use std::io::{self, Read};
use std::env;
use std::process;

fn interpret_brainfuck(code: &str) {
    let mut memory = vec![0u8; 30000]; // 30,000 cells of memory initialized to 0
    let mut pointer: usize = 0; // data pointer starts at the first cell
    let mut pc: usize = 0; // program counter for the Brainfuck code
    let code_chars: Vec<char> = code.chars().collect(); // convert the Brainfuck code to a vector of characters

    // Stack to keep track of [ and ] for loops
    let mut loop_stack: Vec<usize> = Vec::new();

    while pc < code_chars.len() {
        match code_chars[pc] {
            '>' => {
                pointer += 1;
                if pointer >= memory.len() {
                    panic!("Pointer out of bounds (positive)");
                }
            }
            '<' => {
                if pointer == 0 {
                    panic!("Pointer out of bounds (negative)");
                }
                pointer -= 1;
            }
            '+' => {
                memory[pointer] = memory[pointer].wrapping_add(1); // increment with wrap around
            }
            '-' => {
                memory[pointer] = memory[pointer].wrapping_sub(1); // decrement with wrap around
            }
            '.' => {
                print!("{}", memory[pointer] as char);
            }
            ',' => {
                let mut input = [0u8];
                io::stdin().read_exact(&mut input).expect("Failed to read input. Unable to read a single byte from stdin");
                memory[pointer] = input[0];
            }
            '[' => {
                if memory[pointer] == 0 {
                    // Jump to the matching ']' if the current cell is 0
                    let mut open_brackets = 1;
                    while open_brackets > 0 {
                        pc += 1;
                        if pc >= code_chars.len() {
                            panic!("Unmatched [");
                        }
                        if code_chars[pc] == '[' {
                            open_brackets += 1;
                        } else if code_chars[pc] == ']' {
                            open_brackets -= 1;
                        }
                    }
                } else {
                    // Push the current position to the stack if not jumping
                    loop_stack.push(pc);
                }
            }
            ']' => {
                if memory[pointer] != 0 {
                    // Jump back to the matching '[' if the current cell is nonzero
                    if let Some(loop_start) = loop_stack.last() {
                        pc = *loop_start;
                    } else {
                        panic!("Unmatched ]");
                    }
                } else {
                    // Pop the stack if done with the loop
                    loop_stack.pop();
                }
            }
            _ => {} // Ignore any non-Brainfuck commands
        }
        pc += 1; // Move to the next instruction
    }
}

fn main() {
    let args: Vec<String> = env::args().collect();
    if args.len() != 2 || !args[1].ends_with(".bf") {
        eprintln!("Usage: {} <filename.bf>", args[0]);
        process::exit(1);
    }

    let filename = &args[1];
    let code = std::fs::read_to_string(filename).expect("Failed to read the file");
    interpret_brainfuck(&code);
}