//! Virtual machine and errors
use std::{
    cmp::Ordering,
    fmt::{self, Display, Formatter},
};

use crate::{
    compile, value::ConcreteValue, AnnotatedError, Chunk, ChunkError, DustError, FunctionType,
    Instruction, Local, NativeFunction, NativeFunctionError, Operation, Span, Type, Value,
    ValueError,
};

pub fn run(source: &str) -> Result<Option<Value>, DustError> {
    let chunk = compile(source)?;
    let mut vm = Vm::new(chunk, None);

    vm.run()
        .map(|option| option.cloned())
        .map_err(|error| DustError::Runtime { error, source })
}

pub fn run_and_display_output(source: &str) {
    match run(source) {
        Ok(Some(value)) => println!("{}", value),
        Ok(None) => {}
        Err(error) => eprintln!("{}", error.report()),
    }
}

/// Dust virtual machine.
///
/// See the [module-level documentation](index.html) for more information.
#[derive(Debug, Eq, PartialEq)]
pub struct Vm<'parent> {
    ip: usize,
    chunk: Chunk,
    stack: Vec<Register>,
    last_assigned_register: Option<u8>,
    parent: Option<&'parent Vm<'parent>>,
}

impl<'parent> Vm<'parent> {
    const STACK_LIMIT: usize = u16::MAX as usize;

    pub fn new(chunk: Chunk, parent: Option<&'parent Vm<'parent>>) -> Self {
        Self {
            ip: 0,
            chunk,
            stack: Vec::new(),
            last_assigned_register: None,
            parent,
        }
    }

    pub fn run(&mut self) -> Result<Option<&Value>, VmError> {
        // DRY helper to get constant or register values for binary operations
        fn get_arguments<'a>(
            vm: &'a mut Vm,
            instruction: Instruction,
            position: Span,
        ) -> Result<(&'a Value, &'a Value), VmError> {
            let left = if instruction.b_is_constant() {
                vm.get_constant(instruction.b(), position)?
            } else {
                vm.get_register(instruction.b(), position)?
            };
            let right = if instruction.c_is_constant() {
                vm.get_constant(instruction.c(), position)?
            } else {
                vm.get_register(instruction.c(), position)?
            };

            Ok((left, right))
        }

        while let Ok((instruction, position)) = self.read(Span(0, 0)).copied() {
            log::info!(
                "{} | {} | {} | {}",
                self.ip - 1,
                position,
                instruction.operation(),
                instruction.disassembly_info(&self.chunk)
            );

            match instruction.operation() {
                Operation::Move => {
                    let to_register = instruction.a();
                    let from_register = instruction.b();
                    let from_register_has_value = self
                        .stack
                        .get(from_register as usize)
                        .is_some_and(|register| !matches!(register, Register::Empty));

                    if from_register_has_value {
                        self.set_register(
                            to_register,
                            Register::StackPointer(from_register),
                            position,
                        )?;
                    }
                }
                Operation::Close => {
                    let from_register = instruction.b();
                    let to_register = instruction.c();

                    if self.stack.len() < to_register as usize {
                        return Err(VmError::StackUnderflow { position });
                    }

                    for register_index in from_register..to_register {
                        self.stack[register_index as usize] = Register::Empty;
                    }
                }
                Operation::LoadBoolean => {
                    let to_register = instruction.a();
                    let boolean = instruction.b_as_boolean();
                    let jump = instruction.c_as_boolean();
                    let value = Value::boolean(boolean);

                    self.set_register(to_register, Register::Value(value), position)?;

                    if jump {
                        self.ip += 1;
                    }
                }
                Operation::LoadConstant => {
                    let to_register = instruction.a();
                    let from_constant = instruction.b();
                    let jump = instruction.c_as_boolean();

                    self.set_register(
                        to_register,
                        Register::ConstantPointer(from_constant),
                        position,
                    )?;

                    if jump {
                        self.ip += 1
                    }
                }
                Operation::LoadList => {
                    let to_register = instruction.a();
                    let start_register = instruction.b();
                    let item_type = (start_register..to_register)
                        .find_map(|register_index| {
                            if let Ok(value) = self.get_register(register_index, position) {
                                Some(value.r#type())
                            } else {
                                None
                            }
                        })
                        .unwrap_or(Type::Any);
                    let value = Value::abstract_list(start_register, to_register, item_type);

                    self.set_register(to_register, Register::Value(value), position)?;
                }
                Operation::LoadSelf => {
                    let to_register = instruction.a();
                    let value = Value::function(
                        self.chunk.clone(),
                        FunctionType {
                            type_parameters: None,
                            value_parameters: None,
                            return_type: None,
                        },
                    );

                    self.set_register(to_register, Register::Value(value), position)?;
                }
                Operation::DefineLocal => {
                    let from_register = instruction.a();
                    let to_local = instruction.b();

                    self.define_local(to_local, from_register, position)?;
                }
                Operation::GetLocal => {
                    let to_register = instruction.a();
                    let local_index = instruction.b();
                    let local = self.get_local(local_index, position)?;

                    self.set_register(
                        to_register,
                        Register::StackPointer(local.register_index),
                        position,
                    )?;
                }
                Operation::SetLocal => {
                    let register = instruction.a();
                    let local_index = instruction.b();

                    self.define_local(local_index, register, position)?;
                }
                Operation::Add => {
                    let to_register = instruction.a();
                    let (left, right) = get_arguments(self, instruction, position)?;
                    let sum = left
                        .add(right)
                        .map_err(|error| VmError::Value { error, position })?;

                    self.set_register(to_register, Register::Value(sum), position)?;
                }
                Operation::Subtract => {
                    let to_register = instruction.a();
                    let (left, right) = get_arguments(self, instruction, position)?;
                    let difference = left
                        .subtract(right)
                        .map_err(|error| VmError::Value { error, position })?;

                    self.set_register(to_register, Register::Value(difference), position)?;
                }
                Operation::Multiply => {
                    let to_register = instruction.a();
                    let (left, right) = get_arguments(self, instruction, position)?;
                    let product = left
                        .multiply(right)
                        .map_err(|error| VmError::Value { error, position })?;

                    self.set_register(to_register, Register::Value(product), position)?;
                }
                Operation::Divide => {
                    let to_register = instruction.a();
                    let (left, right) = get_arguments(self, instruction, position)?;
                    let quotient = left
                        .divide(right)
                        .map_err(|error| VmError::Value { error, position })?;

                    self.set_register(to_register, Register::Value(quotient), position)?;
                }
                Operation::Modulo => {
                    let to_register = instruction.a();
                    let (left, right) = get_arguments(self, instruction, position)?;
                    let remainder = left
                        .modulo(right)
                        .map_err(|error| VmError::Value { error, position })?;

                    self.set_register(to_register, Register::Value(remainder), position)?;
                }
                Operation::Test => {
                    let register = instruction.a();
                    let test_value = instruction.c_as_boolean();
                    let value = self.get_register(register, position)?;
                    let boolean = if let Value::Concrete(ConcreteValue::Boolean(boolean)) = value {
                        *boolean
                    } else {
                        return Err(VmError::ExpectedBoolean {
                            found: value.clone(),
                            position,
                        });
                    };

                    if boolean != test_value {
                        self.ip += 1;
                    }
                }
                Operation::TestSet => todo!(),
                Operation::Equal => {
                    debug_assert_eq!(
                        self.get_instruction(self.ip, position)?.0.operation(),
                        Operation::Jump
                    );

                    let (left, right) = get_arguments(self, instruction, position)?;
                    let equal_result = left
                        .equal(right)
                        .map_err(|error| VmError::Value { error, position })?;
                    let boolean =
                        if let Value::Concrete(ConcreteValue::Boolean(boolean)) = equal_result {
                            boolean
                        } else {
                            return Err(VmError::ExpectedBoolean {
                                found: equal_result.clone(),
                                position,
                            });
                        };
                    let compare_to = instruction.a_as_boolean();

                    if boolean == compare_to {
                        self.ip += 1;
                    } else {
                        let (jump, _) = self.get_instruction(self.ip, position)?;
                        let jump_distance = jump.a();
                        let is_positive = jump.b_as_boolean();
                        let new_ip = if is_positive {
                            self.ip + jump_distance as usize
                        } else {
                            self.ip - jump_distance as usize
                        };

                        self.ip = new_ip;
                    }
                }
                Operation::Less => {
                    debug_assert_eq!(
                        self.get_instruction(self.ip, position)?.0.operation(),
                        Operation::Jump
                    );

                    let (left, right) = get_arguments(self, instruction, position)?;
                    let less_result = left
                        .less_than(right)
                        .map_err(|error| VmError::Value { error, position })?;
                    let boolean =
                        if let Value::Concrete(ConcreteValue::Boolean(boolean)) = less_result {
                            boolean
                        } else {
                            return Err(VmError::ExpectedBoolean {
                                found: less_result.clone(),
                                position,
                            });
                        };
                    let compare_to = instruction.a_as_boolean();

                    if boolean == compare_to {
                        self.ip += 1;
                    } else {
                        let jump = self.get_instruction(self.ip, position)?.0;
                        let jump_distance = jump.a();
                        let is_positive = jump.b_as_boolean();
                        let new_ip = if is_positive {
                            self.ip + jump_distance as usize
                        } else {
                            self.ip - jump_distance as usize
                        };

                        self.ip = new_ip;
                    }
                }
                Operation::LessEqual => {
                    debug_assert_eq!(
                        self.get_instruction(self.ip, position)?.0.operation(),
                        Operation::Jump
                    );

                    let (left, right) = get_arguments(self, instruction, position)?;
                    let less_or_equal_result = left
                        .less_than_or_equal(right)
                        .map_err(|error| VmError::Value { error, position })?;
                    let boolean = if let Value::Concrete(ConcreteValue::Boolean(boolean)) =
                        less_or_equal_result
                    {
                        boolean
                    } else {
                        return Err(VmError::ExpectedBoolean {
                            found: less_or_equal_result.clone(),
                            position,
                        });
                    };
                    let compare_to = instruction.a_as_boolean();

                    if boolean == compare_to {
                        self.ip += 1;
                    } else {
                        let jump = self.get_instruction(self.ip, position)?.0;
                        let jump_distance = jump.a();
                        let is_positive = jump.b_as_boolean();
                        let new_ip = if is_positive {
                            self.ip + jump_distance as usize
                        } else {
                            self.ip - jump_distance as usize
                        };

                        self.ip = new_ip;
                    }
                }
                Operation::Negate => {
                    let value = if instruction.b_is_constant() {
                        self.get_constant(instruction.b(), position)?
                    } else {
                        self.get_register(instruction.b(), position)?
                    };
                    let negated = value
                        .negate()
                        .map_err(|error| VmError::Value { error, position })?;

                    self.set_register(instruction.a(), Register::Value(negated), position)?;
                }
                Operation::Not => {
                    let value = if instruction.b_is_constant() {
                        self.get_constant(instruction.b(), position)?
                    } else {
                        self.get_register(instruction.b(), position)?
                    };
                    let not = value
                        .not()
                        .map_err(|error| VmError::Value { error, position })?;

                    self.set_register(instruction.a(), Register::Value(not), position)?;
                }
                Operation::Jump => {
                    let jump_distance = instruction.b();
                    let is_positive = instruction.c_as_boolean();
                    let new_ip = if is_positive {
                        self.ip + jump_distance as usize
                    } else {
                        self.ip - jump_distance as usize - 1
                    };
                    self.ip = new_ip;
                }
                Operation::Call => {
                    let to_register = instruction.a();
                    let function_register = instruction.b();
                    let argument_count = instruction.c();
                    let value = self.get_register(function_register, position)?.clone();
                    let function = if let Value::Concrete(ConcreteValue::Function(function)) = value
                    {
                        function
                    } else {
                        return Err(VmError::ExpectedFunction {
                            found: value,
                            position,
                        });
                    };
                    let mut function_vm = Vm::new(function.take_chunk(), Some(self));
                    let first_argument_index = function_register + 1;

                    for argument_index in
                        first_argument_index..first_argument_index + argument_count
                    {
                        let top_of_stack = function_vm.stack.len() as u8;

                        function_vm.set_register(
                            top_of_stack,
                            Register::ParentStackPointer(argument_index),
                            position,
                        )?
                    }

                    let return_value = function_vm.run()?.cloned();

                    if let Some(value) = return_value {
                        self.set_register(to_register, Register::Value(value), position)?;
                    }
                }
                Operation::CallNative => {
                    let native_function = NativeFunction::from(instruction.b());
                    let return_value = native_function.call(instruction, self, position)?;

                    if let Some(value) = return_value {
                        let to_register = instruction.a();

                        self.set_register(to_register, Register::Value(value), position)?;
                    }
                }
                Operation::Return => {
                    let should_return_value = instruction.b_as_boolean();

                    if !should_return_value {
                        return Ok(None);
                    }

                    let return_value = if let Some(register_index) = self.last_assigned_register {
                        self.get_register(register_index, position)?
                    } else {
                        return Err(VmError::StackUnderflow { position });
                    };

                    return Ok(Some(return_value));
                }
            }
        }

        Ok(None)
    }

    fn set_register(
        &mut self,
        to_register: u8,
        register: Register,
        position: Span,
    ) -> Result<(), VmError> {
        self.last_assigned_register = Some(to_register);

        let length = self.stack.len();
        let to_register = to_register as usize;

        if length == Self::STACK_LIMIT {
            return Err(VmError::StackOverflow { position });
        }

        match to_register.cmp(&length) {
            Ordering::Less => {
                log::trace!("Change R{to_register} to {register}");

                self.stack[to_register] = register;

                Ok(())
            }
            Ordering::Equal => {
                log::trace!("Set R{to_register} to {register}");

                self.stack.push(register);

                Ok(())
            }
            Ordering::Greater => {
                let difference = to_register - length;

                for index in 0..difference {
                    log::trace!("Set R{index} to empty");

                    self.stack.push(Register::Empty);
                }

                log::trace!("Set R{to_register} to {register}");

                self.stack.push(register);

                Ok(())
            }
        }
    }

    fn get_constant(&self, index: u8, position: Span) -> Result<&Value, VmError> {
        self.chunk
            .get_constant(index)
            .map_err(|error| VmError::Chunk { error, position })
    }

    pub fn get_register(&self, register_index: u8, position: Span) -> Result<&Value, VmError> {
        let register_index = register_index as usize;
        let register =
            self.stack
                .get(register_index)
                .ok_or_else(|| VmError::RegisterIndexOutOfBounds {
                    index: register_index,
                    position,
                })?;

        match register {
            Register::Value(value) => Ok(value),
            Register::StackPointer(register_index) => self.get_register(*register_index, position),
            Register::ConstantPointer(constant_index) => {
                self.get_constant(*constant_index, position)
            }
            Register::ParentStackPointer(register_index) => {
                let parent = self
                    .parent
                    .as_ref()
                    .ok_or(VmError::ExpectedParent { position })?;

                parent.get_register(*register_index, position)
            }
            Register::ParentConstantPointer(constant_index) => {
                let parent = self
                    .parent
                    .as_ref()
                    .ok_or(VmError::ExpectedParent { position })?;

                parent.get_constant(*constant_index, position)
            }
            Register::Empty => Err(VmError::EmptyRegister {
                index: register_index,
                position,
            }),
        }
    }

    fn read(&mut self, position: Span) -> Result<&(Instruction, Span), VmError> {
        self.chunk
            .expect_not_poisoned()
            .map_err(|error| VmError::Chunk { error, position })?;

        let max_ip = self.chunk.len() - 1;

        if self.ip > max_ip {
            return self.get_instruction(max_ip, position);
        } else {
            self.ip += 1;
        }

        self.get_instruction(self.ip - 1, position)
    }

    fn define_local(
        &mut self,
        local_index: u8,
        register_index: u8,
        position: Span,
    ) -> Result<(), VmError> {
        let local = self
            .chunk
            .get_local_mut(local_index)
            .map_err(|error| VmError::Chunk { error, position })?;

        log::debug!("Define local L{}", local_index);

        local.register_index = register_index;

        Ok(())
    }

    fn get_local(&self, local_index: u8, position: Span) -> Result<&Local, VmError> {
        self.chunk
            .get_local(local_index)
            .map_err(|error| VmError::Chunk { error, position })
    }

    fn get_instruction(
        &self,
        index: usize,
        position: Span,
    ) -> Result<&(Instruction, Span), VmError> {
        self.chunk
            .get_instruction(index)
            .map_err(|error| VmError::Chunk { error, position })
    }
}

#[derive(Debug, Eq, PartialEq)]
enum Register {
    Empty,
    Value(Value),
    StackPointer(u8),
    ConstantPointer(u8),
    ParentStackPointer(u8),
    ParentConstantPointer(u8),
}

impl Display for Register {
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        match self {
            Self::Empty => write!(f, "empty"),
            Self::Value(value) => write!(f, "{}", value),
            Self::StackPointer(index) => write!(f, "R{}", index),
            Self::ConstantPointer(index) => write!(f, "C{}", index),
            Self::ParentStackPointer(index) => write!(f, "PR{}", index),
            Self::ParentConstantPointer(index) => write!(f, "PC{}", index),
        }
    }
}

#[derive(Clone, Debug, PartialEq)]
pub enum VmError {
    // Stack errors
    StackOverflow { position: Span },
    StackUnderflow { position: Span },

    // Register errors
    EmptyRegister { index: usize, position: Span },
    RegisterIndexOutOfBounds { index: usize, position: Span },

    // Execution errors
    ExpectedBoolean { found: Value, position: Span },
    ExpectedFunction { found: Value, position: Span },
    ExpectedParent { position: Span },

    // Wrappers for foreign errors
    Chunk { error: ChunkError, position: Span },
    NativeFunction(NativeFunctionError),
    Value { error: ValueError, position: Span },
}

impl AnnotatedError for VmError {
    fn title() -> &'static str {
        "Runtime Error"
    }

    fn description(&self) -> &'static str {
        match self {
            Self::Chunk { .. } => "Chunk error",
            Self::EmptyRegister { .. } => "Empty register",
            Self::ExpectedBoolean { .. } => "Expected boolean",
            Self::ExpectedFunction { .. } => "Expected function",
            Self::ExpectedParent { .. } => "Expected parent",
            Self::NativeFunction(error) => error.description(),
            Self::RegisterIndexOutOfBounds { .. } => "Register index out of bounds",
            Self::StackOverflow { .. } => "Stack overflow",
            Self::StackUnderflow { .. } => "Stack underflow",
            Self::Value { .. } => "Value error",
        }
    }

    fn details(&self) -> Option<String> {
        match self {
            Self::Chunk { error, .. } => Some(error.to_string()),
            Self::EmptyRegister { index, .. } => Some(format!("Register R{index} is empty")),
            Self::ExpectedFunction { found, .. } => Some(format!("{found} is not a function")),
            Self::RegisterIndexOutOfBounds { index, .. } => {
                Some(format!("Register {index} does not exist"))
            }
            Self::NativeFunction(error) => error.details(),
            Self::Value { error, .. } => Some(error.to_string()),
            _ => None,
        }
    }

    fn position(&self) -> Span {
        match self {
            Self::Chunk { position, .. } => *position,
            Self::EmptyRegister { position, .. } => *position,
            Self::ExpectedBoolean { position, .. } => *position,
            Self::ExpectedFunction { position, .. } => *position,
            Self::ExpectedParent { position } => *position,
            Self::NativeFunction(error) => error.position(),
            Self::RegisterIndexOutOfBounds { position, .. } => *position,
            Self::StackOverflow { position } => *position,
            Self::StackUnderflow { position } => *position,
            Self::Value { position, .. } => *position,
        }
    }
}