move distanceXY and factorial to dot/js/util/, phetsims/models-of-the…

…-hydrogen-atom#118

js/util/distanceXY.ts

@@ -0,0 +1,12 @@
+// Copyright 2025, University of Colorado Boulder
+
+/**
+ * Returns the distance between 2 points, given by (x,y) coordinates.
+ *
+ * @author Chris Malley ([email protected])
+ */
+export default function distanceXY( x1: number, y1: number, x2: number, y2: number ): number {
+  const dx = x1 - x2;
+  const dy = y1 - y2;
+  return Math.sqrt( dx * dx + dy * dy );
+}

js/util/factorial.ts

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+  // Copyright 2025, University of Colorado Boulder
+
+  /**
+   * Computes the factorial of a non-negative integer n without using recursion.
+   * n! = 1 * 2 * ... * ( n - 1 ) * n
+   *
+   * @author Chris Malley ([email protected])
+   */
+  export default function factorial( n: number ): number {
+    assert && assert( Number.isInteger( n ) && n >= 0, `n must be a non-negative integer: ${n}` );
+    let f = 1;
+    for ( let i = 2; i <= n; i++ ) {
+      f *= i;
+    }
+    return f;
+  }