/**
 * Pane layout state — the resizable three-pane shell (spec §01A, persisted per
 * §09D "Panel layout").
 *
 * Model: the two **side** panes (library, preview) carry explicit remembered
 * widths; the **center** editor flexes to fill the remainder. This is what makes
 * a drag "leave the rest of the layout unaffected" (§01A): dragging the left
 * handle trades width between library and editor, the right handle between
 * preview and editor — the opposite side pane never moves.
 *
 * Per-pane visibility / the toggle strip (the other half of §01A) is not here
 * yet; this slice is resize + persistence, pulled forward from M6 because it is
 * coupled to the Live Preview's container sizing.
 *
 * Pure clamp helpers live alongside the store so the resize math is unit-tested
 * without a DOM. Persistence is a startup subscriber (orchestration/panes), not
 * done here — the store stays browser-free.
 */

import { create } from 'zustand';

/** Which side pane a drag handle controls. */
export type PaneSide = 'library' | 'preview';

/** Every pane that can be shown/hidden via the toggle strip (spec §01A). */
export type PaneName = 'library' | 'editor' | 'preview';

/** Minimum widths (px) enforced while resizing, so no pane collapses (§01A). */
export const PANE_MIN = { library: 180, preview: 240, editor: 320 } as const;

/** Initial side-pane widths (px) on first run, before any persisted layout. */
export const PANE_DEFAULT = { library: 280, preview: 360 } as const;

/** Combined width (px) the resize handles occupy between the panes. */
export const HANDLES_TOTAL = 12;

/**
 * The widest a side pane can be while the editor still meets its minimum.
 * `containerWidth` is the full panes-row width; `otherWidth` is the opposite
 * side pane's current width. Never reports below the pane's own minimum.
 */
export function maxSideWidth(side: PaneSide, containerWidth: number, otherWidth: number): number {
  return Math.max(PANE_MIN[side], containerWidth - otherWidth - HANDLES_TOTAL - PANE_MIN.editor);
}

/**
 * Clamp a desired side-pane width so it stays at least its own minimum and
 * leaves the editor at least its minimum. Pure — the single place the resize
 * constraint lives.
 */
export function clampSideWidth(
  side: PaneSide,
  desired: number,
  containerWidth: number,
  otherWidth: number,
): number {
  return Math.max(
    PANE_MIN[side],
    Math.min(desired, maxSideWidth(side, containerWidth, otherWidth)),
  );
}

/**
 * Normalize a side pane's width to the 0–100 position a window splitter reports
 * via `aria-valuenow` (WAI-ARIA APG → Window Splitter): 0 = pane at its minimum
 * size, 100 = pane at its maximum. Returns null when the container width isn't
 * known yet (pre-layout) or the range has collapsed, so the caller omits the
 * attribute rather than emitting NaN. The 0–100 scale is APG's "typical" choice
 * and announces as a percentage — more meaningful than a moving pixel count.
 */
export function sideWidthValue(
  side: PaneSide,
  width: number,
  containerWidth: number,
  otherWidth: number,
): number | null {
  if (containerWidth <= 0) return null;
  const min = PANE_MIN[side];
  const max = maxSideWidth(side, containerWidth, otherWidth);
  if (max <= min) return null;
  const pct = ((width - min) / (max - min)) * 100;
  return Math.round(Math.min(100, Math.max(0, pct)));
}

/** Per-pane visibility (spec §01A). Widths are kept *independently* of visibility,
 * so a hidden pane keeps its remembered width and re-shows at it (not a default). */
export interface PaneVisibility {
  library?: boolean;
  editor?: boolean;
  preview?: boolean;
}

export interface PanesState {
  libraryWidth: number;
  previewWidth: number;
  /** Whether each pane is currently shown. All visible by default (§01A). */
  libraryVisible: boolean;
  editorVisible: boolean;
  previewVisible: boolean;
  /** Set a side pane's width (already clamped by the caller). */
  setWidth: (side: PaneSide, width: number) => void;
  /** Show/hide one pane. Hiding all panes is permitted (§01A) — the strip stays. */
  togglePane: (pane: PaneName) => void;
  /** Restore persisted widths + visibility on startup; missing values keep defaults. */
  hydrate: (
    layout: { libraryWidth?: number; previewWidth?: number },
    visibility?: PaneVisibility,
  ) => void;
}

const VISIBLE_KEY = {
  library: 'libraryVisible',
  editor: 'editorVisible',
  preview: 'previewVisible',
} as const;

export const usePanesStore = create<PanesState>((set) => ({
  libraryWidth: PANE_DEFAULT.library,
  previewWidth: PANE_DEFAULT.preview,
  libraryVisible: true,
  editorVisible: true,
  previewVisible: true,

  setWidth: (side, width) =>
    set(side === 'library' ? { libraryWidth: width } : { previewWidth: width }),

  togglePane: (pane) => set((s) => ({ [VISIBLE_KEY[pane]]: !s[VISIBLE_KEY[pane]] })),

  hydrate: (layout, visibility) =>
    set({
      libraryWidth: layout.libraryWidth ?? PANE_DEFAULT.library,
      previewWidth: layout.previewWidth ?? PANE_DEFAULT.preview,
      libraryVisible: visibility?.library ?? true,
      editorVisible: visibility?.editor ?? true,
      previewVisible: visibility?.preview ?? true,
    }),
}));