/**
 * Tap-gesture interpretation for the video player surface.
 *
 * Pulled out of `VideoPlayer.svelte` so the timing rules are unit-testable:
 * a tap cannot be classified at the moment it lands, because it may still turn
 * out to be the first half of a double tap. Play/pause is therefore *deferred*
 * until the double-tap window closes, and cancelled outright if a second tap
 * arrives — otherwise a double tap both toggles pause and seeks.
 *
 * TRACES: UR-005, UR-061 | DR-092 | UT-085, UT-086, UT-087, UT-088
 */

/** A second tap within this window makes a double tap. */
export const DOUBLE_TAP_WINDOW_MS = 300;

/** Double tap on the right half: skip forward. */
export const SEEK_FORWARD_SECONDS = 30;

/** Double tap on the left half: skip back. */
export const SEEK_BACKWARD_SECONDS = -10;

export type TapFeedback = "left" | "right";

export type TapOutcome =
  /** Deferred: play/pause fires only if no second tap lands within the window. */
  | { action: "pending"; pendingAfterMs: number }
  | { action: "seek"; seekSeconds: number; feedback: TapFeedback };

export interface TapInput {
  /** Tap x position, viewport pixels. */
  x: number;
  screenWidth: number;
  now: number;
}

export interface TapGestureState {
  /**
   * Resolve a still-pending single tap. Returns the play/pause action once the
   * double-tap window has elapsed, or null if there is nothing pending (the tap
   * became a double tap, or was cancelled).
   */
  resolvePending(now: number): { action: "togglePlayPause" } | null;
  /** Drop any pending tap — used when the gesture turns into a swipe. */
  cancel(): void;
}

interface InternalState extends TapGestureState {
  lastTapTime: number;
  pendingSince: number | null;
}

export function createTapGestureState(): TapGestureState {
  const state: InternalState = {
    lastTapTime: 0,
    pendingSince: null,
    resolvePending(now: number) {
      if (state.pendingSince === null) return null;
      if (now - state.pendingSince < DOUBLE_TAP_WINDOW_MS) return null;
      state.pendingSince = null;
      return { action: "togglePlayPause" };
    },
    cancel() {
      state.pendingSince = null;
      state.lastTapTime = 0;
    },
  };
  return state;
}

/**
 * Classify a tap. The first tap of a potential pair returns `pending` — the
 * caller schedules `resolvePending` after `pendingAfterMs`. A second tap inside
 * the window returns the seek and clears the pending play/pause.
 */
export function registerTap(state: TapGestureState, input: TapInput): TapOutcome {
  const s = state as InternalState;
  const sinceLastTap = input.now - s.lastTapTime;

  if (s.lastTapTime > 0 && sinceLastTap > 0 && sinceLastTap < DOUBLE_TAP_WINDOW_MS) {
    // Second tap: cancel the deferred play/pause and seek instead.
    s.pendingSince = null;
    s.lastTapTime = 0; // consumed, so a third tap starts fresh
    const isLeftSide = input.x < input.screenWidth / 2;
    return isLeftSide
      ? { action: "seek", seekSeconds: SEEK_BACKWARD_SECONDS, feedback: "left" }
      : { action: "seek", seekSeconds: SEEK_FORWARD_SECONDS, feedback: "right" };
  }

  s.lastTapTime = input.now;
  s.pendingSince = input.now;
  return { action: "pending", pendingAfterMs: DOUBLE_TAP_WINDOW_MS };
}

export interface SeekTargetInput {
  /** Relative offset in seconds (negative rewinds). */
  delta: number;
  /** Latest position reported by the player — the authoritative source. */
  reportedPosition: number;
  /** Media duration; 0/unknown disables the upper clamp. */
  duration: number;
  /**
   * Target of a seek already requested but not yet reflected in
   * `reportedPosition`. Consecutive double taps chain off this so they add up
   * instead of all resolving against the same stale position.
   */
  pendingTarget?: number | null;
}

/**
 * Resolve a relative skip to the absolute position the facade expects.
 *
 * The player facade seeks by absolute position only (the backend picks the seek
 * strategy), so the delta is applied here — against the pending target when one
 * is still in flight and still ahead of what the player has reported.
 */
export function resolveSeekTarget(input: SeekTargetInput): number {
  const { delta, reportedPosition, duration, pendingTarget } = input;

  const base =
    pendingTarget != null && Math.abs(pendingTarget - reportedPosition) > 0.5 && pendingTarget > reportedPosition
      ? pendingTarget
      : reportedPosition;

  const target = base + delta;
  if (target < 0) return 0;
  if (duration > 0 && target > duration) return duration;
  return target;
}