#include "my_str.h"
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>

/*
  Use this command to run

  gcc -DMY_STR_BUILD_DLL -shared my_str.c -o my_str.dll -Wl,--out-implib,libmy_str.a

*/


// Define maximum capacity for stacks
#define MAX 100

// Operand stack to store numbers
static double numStack[MAX];
static int numTop = -1;

// Operator stack to store characters (+, -, *, /, ()
static char opStack[MAX];
static int opTop = -1;

// Stack operations
static void pushNum(double val) { numStack[++numTop] = val; }
static double popNum() { return numStack[numTop--]; }
static void pushOp(char op) { opStack[++opTop] = op; }
static char popOp() { return opStack[opTop--]; }
static char peekOp() { return opStack[opTop]; }

// Returns priority of operators: Higher value means higher precedence
static int priority(char op) {
    if (op == '+' || op == '-') return 1;
    if (op == '*' || op == '/') return 2;
    return 0; // Parentheses have the lowest priority inside the stack
}

// Performs arithmetic operations
static double applyOp(double a, double b, char op) {
    switch (op) {
        case '+': return a + b;
        case '-': return a - b;
        case '*': return a * b;
        case '/': 
            if (b == 0) {
                printf("Error: Division by zero\n");
                return 0;
            }
            return a / b;
    }
    return 0;
}

// Main function to evaluate infix expression
double evaluate(const char* exp) {
    for (int i = 0; exp[i] != '\0'; i++) {
        // 1. Skip whitespace characters
        if (exp[i] == ' ' || exp[i] == '\t') continue;

        // 2. If the character is a digit, parse the full number (supports multi-digit)
        if (isdigit(exp[i])) {
            double val = 0;
            while (i < strlen(exp) && isdigit(exp[i])) {
                val = (val * 10) + (exp[i] - '0');
                i++;
            }
            pushNum(val);
            i--; // Decrement index because the loop increments it
        } 
        // 3. If left parenthesis, push it to the operator stack
        else if (exp[i] == '(') {
            pushOp(exp[i]);
        } 
        // 4. If right parenthesis, solve the entire bracket
        else if (exp[i] == ')') {
            while (opTop != -1 && peekOp() != '(') {
                double val2 = popNum();
                double val1 = popNum();
                char op = popOp();
                pushNum(applyOp(val1, val2, op));
            }
            if (opTop != -1) popOp(); // Remove '(' from stack
        } 
        // 5. If character is an operator
        else {
            // While the top of the stack has same or higher precedence, calculate
            while (opTop != -1 && priority(peekOp()) >= priority(exp[i])) {
                double val2 = popNum();
                double val1 = popNum();
                char op = popOp();
                pushNum(applyOp(val1, val2, op));
            }
            // Push current operator to stack
            pushOp(exp[i]);
        }
    }

    // 6. Entire expression parsed, calculate remaining operations in stack
    while (opTop != -1) {
        double val2 = popNum();
        double val1 = popNum();
        char op = popOp();
        pushNum(applyOp(val1, val2, op));
    }

    // The final result is the only element left in the operand stack
    return popNum();
}