From d34d5b5572aa0cde9c567ccd3b878101295864fd Mon Sep 17 00:00:00 2001
From: "frinad.8555" <frinadvk2006@gmail.com>
Date: Sat, 11 Oct 2025 22:47:47 +0530
Subject: [PATCH] Add Armijo Backtracking Line Search Algorithm

---
 numerical_methods/armijo_backtrack.cpp | 155 +++++++++++++++++++++++++
 1 file changed, 155 insertions(+)
 create mode 100644 numerical_methods/armijo_backtrack.cpp

diff --git a/numerical_methods/armijo_backtrack.cpp b/numerical_methods/armijo_backtrack.cpp
new file mode 100644
index 00000000000..836534e9bbe
--- /dev/null
+++ b/numerical_methods/armijo_backtrack.cpp
@@ -0,0 +1,155 @@
+/**
+ * @file
+ * @brief Demonstration of 1D optimization using the Armijo backtracking
+ * line search method (adaptive step size gradient descent).
+ *
+ * @author [Frinad Kandoriya](https://github.com/frinad8555)
+ * @see https://en.wikipedia.org/wiki/Backtracking_line_search
+ */
+
+#include <cassert>
+#include <cmath>
+#include <cstdint>
+#include <functional>
+#include <iostream>
+#include <limits>
+
+#define EPSILON 1e-7 // convergence tolerance
+
+/**
+ * @brief Perform Armijo backtracking line search for a 1D function
+ *
+ * The method adaptively reduces the step size until the Armijo condition is satisfied:
+ * \f[
+ * f(x + \alpha d) \le f(x) + c \alpha f'(x)d
+ * \f]
+ *
+ * @param f the function to minimize
+ * @param df first derivative of f
+ * @param xk current position
+ * @param dk descent direction
+ * @param c Armijo constant (usually 1e-4)
+ * @param tau backtracking reduction factor (usually 0.5)
+ * @param alpha_init initial trial step size
+ * @return acceptable step length satisfying Armijo condition
+ */
+
+double armijo_backtrack(const std::function<double(double)> &f,
+                        const std::function<double(double)> &df, double xk,
+                        double dk, double c = 1e-4, double tau = 0.5,
+                        double alpha_init = 1.0) {
+    double alpha = alpha_init;
+    double fxk = f(xk);
+    double grad_fxk = df(xk);
+
+    while (f(xk + alpha * dk) > fxk + c * alpha * grad_fxk * dk) {
+        alpha *= tau;  // reduce step size
+    }
+
+    return alpha;
+}
+
+/**
+ * @brief Perform gradient descent using Armijo line search on a 1D function.
+ *
+ * @param f target function
+ * @param df first derivative of f
+ * @param x0 initial guess
+ * @param max_iter maximum number of iterations
+ * @return estimated minimum point
+ */
+
+double gradient_descent_armijo(const std::function<double(double)> &f,
+                               const std::function<double(double)> &df,
+                               double x0, uint32_t max_iter = 50) {
+    double x = x0;
+    double grad = df(x);
+    double d = -grad;
+    int iter = 0;
+
+    while (std::abs(grad) > EPSILON && iter < max_iter) {
+        double alpha = armijo_backtrack(f, df, x, d);
+        x += alpha * d;
+        grad = df(x);
+        d = -grad;
+        std::cout << "Iter " << iter << ": x = " << x << ", f(x) = " << f(x)
+                  << ", alpha = " << alpha << "\n";
+        iter++;
+    }
+
+    std::cout << "Converged in " << iter << " iterations.\n";
+    return x;
+}
+
+/**
+ * @brief Test case 1: minimize f(x) = (x - 2)^2
+ * \n Expected minimum at x = 2
+ */
+
+void test1() {
+    std::cout << "Test 1... ";
+
+    std::function<double(double)> f = [](double x) { return pow(x - 2.0, 2); };
+    std::function<double(double)> df = [](double x) { return 2.0 * (x - 2.0); };
+
+    double xmin = gradient_descent_armijo(f, df, 0.0);
+
+    std::cout << "Result = " << xmin << "\n";
+    assert(std::abs(xmin - 2.0) < 1e-5);
+    std::cout << "Passed.\n";
+    std::cout << "\n";
+}
+
+/**
+ * @brief Test case 2: minimize f(x) = (x + 3)^2 + 1
+ * \n Expected minimum at x = -3
+ */
+
+void test2() {
+    std::cout << "Test 2... ";
+
+    std::function<double(double)> f = [](double x) { return pow(x + 3.0, 2) + 1; };
+    std::function<double(double)> df = [](double x) { return 2.0 * (x + 3.0); };
+
+    double xmin = gradient_descent_armijo(f, df, 5.0);
+
+    std::cout << "Result = " << xmin << "\n";
+    assert(std::abs(xmin + 3.0) < 1e-5);
+    std::cout << "Passed.\n";
+    std::cout << "\n";
+}
+
+/**
+ * @brief Test case 3: minimize non-convex f(x) = x^4 - 3x^3 + 2
+ * \n Expected local minimum near x ≈ 2.25
+ */
+
+void test3() {
+    std::cout << "Test 3... ";
+
+    std::function<double(double)> f = [](double x) {
+        return pow(x, 4) - 3 * pow(x, 3) + 2;
+    };
+    std::function<double(double)> df = [](double x) {
+        return 4 * pow(x, 3) - 9 * pow(x, 2);
+    };
+
+    double xmin = gradient_descent_armijo(f, df, 1.0);
+
+    std::cout << "Result = " << xmin << "\n";
+    std::cout << "Passed.\n";
+    std::cout << "\n";
+}
+
+/** @brief Main function */
+int main() {
+    std::cout.precision(9);
+    std::cout << "Armijo Backtracking Line Search Example\n";
+    std::cout << "Machine epsilon: " << EPSILON << "\n\n";
+
+    test1();
+    test2();
+    test3();
+
+    return 0;
+}