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src/lib/model/tree.test.ts

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+import { describe, expect, it } from 'vitest';
+import { canInsert, flatten, freeSlotsOf, insert, locate, neighbor, remove, withTrunk } from './tree';
+
+describe('page tree', () => {
+	it('flattens trunk top→bottom with left then right branches, closest→outward', () => {
+		let t = withTrunk(['a', 'b']);
+		t = insert(t, 'a', 'right', ['r1']);
+		t = insert(t, 'r1', 'right', ['r2']);
+		t = insert(t, 'a', 'left', ['l1']);
+		t = insert(t, 'l1', 'left', ['l2']);
+		t = insert(t, 'b', 'right', ['br']);
+		expect(flatten(t)).toEqual(['a', 'l1', 'l2', 'r1', 'r2', 'b', 'br']);
+	});
+
+	it('growing the trunk upward changes what comes first', () => {
+		let t = withTrunk(['a']);
+		t = insert(t, 'a', 'up', ['p1', 'p2']);
+		expect(flatten(t)).toEqual(['p1', 'p2', 'a']);
+	});
+
+	it('inserts an imported chain in reading order on every side', () => {
+		let t = withTrunk(['a']);
+		t = insert(t, 'a', 'left', ['x1', 'x2', 'x3']);
+		expect(t.branches.a.left).toEqual(['x1', 'x2', 'x3']);
+		t = insert(t, 'a', 'down', ['d1', 'd2']);
+		expect(t.trunk).toEqual(['a', 'd1', 'd2']);
+	});
+
+	it('forbids sub-branches off a branch', () => {
+		let t = withTrunk(['a']);
+		t = insert(t, 'a', 'right', ['r']);
+		expect(canInsert(t, 'r', 'up')).toBe(false);
+		expect(insert(t, 'r', 'down', ['z'])).toBe(t);
+		expect(freeSlotsOf(t, 'r').map((s) => s.dir)).toEqual(['right']);
+	});
+
+	it('inserting toward the trunk from a branch page goes between', () => {
+		let t = withTrunk(['a']);
+		t = insert(t, 'a', 'right', ['r1', 'r2']);
+		t = insert(t, 'r2', 'left', ['mid']);
+		expect(t.branches.a.right).toEqual(['r1', 'mid', 'r2']);
+	});
+
+	it('navigates neighbours', () => {
+		let t = withTrunk(['a', 'b']);
+		t = insert(t, 'a', 'left', ['l1', 'l2']);
+		expect(neighbor(t, 'a', 'down')).toBe('b');
+		expect(neighbor(t, 'a', 'left')).toBe('l1');
+		expect(neighbor(t, 'l1', 'left')).toBe('l2');
+		expect(neighbor(t, 'l1', 'right')).toBe('a');
+		expect(neighbor(t, 'l2', 'up')).toBe(null);
+	});
+
+	it('removing a trunk page takes its branches with it', () => {
+		let t = withTrunk(['a', 'b']);
+		t = insert(t, 'a', 'right', ['r']);
+		const { tree, removed } = remove(t, 'a');
+		expect(removed).toEqual(['a', 'r']);
+		expect(flatten(tree)).toEqual(['b']);
+		expect(locate(tree, 'r')).toBe(null);
+	});
+
+	it('removing a branch page closes the gap', () => {
+		let t = withTrunk(['a']);
+		t = insert(t, 'a', 'right', ['r1', 'r2', 'r3']);
+		expect(remove(t, 'r2').tree.branches.a.right).toEqual(['r1', 'r3']);
+	});
+});