 */

/**
 * Breathing room left under the last letter, in px. Mirrors the `top-2` sticky
 * offset at the other end so the strip sits symmetrically in the scrollport.
 */
export const STRIP_BOTTOM_GAP = 8;

export interface StripHeightInput {
  /** Viewport-relative top of the strip container (`getBoundingClientRect().top`). */
  stripTop: number;
  /**
   * Viewport-relative bottom edge of the scroll container the strip lives in,
   * or `null` when the strip has no scrollable ancestor to measure.
   */
  scrollerBottom: number | null;
  /** Viewport height — the fallback floor when there is no scroll container. */
  viewportHeight: number;
  /** Override for {@link STRIP_BOTTOM_GAP}, in px. */
  gap?: number;
}

/** How tall the A-Z strip may be without running under the bottom bars. */
export function stripHeightFor({
  stripTop,
  scrollerBottom,
  viewportHeight,
  gap = STRIP_BOTTOM_GAP,
}: StripHeightInput): number {
  const floor = scrollerBottom ?? viewportHeight;
  return Math.max(0, floor - gap - stripTop);
}