♻️ refactor: Assert input children are black in insert case analysis.

Author: make-github-pseudonymous-again

src/insertion/insert_case1.js

@@ -7,12 +7,15 @@ import insert_case2 from './insert_case2.js';
 /**
  * Preconditions:
  *   - n is red.
+ *   - n's children are BLACK
  *
  * @param {Node} n - The input node.
  */
 const insert_case1 = (n) => {
 	assert(n instanceof Node);
 	assert(n._color === RED);
+	assert(n.left._color === BLACK);
+	assert(n.right._color === BLACK);
 	/**
 	 * If n is the root of the tree, paint it black and we are done.
 	 *

src/insertion/insert_case2.js

@@ -7,13 +7,16 @@ import insert_case3 from './insert_case3.js';
 /**
  * Preconditions:
  *   - n is red.
+ *   - n's children are BLACK
  *   - n is not the root of the tree.
  *
  * @param {Node} n - The input node.
  */
 const insert_case2 = (n) => {
 	assert(n instanceof Node);
 	assert(n._color === RED);
+	assert(n.left._color === BLACK);
+	assert(n.right._color === BLACK);
 	assert(n.parent !== null);
 
 	/**

src/insertion/insert_case3.js

@@ -10,6 +10,7 @@ import insert_case4 from './insert_case4.js';
 /**
  * Preconditions:
  *   - n is red.
+ *   - n's children are BLACK
  *   - n is not the root of the tree.
  *   - n's parent is red.
  *
@@ -18,6 +19,8 @@ import insert_case4 from './insert_case4.js';
 const insert_case3 = (n) => {
 	assert(n instanceof Node);
 	assert(n._color === RED);
+	assert(n.left._color === BLACK);
+	assert(n.right._color === BLACK);
 	assert(n.parent !== null);
 	assert(n.parent._color === RED);
 	const u = uncle(n);

src/insertion/insert_case4.js

@@ -1,5 +1,6 @@
 import assert from 'assert';
 import Node from '../adt/Node.js';
+import BLACK from '../color/BLACK.js';
 import RED from '../color/RED.js';
 import rotate_left from '../rotate/rotate_left.js';
 import rotate_right from '../rotate/rotate_right.js';
@@ -9,6 +10,7 @@ import insert_case5 from './insert_case5.js';
 /**
  * Preconditions:
  *   - n is red.
+ *   - n's children are BLACK
  *   - n is not the root of the tree.
  *   - n's parent is red.
  *   - n's uncle is black.
@@ -20,6 +22,8 @@ import insert_case5 from './insert_case5.js';
 const insert_case4 = (n) => {
 	assert(n instanceof Node);
 	assert(n._color === RED);
+	assert(n.left._color === BLACK);
+	assert(n.right._color === BLACK);
 	assert(n.parent !== null);
 	assert(n.parent._color === RED);
 	const g = grandparent(n);

src/insertion/insert_case5.js

@@ -9,6 +9,7 @@ import grandparent from '../family/grandparent.js';
 /**
  * Preconditions:
  *   - n is red.
+ *   - n's children are BLACK
  *   - n is not the root of the tree.
  *   - n's parent is red.
  *   - n's uncle is black.
@@ -19,6 +20,8 @@ import grandparent from '../family/grandparent.js';
 const insert_case5 = (n) => {
 	assert(n instanceof Node);
 	assert(n._color === RED);
+	assert(n.left._color === BLACK);
+	assert(n.right._color === BLACK);
 	assert(n.parent !== null);
 	assert(n.parent._color === RED);
 	const g = grandparent(n);