//! tree walking interpreter for tbsp

use crate::{ast, Wrap};
use std::{collections::HashMap, fmt, io};

mod analysis;
mod builtins;
#[cfg(test)]
mod test;

#[derive(Debug, PartialEq, Eq, Clone)]
pub struct Variable {
    pub ty: ast::Type,
    pub name: ast::Identifier,
    pub value: Value,
}

impl Variable {
    fn value(&self) -> &Value {
        &self.value
    }

    pub(crate) fn ty(&self) -> ast::Type {
        self.ty
    }

    fn assign(&mut self, value: Value) -> Result {
        if self.ty() == value.ty() {
            self.value = value;
            Ok(self.value.clone())
        } else {
            Err(value.ty().expected([self.ty()]))
        }
    }

    pub(crate) fn mutate(&mut self, f: impl FnOnce(&mut Self) -> Result) -> Result {
        f(self)
    }
}

#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Clone)]
pub enum Value {
    Unit,
    Integer(i128),
    String(String),
    Boolean(bool),
    Node(NodeId),
    List(Vec<Value>),
}

type NodeId = usize;

impl Value {
    pub(crate) fn ty(&self) -> ast::Type {
        match self {
            Self::Unit => ast::Type::Unit,
            Self::Integer(_) => ast::Type::Integer,
            Self::String(_) => ast::Type::String,
            Self::Boolean(_) => ast::Type::Boolean,
            Self::Node(_) => ast::Type::Node,
            Self::List(_) => ast::Type::List,
        }
    }

    fn default(ty: ast::Type) -> Self {
        match ty {
            ast::Type::Unit => Self::Unit,
            ast::Type::Integer => Self::default_int(),
            ast::Type::String => Self::default_string(),
            ast::Type::Boolean => Self::default_bool(),
            ast::Type::Node => unreachable!(),
            ast::Type::List => Self::default_list(),
        }
    }

    fn default_int() -> Self {
        Self::Integer(0)
    }

    fn default_bool() -> Self {
        Self::Boolean(false)
    }

    fn default_string() -> Self {
        Self::String(String::default())
    }

    fn default_list() -> Self {
        Self::List(Vec::new())
    }

    fn as_boolean(&self) -> std::result::Result<bool, Error> {
        match self {
            Self::Boolean(b) => Ok(*b),
            v => Err(v.ty().expected([ast::Type::Boolean])),
        }
    }

    pub(crate) fn as_str(&self) -> std::result::Result<&str, Error> {
        match self {
            Self::String(s) => Ok(s.as_str()),
            v => Err(v.ty().expected([ast::Type::String])),
        }
    }

    pub(crate) fn as_int(&self) -> std::result::Result<i128, Error> {
        match self {
            Self::Integer(i) => Ok(*i),
            v => Err(v.ty().expected([ast::Type::Integer])),
        }
    }

    pub(crate) fn as_node(&self) -> std::result::Result<NodeId, Error> {
        match self {
            Self::Node(id) => Ok(*id),
            v => Err(v.ty().expected([ast::Type::Node])),
        }
    }

    pub(crate) fn as_list(&self) -> std::result::Result<Vec<Value>, Error> {
        match self {
            Self::List(values) => Ok(values.clone()),
            v => Err(v.ty().expected([ast::Type::List])),
        }
    }

    fn add(&self, other: &Self) -> Result {
        match (self, other) {
            (Self::Integer(s), Self::Integer(o)) => Ok(Self::Integer(*s + *o)),
            (Self::String(s), Self::String(o)) => Ok(Self::String(format!("{s}{o}"))),
            (Self::List(l), o) => Ok(Self::List(l.iter().cloned().chain([o.clone()]).collect())),
            _ => Err(Error::UndefinedBinOp(
                ast::BinOp::Arith(ast::ArithOp::Add),
                self.ty(),
                other.ty(),
            )),
        }
    }

    fn sub(&self, other: &Self) -> Result {
        match (self, other) {
            (Self::Integer(s), Self::Integer(o)) => Ok(Self::Integer(*s - *o)),
            (Self::String(s), Self::String(o)) => {
                Ok(Self::String(s.strip_suffix(o).unwrap_or(s).to_owned()))
            }
            _ => Err(Error::UndefinedBinOp(
                ast::BinOp::Arith(ast::ArithOp::Sub),
                self.ty(),
                other.ty(),
            )),
        }
    }

    fn mul(&self, other: &Self) -> Result {
        match (self, other) {
            (Self::Integer(s), Self::Integer(o)) => Ok(Self::Integer(*s * *o)),
            (Self::Integer(s), Self::String(o)) => Ok(Self::String(o.repeat(*s as usize))),
            (Self::String(_), Self::Integer(_)) => other.mul(self),
            _ => Err(Error::UndefinedBinOp(
                ast::BinOp::Arith(ast::ArithOp::Mul),
                self.ty(),
                other.ty(),
            )),
        }
    }

    fn div(&self, other: &Self) -> Result {
        match (self, other) {
            (Self::Integer(s), Self::Integer(o)) => Ok(Self::Integer(*s / *o)),
            _ => Err(Error::UndefinedBinOp(
                ast::BinOp::Arith(ast::ArithOp::Div),
                self.ty(),
                other.ty(),
            )),
        }
    }

    fn mod_(&self, other: &Self) -> Result {
        match (self, other) {
            (Self::Integer(s), Self::Integer(o)) => Ok(Self::Integer(*s % *o)),
            _ => Err(Error::UndefinedBinOp(
                ast::BinOp::Arith(ast::ArithOp::Mod),
                self.ty(),
                other.ty(),
            )),
        }
    }

    fn equals(&self, other: &Self) -> Result {
        match (self, other) {
            (Self::Integer(s), Self::Integer(o)) => Ok(Self::Boolean(s == o)),
            (Self::String(s), Self::String(o)) => Ok(Self::Boolean(s == o)),
            (Self::Boolean(s), Self::Boolean(o)) => Ok(Self::Boolean(s == o)),
            _ => Err(Error::UndefinedBinOp(
                ast::BinOp::Cmp(ast::CmpOp::Eq),
                self.ty(),
                other.ty(),
            )),
        }
    }

    fn greater_than(&self, other: &Self) -> Result {
        match (self, other) {
            (Self::Integer(s), Self::Integer(o)) => Ok(Self::Boolean(s > o)),
            (Self::String(s), Self::String(o)) => Ok(Self::Boolean(s.cmp(o).is_gt())),
            _ => Err(Error::UndefinedBinOp(
                ast::BinOp::Cmp(ast::CmpOp::Gt),
                self.ty(),
                other.ty(),
            )),
        }
    }

    fn less_than(&self, other: &Self) -> Result {
        match (self, other) {
            (Self::Integer(s), Self::Integer(o)) => Ok(Self::Boolean(s < o)),
            (Self::String(s), Self::String(o)) => Ok(Self::Boolean(s.cmp(o).is_lt())),
            _ => Err(Error::UndefinedBinOp(
                ast::BinOp::Cmp(ast::CmpOp::Lt),
                self.ty(),
                other.ty(),
            )),
        }
    }

    fn greater_than_equals(&self, other: &Self) -> Result {
        match (self, other) {
            (Self::Integer(s), Self::Integer(o)) => Ok(Self::Boolean(s >= o)),
            (Self::String(s), Self::String(o)) => Ok(Self::Boolean(s.cmp(o).is_ge())),
            (Self::Boolean(s), Self::Boolean(o)) => Ok(Self::Boolean(s == o)),
            _ => Err(Error::UndefinedBinOp(
                ast::BinOp::Cmp(ast::CmpOp::Gte),
                self.ty(),
                other.ty(),
            )),
        }
    }

    fn less_than_equals(&self, other: &Self) -> Result {
        match (self, other) {
            (Self::Integer(s), Self::Integer(o)) => Ok(Self::Boolean(s <= o)),
            (Self::String(s), Self::String(o)) => Ok(Self::Boolean(s.cmp(o).is_le())),
            (Self::Boolean(s), Self::Boolean(o)) => Ok(Self::Boolean(s == o)),
            _ => Err(Error::UndefinedBinOp(
                ast::BinOp::Cmp(ast::CmpOp::Lte),
                self.ty(),
                other.ty(),
            )),
        }
    }

    fn not(&self) -> Result {
        match self {
            Self::Boolean(s) => Ok(Self::Boolean(!s)),
            _ => Err(Error::UndefinedUnaryOp(ast::UnaryOp::Not, self.ty())),
        }
    }

    fn and(&self, other: &Self) -> Result {
        match (self, other) {
            (Self::Boolean(s), Self::Boolean(o)) => Ok(Self::Boolean(*s && *o)),
            _ => Err(Error::UndefinedBinOp(
                ast::BinOp::Logic(ast::LogicOp::And),
                self.ty(),
                other.ty(),
            )),
        }
    }

    fn or(&self, other: &Self) -> Result {
        match (self, other) {
            (Self::Boolean(s), Self::Boolean(o)) => Ok(Self::Boolean(*s || *o)),
            _ => Err(Error::UndefinedBinOp(
                ast::BinOp::Logic(ast::LogicOp::Or),
                self.ty(),
                other.ty(),
            )),
        }
    }
}

impl fmt::Display for Value {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Self::Unit => write!(f, "()"),
            Self::Integer(i) => write!(f, "{i}"),
            Self::String(s) => write!(f, "{s}"),
            Self::Boolean(b) => write!(f, "{b}"),
            Self::Node(id) => write!(f, "<node #{id}>"),
            Self::List(items) => {
                write!(f, "[")?;
                let mut iterable = items.iter().peekable();
                while let Some(item) = iterable.next() {
                    if iterable.peek().is_none() {
                        write!(f, "{item}")?;
                    } else {
                        write!(f, "{item}, ")?;
                    }
                }
                write!(f, "]")
            }
        }
    }
}

impl From<bool> for Value {
    fn from(value: bool) -> Self {
        Self::Boolean(value)
    }
}

impl From<i128> for Value {
    fn from(value: i128) -> Self {
        Self::Integer(value)
    }
}

impl From<usize> for Value {
    fn from(value: usize) -> Self {
        (value as i128).into()
    }
}

impl From<&str> for Value {
    fn from(value: &str) -> Self {
        Self::String(value.to_owned())
    }
}

impl From<String> for Value {
    fn from(value: String) -> Self {
        Self::String(value.to_owned())
    }
}

impl From<Vec<Value>> for Value {
    fn from(value: Vec<Value>) -> Self {
        Self::List(value)
    }
}

#[derive(Debug, PartialEq, Eq)]
pub enum Error {
    FailedLookup(ast::Identifier),
    TypeMismatch {
        expected: Vec<ast::Type>,
        got: ast::Type,
    },
    UndefinedBinOp(ast::BinOp, ast::Type, ast::Type),
    UndefinedUnaryOp(ast::UnaryOp, ast::Type),
    AlreadyBound(ast::Identifier),
    MalformedExpr(String),
    InvalidNodeKind(String),
    NoParentNode(tree_sitter::Node<'static>),
    IncorrectArgFormat {
        wanted: usize,
        got: usize,
    },
    InvalidStringSlice {
        length: usize,
        start: i128,
        end: i128,
    },
    ArrayOutOfBounds {
        idx: i128,
        len: usize,
    },
    // current node is only set in visitors, not in BEGIN or END blocks
    CurrentNodeNotPresent,
    Analysis,
}

impl Error {
    pub fn code(self) -> i32 {
        match self {
            Self::FailedLookup(_) => 30,
            Self::TypeMismatch { .. } => 31,
            Self::UndefinedBinOp(..) => 32,
            Self::UndefinedUnaryOp(..) => 33,
            Self::AlreadyBound(..) => 34,
            Self::MalformedExpr(..) => 35,
            Self::InvalidNodeKind(..) => 36,
            Self::NoParentNode(..) => 37,
            Self::IncorrectArgFormat { .. } => 38,
            Self::InvalidStringSlice { .. } => 39,
            Self::ArrayOutOfBounds { .. } => 40,
            Self::CurrentNodeNotPresent => 41,
            Self::Analysis => 50,
        }
    }
}

impl std::fmt::Display for Error {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Self::FailedLookup(ident) => write!(f, "failed to lookup variable `{ident}`"),
            Self::TypeMismatch { expected, got } => write!(
                f,
                "mismatched types, expected one of `{}`, got `{got}`",
                expected
                    .iter()
                    .map(|t| t.to_string())
                    .collect::<Vec<_>>()
                    .join(", ")
            ),
            Self::UndefinedBinOp(op, ltype, rtype) => write!(
                f,
                "undefined operation `{op}` between `{ltype}` and `{rtype}`"
            ),
            Self::UndefinedUnaryOp(op, ty) => write!(f, "undefined operation `{op}` on `{ty}`"),
            Self::AlreadyBound(ident) => write!(f, "variable `{ident}` is aleady declared"),
            Self::MalformedExpr(s) => write!(f, "{s}"),
            Self::InvalidNodeKind(s) => write!(f, "invalid node kind `{s}`"),
            Self::NoParentNode(n) => write!(
                f,
                "current node (with kind {}) has no parent node ",
                n.kind()
            ),
            Self::IncorrectArgFormat { wanted, got } => write!(
                f,
                "incorrect number of arguments, wanted {wanted} but got {got}"
            ),
            Self::InvalidStringSlice { length, start, end } => write!(
                f,
                "invalid string-slice operation, length was {length}, but slice was {start}:{end}"
            ),
            Self::ArrayOutOfBounds { idx, len } => write!(
                f,
                "array index out of bounds, length was {len} but index was {idx}"
            ),
            Self::CurrentNodeNotPresent => write!(f, "no current node captured in stanza"),
            Self::Analysis => write!(f, "encountered analysis errors"),
        }
    }
}

impl ast::Type {
    pub fn expected<const N: usize>(self, expected: [Self; N]) -> Error {
        Error::TypeMismatch {
            expected: expected.to_vec(),
            got: self,
        }
    }
}

pub type Result = std::result::Result<Value, Error>;

pub struct Context<'s> {
    variables: HashMap<ast::Identifier, Variable>,
    language: tree_sitter::Language,
    program: ast::Program,
    pub(crate) input_src: Option<String>,
    cursor: Option<tree_sitter::TreeCursor<'static>>,
    tree: Option<&'static tree_sitter::Tree>,
    cache: HashMap<NodeId, tree_sitter::Node<'static>>,
    output_stream: Box<dyn io::Write + 's>,
    error_stream: Box<dyn io::Write + 's>,
}

impl<'s> fmt::Debug for Context<'s> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Context")
            .field("variables", &self.variables)
            .field("language", &self.language)
            .field("input_src", &self.input_src)
            .field(
                "cursor",
                if self.cursor.is_some() {
                    &"Some(<cursor>)"
                } else {
                    &"None"
                },
            )
            .finish()
    }
}

impl<'s> Context<'s> {
    pub fn new(language: tree_sitter::Language) -> Self {
        Self {
            program: Default::default(),
            variables: Default::default(),
            language,
            input_src: None,
            cursor: None,
            tree: None,
            cache: HashMap::default(),
            output_stream: Box::new(io::stdout()) as Box<dyn io::Write + 's>,
            error_stream: Box::new(io::stderr()) as Box<dyn io::Write + 's>,
        }
    }

    pub fn cache_node(&mut self, node: tree_sitter::Node<'static>) {
        self.cache.entry(node.id()).or_insert(node);
    }

    pub fn get_node_by_id(&mut self, id: usize) -> Option<tree_sitter::Node<'static>> {
        let root_node = self.tree.as_ref().map(|t| t.root_node())?;
        self.get_node_by_id_helper(root_node, id)
    }

    fn get_node_by_id_helper(
        &mut self,
        start: tree_sitter::Node<'static>,
        id: usize,
    ) -> Option<tree_sitter::Node<'static>> {
        self.cache_node(start);

        if let Some(found) = self.cache.get(&id) {
            return Some(*found);
        }

        if start.id() == id {
            return Some(start);
        } else {
            for child in start.children(&mut start.walk()) {
                if let Some(n) = self.get_node_by_id_helper(child, id) {
                    return Some(n);
                };
            }
        }

        None
    }

    pub fn with_program(mut self, program: ast::Program) -> Self {
        self.program = program;
        self
    }

    pub fn with_input(mut self, src: String) -> Self {
        self.input_src = Some(src);
        self
    }

    pub fn with_tree(mut self, tree: tree_sitter::Tree) -> Self {
        let tree = Box::leak(Box::new(tree));
        self.cursor = Some(tree.walk());
        self.tree = Some(tree);
        self
    }

    pub fn with_output_stream(mut self, stream: Box<dyn io::Write + 's>) -> Self {
        self.output_stream = stream;
        self
    }

    pub fn with_error_stream(mut self, stream: Box<dyn io::Write + 's>) -> Self {
        self.error_stream = stream;
        self
    }

    pub(crate) fn eval_expr(&mut self, expr: &ast::Expr) -> Result {
        match expr {
            ast::Expr::Unit => Ok(Value::Unit),
            ast::Expr::Lit(lit) => self.eval_lit(lit),
            ast::Expr::Ident(ident) => self.lookup(ident).map(Variable::value).cloned(),
            ast::Expr::Bin(lhs, op, rhs) => self.eval_bin(lhs, *op, rhs),
            ast::Expr::Unary(expr, op) => self.eval_unary(expr, *op),
            ast::Expr::Call(call) => self.eval_call(call),
            ast::Expr::List(list) => self.eval_list(list),
            ast::Expr::Index(target, idx) => self.eval_index(target, idx),
            ast::Expr::If(if_expr) => self.eval_if(if_expr),
            ast::Expr::Block(block) => self.eval_block(block),
            ast::Expr::Node => self.eval_node(),
            ast::Expr::FieldAccess(expr, items) => self.eval_field_access(expr, items),
        }
    }

    fn eval_lit(&mut self, lit: &ast::Literal) -> Result {
        match lit {
            ast::Literal::Str(s) => Ok(Value::String(s.to_owned())),
            ast::Literal::Int(i) => Ok(Value::Integer(*i)),
            ast::Literal::Bool(b) => Ok(Value::Boolean(*b)),
        }
    }

    fn eval_node(&mut self) -> Result {
        self.cursor
            .as_ref()
            .ok_or(Error::CurrentNodeNotPresent)?
            .node()
            .id()
            .wrap(Value::Node)
            .wrap_ok()
    }

    pub(crate) fn lookup(
        &mut self,
        ident: &ast::Identifier,
    ) -> std::result::Result<&Variable, Error> {
        self.variables
            .get(ident)
            .ok_or_else(|| Error::FailedLookup(ident.to_owned()))
    }

    pub(crate) fn lookup_mut(
        &mut self,
        ident: &ast::Identifier,
    ) -> std::result::Result<&mut Variable, Error> {
        self.variables
            .get_mut(ident)
            .ok_or_else(|| Error::FailedLookup(ident.to_owned()))
    }

    fn bind(
        &mut self,
        ident: &ast::Identifier,
        ty: ast::Type,
    ) -> std::result::Result<&mut Variable, Error> {
        if self.lookup(ident).is_err() {
            self.variables
                .entry(ident.to_owned())
                .or_insert_with(|| Variable {
                    name: ident.to_owned(),
                    value: Value::default(ty),
                    ty,
                })
                .wrap_ok()
        } else {
            Err(Error::AlreadyBound(ident.to_owned()))
        }
    }

    fn eval_bin(&mut self, lhs: &ast::Expr, op: ast::BinOp, rhs: &ast::Expr) -> Result {
        match op {
            ast::BinOp::Assign(op) => self.eval_assign(lhs, op, rhs),
            ast::BinOp::Arith(op) => self.eval_arith(lhs, op, rhs),
            ast::BinOp::Cmp(op) => self.eval_cmp(lhs, op, rhs),
            ast::BinOp::Logic(op) => self.eval_logic(lhs, op, rhs),
        }
    }

    fn eval_assign(
        &mut self,
        lhs: &ast::Expr,
        ast::AssignOp { op }: ast::AssignOp,
        rhs: &ast::Expr,
    ) -> Result {
        let ast::Expr::Ident(ident) = lhs else {
            return Err(Error::MalformedExpr(format!(
                "malformed assigment, lhs: {:?}",
                lhs
            )));
        };
        let value = self.eval_expr(rhs)?;
        let variable = self.lookup_mut(ident)?;
        match op {
            None => variable.assign(value),
            Some(ast::ArithOp::Add) => variable.assign(variable.value().add(&value)?),
            Some(ast::ArithOp::Sub) => variable.assign(variable.value().sub(&value)?),
            Some(ast::ArithOp::Mul) => variable.assign(variable.value().mul(&value)?),
            Some(ast::ArithOp::Div) => variable.assign(variable.value().div(&value)?),
            Some(ast::ArithOp::Mod) => variable.assign(variable.value().mod_(&value)?),
        }
    }

    fn eval_arith(&mut self, lhs: &ast::Expr, op: ast::ArithOp, rhs: &ast::Expr) -> Result {
        let l = self.eval_expr(lhs)?;
        let r = self.eval_expr(rhs)?;
        match op {
            ast::ArithOp::Add => l.add(&r),
            ast::ArithOp::Sub => l.sub(&r),
            ast::ArithOp::Mul => l.mul(&r),
            ast::ArithOp::Div => l.div(&r),
            ast::ArithOp::Mod => l.mod_(&r),
        }
    }

    fn eval_cmp(&mut self, lhs: &ast::Expr, op: ast::CmpOp, rhs: &ast::Expr) -> Result {
        let l = self.eval_expr(lhs)?;
        let r = self.eval_expr(rhs)?;

        match op {
            ast::CmpOp::Eq => l.equals(&r),
            ast::CmpOp::Gt => l.greater_than(&r),
            ast::CmpOp::Lt => l.less_than(&r),
            ast::CmpOp::Neq => l.equals(&r).and_then(|v| v.not()),
            ast::CmpOp::Gte => l.greater_than_equals(&r),
            ast::CmpOp::Lte => l.less_than_equals(&r),
        }
    }

    fn eval_logic(&mut self, lhs: &ast::Expr, op: ast::LogicOp, rhs: &ast::Expr) -> Result {
        let l = self.eval_expr(lhs)?;

        // short-circuit
        let l_value = l.as_boolean()?;

        match op {
            ast::LogicOp::Or => {
                if l_value {
                    Ok(l)
                } else {
                    let r = self.eval_expr(rhs)?;
                    l.or(&r)
                }
            }
            ast::LogicOp::And => {
                if !l_value {
                    Ok(l)
                } else {
                    let r = self.eval_expr(rhs)?;
                    l.and(&r)
                }
            }
        }
    }

    fn eval_unary(&mut self, expr: &ast::Expr, op: ast::UnaryOp) -> Result {
        let val = self.eval_expr(expr)?;
        match op {
            ast::UnaryOp::Not => val.not(),
        }
    }

    fn eval_if(&mut self, if_expr: &ast::IfExpr) -> Result {
        let cond = self.eval_expr(&if_expr.condition)?;

        if cond.as_boolean()? {
            self.eval_block(&if_expr.then)
        } else {
            self.eval_block(&if_expr.else_)
        }
    }

    fn eval_call(&mut self, call: &ast::Call) -> Result {
        ((*builtins::BUILTINS)
            .get(call.function.as_str())
            .ok_or_else(|| Error::FailedLookup(call.function.to_owned()))?)(
            self,
            call.parameters.as_slice(),
        )
    }

    fn eval_list(&mut self, list: &ast::List) -> Result {
        let mut vals = vec![];
        for i in &list.items {
            vals.push(self.eval_expr(i)?);
        }
        Ok(vals.into())
    }

    fn eval_index(&mut self, target: &ast::Expr, idx: &ast::Expr) -> Result {
        let mut target = self.eval_expr(target)?.as_list()?;
        let idx = self.eval_expr(idx)?.as_int()?;
        if idx < 0 || idx >= target.len() as i128 {
            Err(Error::ArrayOutOfBounds {
                idx,
                len: target.len(),
            })
        } else {
            Ok(target.remove(idx as usize))
        }
    }

    fn eval_declaration(&mut self, decl: &ast::Declaration) -> Result {
        let initial_value = match decl.init.as_ref() {
            Some(init) => Some(self.eval_expr(init)?),
            None => None,
        };
        let variable = self.bind(&decl.name, decl.ty)?;

        if let Some(init) = initial_value {
            variable.assign(init)?;
        }

        Ok(Value::Unit)
    }

    fn eval_statement(&mut self, stmt: &ast::Statement) -> Result {
        match stmt {
            ast::Statement::Bare(expr) => self.eval_expr(expr),
            ast::Statement::Declaration(decl) => self.eval_declaration(decl),
        }
    }

    fn eval_block(&mut self, block: &ast::Block) -> Result {
        for stmt in block.body.iter() {
            self.eval_statement(stmt)?;
        }
        Ok(Value::Unit)
    }

    fn eval_field_access(&mut self, expr: &ast::Expr, field: &ast::Identifier) -> Result {
        let v = self.eval_expr(expr)?;
        let base = v.as_node()?;
        let base_node = self.get_node_by_id(base).unwrap();
        base_node
            .child_by_field_name(field)
            .ok_or(Error::InvalidNodeKind(field.clone()))
            .map(|n| n.id())
            .map(Value::Node)
    }

    fn goto_first_child(&mut self) -> bool {
        self.cursor
            .as_mut()
            .map(|c| c.goto_first_child())
            .unwrap_or_default()
    }

    pub fn analyze(&mut self) -> analysis::Analysis {
        analysis::run(&*self)
    }

    fn eval_program(&mut self) -> Result {
        let program = std::mem::take(&mut self.program);
        let mut has_next = true;
        let mut postorder = Vec::new();

        // BEGIN block
        if let Some(block) = program.begin() {
            self.eval_block(block)?;
        }

        while has_next {
            let current_node = self.cursor.as_ref().unwrap().node();
            postorder.push(current_node);

            if let Some(block) = program.stanza_by_node(current_node, ast::Modifier::Enter) {
                self.eval_block(block)?;
            }

            has_next = self.goto_first_child();

            if !has_next {
                has_next = self.cursor.as_mut().unwrap().goto_next_sibling();
                if let Some(block) = postorder
                    .pop()
                    .and_then(|n| program.stanza_by_node(n, ast::Modifier::Leave))
                {
                    self.eval_block(block)?;
                };
            }

            while !has_next && self.cursor.as_mut().unwrap().goto_parent() {
                has_next = self.cursor.as_mut().unwrap().goto_next_sibling();
                if let Some(block) = postorder
                    .pop()
                    .and_then(|n| program.stanza_by_node(n, ast::Modifier::Leave))
                {
                    self.eval_block(block)?;
                };
            }
        }

        // END block
        if let Some(block) = program.end() {
            self.eval_block(block)?;
        }

        Ok(Value::Unit)
    }

    pub fn eval(&mut self) -> Result {
        let analysis = self.analyze();
        write!(self.error_stream, "{analysis}").unwrap();
        if analysis.contains_error() {
            return Err(Error::Analysis);
        }
        self.eval_program()
    }
}

pub fn evaluate(file: &str, program: &str, language: tree_sitter::Language) -> Result {
    let mut parser = tree_sitter::Parser::new();
    let _ = parser.set_language(&language);

    let tree = parser.parse(file, None).unwrap();

    let program = ast::Program::new().with_file(program).unwrap();
    let mut ctx = Context::new(language)
        .with_input(file.to_owned())
        .with_tree(tree)
        .with_output_stream(Box::new(io::stdout()))
        .with_error_stream(Box::new(io::stderr()))
        .with_program(program);

    ctx.eval()
}