#[cfg(feature = "debug-output")]
use std::env;

#[derive(Copy, Clone, PartialEq)]
pub struct Registers {
    pub values: [u64; 6],
}

#[derive(Debug, Copy, Clone)]
pub struct Instruction {
    opcode: Opcode,
    pub a: u64,
    pub b: u64,
    pub c: u64,
}

pub struct Program {
    pub instruction_pointer_index: u8,
    pub instructions: Vec<Instruction>,
    pub registers: Registers,
}

impl Program {
    pub fn instruction_pointer(&self) -> Result<u64, String> {
        self.registers
            .values
            .get(self.instruction_pointer_index as usize)
            .copied()
            .ok_or_else(|| "Invalid instruction pointer".to_string())
    }

    pub fn execute_one_instruction(&mut self) -> Result<bool, String> {
        let ip = self.instruction_pointer()?;
        if ip as usize >= self.instructions.len() {
            return Ok(false);
        }
        let instruction = self.instructions[ip as usize];
        self.registers.apply(
            instruction.opcode,
            instruction.a,
            instruction.b,
            instruction.c,
        );
        self.registers.values[self.instruction_pointer_index as usize] += 1;
        Ok(true)
    }

    pub fn execute_until_halt(&mut self, max_instructions: u32) -> Result<u64, String> {
        let mut loop_count = 0;
        while self.execute_one_instruction()? {
            loop_count += 1;
            if loop_count > max_instructions {
                return Err(format!("Aborting after {} instructions", max_instructions));
            }
        }
        Ok(self.registers.values[0])
    }

    pub fn parse(input_string: &str) -> Result<Self, String> {
        let mut lines = input_string.lines();
        let first_line = lines.next().ok_or("Empty input")?;

        if first_line.len() < 5 {
            return Err("Invalid input".to_string());
        }
        let error = |_| "Invalid input";
        let instruction_pointer_index = (&first_line[4..]).parse::<u8>().map_err(error)?;

        let mut instructions = Vec::new();
        for line in lines {
            let parts: Vec<&str> = line.split_whitespace().collect();
            let opcode = opcode_from_str(parts[0])?;
            let a = parts[1].parse::<u64>().map_err(error)?;
            let b = parts[2].parse::<u64>().map_err(error)?;
            let c = parts[3].parse::<u64>().map_err(error)?;
            instructions.push(Instruction { opcode, a, b, c });
        }

        Ok(Self {
            instruction_pointer_index,
            instructions,
            registers: Registers::new(),
        })
    }

    pub fn optimize(&mut self) {
        for (line, instruction) in self.instructions.iter_mut().enumerate() {
            match instruction.opcode {
                Opcode::Addi => {
                    if instruction.a as u8 == self.instruction_pointer_index {
                        instruction.opcode = Opcode::Seti;
                        instruction.a = line as u64 + instruction.b;
                        instruction.b = 0; // ignored
                    }
                }
                Opcode::Mulr => {
                    if instruction.a as u8 == self.instruction_pointer_index
                        && instruction.b as u8 == self.instruction_pointer_index
                    {
                        instruction.opcode = Opcode::Seti;
                        instruction.a = line as u64 * line as u64;
                        instruction.b = 0; // ignored
                    }
                }
                Opcode::Muli => {
                    if instruction.a as u8 == self.instruction_pointer_index {
                        instruction.opcode = Opcode::Seti;
                        instruction.a = line as u64 * instruction.b;
                        instruction.b = 0; // ignored
                    }
                }
                _ => {}
            }
        }
    }

    #[cfg(feature = "debug-output")]
    pub fn pretty_print(&self, title: &str) {
        if env::var("ADVENT_DEBUG").is_err() {
            return;
        }

        println!("# {}", title);

        for (line, &instruction) in self.instructions.iter().enumerate() {
            print!("{:02}: ", line);

            // If target register is the instruction pointer:
            let goto = instruction.c as u8 == self.instruction_pointer_index;
            if goto {
                print!("goto ");
            } else {
                print!("r{} = ", instruction.c)
            }

            // "When the instruction pointer is bound to a register, its value is written to that register
            // just before each instruction is executed, and the value of that register is written back to
            // the instruction pointer immediately after each instruction finishes execution. Afterward,
            // move to the next instruction by adding one to the instruction pointer, even if the value
            // in the instruction pointer was just updated by an instruction. (Because of this, instructions
            // must effectively set the instruction pointer to the instruction before the one they want
            // executed next.)" - Day 19 instructions
            // So inline value if possible.
            let mut append_to_value = if goto { 1 } else { 0 };
            let mut appender = || {
                let result = append_to_value;
                append_to_value = 0;
                result
            };

            let b_is_ignored = matches!(instruction.opcode, Opcode::Setr | Opcode::Seti);

            let a_is_value = matches!(
                instruction.opcode,
                Opcode::Seti | Opcode::Gtir | Opcode::Eqir
            );
            let b_is_value = matches!(
                instruction.opcode,
                Opcode::Addi
                    | Opcode::Muli
                    | Opcode::Bani
                    | Opcode::Bori
                    | Opcode::Gtri
                    | Opcode::Eqri
            );

            let a = if a_is_value {
                format!(
                    "{}",
                    instruction.a + if b_is_ignored { appender() } else { 0 }
                )
            } else if instruction.a as u8 == self.instruction_pointer_index {
                format!("{}", line)
            } else {
                format!("r{}", instruction.a)
            };

            let b = if b_is_value {
                format!("{}", instruction.b + appender() as u64)
            } else if instruction.b as u8 == self.instruction_pointer_index {
                format!("{}", (line + appender() as usize))
            } else {
                format!("r{}", instruction.b)
            };

            let pretty = match instruction.opcode {
                Opcode::Addr | Opcode::Addi => {
                    if a_is_value && b_is_value {
                        format!("{}", instruction.a + instruction.b)
                    } else {
                        format!("{} + {}", a, b)
                    }
                }
                Opcode::Mulr | Opcode::Muli => {
                    if a_is_value && b_is_value {
                        format!("{}", instruction.a * instruction.b)
                    } else {
                        format!("{} * {}", a, b)
                    }
                }
                Opcode::Setr | Opcode::Seti => a.to_string(),
                Opcode::Gtir | Opcode::Gtri | Opcode::Gtrr => format!("({} > {}) ? 1 : 0", a, b),
                Opcode::Eqrr | Opcode::Eqri => format!("({} == {}) ? 1 : 0", a, b),
                Opcode::Bani => format!("{} & {}", a, b),
                Opcode::Bori => format!("{} | {}", a, b),
                _ => format!(
                    "Unhandled opcode at line {}: {:?}",
                    line, instruction.opcode
                ),
            };
            print!("{}", pretty);
            if append_to_value != 0 {
                print!(" + 1");
            }
            println!();
        }
    }
}

#[derive(Copy, Clone, Debug, Hash, Eq, PartialEq)]
pub enum Opcode {
    Addr, // (add register) stores into register C the result of adding register A and register B
    Addi, // (add immediate) stores into register C the result of adding register A and value B.
    Mulr, // (multiply register) stores into register C the result of multiplying register A and register B.
    Muli, // (multiply immediate) stores into register C the result of multiplying register A and value B.
    Banr, // (bitwise AND register) stores into register C the result of the bitwise AND of register A and register B.
    Bani, // (bitwise AND immediate) stores into register C the result of the bitwise AND of register A and value B.
    Borr, // (bitwise OR register) stores into register C the result of the bitwise OR of register A and register B.
    Bori, // (bitwise OR immediate) stores into register C the result of the bitwise OR of register A and value B.
    Setr, // (set register) copies the contents of register A into register C. (Input B is ignored.)
    Seti, // (set immediate) stores value A into register C. (Input B is ignored.)
    Gtir, // (greater-than immediate/register) sets register C to 1 if value A is greater than register B. Otherwise, register C is set to 0.
    Gtri, // (greater-than register/immediate) sets register C to 1 if register A is greater than value B. Otherwise, register C is set to 0.
    Gtrr, // (greater-than register/register) sets register C to 1 if register A is greater than register B. Otherwise, register C is set to 0.
    Eqir, // (equal immediate/register) sets register C to 1 if value A is equal to register B. Otherwise, register C is set to 0.
    Eqri, // (equal register/immediate) sets register C to 1 if register A is equal to value B. Otherwise, register C is set to 0.
    Eqrr, // (equal register/register) sets register C to 1 if register A is equal to register B. Otherwise, register C is set to 0.
}

fn opcode_from_str(name: &str) -> Result<Opcode, String> {
    Ok(match name {
        "addr" => Opcode::Addr,
        "addi" => Opcode::Addi,
        "mulr" => Opcode::Mulr,
        "muli" => Opcode::Muli,
        "banr" => Opcode::Banr,
        "bani" => Opcode::Bani,
        "borr" => Opcode::Borr,
        "bori" => Opcode::Bori,
        "setr" => Opcode::Setr,
        "seti" => Opcode::Seti,
        "gtir" => Opcode::Gtir,
        "gtri" => Opcode::Gtri,
        "gtrr" => Opcode::Gtrr,
        "eqir" => Opcode::Eqir,
        "eqri" => Opcode::Eqri,
        "eqrr" => Opcode::Eqrr,
        _ => {
            return Err(format!("No matching opcode: {}", name));
        }
    })
}

impl Registers {
    const fn new() -> Self {
        Self {
            values: [0, 0, 0, 0, 0, 0],
        }
    }

    fn reg(&mut self, index: u64) -> u64 {
        self.values[index as usize]
    }

    fn apply(&mut self, opcode: Opcode, a: u64, b: u64, c: u64) {
        let c = c as usize;
        self.values[c] = match opcode {
            Opcode::Addr => self.reg(a) + self.reg(b),
            Opcode::Addi => self.reg(a) + b,
            Opcode::Mulr => self.reg(a) * self.reg(b),
            Opcode::Muli => self.reg(a) * b,
            Opcode::Banr => self.reg(a) & self.reg(b),
            Opcode::Bani => self.reg(a) & b,
            Opcode::Borr => self.reg(a) | self.reg(b),
            Opcode::Bori => self.reg(a) | b,
            Opcode::Setr => self.reg(a),
            Opcode::Seti => a,
            Opcode::Gtir => (a > self.reg(b)) as u64,
            Opcode::Gtri => (self.reg(a) > b) as u64,
            Opcode::Gtrr => (self.reg(a) > self.reg(b)) as u64,
            Opcode::Eqir => (a == self.reg(b)) as u64,
            Opcode::Eqri => (self.reg(a) == b) as u64,
            Opcode::Eqrr => (self.reg(a) == self.reg(b)) as u64,
        }
    }
}