fn main() {
  let program_data = "++++++++[>++++[>++>+++>+++>+<<<<-]>+>+>->>+[<]<-]>>.>---.+++++++..+++.>>.<-.<.+++.------.--------.>>+.>++.";
  let mut program = Program::from_chars(program_data);
  program.step();
  println!("OUTPUT:");
  println!("    pointer: {}", program.pointer);
  println!("    data: {:?}", program.data);
}

#[derive(Debug)]
pub enum Instruction {
  // > : Move pointer forward
  Forward,

  // < : Move pointer backward
  Backward,

  // + : Increment current value by 1
  Increment,

  // - : Decrement current value by 1
  Decrement,

  // . : Output current value
  Output,

  // , : Input value, store at current pointer
  Input,

  // [ : Begin loop
  Begin,

  // ] : End loop; if current value is not zero, jump back to ]
  End
}

impl Instruction {

  fn new(c: char) -> Instruction {
    match c {
      '>' => Instruction::Forward,
      '<' => Instruction::Backward,
      '+' => Instruction::Increment,
      '-' => Instruction::Decrement,
      '.' => Instruction::Output,
      ',' => Instruction::Input,
      '[' => Instruction::Begin,
      ']' => Instruction::End,
      _   => panic!("Invalid character: {}", c)
    }
  }

}

struct Program {
  instructions: Vec<Instruction>,
  step: usize,
  pointer: usize,
  begin_stack: Vec<usize>,
  data: Vec<i32>
}

impl Program {
  fn from_chars(characters: &str) -> Program {
    let instructions = characters.chars().map(Instruction::new).collect();
    Program {
      instructions: instructions,
      step: 0 as usize,
      pointer: 0 as usize,
      begin_stack: Vec::new(),
      data: vec![0] // reserve
    }
  }

  fn step(&mut self) {
    let step = self.step;
    self.step += 1;
    if step < self.instructions.len() {
      match self.instructions[step] {
        Instruction::Forward => self.forward(),
        Instruction::Backward => self.backward(),
        Instruction::Increment => self.increment(),
        Instruction::Decrement => self.decrement(),
        Instruction::Input => self.input(),
        Instruction::Output => self.output(),
        Instruction::Begin => self.begin(),
        Instruction::End => self.end()
      }
    }
  }

  fn forward(&mut self) {
    println!("forward");
    self.pointer += 1;
    println!("pointer: {}", self.pointer);
    if self.pointer >= self.data.len() {
      self.data.push(0);
    }
    println!("data: {:?}", self.data);
    self.step();
  }

  fn backward(&mut self) {
    println!("backward");
    self.pointer -= 1;
    println!("pointer: {}", self.pointer);
    println!("data: {:?}", self.data);
    self.step();
  }

  fn increment(&mut self) {
    println!("increment");
    self.data[self.pointer] += 1;
    println!("{:?}", self.pointer);
    println!("{:?}", self.data);
    self.step();
  }

  fn decrement(&mut self) {
    println!("decrement");
    self.data[self.pointer] -= 1;
    self.step();
  }

  fn input(&mut self) {
    println!("input");
    self.step();
  }

  fn output(&mut self) {
    println!("output");
    println!("{}", self.data[self.pointer]);
    self.step();
  }

  fn begin(&mut self) {
    println!("begin");
    self.begin_stack.push(self.step);
    self.step();
  }

  fn end(&mut self) {
    println!("end");
    println!("{:?}", self.pointer);
    if (self.data[self.pointer] == 0) {
      self.begin_stack.pop();
      self.step();
    } else {
      self.step = match self.begin_stack.last() {
        Some(v) => *v,
        None    => panic!("Unmatched loop bracket")
      };
      self.step();
    }
  }
}