vulcan/forge_core/src/compiler.rs

use crate::ast::*;

use std::collections::btree_map::Entry::Vacant;
use std::collections::{BTreeMap, HashMap};
use std::fmt::{Display, Formatter};

#[derive(Eq, Clone, PartialEq, Debug)]
pub struct CompileError(usize, usize, String);

impl Display for CompileError {
    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
        write!(f, "({}:{}) {}", self.0, self.1, self.2)
    }
}

/// A variable, of various different types:
/// - Literals have static values and can just be pushed to the stack
/// - Labels contain the label pointing to the value
/// - Locals contain an index into the local frame
#[derive(Clone, PartialEq, Debug)]
pub enum Variable {
    Literal(i32),
    Label(String),
    Local(usize),
}

/// The data associated with a function signature:
/// - The frame size is in bytes
/// - The local scope is full of `Variable::Local`s, and contains args and local vars
/// - The body is just the statements of the function, not the code to move the frame
/// pointer around (which can't be generated until the variable declarations are all found)
///
/// A complete function implementation consists of:
/// - A label for the entrypoint
/// - Preamble code to set up the stack frame
/// - The function body
///
/// The stack frame is managed by the function through a global pointer called "frame". When
/// the function is called, it can assume that all memory after "frame" is free for use (this
/// isn't actually true because you can blow out the stack, but within reason it is). So when
/// you make a call to another function, you need to increment frame by the current frame size,
/// and then after the other function has returned, decrement it back so that frame again points
/// at your stack frame. Locals can be found by adding some offset from the frame pointer.
#[derive(Clone, PartialEq, Debug, Default)]
pub struct Signature {
    pub label: String,
    pub frame_size: usize,
    pub local_scope: Scope,
    pub body: Vec<String>,
}

impl Signature {
    /// Add a name to the local scope, which:
    /// - Increases the size of the stack frame by that much
    /// - Records the offset into the local stack frame where that variable is stored
    fn add_local(&mut self, name: &str) -> Result<(), CompileError> {
        if let Vacant(e) = self.local_scope.entry(name.into()) {
            e.insert(Variable::Local(self.frame_size));
            self.frame_size += 3; // todo: variously-sized structs
            Ok(())
        } else {
            Err(CompileError(0, 0, format!("Duplicate name {}", name)))
        }
    }

    /// Emit a string (ideally one instruction, but whatever) to the function body.
    /// This doesn't actually emit anything to output, the body will eventually be emitted
    /// in a final pass by the compiler once all functions are compiled.
    fn emit(&mut self, opcode: &str) {
        self.body.push(String::from(opcode))
    }

    /// A shorthand method to emit something with a `Display` arg, because emitting a
    /// single instruction with a variable (numeric or label) arg is very common.
    fn emit_arg<T: Display>(&mut self, opcode: &str, arg: T) {
        self.body.push(format!("{} {}", opcode, arg))
    }
}

/// Maps from names to the variables they represent
pub type Scope = BTreeMap<String, Variable>;

/// The compiler state:
#[derive(Clone, PartialEq, Debug, Default)]
struct State {
    /// Used by gensym to generate unique symbols
    pub gensym_index: usize,
    /// The globally-defined names
    pub global_scope: Scope,
    /// The functions
    pub functions: BTreeMap<String, Signature>,
}

impl State {
    /// Generate a guaranteed-unique symbolic name
    fn gensym(&mut self) -> String {
        self.gensym_index += 1;
        format!("_gensym_{}", self.gensym_index)
    }

    /// Return whether a name exists in the global scope already
    fn defined(&self, name: &str) -> bool {
        self.global_scope.contains_key(name)
    }

    fn add_global<F: Fn(&mut State) -> Variable>(
        &mut self,
        name: &str,
        val: F,
    ) -> Result<(), CompileError> {
        if self.defined(name) {
            Err(CompileError(0, 0, format!("name {} already defined", name)))
        } else {
            let val = val(self);
            self.global_scope.insert(name.into(), val);
            Ok(())
        }
    }
}

trait Compilable {
    // Every AST node we visit can see the global compiler state (to make unique
    // symbols and reach for global names) as well as optionally the current
    // function (for AST nodes within functions)
    fn process(
        self,
        state: &mut State,
        function: Option<&mut Signature>,
    ) -> Result<(), CompileError>;
}

///////////////////////////////////////////////////////////

impl Compilable for Program {
    fn process(self, state: &mut State, _: Option<&mut Signature>) -> Result<(), CompileError> {
        for decl in self.0 {
            decl.process(state, None)?
        }
        Ok(())
    }
}

///////////////////////////////////////////////////////////

impl Compilable for Declaration {
    fn process(self, state: &mut State, _: Option<&mut Signature>) -> Result<(), CompileError> {
        match self {
            Declaration::Function(f) => f.process(state, None),
            Declaration::Global(g) => g.process(state, None),
            Declaration::Struct(_) => todo!("Structs are not yet supported"),
            Declaration::Const(c) => c.process(state, None),
        }
    }
}

///////////////////////////////////////////////////////////

impl Compilable for Function {
    fn process(self, state: &mut State, _: Option<&mut Signature>) -> Result<(), CompileError> {
        if self.typename.is_some() {
            todo!("Structs are not yet supported")
        }
        if self.org.is_some() || self.inline {
            todo!("Attributes are not yet supported")
        }

        // The signature for this function, which will eventually get added to the state
        let mut sig = Signature {
            label: state.gensym(),
            ..Default::default()
        };

        // Add each argument as a local
        for arg in self.args {
            if arg.typename.is_some() {
                todo!("Structs are not yet supported")
            }
            sig.add_local(&arg.name)?
        }

        // Compile each statement: todo this should be broken into several different Compilables
        for stmt in self.body.0 {
            match stmt {
                Statement::Return(_) => {}
                Statement::Assignment(assign) => assign.process(state, Some(&mut sig))?,
                Statement::Call(_) => todo!(),
                Statement::VarDecl(v) => {
                    if v.typename.is_some() || v.size.is_some() {
                        todo!("Structs and arrays are not yet supported")
                    }
                    if let Some(initial) = v.initial {
                        // If it's got an initial value, we have to compile that before we add
                        // the name to scope, or else UB will ensue if it refers to itself:
                        initial.process(state, Some(&mut sig))?;
                        // But then add it to scope and assign:
                        sig.add_local(&v.name)?;
                        // We'll just whip up an lvalue real quick...
                        Lvalue::Name(v.name).process(state, Some(&mut sig))?;
                        sig.emit("storew"); // And store the initial value there
                    } else {
                        // Otherwise, just add it to scope and leave garbage in there:
                        sig.add_local(&v.name)?;
                    }
                }
                Statement::Conditional(_) | Statement::WhileLoop(_) | Statement::RepeatLoop(_) => {
                    todo!()
                }
            }
        }

        state.add_global(&self.name, |_| Variable::Label(sig.label.clone()))?;
        // This can't fail because if it were a dupe name, the one before it would fail
        state.functions.insert(self.name.clone(), sig);
        Ok(())
    }
}

/// Look up a name first in the local scope, and failing that in the global scope.
fn lookup<'a>(name: &str, global_scope: &'a Scope, local_scope: &'a Scope) -> Option<&'a Variable> {
    if let Some(var) = local_scope.get(name) {
        Some(var)
    } else if let Some(var) = global_scope.get(name) {
        Some(var)
    } else {
        None
    }
}

///////////////////////////////////////////////////////////

impl Compilable for Assignment {
    fn process(self, state: &mut State, sig: Option<&mut Signature>) -> Result<(), CompileError> {
        let Assignment { lvalue, rvalue } = self;
        let sig = sig.expect("Assignment outside function");

        match rvalue {
            Rvalue::Expr(rvalue) => {
                rvalue.process(state, Some(sig))?;
                lvalue.process(state, Some(sig))?;
                sig.emit("storew");
            }
            Rvalue::String(_) => todo!(),
        }
        Ok(())
    }
}

///////////////////////////////////////////////////////////

/// Evaluate an lvalue and leave its address on the stack (ready to be consumed by storew)
impl Compilable for Lvalue {
    fn process(self, state: &mut State, sig: Option<&mut Signature>) -> Result<(), CompileError> {
        let global_scope = &state.global_scope;
        let mut sig = sig.expect("lvalue outside a function");
        match self {
            Lvalue::ArrayRef(_) => todo!("Arrays are not implemented yet"),
            Lvalue::Name(name) => {
                if let Some(var) = lookup(&name, global_scope, &sig.local_scope) {
                    match var {
                        Variable::Literal(_) => {
                            Err(CompileError(0, 0, format!("Invalid lvalue {}", name)))
                        }
                        Variable::Label(label) => {
                            let label = label.clone();
                            sig.emit_arg("push", label);
                            Ok(())
                        }
                        Variable::Local(offset) => {
                            let offset = *offset;
                            sig.emit_arg("loadw", "frame");
                            if offset > 0 {
                                sig.emit_arg("add", offset);
                            }
                            Ok(())
                        }
                    }
                } else {
                    Err(CompileError(0, 0, format!("Unknown name {}", name)))
                }
            }
        }
    }
}

///////////////////////////////////////////////////////////

/// Evaluate an expression in the context of a local scope. The runtime brother to eval_const.
/// This recursively evaluates a Node and leaves its value on the stack.
impl Compilable for Node {
    fn process(self, state: &mut State, sig: Option<&mut Signature>) -> Result<(), CompileError> {
        let mut sig = sig.expect("Non-const expression outside a function");
        let global_scope = &state.global_scope;

        // First, a sanity check: try and eval_const this. If it's something incredibly basic
        // that just becomes an i32, then we don't need to do anything else:
        if let Ok(val) = eval_const(self.clone(), &state.global_scope) {
            sig.emit_arg("push", val);
            return Ok(());
        }

        // Okay, looks like we need something that's in scope. Let's recurse:
        match self {
            Node::Number(n) => {
                // Numbers are just pushed as literals
                sig.body.push(format!("push {}", n));
                Ok(())
            }
            Node::Call(_) | Node::ArrayRef(_) | Node::Address(_) => todo!(),
            Node::Name(name) => match lookup(&name, global_scope, &sig.local_scope) {
                // Names are treated differently depending on what they are
                Some(Variable::Literal(val)) => {
                    // Names of constants are just that number
                    sig.emit_arg("push", *val);
                    Ok(())
                }
                Some(Variable::Label(label)) => {
                    // Names pointing at labels are loaded (rvalue; for lvalues they aren't)
                    sig.emit_arg("loadw", label.clone());
                    Ok(())
                }
                Some(Variable::Local(offset)) => {
                    // Names of locals are added from the frame pointer
                    let offset = *offset;
                    sig.emit("loadw frame");
                    if offset > 0 {
                        sig.emit_arg("add", offset);
                        sig.emit("loadw");
                    }
                    Ok(())
                }
                None => Err(CompileError(0, 0, format!("Unknown name {}", name))),
            },
            Node::Expr(lhs, op, rhs) => {
                // Recurse on expressions, handling operators
                lhs.0.process(state, Some(&mut sig))?;
                rhs.0.process(state, Some(&mut sig))?;
                match op {
                    // Basic math
                    Operator::Add => sig.emit("add"),
                    Operator::Sub => sig.emit("sub"),
                    Operator::Mul => sig.emit("mul"),
                    Operator::Div => sig.emit("div"),
                    Operator::Mod => sig.emit("mod"),
                    Operator::And => {
                        // Vulcan "and" is bitwise, so we need to flag-ify both args to make it logical
                        sig.emit("gt 0");
                        sig.emit("swap");
                        sig.emit("gt 0");
                        sig.emit("and");
                    }
                    Operator::Or => {
                        // Same as and, flag-ify both args
                        sig.emit("gt 0");
                        sig.emit("swap");
                        sig.emit("gt 0");
                        sig.emit("or");
                    }
                    Operator::BitAnd => sig.emit("and"),
                    Operator::BitOr => sig.emit("or"),
                    Operator::Xor => sig.emit("xor"),
                    Operator::Lt => sig.emit("alt"),
                    Operator::Le => {
                        // LE and GE are the inverses of GT and LT (arithmetic versions)
                        sig.emit("agt");
                        sig.emit("not");
                    }
                    Operator::Gt => sig.emit("agt"),
                    Operator::Ge => {
                        sig.emit("alt");
                        sig.emit("not");
                    }
                    Operator::Eq => {
                        sig.emit("xor");
                        sig.emit("not");
                    }
                    Operator::Ne => sig.emit("xor"),
                    Operator::Lshift => sig.emit("lshift"),
                    Operator::Rshift => sig.emit("arshift"),
                }
                Ok(())
            }
            Node::Prefix(prefix, node) => {
                node.0.process(state, Some(sig))?;
                match prefix {
                    Prefix::Neg => {
                        // To arithmetically negate something, invert and increment (2s complement)
                        sig.emit("xor -1");
                        sig.emit("add 1");
                    }
                    Prefix::Not => sig.emit("not"),
                }
                Ok(())
            }
        }
    }
}

///////////////////////////////////////////////////////////

impl Compilable for Global {
    fn process(self, state: &mut State, _: Option<&mut Signature>) -> Result<(), CompileError> {
        if self.typename.is_some() || self.size.is_some() {
            todo!("Structs and arrays are not yet supported")
        }
        state.add_global(&self.name, |s| Variable::Label(s.gensym()))
    }
}

///////////////////////////////////////////////////////////

impl Compilable for Const {
    fn process(self, state: &mut State, _: Option<&mut Signature>) -> Result<(), CompileError> {
        if self.string.is_some() {
            todo!("Strings are not yet supported")
        }

        if let Some(expr) = self.value {
            let val = eval_const(expr, &state.global_scope)?;
            state.add_global(&self.name, |_| Variable::Literal(val))
        } else {
            unreachable!()
        }
    }
}

///////////////////////////////////////////////////////////

fn to_flag(val: bool) -> i32 {
    if val {
        1
    } else {
        0
    }
}

/// Evaluate a node in a static context, for const definitions and array sizes, that sort of thing.
pub fn eval_const(expr: Node, scope: &Scope) -> Result<i32, CompileError> {
    match expr {
        Node::Number(n) => Ok(n),
        Node::Address(_) | Node::ArrayRef(_) | Node::Call(_) => Err(CompileError(
            0,
            0,
            String::from("Constants must be statically defined"),
        )),

        Node::Name(n) => {
            if let Some(Variable::Literal(val)) = scope.get(&n) {
                Ok(*val)
            } else {
                Err(CompileError(0, 0, format!("Unknown const {}", n)))
            }
        }

        Node::Expr(lhs, op, rhs) => {
            let lhs = eval_const(lhs.into(), scope)?;
            let rhs = eval_const(rhs.into(), scope)?;
            match op {
                Operator::Add => Ok(lhs + rhs),
                Operator::Sub => Ok(lhs - rhs),
                Operator::Mul => Ok(lhs * rhs),
                Operator::Div => Ok(lhs / rhs),
                Operator::Mod => Ok(lhs % rhs),
                Operator::And => Ok(to_flag(lhs != 0 && rhs != 0)),
                Operator::Or => Ok(to_flag(lhs != 0 || rhs != 0)),
                Operator::BitAnd => Ok(lhs & rhs),
                Operator::BitOr => Ok(lhs | rhs),
                Operator::Xor => Ok(lhs ^ rhs),
                Operator::Lt => Ok(to_flag(lhs < rhs)),
                Operator::Le => Ok(to_flag(lhs <= rhs)),
                Operator::Gt => Ok(to_flag(lhs > rhs)),
                Operator::Ge => Ok(to_flag(lhs >= rhs)),
                Operator::Eq => Ok(to_flag(lhs == rhs)),
                Operator::Ne => Ok(to_flag(lhs != rhs)),
                Operator::Lshift => Ok(lhs << rhs),
                Operator::Rshift => Ok(lhs >> rhs),
            }
        }
        Node::Prefix(p, child) => {
            let val = eval_const(child.into(), scope)?;
            match p {
                Prefix::Neg => Ok(-val),
                Prefix::Not => {
                    if val == 0 {
                        Ok(1)
                    } else {
                        Ok(0)
                    }
                }
            }
        }
    }
}

#[cfg(test)]
mod test {
    use super::*;
    use crate::forge_parser::parse;

    #[test]
    fn test_eval_const() {
        let empty_scope = Scope::new();
        let to_node = |s| Node::parse(s).unwrap();

        // Basic arithmetic
        assert_eq!(eval_const(to_node("2 * 3 + 4"), &empty_scope), Ok(10));
        assert_eq!(eval_const(to_node("1 + -2"), &empty_scope), Ok(-1));
        assert_eq!(eval_const(to_node("1 << 3"), &empty_scope), Ok(8));

        // Names
        let scope: Scope = [
            ("foo".into(), Variable::Literal(10)),
            ("bar".into(), Variable::Literal(5)),
        ]
        .into();
        assert_eq!(eval_const(to_node("foo + 5"), &scope), Ok(15));
        assert_eq!(eval_const(to_node("bar * foo"), &scope), Ok(50));

        // Error
        assert_eq!(
            eval_const(to_node("nope"), &scope),
            Err(CompileError(0, 0, String::from("Unknown const nope")))
        );
    }

    #[test]
    fn test_const_decl() {
        let mut state = State::default();
        parse("const foo = 17 + 3;")
            .unwrap()
            .process(&mut state, None)
            .unwrap();
        assert_eq!(
            state.global_scope,
            [("foo".into(), Variable::Literal(20))].into()
        )
    }

    #[test]
    fn test_global_decl() {
        let mut state = State::default();
        parse("global a;")
            .unwrap()
            .process(&mut state, None)
            .unwrap();
        assert_eq!(
            state.global_scope,
            [("a".into(), Variable::Label("_gensym_1".into()))].into()
        )
    }

    #[test]
    fn test_name_collision() {
        let mut state = State::default();
        assert!(parse("const a = 7; global a;")
            .unwrap()
            .process(&mut state, None)
            .is_err());
    }

    fn body_as_string(sig: &Signature) -> String {
        sig.body.join("\n")
    }

    #[test]
    fn test_basic_fns() {
        let mut state = State::default();
        parse("fn blah(a, b) { b = 17 + a; }")
            .unwrap()
            .process(&mut state, None)
            .expect("Failed to compile");
        let body = body_as_string(state.functions.get("blah").unwrap());
        assert_eq!(
            body,
            vec![
                "push 17",     // Start calculating the rvalue, push the literal
                "loadw frame", // This is looking up the "a" arg, at frame + 0
                "add",         // 17 + a
                "loadw frame", // Calculate the lvalue
                "add 3",       // "b" arg is frame + 3
                "storew",      // Finally store
            ]
            .join("\n")
        )
    }

    #[test]
    fn test_var_decls() {
        let mut state = State::default();
        parse("fn blah() { var a; var b = 7; a = b * 2; }")
            .unwrap()
            .process(&mut state, None)
            .expect("Failed to compile");
        let body = body_as_string(state.functions.get("blah").unwrap());
        assert_eq!(
            body,
            vec![
                "push 7",      // Start calculating the rvalue, push the literal
                "loadw frame", // "b" is the second local var at frame - 3
                "add 3",
                "storew",      // Do the initialization
                "loadw frame", // Now we're evaluating b * 2
                "add 3",       // b is at frame + 3...
                "loadw",       // load it to the stack
                "push 2",
                "mul",         // b * 2 evaluated
                "loadw frame", // Loading "a" as an lvalue
                "storew",      // doing the assignment
            ]
            .join("\n")
        )
    }
}