refactor: ♻️ move derivative from ootk-core

src/utils/functions.ts

@@ -0,0 +1,21 @@
+import { DifferentiableFunction } from '../main';
+
+/**
+ * Calculates the derivative of a differentiable function.
+ * @param f The differentiable function.
+ * @param h The step size for numerical differentiation. Default value is 1e-3.
+ * @returns The derivative function.
+ */
+export function derivative(f: DifferentiableFunction, h = 1e-3): DifferentiableFunction {
+  /**
+   * @param x The value at which to calculate the derivative.
+   * @returns The derivative of the function at the given value.
+   */
+  function df(x: number): number {
+    const hh = h * 0.5;
+
+    return (f(x + hh) - f(x - hh)) / h;
+  }
+
+  return df;
+}

src/utils/index.ts

@@ -1 +1,2 @@
 export * from './jacobian';
+export * from './functions';

test/utils/functions.test.ts

@@ -1,6 +1,11 @@
-import { jacobian } from '../../src/main';
+import { derivative, jacobian } from '../../src/main';
 
 // jacobian
 it('should be calculate jacobian', () => {
   expect(jacobian((xs: Float64Array) => xs, 1, new Float64Array([1, 2, 3]))).toMatchSnapshot();
 });
+
+// derivative
+it('should be calculate derivative', () => {
+  expect(derivative((x: number) => x * x, 1)).toMatchSnapshot();
+});