mest-core 0.1.0

use bumpalo::Bump;
use chumsky::span::SimpleSpan;
use lasso::{Rodeo, Spur};
use std::{cell::RefCell, ops::Deref, rc::Rc};

use crate::thunk::Thunk;

#[derive(Debug, Clone)]
pub enum Literal {
    Int(i64),
    Float(f64),
    Bool(bool),
}

#[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
pub struct Ident(pub Spur);

#[derive(Debug, Clone)]
pub enum BinOp {
    Eq,
    NotEq,
    Lt,
    Gt,
    Le,
    Ge,
    And,
    Or,
    Add,
    Sub,
    Mul,
    Div,
    Pow,
}

#[derive(Debug, Clone)]
pub enum UnaryOp {
    Neg,
    Not,
}

#[derive(Debug, Clone)]
pub enum Pat<'bump> {
    Wildcard,
    Var(Ident),
    Lit(Literal),
    Or(&'bump Pat<'bump>, &'bump Pat<'bump>),
}

#[derive(Debug, Clone, Copy)]
pub struct Expr<'bump> {
    pub kind: &'bump ExprKind<'bump>,
    pub span: SimpleSpan,
}

impl<'bump> Deref for Expr<'bump> {
    type Target = ExprKind<'bump>;

    fn deref(&self) -> &Self::Target {
        &self.kind
    }
}

#[derive(Debug, Clone)]
pub enum ExprKind<'bump> {
    Literal(Literal),
    Ident(Ident),
    If {
        cond: Expr<'bump>,
        then_expr: Expr<'bump>,
        else_expr: Expr<'bump>,
    },
    BinOp {
        op: BinOp,
        lhs: Expr<'bump>,
        rhs: Expr<'bump>,
    },
    UnaryOp {
        op: UnaryOp,
        rhs: Expr<'bump>,
    },
    Let {
        name: Ident,
        value: Expr<'bump>,
        body: Expr<'bump>,
        rec: bool,
    },
    Match {
        scrutinee: Expr<'bump>,
        arms: &'bump [(Pat<'bump>, Expr<'bump>)],
    },
    Abs {
        param: Ident,
        body: Expr<'bump>,
    },
    App {
        func: Expr<'bump>,
        arg: Expr<'bump>,
    },
}

pub type Env<'bump> = im::HashMap<Ident, Thunk<'bump>>;

#[derive(Debug, Clone)]
pub enum Value<'bump> {
    Int(i64),
    Float(f64),
    Bool(bool),
    Closure {
        param: Ident,
        body: Expr<'bump>,
        env: Env<'bump>,
    },
}

impl std::fmt::Display for Value<'_> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Value::Int(n) => write!(f, "{n}"),
            Value::Float(n) => write!(f, "{n}"),
            Value::Bool(b) => write!(f, "{b}"),
            Value::Closure { .. } => write!(f, "<closure>"),
        }
    }
}

#[derive(Debug, Clone)]
pub enum EvalError {
    UnboundVariable(String),
    TypeMismatch {
        expected: &'static str,
        got: &'static str,
    },
    DivisionByZero,
    NotAFunction,
    NonExhaustiveMatch,
}

impl std::fmt::Display for EvalError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            EvalError::NonExhaustiveMatch => write!(f, "non-exhaustive match"),
            EvalError::UnboundVariable(name) => write!(f, "unbound variable `{name}`"),
            EvalError::TypeMismatch { expected, got } => {
                write!(f, "type mismatch: expected {expected}, got {got}")
            }
            EvalError::DivisionByZero => write!(f, "division by zero"),
            EvalError::NotAFunction => write!(f, "applied a non-function value"),
        }
    }
}

fn type_name(v: &Value) -> &'static str {
    match v {
        Value::Int(_) => "int",
        Value::Float(_) => "float",
        Value::Bool(_) => "bool",
        Value::Closure { .. } => "closure",
    }
}

impl<'bump> ExprKind<'bump> {
    fn node(expr: &'bump ExprKind<'bump>, span: SimpleSpan) -> Expr<'bump> {
        Expr { kind: expr, span }
    }

    pub fn literal(bump: &'bump Bump, span: SimpleSpan, lit: Literal) -> Expr<'bump> {
        Self::node(bump.alloc(ExprKind::Literal(lit)), span)
    }

    pub fn ident(bump: &'bump Bump, span: SimpleSpan, name: Ident) -> Expr<'bump> {
        Self::node(bump.alloc(ExprKind::Ident(name)), span)
    }

    pub fn if_expr(
        bump: &'bump Bump,
        span: SimpleSpan,
        cond: Expr<'bump>,
        then_expr: Expr<'bump>,
        else_expr: Expr<'bump>,
    ) -> Expr<'bump> {
        Self::node(
            bump.alloc(ExprKind::If {
                cond,
                then_expr,
                else_expr,
            }),
            span,
        )
    }

    pub fn binop(
        bump: &'bump Bump,
        span: SimpleSpan,
        op: BinOp,
        lhs: Expr<'bump>,
        rhs: Expr<'bump>,
    ) -> Expr<'bump> {
        Self::node(bump.alloc(ExprKind::BinOp { op, lhs, rhs }), span)
    }

    pub fn unaryop(
        bump: &'bump Bump,
        span: SimpleSpan,
        op: UnaryOp,
        rhs: Expr<'bump>,
    ) -> Expr<'bump> {
        Self::node(bump.alloc(ExprKind::UnaryOp { op, rhs }), span)
    }

    pub fn let_expr(
        bump: &'bump Bump,
        span: SimpleSpan,
        name: Ident,
        value: Expr<'bump>,
        body: Expr<'bump>,
        rec: bool,
    ) -> Expr<'bump> {
        Self::node(
            bump.alloc(ExprKind::Let {
                name,
                value,
                body,
                rec,
            }),
            span,
        )
    }

    pub fn match_expr(
        bump: &'bump Bump,
        span: SimpleSpan,
        scrutinee: Expr<'bump>,
        arms: &'bump [(Pat<'bump>, Expr<'bump>)],
    ) -> Expr<'bump> {
        Self::node(bump.alloc(ExprKind::Match { scrutinee, arms }), span)
    }

    pub fn lambda(
        bump: &'bump Bump,
        span: SimpleSpan,
        param: Ident,
        body: Expr<'bump>,
    ) -> Expr<'bump> {
        Self::node(bump.alloc(ExprKind::Abs { param, body }), span)
    }

    pub fn app(
        bump: &'bump Bump,
        span: SimpleSpan,
        func: Expr<'bump>,
        arg: Expr<'bump>,
    ) -> Expr<'bump> {
        Self::node(bump.alloc(ExprKind::App { func, arg }), span)
    }
}

impl<'bump> ExprKind<'bump> {
    fn force(thunk: &Thunk<'bump>, rodeo: &Rodeo) -> Result<Value<'bump>, EvalError> {
        thunk.force(rodeo)
    }

    pub fn thunk(expr: &'bump ExprKind<'bump>, env: &Env<'bump>) -> Thunk<'bump> {
        Thunk::new(expr, env.clone())
    }

    pub fn eval_lazy(
        &'bump self,
        env: &Env<'bump>,
        rodeo: &Rodeo,
    ) -> Result<Value<'bump>, EvalError> {
        match self {
            ExprKind::Literal(Literal::Bool(b)) => Ok(Value::Bool(*b)),
            ExprKind::Literal(Literal::Int(v)) => Ok(Value::Int(*v)),
            ExprKind::Literal(Literal::Float(v)) => Ok(Value::Float(*v)),

            ExprKind::Ident(ident) => {
                let thunk = env.get(ident).ok_or_else(|| {
                    EvalError::UnboundVariable(rodeo.resolve(&ident.0).to_owned())
                })?;
                Self::force(thunk, rodeo)
            }

            ExprKind::UnaryOp { op, rhs } => {
                let rhs = rhs.kind.eval_lazy(env, rodeo)?;
                match (op, rhs) {
                    (UnaryOp::Neg, Value::Int(n)) => Ok(Value::Int(-n)),
                    (UnaryOp::Neg, Value::Float(f)) => Ok(Value::Float(-f)),
                    (UnaryOp::Not, Value::Bool(b)) => Ok(Value::Bool(!b)),
                    (UnaryOp::Neg, v) => Err(EvalError::TypeMismatch {
                        expected: "number",
                        got: type_name(&v),
                    }),
                    (UnaryOp::Not, v) => Err(EvalError::TypeMismatch {
                        expected: "bool",
                        got: type_name(&v),
                    }),
                }
            }

            ExprKind::BinOp { op, lhs, rhs } => {
                let lhs = lhs.kind.eval_lazy(env, rodeo)?;
                let rhs = rhs.kind.eval_lazy(env, rodeo)?;
                Self::eval_binop(op, lhs, rhs)
            }

            ExprKind::Let {
                name,
                value,
                body,
                rec: true,
            } => {
                let rec_env = Rc::new(RefCell::new(env.clone()));
                let thunk = Thunk::new_shared(value.kind, Rc::clone(&rec_env));
                rec_env.borrow_mut().insert(*name, thunk.clone());
                let mut body_env = env.clone();
                body_env.insert(*name, thunk);
                body.kind.eval_lazy(&body_env, rodeo)
            }

            ExprKind::Let {
                name,
                value,
                body,
                rec: false,
            } => {
                let mut env = env.clone();
                env.insert(*name, Self::thunk(value.kind, &env));
                body.kind.eval_lazy(&env, rodeo)
            }

            ExprKind::Match { scrutinee, arms } => {
                let scrutinee_thunk = Thunk::new(scrutinee.kind, env.clone());
                for (pat, body) in arms.iter() {
                    let mut arm_env = env.clone();
                    if Self::match_pat(pat, &scrutinee_thunk, &mut arm_env, rodeo)? {
                        return body.kind.eval_lazy(&arm_env, rodeo);
                    }
                }
                Err(EvalError::NonExhaustiveMatch)
            }

            ExprKind::If {
                cond,
                then_expr,
                else_expr,
            } => match cond.kind.eval_lazy(env, rodeo)? {
                Value::Bool(true) => then_expr.kind.eval_lazy(env, rodeo),
                Value::Bool(false) => else_expr.kind.eval_lazy(env, rodeo),
                v => Err(EvalError::TypeMismatch {
                    expected: "bool",
                    got: type_name(&v),
                }),
            },

            ExprKind::Abs { param, body } => Ok(Value::Closure {
                param: *param,
                body: *body,
                env: env.clone(),
            }),

            ExprKind::App { func, arg } => {
                let func = func.kind.eval_lazy(env, rodeo)?;
                match func {
                    Value::Closure {
                        param,
                        body,
                        env: mut closure_env,
                    } => {
                        closure_env.insert(param, Self::thunk(arg.kind, env));
                        body.kind.eval_lazy(&closure_env, rodeo)
                    }
                    _ => Err(EvalError::NotAFunction),
                }
            }
        }
    }

    fn match_pat(
        pat: &Pat<'bump>,
        thunk: &Thunk<'bump>,
        env: &mut Env<'bump>,
        rodeo: &Rodeo,
    ) -> Result<bool, EvalError> {
        match pat {
            Pat::Wildcard => Ok(true),
            Pat::Var(name) => {
                env.insert(*name, thunk.clone());
                Ok(true)
            }
            Pat::Lit(lit) => {
                let val = thunk.force(rodeo)?;
                Ok(match (lit, &val) {
                    (Literal::Int(a), Value::Int(b)) => a == b,
                    (Literal::Float(a), Value::Float(b)) => a == b,
                    (Literal::Bool(a), Value::Bool(b)) => a == b,
                    _ => false,
                })
            }
            Pat::Or(a, b) => {
                let mut env_a = env.clone();
                if Self::match_pat(a, thunk, &mut env_a, rodeo)? {
                    *env = env_a;
                    Ok(true)
                } else {
                    Self::match_pat(b, thunk, env, rodeo)
                }
            }
        }
    }

    fn eval_binop(
        op: &BinOp,
        lhs: Value<'bump>,
        rhs: Value<'bump>,
    ) -> Result<Value<'bump>, EvalError> {
        match (op, &lhs, &rhs) {
            (BinOp::And, Value::Bool(l), Value::Bool(r)) => return Ok(Value::Bool(*l && *r)),
            (BinOp::Or, Value::Bool(l), Value::Bool(r)) => return Ok(Value::Bool(*l || *r)),
            (BinOp::And, _, _) | (BinOp::Or, _, _) => {
                return Err(EvalError::TypeMismatch {
                    expected: "bool",
                    got: type_name(&lhs),
                });
            }
            _ => {}
        }

        match op {
            BinOp::Eq => return Ok(Value::Bool(Self::values_equal(&lhs, &rhs)?)),
            BinOp::NotEq => return Ok(Value::Bool(!Self::values_equal(&lhs, &rhs)?)),
            _ => {}
        }

        match (lhs, rhs) {
            (Value::Int(l), Value::Int(r)) => Self::eval_int_binop(op, l, r),
            (Value::Float(l), Value::Float(r)) => Self::eval_float_binop(op, l, r),
            (Value::Int(l), Value::Float(r)) => Self::eval_float_binop(op, l as f64, r),
            (Value::Float(l), Value::Int(r)) => Self::eval_float_binop(op, l, r as f64),
            (lhs, _) => Err(EvalError::TypeMismatch {
                expected: "number",
                got: type_name(&lhs),
            }),
        }
    }

    fn values_equal(lhs: &Value, rhs: &Value) -> Result<bool, EvalError> {
        match (lhs, rhs) {
            (Value::Int(l), Value::Int(r)) => Ok(l == r),
            (Value::Float(l), Value::Float(r)) => Ok(l == r),
            (Value::Bool(l), Value::Bool(r)) => Ok(l == r),
            (Value::Int(l), Value::Float(r)) => Ok((*l as f64) == *r),
            (Value::Float(l), Value::Int(r)) => Ok(*l == (*r as f64)),
            (l, r) => Err(EvalError::TypeMismatch {
                expected: type_name(l),
                got: type_name(r),
            }),
        }
    }

    fn eval_int_binop(op: &BinOp, lhs: i64, rhs: i64) -> Result<Value<'bump>, EvalError> {
        match op {
            BinOp::Lt => return Ok(Value::Bool(lhs < rhs)),
            BinOp::Gt => return Ok(Value::Bool(lhs > rhs)),
            BinOp::Le => return Ok(Value::Bool(lhs <= rhs)),
            BinOp::Ge => return Ok(Value::Bool(lhs >= rhs)),
            _ => {}
        }

        Ok(Value::Int(match op {
            BinOp::Add => lhs + rhs,
            BinOp::Sub => lhs - rhs,
            BinOp::Mul => lhs * rhs,
            BinOp::Div => {
                if rhs == 0 {
                    return Err(EvalError::DivisionByZero);
                }
                lhs / rhs
            }
            BinOp::Pow => lhs.pow(rhs as u32),
            _ => unreachable!(),
        }))
    }

    fn eval_float_binop(op: &BinOp, lhs: f64, rhs: f64) -> Result<Value<'bump>, EvalError> {
        match op {
            BinOp::Lt => return Ok(Value::Bool(lhs < rhs)),
            BinOp::Gt => return Ok(Value::Bool(lhs > rhs)),
            BinOp::Le => return Ok(Value::Bool(lhs <= rhs)),
            BinOp::Ge => return Ok(Value::Bool(lhs >= rhs)),
            _ => {}
        }

        Ok(Value::Float(match op {
            BinOp::Add => lhs + rhs,
            BinOp::Sub => lhs - rhs,
            BinOp::Mul => lhs * rhs,
            BinOp::Div => {
                if rhs == 0.0 {
                    return Err(EvalError::DivisionByZero);
                }
                lhs / rhs
            }
            BinOp::Pow => lhs.powf(rhs),
            _ => unreachable!(),
        }))
    }
}