Create ast.cxx for AST implementation and tests

Commit details:
- Add new file ast.cxx with ASTNode, Identifier, Divide, Multiply, Subtract, Power classes.
- Include necessary headers and static initialization of variableTable in Identifier class.
- Add a test function runTest() for evaluating a complex expression.
- Add printHelpMessage() function for displaying program usage.
- Update main function to handle "--run-test" argument.

src/ast.cxx

@@ -0,0 +1,63 @@
+#include "ast.hxx"
+#include <cstring>
+
+// Static initialization of variableTable in Identifier class.
+std::unordered_map<std::string, double> Identifier::variableTable;
+
+#ifdef ENABLE_TESTS
+// Function to run a test expression evaluation.
+void runTest() {
+    // Test with a complex expression: (2 * (a + b)) / (c - 1) ^ (d + 1)
+    Identifier::setVariable("a", 3.0);
+    Identifier::setVariable("b", 1.0);
+    Identifier::setVariable("c", 5.0);
+    Identifier::setVariable("d", 2.0);
+
+    std::unique_ptr<const ASTNode> variableA = std::make_unique<Identifier>("a");
+    std::unique_ptr<const ASTNode> variableB = std::make_unique<Identifier>("b");
+    std::unique_ptr<const ASTNode> variableC = std::make_unique<Identifier>("c");
+    std::unique_ptr<const ASTNode> variableD = std::make_unique<Identifier>("d");
+
+    std::unique_ptr<const ASTNode> expression = std::make_unique<Divide>(
+        std::make_unique<Multiply>(std::make_unique<Constant>(2.0),
+                                   std::make_unique<Add>(std::move(variableA), std::move(variableB))),
+        std::make_unique<Power>(std::make_unique<Subtract>(std::move(variableC), std::make_unique<Constant>(1.0)),
+                                std::make_unique<Add>(std::move(variableD), std::make_unique<Constant>(1.0))));
+
+    // Evaluate the expression
+    double result = expression->evaluate();
+
+    // Expected result: (2 * (3 + 1)) / (5 - 1) ^ (2 + 1) = 8 / 4 ^ 3 = 8 / 64 = 0.125
+    assert(result == 0.125);
+
+    std::cout << "Test passed successfully. Result: " << result << std::endl;
+
+    // Clear variables for the next test
+    Identifier::clearVariables();
+}
+#endif // ENABLE_TESTS
+
+// Function to print the help message.
+void printHelpMessage(const char *programName) {
+    std::cout << "Usage: " << programName << " [--run-test]\n"
+              << "Options:\n"
+              << "  --run-test  Run the test for the expression evaluation code.\n"
+              << "              This option should be used without any additional arguments.\n"
+              << "              Example: " << programName << " --run-test\n";
+}
+
+// Main function
+int main(int argc, char *argv[]) {
+    // Check if the "--run-test" argument is provided
+    if (argc == 2 && std::strcmp(argv[1], "--run-test") == 0) {
+#ifdef ENABLE_TESTS
+        // Run the test if ENABLE_TESTS is defined
+        runTest();
+#endif // ENABLE_TESTS
+    } else {
+        // Print help message if no valid arguments are provided
+        printHelpMessage(argv[0]);
+    }
+
+    return 0;
+}