maestro/web/src/layout.ts

// Multi-view layout model. The layout tree is the single source of truth for
// what's on screen: a set of tabs, each holding a binary split tree whose leaves
// hold a session (or nothing yet). Desktop renders the tree as nested splits;
// mobile (issue #35) projects its leaves to tabs.
//
// A session lives in at most one leaf at a time (the connection pool is
// one-socket-per-session), so a leaf's `sessionId` is unique across the whole
// layout — or null for a freshly-split, not-yet-filled pane.

import type { Uuid } from "./types.js";

export type Dir = "row" | "col";

export type Node =
  | { kind: "leaf"; id: string; sessionId: Uuid | null }
  | { kind: "split"; dir: Dir; a: Node; b: Node; ratio: number };

export type Leaf = Extract<Node, { kind: "leaf" }>;

export interface Tab {
  id: string;
  title: string;
  /** The split tree, or null for an empty tab. */
  root: Node | null;
  /** Which leaf has focus (drives keyboard input + the strong highlight). */
  focusedLeafId: string | null;
}

export interface Layout {
  tabs: Tab[];
  activeTabId: string;
}

let seq = 0;
export function freshId(prefix = "n"): string {
  return `${prefix}${++seq}`;
}

/** Every leaf under `node`, left-to-right. */
export function leaves(node: Node | null): Leaf[] {
  if (!node) return [];
  if (node.kind === "leaf") return [node];
  return [...leaves(node.a), ...leaves(node.b)];
}

export function findLeaf(node: Node | null, leafId: string): Leaf | null {
  return leaves(node).find((l) => l.id === leafId) ?? null;
}

export function activeTab(layout: Layout): Tab {
  return layout.tabs.find((t) => t.id === layout.activeTabId) ?? layout.tabs[0];
}

/** Distinct sessions shown in any pane of any tab. */
export function openSessions(layout: Layout): Set<Uuid> {
  const out = new Set<Uuid>();
  for (const t of layout.tabs) {
    for (const l of leaves(t.root)) if (l.sessionId) out.add(l.sessionId);
  }
  return out;
}

/** Find the tab + leaf currently showing `sessionId`, if any. */
export function locateSession(
  layout: Layout,
  sessionId: Uuid,
): { tab: Tab; leaf: Leaf } | null {
  for (const tab of layout.tabs) {
    const leaf = leaves(tab.root).find((l) => l.sessionId === sessionId);
    if (leaf) return { tab, leaf };
  }
  return null;
}

/** A layout with one empty tab. */
export function emptyLayout(): Layout {
  const tab: Tab = { id: freshId("t"), title: "Tab 1", root: null, focusedLeafId: null };
  return { tabs: [tab], activeTabId: tab.id };
}

// ---- mutations (return the affected leaf id where useful) ----

/** Replace `targetId` with a split of [target, fresh] (or reversed when
 *  `after` is false). Returns the new leaf's id. */
export function splitLeaf(
  tab: Tab,
  targetId: string,
  sessionId: Uuid | null,
  dir: Dir,
  after: boolean,
): string {
  const fresh: Leaf = { kind: "leaf", id: freshId("l"), sessionId };
  const rec = (n: Node): Node => {
    if (n.kind === "leaf") {
      if (n.id !== targetId) return n;
      return { kind: "split", dir, ratio: 0.5, a: after ? n : fresh, b: after ? fresh : n };
    }
    n.a = rec(n.a);
    n.b = rec(n.b);
    return n;
  };
  tab.root = tab.root ? rec(tab.root) : fresh;
  return fresh.id;
}

/** Remove a leaf, collapsing its parent split into the sibling. */
export function removeLeaf(tab: Tab, leafId: string): void {
  const rec = (n: Node): Node | null => {
    if (n.kind === "leaf") return n.id === leafId ? null : n;
    const a = rec(n.a);
    const b = rec(n.b);
    if (!a) return b;
    if (!b) return a;
    n.a = a;
    n.b = b;
    return n;
  };
  tab.root = tab.root ? rec(tab.root) : null;
}

/** Reset every split ratio under `node` so all leaves get equal width,
 *  regardless of split direction or nesting order. Returns the leaf count. */
export function equalizeRatios(node: Node | null): number {
  if (!node) return 0;
  if (node.kind === "leaf") return 1;
  const na = equalizeRatios(node.a);
  const nb = equalizeRatios(node.b);
  node.ratio = na / (na + nb);
  return na + nb;
}

export function addTab(layout: Layout): Tab {
  const tab: Tab = {
    id: freshId("t"),
    title: `Tab ${layout.tabs.length + 1}`,
    root: null,
    focusedLeafId: null,
  };
  layout.tabs.push(tab);
  layout.activeTabId = tab.id;
  return tab;
}

export function closeTab(layout: Layout, tabId: string): void {
  if (layout.tabs.length <= 1) {
    // Keep at least one tab; just empty it.
    const t = layout.tabs[0];
    t.root = null;
    t.focusedLeafId = null;
    return;
  }
  const i = layout.tabs.findIndex((t) => t.id === tabId);
  if (i < 0) return;
  layout.tabs.splice(i, 1);
  if (layout.activeTabId === tabId) {
    layout.activeTabId = layout.tabs[Math.max(0, i - 1)].id;
  }
}