/**
 * 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';

/** 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;

/**
 * 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. `containerWidth` is the full panes-row width; `otherWidth`
 * is the opposite side pane's current width.
 */
export function clampSideWidth(
  side: PaneSide,
  desired: number,
  containerWidth: number,
  otherWidth: number,
): number {
  const min = PANE_MIN[side];
  // The widest this pane can be while the editor still meets its minimum.
  const max = containerWidth - otherWidth - HANDLES_TOTAL - PANE_MIN.editor;
  // If the container is too narrow for everyone, the min wins (never below it).
  return Math.max(min, Math.min(desired, Math.max(min, max)));
}

export interface PanesState {
  libraryWidth: number;
  previewWidth: number;
  /** Set a side pane's width (already clamped by the caller). */
  setWidth: (side: PaneSide, width: number) => void;
  /** Restore persisted widths on startup; missing values keep their defaults. */
  hydrate: (layout: { libraryWidth?: number; previewWidth?: number }) => void;
}

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

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

  hydrate: (layout) =>
    set({
      libraryWidth: layout.libraryWidth ?? PANE_DEFAULT.library,
      previewWidth: layout.previewWidth ?? PANE_DEFAULT.preview,
    }),
}));