const INPUT: &str = include_str!("../../data/day8");

#[derive(Clone, Copy, PartialEq, Eq, Debug)]
enum Instruction {
	Acc(i64),
	Jmp(isize),
	Nop(isize),
}

impl Instruction {
	fn parse(line: &str) -> Self {
		let line = line.trim();
		if let Some(param) = line.strip_prefix("acc ") {
			Instruction::Acc(param.parse().unwrap())
		} else if let Some(param) = line.strip_prefix("jmp ") {
			Instruction::Jmp(param.parse().unwrap())
		} else if let Some(param) = line.strip_prefix("nop ") {
			Instruction::Nop(param.parse().unwrap())
		} else {
			panic!("Unknown instructin: {:?}", line);
		}
	}

	fn parse_list(input: &str) -> Vec<Self> {
		input.lines().map(Self::parse).collect()
	}
}

#[derive(Clone, Copy, PartialEq, Eq, Debug)]
enum EmulationStepResult {
	Success,
	EndOfCode,
	InfiniteLoop,
}

#[derive(Clone, Debug)]
struct Emulator<'a> {
	instructions: &'a [Instruction],
	visited: Vec<bool>,
	accumulator: i64,
	next_instruction: usize,
}

impl<'a> Emulator<'a> {
	fn new(instructions: &'a [Instruction]) -> Self {
		let mut visited = Vec::new();
		visited.resize(instructions.len(), false);
		Self { instructions, visited, accumulator: 0, next_instruction: 0 }
	}

	fn step(&mut self) -> EmulationStepResult {
		if self.next_instruction >= self.instructions.len() {
			return EmulationStepResult::EndOfCode;
		}
		if self.visited[self.next_instruction] {
			return EmulationStepResult::InfiniteLoop;
		}
		self.visited[self.next_instruction] = true;
		match self.instructions[self.next_instruction] {
			Instruction::Acc(inc) => {
				self.accumulator += inc;
			},
			Instruction::Jmp(offset) => {
				let ip = self.next_instruction as isize + offset;
				assert!(ip >= 0);
				self.next_instruction = ip as usize;
				return EmulationStepResult::Success; // don't increment again below
			},
			Instruction::Nop(_) => (),
		}
		self.next_instruction += 1;
		EmulationStepResult::Success
	}

	fn run(&mut self) -> bool {
		loop {
			match self.step() {
				EmulationStepResult::Success => (), // continue
				EmulationStepResult::InfiniteLoop => return false,
				EmulationStepResult::EndOfCode => return true,
			}
		}
	}
}

fn main() {
	let mut instructions = Instruction::parse_list(INPUT);
	{
		let mut emu = Emulator::new(&instructions);
		assert!(!emu.run()); // expecting infinite loop
		println!("Accumulator at loop: {}", emu.accumulator);
	}
	for i in 0..instructions.len() {
		let orig = instructions[i];
		instructions[i] = match orig {
			Instruction::Acc(_) => continue,
			Instruction::Jmp(a) => Instruction::Nop(a),
			Instruction::Nop(a) => Instruction::Jmp(a),
		};
		{
			let mut emu = Emulator::new(&instructions);
			if emu.run() {
				println!("Found fixed program (modified at {}), accumulator at end: {}", i, emu.accumulator);
				break;
			}
		}
		instructions[i] = orig;
	}
}