use std::collections::HashMap;
use std::rc::Rc;

use crate::{
    parser::{Child, SyntaxTree, Tree, TreeKind, TreeRef},
    semantics::{string_constant_to_string, Declaration, Environment, Location, Semantics, Type},
    tokens::TokenKind,
};

pub const EXTERN_BUILTIN_NOOP: usize = 0;
pub const EXTERN_BUILTIN_LIST_GET_ITERATOR: usize = 1;
pub const EXTERN_BUILTIN_LIST_ITERATOR_NEXT: usize = 2;

pub const EXTERN_USER_FIRST: usize = 100000;

// TODO: If I were cool this would by actual bytecode.
//       But I'm not cool.
#[derive(Debug, Clone, Copy)]
pub enum Instruction {
    Panic,

    BoolNot,
    Call(usize),
    Discard,
    Dup,
    EqBool,
    EqFloat,
    EqString,
    FloatAdd,
    FloatDivide,
    FloatMultiply,
    FloatSubtract,
    GreaterFloat,
    GreaterString,
    IsBool,
    IsClass(i64),
    IsFloat,
    IsNothing,
    IsString,
    Jump(usize),
    JumpFalse(usize),
    JumpTrue(usize), // TODO: Only one of these, and use BoolNot?
    LessFloat,
    LessString,
    LoadArgument(usize),
    LoadExternFunction(usize), // NOTE: FUNKY, might want to indirect this index.
    LoadFunction(usize),
    LoadLocal(usize),
    LoadModule(usize),
    LoadSlot(usize),
    NewObject(usize),
    PushFalse,
    PushFloat(f64),
    PushInt(i64),
    PushNothing,
    PushString(usize),
    PushTrue,
    Return,
    StoreArgument(usize),
    StoreLocal(usize),
    StoreModule(usize),
    StoreSlot(usize),
    StringAdd,
    NewList(usize),
}

pub enum Export {
    Function(usize),
    Global(usize),
}

pub struct Module {
    pub functions: Vec<Rc<Function>>,     // Functions
    pub globals: usize,                   // The number of global variables
    pub exports: HashMap<String, Export>, // Exports by name
    pub init: usize,                      // The index of the initialization function
}

impl Module {
    pub fn new() -> Self {
        Module {
            functions: Vec::new(),
            globals: 0,
            exports: HashMap::new(),
            init: 0,
        }
    }

    pub fn functions(&self) -> &[Rc<Function>] {
        &self.functions
    }
}

// TODO: Debug information.
pub struct Function {
    name: String,
    instructions: Vec<Instruction>,
    strings: Vec<Rc<str>>,
    args: usize,   // TODO: Probably type information too?
    locals: usize, // TODO: Same?
}

impl Function {
    pub fn new(name: &str, args: usize) -> Self {
        Function {
            name: name.to_string(),
            instructions: Vec::new(),
            strings: Vec::new(),
            args,
            locals: 0,
        }
    }

    pub fn name(&self) -> &str {
        &self.name
    }

    pub fn args(&self) -> usize {
        self.args
    }

    pub fn locals(&self) -> usize {
        self.locals
    }

    pub fn strings(&self) -> &[Rc<str>] {
        &self.strings
    }

    pub fn instructions(&self) -> &[Instruction] {
        &self.instructions
    }
}

impl std::fmt::Debug for Function {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(
            f,
            "fn {} ({} args, {} locals) ...",
            self.name, self.args, self.locals
        )
    }
}

#[derive(Eq, PartialEq, Hash, Clone)]
struct FunctionKey {
    tree: TreeRef,
}

struct Compiler<'a> {
    source: &'a str,
    semantics: &'a Semantics,
    syntax: &'a SyntaxTree,

    function_bindings: HashMap<FunctionKey, usize>,
    pending_functions: Vec<(FunctionKey, usize, Function)>,
    temp_functions: Vec<Option<Rc<Function>>>,

    module: Module,
    function: Function,
}

impl<'a> Compiler<'a> {
    fn add_string(&mut self, result: String) -> usize {
        let index = self.function.strings.len();
        self.function.strings.push(result.into());
        index
    }

    fn push(&mut self, inst: Instruction) -> usize {
        let index = self.function.instructions.len();
        self.function.instructions.push(inst);
        index
    }

    fn patch(&mut self, i: usize, f: impl FnOnce(usize) -> Instruction) {
        let index = self.function.instructions.len();
        self.function.instructions[i] = f(index);
    }
}

macro_rules! compiler_assert_eq {
    ($compiler:expr, $tr:expr, $ll:expr, $rr:expr $(,)?) => {{
        let left = &$ll;
        let right = &$rr;
        if left != right {
            $compiler.semantics.dump_compiler_state(Some($tr));

            assert_eq!(left, right);
        }
    }};

    ($compiler:expr, $tr:expr, $ll:expr, $rr:expr, $($t:tt)+) => {{
        let left = &$ll;
        let right = &$rr;
        if left != right {
            $compiler.semantics.dump_compiler_state(Some($tr));

            assert_eq!(left, right, $($t)*);
        }
    }};
}

macro_rules! compiler_assert {
    ($compiler:expr, $tr:expr, $cond:expr $(,)?) => {{
        if !$cond {
            $compiler.semantics.dump_compiler_state(Some($tr));

            assert!($cond);
        }
    }};

    ($compiler:expr, $tr:expr, $cond:expr, $($arg:tt)+) => {{
        if !$cond {
            $compiler.semantics.dump_compiler_state(Some($tr));

            assert!($cond, $($arg)*);
        }
    }};
}

macro_rules! ice {
    ($compiler:expr, $tr:expr, $($t:tt)+) => {{
        $compiler.semantics.dump_compiler_state(Some($tr));
        panic!($($t)*)
    }}
}

macro_rules! inst_panic {
    ($($t:tt)+) => {{
        // eprintln!($($t)*);
        Instruction::Panic
    }};
}

// macro_rules! ice {
//     ($compiler:expr, $tr:expr, $($t:tt)*) => {{}};
// }

pub fn compile(semantics: &Semantics) -> Rc<Module> {
    let source = semantics.source();
    let syntax_tree = semantics.tree();

    let mut compiler = Compiler {
        source: &source,
        semantics: &semantics,
        syntax: &syntax_tree,

        function_bindings: HashMap::new(),
        pending_functions: Vec::new(),
        temp_functions: Vec::new(),

        module: Module::new(),
        function: Function::new("<< module >>", 0),
    };

    if let Some(t) = semantics.tree().root() {
        compiler.temp_functions.push(None);
        file(&mut compiler, t);
        compiler.temp_functions[0] = Some(Rc::new(compiler.function));
        compiler.module.init = 0;
    }

    while let Some((fk, idx, func)) = compiler.pending_functions.pop() {
        if idx >= compiler.temp_functions.len() {
            compiler.temp_functions.resize(idx + 1, None);
        }
        compiler.function = func;
        compile_function(&mut compiler, fk.tree);
        compiler.temp_functions[idx] = Some(Rc::new(compiler.function));
    }

    let mut module = compiler.module;
    for f in compiler.temp_functions {
        module.functions.push(f.unwrap());
    }

    Rc::new(module)
}

fn file(c: &mut Compiler, t: TreeRef) {
    let tree = &c.syntax[t];
    compiler_assert_eq!(c, t, tree.kind, TreeKind::File, "must be compiling a file");

    let children: Vec<_> = tree.child_trees().collect();
    if children.len() == 0 {
        c.push(Instruction::PushNothing);
    } else {
        for i in 0..children.len() - 1 {
            compile_statement(c, children[i], false);
        }
        compile_statement(c, *children.last().unwrap(), true);
    }
    c.push(Instruction::Return);
}

type CR = Option<()>;
const OK: CR = CR::Some(());

fn compile_expression(c: &mut Compiler, t: TreeRef) {
    let tree = &c.syntax[t];
    let cr = match tree.kind {
        TreeKind::Error => None,

        TreeKind::Argument => compile_argument(c, tree),
        TreeKind::BinaryExpression => compile_binary_expression(c, t, tree),
        TreeKind::Block => compile_block_expression(c, tree),
        TreeKind::CallExpression => compile_call_expression(c, tree),
        TreeKind::ConditionalExpression => compile_condition_expression(c, tree),
        TreeKind::FieldValue => compile_field_value(c, t, tree),
        TreeKind::GroupingExpression => compile_grouping(c, tree),
        TreeKind::Identifier => compile_identifier_expression(c, t, tree),
        TreeKind::IsExpression => compile_is_expression(c, tree),
        TreeKind::ListConstructor => compile_list_constructor(c, tree),
        TreeKind::ListConstructorElement => compile_list_constructor_element(c, tree),
        TreeKind::LiteralExpression => compile_literal(c, t, tree),
        TreeKind::MemberAccess => compile_member_access(c, t, tree),
        TreeKind::NewObjectExpression => compile_new_object_expression(c, t, tree),
        TreeKind::SelfReference => compile_self_reference(c),
        TreeKind::UnaryExpression => compile_unary_operator(c, t, tree),

        _ => ice!(c, t, "{tree:?} is not an expression, cannot compile"),
    };
    if matches!(cr, None) {
        c.push(inst_panic!("panic compiling expression {:?}", tree));
    }
}

fn compile_literal(c: &mut Compiler, t: TreeRef, tr: &Tree) -> CR {
    let tok = tr.nth_token(0)?;
    match c.semantics.type_of(t) {
        Type::F64 => c.push(Instruction::PushFloat(
            tok.as_str(c.source).parse().unwrap(),
        )),
        Type::Bool => c.push(if tok.kind == TokenKind::True {
            Instruction::PushTrue
        } else {
            Instruction::PushFalse
        }),
        Type::String => {
            let result = string_constant_to_string(tok.as_str(c.source));
            let index = c.add_string(result);
            c.push(Instruction::PushString(index))
        }
        Type::Error => c.push(inst_panic!("compiling literal {:?}", tr)),
        _ => ice!(c, t, "unsupported literal type: {t:?}"),
    };
    OK
}

fn compile_grouping(c: &mut Compiler, t: &Tree) -> CR {
    compile_expression(c, t.nth_tree(1)?);
    OK
}

fn compile_unary_operator(c: &mut Compiler, t: TreeRef, tr: &Tree) -> CR {
    compile_expression(c, tr.nth_tree(1)?);

    let tok = tr.nth_token(0)?;
    match tok.kind {
        TokenKind::Minus => {
            c.push(Instruction::PushFloat(-1.0));
            c.push(Instruction::FloatMultiply);
        }
        TokenKind::Bang => {
            c.push(Instruction::BoolNot);
        }
        _ => ice!(c, t, "unsupported unary operator"),
    }
    OK
}

fn compile_condition_expression(c: &mut Compiler, t: &Tree) -> CR {
    let condition = t.nth_tree(1)?;
    compile_expression(c, condition);

    let jump_else_index = c.push(Instruction::JumpFalse(0));

    let then_branch = t.nth_tree(2)?;
    compile_expression(c, then_branch);

    let jump_end_index = c.push(Instruction::Jump(0));
    c.patch(jump_else_index, |i| Instruction::JumpFalse(i));

    if let Some(else_branch) = t.nth_tree(4) {
        compile_expression(c, else_branch);
    } else {
        c.push(Instruction::PushNothing);
    }

    c.patch(jump_end_index, |i| Instruction::Jump(i));
    OK
}

fn compile_simple_binary_expression<T>(c: &mut Compiler, tr: &Tree, f: T) -> CR
where
    T: FnOnce(&mut Compiler, &Type) -> Instruction,
{
    compile_expression(c, tr.nth_tree(0)?);

    let arg_tree = tr.nth_tree(2)?;
    let arg_type = c.semantics.type_of(arg_tree);

    compile_expression(c, arg_tree);

    let inst = f(c, &arg_type);
    c.push(inst);

    OK
}

fn compile_binary_expression(c: &mut Compiler, t: TreeRef, tr: &Tree) -> CR {
    let op = tr.nth_token(1)?;
    match op.kind {
        TokenKind::Plus => compile_simple_binary_expression(c, tr, |_, t| match t {
            Type::F64 => Instruction::FloatAdd,
            Type::String => Instruction::StringAdd,
            _ => inst_panic!("panic adding {}", t),
        }),
        TokenKind::Minus => {
            compile_simple_binary_expression(c, tr, |_, _| Instruction::FloatSubtract)
        }
        TokenKind::Star => {
            compile_simple_binary_expression(c, tr, |_, _| Instruction::FloatMultiply)
        }
        TokenKind::Slash => {
            compile_simple_binary_expression(c, tr, |_, _| Instruction::FloatDivide)
        }

        TokenKind::Less => compile_simple_binary_expression(c, tr, |_, t| match t {
            Type::F64 => Instruction::LessFloat,
            Type::String => Instruction::LessString,
            _ => inst_panic!("panic less {}", t),
        }),
        TokenKind::LessEqual => {
            compile_simple_binary_expression(c, tr, |_, t| match t {
                Type::F64 => Instruction::GreaterFloat,
                Type::String => Instruction::GreaterString,
                _ => inst_panic!("panic less equal {}", t),
            });
            c.push(Instruction::BoolNot);
            OK
        }
        TokenKind::Greater => compile_simple_binary_expression(c, tr, |_, t| match t {
            Type::F64 => Instruction::GreaterFloat,
            Type::String => Instruction::GreaterString,
            _ => inst_panic!("panic greater {}", t),
        }),
        TokenKind::GreaterEqual => {
            compile_simple_binary_expression(c, tr, |_, t| match t {
                Type::F64 => Instruction::LessFloat,
                Type::String => Instruction::LessString,
                _ => inst_panic!("panic greater equal {}", t),
            });
            c.push(Instruction::BoolNot);
            OK
        }

        TokenKind::And => {
            // Compile the left hand side, it leaves a bool on the stack
            compile_expression(c, tr.nth_tree(0)?);

            // If the first part is true (hooray!) then we need to evaluate
            // the right hand side, so jump around the short circuit...
            let jump_true_index = c.push(Instruction::JumpTrue(0));

            // ...but if the first part is false then we stop here. We need
            // to leave a value on the stack (it was consumed by jump above)
            // so we push an extra False here, and jump to the end.
            c.push(Instruction::PushFalse);
            let jump_end_index = c.push(Instruction::Jump(0));

            // Here we are, we consumed the `true` off the stack now time to
            // do the right hand side.
            c.patch(jump_true_index, |i| Instruction::JumpTrue(i));

            // The right hand side leaves true or false on the stack, it's
            // the result of the expression.
            compile_expression(c, tr.nth_tree(2)?);

            // (here's where you go after you leave the "false" on the stack.)
            c.patch(jump_end_index, |i| Instruction::Jump(i));
            OK
        }
        TokenKind::Or => {
            // Compile the left hand side, it leaves a bool on the stack
            compile_expression(c, tr.nth_tree(0)?);

            // If the first part is false (boo!) then we need to evaluate the
            // right hand side, so jump around the short circuit...
            let jump_false_index = c.push(Instruction::JumpFalse(0));

            // ...but if the first part os true then we stop here. We need to
            // leave a value on the stack (it was consumed by jump above) so
            // we push an extra True here and jump to the end.
            c.push(Instruction::PushTrue);
            let jump_end_index = c.push(Instruction::Jump(0));

            // Here we are, we consumed the `false` off the stack now time to
            // do the right hand side.
            c.patch(jump_false_index, |i| Instruction::JumpFalse(i));

            // The right hand side leaves true or false on the stack, it's
            // the result of the expression.
            compile_expression(c, tr.nth_tree(2)?);

            // (here's where you go after you leave "true" on the stack.)
            c.patch(jump_end_index, |i| Instruction::Jump(i));
            OK
        }
        TokenKind::EqualEqual => {
            compile_simple_binary_expression(c, tr, |c, arg_type| {
                if c.semantics.can_convert(&arg_type, &Type::Nothing) {
                    c.push(Instruction::Discard);
                    c.push(Instruction::Discard);
                    Instruction::PushTrue
                } else {
                    match arg_type {
                        Type::F64 => Instruction::EqFloat,
                        Type::String => Instruction::EqString,
                        Type::Bool => Instruction::EqBool, // ?
                        _ => inst_panic!("panic comparing {}", arg_type),
                    }
                }
            })
        }
        TokenKind::Equal => {
            compile_expression(c, tr.nth_tree(2)?);
            c.push(Instruction::Dup);

            let lvalue = tr.nth_tree(0)?;
            let ltree = &c.syntax[lvalue];

            #[allow(unused_assignments)]
            let mut environment = Environment::error();

            let declaration = match ltree.kind {
                // TODO: Assign to list access
                TreeKind::Identifier => {
                    let id = ltree.nth_token(0)?.as_str(&c.source);
                    environment = c.semantics.environment_of(lvalue);
                    environment.bind(id)?
                }
                TreeKind::MemberAccess => {
                    let id = ltree.nth_token(2)?.as_str(&c.source);

                    let t = ltree.nth_tree(0)?;
                    let typ = c.semantics.type_of(t);
                    environment = c.semantics.member_environment(t, &typ);
                    environment.bind(id)?
                }
                _ => return None,
            };

            let instruction = match declaration {
                Declaration::Variable {
                    location, index, ..
                } => {
                    let index = *index;
                    match location {
                        Location::Argument => {
                            compiler_assert!(c, t, index < c.function.args);
                            Instruction::StoreArgument(index)
                        }
                        Location::Local => {
                            if index >= c.function.locals {
                                c.function.locals = index + 1;
                            }
                            Instruction::StoreLocal(index)
                        }
                        Location::Module => {
                            compiler_assert!(c, t, index < c.module.globals);
                            Instruction::StoreModule(index)
                        }
                        Location::Slot => {
                            compile_expression(c, ltree.nth_tree(0)?);
                            Instruction::StoreSlot(index)
                        }
                    }
                }

                Declaration::ExternFunction { .. } => inst_panic!("store ext"),
                Declaration::Function { .. } => inst_panic!("store func"),
                Declaration::Class { .. } => inst_panic!("store class"),
                Declaration::Import { .. } => inst_panic!("store import"),
            };
            c.push(instruction);
            OK
        }
        _ => ice!(
            c,
            t,
            "Unsupported binary expression '{}'",
            op.as_str(&c.source)
        ),
    }
}

fn compile_identifier_expression(c: &mut Compiler, t: TreeRef, tree: &Tree) -> Option<()> {
    let ident = tree.nth_token(0)?.as_str(&c.source);
    let environment = c.semantics.environment_of(t);
    let declaration = environment.bind(ident)?;

    compile_load_declaration(c, t, declaration)
}

fn compile_load_declaration(c: &mut Compiler, t: TreeRef, declaration: &Declaration) -> CR {
    let instruction = match declaration {
        Declaration::Variable {
            location, index, ..
        } => {
            let index = *index;
            match location {
                Location::Local => {
                    if index >= c.function.locals {
                        c.function.locals = index + 1;
                    }
                    Instruction::LoadLocal(index)
                }
                Location::Argument => {
                    compiler_assert!(c, t, index < c.function.args);
                    Instruction::LoadArgument(index)
                }
                Location::Module => {
                    compiler_assert!(c, t, index < c.module.globals);
                    Instruction::LoadModule(index)
                }
                Location::Slot => {
                    // TODO: Assert slot is in field range?
                    Instruction::LoadSlot(index)
                }
            }
        }
        Declaration::Function { declaration, .. } => {
            let key = FunctionKey { tree: *declaration };
            let index = match c.function_bindings.get(&key) {
                Some(index) => *index,
                None => {
                    let tree = &c.syntax[*declaration];
                    compiler_assert_eq!(c, t, tree.kind, TreeKind::FunctionDecl);

                    compile_function_declaration(c, *declaration, tree, false)?;

                    match c.function_bindings.get(&key) {
                        Some(index) => *index,
                        None => {
                            ice!(
                                c,
                                t,
                                "did not compile the function with key {:?}!",
                                declaration
                            )
                        }
                    }
                }
            };
            Instruction::LoadFunction(index)
        }
        Declaration::ExternFunction { id, .. } => Instruction::LoadExternFunction(id.id()),

        // Must be a static don't worry about it.
        Declaration::Class { .. } => return OK,

        // fix later
        Declaration::Import { .. } => ice!(c, t, "import compile not supported"),
    };

    c.push(instruction);

    OK
}

fn compile_is_expression(c: &mut Compiler, tree: &Tree) -> Option<()> {
    compile_expression(c, tree.nth_tree(0)?);

    compile_pattern(c, tree.nth_tree(2)?)
}

fn compile_pattern(c: &mut Compiler, t: TreeRef) -> Option<()> {
    let tree = &c.syntax[t];

    // Let's *try* to generate good code in the presence of a wildcard pattern....
    let is_wildcard = tree
        .child_tree_of_kind(&c.syntax, TreeKind::WildcardPattern)
        .is_some();

    let type_expr = tree.child_tree_of_kind(&c.syntax, TreeKind::TypeExpression);

    let and_index = tree.children.iter().position(|c| match c {
        Child::Token(t) => t.kind == TokenKind::And,
        _ => false,
    });

    // If you have a binding, dup and store now, it is in scope.
    if let Some(binding) = tree.child_tree_of_kind(&c.syntax, TreeKind::VariableBinding) {
        if let Some(variable) = binding.nth_token(0) {
            let environment = c.semantics.environment_of(t);
            let declaration = environment.bind(variable.as_str(&c.source))?;

            let Declaration::Variable {
                location: Location::Local,
                index,
                ..
            } = declaration
            else {
                ice!(c, t, "is cannot make a non-local, non-variable declaration")
            };

            // If we aren't a wildcard or we have an attached predicate then
            // we will need the value on the stack, otherwise we can discard
            // it.
            if and_index.is_some() || !is_wildcard {
                c.push(Instruction::Dup);
            }
            c.push(Instruction::StoreLocal(*index));
        }
    }

    if !is_wildcard {
        let type_expr = type_expr?;
        compile_type_expr_eq(c, type_expr.nth_tree(0)?);
    }

    if let Some(and_index) = and_index {
        let jump_end_index = if is_wildcard {
            // If the pattern was a wildcard then don't bother with this jump
            // nonsense; we know the pattern matched, all we need to do is
            // evaluate the predicate.
            None
        } else {
            // Otherwise test the pattern to see if it passed; if it did then
            // we need to run the predicate. (This is the back half of an AND
            // expression.)
            let jump_true_index = c.push(Instruction::JumpTrue(0));

            c.push(Instruction::PushFalse);
            let jump_end_index = c.push(Instruction::Jump(0));

            c.patch(jump_true_index, |i| Instruction::JumpTrue(i));
            Some(jump_end_index)
        };

        compile_expression(c, tree.nth_tree(and_index + 1)?);

        // If we wound up with a jump what needs patching, patch it.
        if let Some(jump_end_index) = jump_end_index {
            c.patch(jump_end_index, |i| Instruction::Jump(i));
        }
    } else if is_wildcard {
        // If there was no predicate *and* the pattern was a wildcard then
        // I'll just need to push true here.
        c.push(Instruction::PushTrue);
    }

    OK
}

fn compile_type_expr_eq(c: &mut Compiler, t: TreeRef) {
    let tree = &c.syntax[t];
    let result = match tree.kind {
        TreeKind::TypeIdentifier => compile_type_identifier_eq(c, t, tree),
        TreeKind::AlternateType => compile_type_alternate_eq(c, tree),
        _ => ice!(c, t, "tree is not a type expression"),
    };

    if result.is_none() {
        c.push(inst_panic!("panic compiling type expression eq {:?}", tree));
    }
}

fn compile_type_identifier_eq(c: &mut Compiler, t: TreeRef, tree: &Tree) -> CR {
    let identifier = tree.nth_token(0)?.as_str(&c.source);
    match identifier {
        "f64" => {
            c.push(Instruction::IsFloat);
        }
        "string" => {
            c.push(Instruction::IsString);
        }
        "bool" => {
            c.push(Instruction::IsBool);
        }
        "nothing" => {
            c.push(Instruction::IsNothing);
        }
        _ => {
            let environment = c.semantics.environment_of(t);
            match environment.bind(identifier)? {
                Declaration::Class { declaration, .. } => {
                    // The runtime identifier of the class is the tree index of the
                    // class declaration sure why not.
                    let index = declaration.index();
                    c.push(Instruction::IsClass(index.try_into().unwrap()));
                }

                _ => return None,
            }
        }
    };

    OK
}

fn compile_type_alternate_eq(c: &mut Compiler, tree: &Tree) -> CR {
    // Compile the left hand side, it leaves a bool on the stack
    compile_type_expr_eq(c, tree.nth_tree(0)?);

    // If the first part is false (boo!) then we need to evaluate the
    // right hand side, so jump around the short circuit...
    let jump_false_index = c.push(Instruction::JumpFalse(0));

    // ...but if the first part is true then we stop here. We need to
    // leave a value on the stack (it was consumed by jump above) so
    // we push an extra True here and jump to the end.
    c.push(Instruction::PushTrue);
    let jump_end_index = c.push(Instruction::Jump(0));

    // Here we are, we consumed the `false` off the stack now time to
    // do the right hand side.
    c.patch(jump_false_index, |i| Instruction::JumpFalse(i));

    // The right hand side leaves true or false on the stack, it's
    // the result of the expression.
    compile_type_expr_eq(c, tree.nth_tree(2)?);

    // (here's where you go after you leave "true" on the stack.)
    c.patch(jump_end_index, |i| Instruction::Jump(i));
    OK
}

fn compile_call_expression(c: &mut Compiler, tree: &Tree) -> CR {
    let arg_list = tree.child_tree_of_kind(&c.syntax, TreeKind::ArgumentList)?;
    let mut args: Vec<_> = arg_list.child_trees().collect();
    let arg_count = args.len();

    args.reverse();
    for arg in args {
        compile_expression(c, arg);
    }

    let func = tree.nth_tree(0)?;
    let func_type = c.semantics.type_of(func);
    let arg_count = match func_type {
        // TODO: Consider being guided by syntax here?
        Type::Method(..) => arg_count + 1,
        _ => arg_count,
    };
    compile_expression(c, func);

    c.push(Instruction::Call(arg_count));
    OK
}

fn compile_block_expression(c: &mut Compiler, tree: &Tree) -> CR {
    if tree.children.len() == 2 {
        c.push(Instruction::PushNothing);
        return OK;
    }

    let last_is_brace = tree.nth_token(tree.children.len() - 1).is_some();
    let last_index = tree.children.len() - if last_is_brace { 2 } else { 1 };

    for i in 1..last_index {
        compile_statement(c, tree.nth_tree(i)?, false);
    }
    compile_statement(c, tree.nth_tree(last_index)?, true);
    OK
}

fn compile_argument(c: &mut Compiler, tree: &Tree) -> CR {
    compile_expression(c, tree.nth_tree(0)?);
    OK
}

fn compile_new_object_expression(c: &mut Compiler, t: TreeRef, tree: &Tree) -> CR {
    // We pass in the arguments.... by... field order?
    let Type::Object(ct, _) = c.semantics.type_of(t) else {
        c.push(inst_panic!("new obj not ob"));
        return OK;
    };
    let class = c.semantics.class_of(ct);

    let field_list = tree.child_tree_of_kind(&c.syntax, TreeKind::FieldList)?;
    let mut field_bindings = HashMap::new();
    for field in field_list.children_of_kind(&c.syntax, TreeKind::FieldValue) {
        let f = &c.syntax[field];
        let name = f.nth_token(0)?;
        field_bindings.insert(name.as_str(&c.source), field);
    }

    // The fields come in this order and since arguments are backwards
    // (stack!) we compile them in reverse order. Missing fields panic,
    // obviously.
    for field in class.fields.iter().rev() {
        let binding = field_bindings.get(&*field.name)?;
        compile_expression(c, *binding);
    }

    // Fetch the correct constructor.
    // TODO: Binding this type should be done by semantics, and we should borrow it.
    let type_reference = tree.child_tree_of_kind(&c.syntax, TreeKind::TypeIdentifier)?;
    let identifier = type_reference.nth_token(0)?.as_str(&c.source);
    let environment = c.semantics.environment_of(t);
    match environment.bind(identifier)? {
        Declaration::Class { declaration, .. } => {
            let key = FunctionKey { tree: *declaration };
            let index = match c.function_bindings.get(&key) {
                Some(index) => *index,
                None => {
                    let tree = &c.syntax[*declaration];
                    compiler_assert_eq!(c, t, tree.kind, TreeKind::ClassDecl);

                    compile_class_declaration(c, t, tree, false)?;

                    *c.function_bindings
                        .get(&key)
                        .expect("did not compile the class constructor!")
                }
            };
            c.push(Instruction::LoadFunction(index));
        }
        _ => return None,
    }
    c.push(Instruction::Call(class.fields.len()));
    OK
}

fn compile_field_value(c: &mut Compiler, t: TreeRef, tree: &Tree) -> CR {
    if let Some(colon) = tree.nth_token(1) {
        if colon.kind == TokenKind::Colon {
            compile_expression(c, tree.nth_tree(2)?);
            return OK;
        }
    }

    // Form 2: { x, ... }
    let environment = c.semantics.environment_of(t);
    let id = tree.nth_token(0)?.as_str(&c.source);
    let declaration = environment.bind(id)?;

    compile_load_declaration(c, t, declaration)
}

fn compile_member_access(c: &mut Compiler, t: TreeRef, tree: &Tree) -> CR {
    // In member access; the lhs sets up the object and in theory the rhs
    // binds against it. ::shrug::
    //
    compile_expression(c, tree.nth_tree(0)?);

    let typ = c.semantics.type_of(tree.nth_tree(0)?);
    let ident = tree.nth_token(2)?.as_str(&c.source);

    let environment = match &typ {
        Type::Object(ct, _) => {
            let class = c.semantics.class_of(*ct);
            class.env.clone()
        }
        Type::Class(ct, _) => {
            let class = c.semantics.class_of(*ct);
            class.static_env.clone()
        }
        _ => {
            c.push(inst_panic!("cannot get environment of {typ}"));
            return None;
        }
    };
    let declaration = environment.bind(ident)?;

    // NOTE: If this is a method call we still don't have to do anything
    //       special here, since the load of the member function will *not*
    //       consume the self pointer from the stack.
    compile_load_declaration(c, t, declaration);
    OK
}

fn compile_self_reference(c: &mut Compiler) -> CR {
    c.push(Instruction::LoadArgument(0));
    OK
}

fn compile_list_constructor(c: &mut Compiler, tree: &Tree) -> CR {
    let mut children: Vec<_> = tree
        .children_of_kind(&c.syntax, TreeKind::ListConstructorElement)
        .collect();
    children.reverse();
    let count = children.len();
    for child in children {
        compile_expression(c, child);
    }
    c.push(Instruction::NewList(count));
    OK
}

fn compile_list_constructor_element(c: &mut Compiler, tree: &Tree) -> CR {
    compile_expression(c, tree.nth_tree(0)?);
    OK
}

fn compile_statement(c: &mut Compiler, t: TreeRef, gen_value: bool) {
    let tree = &c.semantics.tree()[t];
    let cr = match tree.kind {
        TreeKind::Error => None,

        TreeKind::Import => compile_import_statement(c, gen_value),
        TreeKind::Block => compile_block_statement(c, t, gen_value),
        TreeKind::ClassDecl => compile_class_declaration(c, t, tree, gen_value),
        TreeKind::ExpressionStatement => compile_expression_statement(c, tree, gen_value),
        TreeKind::ForStatement => compile_for_statement(c, tree, gen_value),
        TreeKind::FunctionDecl => compile_function_declaration(c, t, tree, gen_value),
        TreeKind::IfStatement => compile_if_statement(c, tree, gen_value),
        TreeKind::LetStatement => compile_let_statement(c, t, tree, gen_value),
        TreeKind::ReturnStatement => compile_return_statement(c, tree),
        TreeKind::WhileStatement => compile_while_statement(c, tree, gen_value),

        _ => ice!(c, t, "unsupported statement tree kind {:?}", tree.kind),
    };
    if matches!(cr, None) {
        c.push(inst_panic!("stat {:?}", tree));
    }
}

fn compile_if_statement(c: &mut Compiler, tree: &Tree, gen_value: bool) -> CR {
    compile_expression(c, tree.nth_tree(0)?);
    if !gen_value {
        c.push(Instruction::Discard);
    }

    OK
}

fn compile_expression_statement(c: &mut Compiler, tree: &Tree, gen_value: bool) -> CR {
    if let Some(expr) = tree.nth_tree(0) {
        compile_expression(c, expr);

        if tree
            .nth_token(1)
            .is_some_and(|t| t.kind == TokenKind::Semicolon)
        {
            c.push(Instruction::Discard);
            if gen_value {
                c.push(Instruction::PushNothing);
            }
        } else if !gen_value {
            c.push(Instruction::Discard);
        }
    } else if gen_value {
        c.push(Instruction::PushNothing);
    };

    OK
}

fn compile_let_statement(c: &mut Compiler, t: TreeRef, tree: &Tree, gen_value: bool) -> CR {
    compile_expression(c, tree.nth_tree(3)?);
    let environment = c.semantics.environment_of(t);
    let declaration = environment.bind(tree.nth_token(1)?.as_str(&c.source))?;

    let Declaration::Variable {
        location, index, ..
    } = declaration
    else {
        ice!(c, t, "let cannot make a non-variable declaration")
    };

    let index = *index;
    let instruction = match location {
        Location::Local => {
            if index >= c.function.locals {
                c.function.locals = index + 1;
            }
            Instruction::StoreLocal(index)
        }
        Location::Module => {
            if index >= c.module.globals {
                c.module.globals = index + 1;
            }
            Instruction::StoreModule(index)
        }
        _ => ice!(c, t, "unsuitable location for let declaration"),
    };
    c.push(instruction);
    if gen_value {
        c.push(Instruction::PushNothing);
    }

    OK
}

fn compile_function_declaration(c: &mut Compiler, t: TreeRef, tree: &Tree, gen_value: bool) -> CR {
    // Only compile a given function once.
    //
    // TODO: When it's time for generics, this should only actually compile
    // if we have no unbound type variables.
    let fk = FunctionKey { tree: t };
    if !c.function_bindings.contains_key(&fk) {
        // TODO: If this is a method the name should be different.
        let name = tree.nth_token(1)?.as_str(&c.source);

        let param_list = tree.child_tree_of_kind(&c.syntax, TreeKind::ParamList)?;
        let param_count = param_list.children.len() - 2;

        let function_index = c.temp_functions.len();
        c.temp_functions.push(None);

        c.pending_functions
            .push((fk.clone(), function_index, Function::new(name, param_count)));
        c.function_bindings.insert(fk, function_index);
        c.module
            .exports
            .insert(name.to_string(), Export::Function(function_index));
    }

    if gen_value {
        c.push(Instruction::PushNothing);
    }

    OK
}

fn compile_class_declaration(c: &mut Compiler, t: TreeRef, tree: &Tree, gen_value: bool) -> CR {
    // Only compile a given function once.
    // Classes get compiled as constructor functions which get called.
    let fk = FunctionKey { tree: t };
    if !c.function_bindings.contains_key(&fk) {
        let name = tree.nth_token(1)?.as_str(&c.source);

        let field_count = tree.children.len() - 2;

        let function_index = c.temp_functions.len();
        c.temp_functions.push(None);

        c.pending_functions
            .push((fk.clone(), function_index, Function::new(name, field_count)));
        c.function_bindings.insert(fk, function_index);
        c.module
            .exports
            .insert(name.to_string(), Export::Function(function_index));
    }

    if gen_value {
        c.push(Instruction::PushNothing);
    }

    OK
}

fn compile_function(c: &mut Compiler, t: TreeRef) -> CR {
    let tree = &c.syntax[t];
    match tree.kind {
        TreeKind::FunctionDecl => {
            let block = tree.child_of_kind(&c.syntax, TreeKind::Block)?;
            compile_expression(c, block);
        }
        TreeKind::ClassDecl => {
            let count = tree
                .children_of_kind(&c.syntax, TreeKind::FieldDecl)
                .count();
            for i in 0..count {
                c.push(Instruction::LoadArgument(count - 1 - i));
            }

            let name = tree.nth_token(1)?.as_str(&c.source);
            let name_index = c.add_string(name.to_string());
            c.push(Instruction::PushString(name_index));
            c.push(Instruction::PushInt(t.index().try_into().unwrap()));
            c.push(Instruction::NewObject(count));
        }
        _ => ice!(c, t, "what is this tree doing in compile_function?"),
    }

    c.push(Instruction::Return);
    OK
}

fn compile_block_statement(c: &mut Compiler, t: TreeRef, gen_value: bool) -> CR {
    compile_expression(c, t);
    if !gen_value {
        c.push(Instruction::Discard);
    }

    OK
}

fn compile_while_statement(c: &mut Compiler, tree: &Tree, gen_value: bool) -> CR {
    let start_index = c.function.instructions.len();
    compile_expression(c, tree.nth_tree(1)?);

    let jump_end_index = c.push(Instruction::JumpFalse(0));

    compile_block_statement(c, tree.nth_tree(2)?, false);
    c.push(Instruction::Jump(start_index));

    c.patch(jump_end_index, |i| Instruction::JumpFalse(i));
    if gen_value {
        c.push(Instruction::PushNothing);
    }

    OK
}

fn compile_return_statement(c: &mut Compiler, tree: &Tree) -> CR {
    if let Some(expr) = tree.nth_tree(1) {
        compile_expression(c, expr);
    } else {
        c.push(Instruction::PushNothing);
    }
    c.push(Instruction::Return);
    OK
}

fn compile_for_statement(c: &mut Compiler, tree: &Tree, gen_value: bool) -> CR {
    // Figure out the variable.
    let vt = tree.nth_tree(1)?;
    let var = &c.syntax[vt];
    let id = var.nth_token(0)?.as_str(&c.source);

    let body = tree.nth_tree(4)?;
    let env = c.semantics.environment_of(body);
    let Some(variable_decl) = env.bind(id) else {
        ice!(c, body, "Unable to bind {id} in loop body");
    };
    let variable_slot = match variable_decl {
        Declaration::Variable {
            location, index, ..
        } => {
            compiler_assert_eq!(
                c,
                vt,
                *location,
                Location::Local,
                "expected loop variable to be local"
            );

            *index
        }
        _ => ice!(c, vt, "loop variable was not a variable"),
    };

    // Figure out the generator.
    let iterable = tree.nth_tree(3)?;
    compile_expression(c, iterable);

    // call 'get_iterator'
    let iterable_typ = c.semantics.type_of(iterable);
    match iterable_typ {
        Type::List(_) => {
            c.push(Instruction::LoadExternFunction(
                EXTERN_BUILTIN_LIST_GET_ITERATOR,
            ));
            c.push(Instruction::Call(1));
        }
        _ => return None, // TODO: Bind and call get_iterator() on type of iterable
    }

    // iterate
    // (Stack is clear except for the iterator.)

    let loop_top = c.push(Instruction::Dup); // Save the iterator
    match iterable_typ {
        Type::List(_) => {
            c.push(Instruction::LoadExternFunction(
                EXTERN_BUILTIN_LIST_ITERATOR_NEXT,
            ));
        }
        _ => return None, // TODO: Bind and call next() on type of iterator
    }
    c.push(Instruction::Call(1)); // Call 'next'

    c.push(Instruction::Dup); // Save the result
    c.push(Instruction::IsNothing); // Check to see if iteration is done
    let jump_to_end = c.push(Instruction::JumpTrue(0));
    c.push(Instruction::StoreLocal(variable_slot)); // Store the value

    // (Stack is clear except for the iterator.)

    compile_statement(c, body, false); // Run the body

    // (Stack is clear except for the iterator.)

    c.push(Instruction::Jump(loop_top));

    // Clean up the loop.
    c.patch(jump_to_end, |i| Instruction::JumpTrue(i));
    c.push(Instruction::Discard); // Drop the unused value
    c.push(Instruction::Discard); // Drop the iterator

    if gen_value {
        c.push(Instruction::PushNothing);
    }

    OK
}

fn compile_import_statement(c: &mut Compiler, gen_value: bool) -> CR {
    if gen_value {
        c.push(Instruction::PushNothing);
    }
    OK
}