//! This module implements a compiler for compiling the rnix AST
//! representation to Tvix bytecode.

use crate::chunk::Chunk;
use crate::errors::EvalResult;
use crate::opcode::OpCode;
use crate::value::{NixString, Value};

use rnix;
use rnix::types::{EntryHolder, TokenWrapper, TypedNode, Wrapper};

struct Compiler {
    chunk: Chunk,
}

impl Compiler {
    fn compile(&mut self, node: rnix::SyntaxNode) -> EvalResult<()> {
        match node.kind() {
            // Root of a file contains no content, it's just a marker
            // type.
            rnix::SyntaxKind::NODE_ROOT => self.compile(node.first_child().expect("TODO")),

            // Literals contain a single token comprising of the
            // literal itself.
            rnix::SyntaxKind::NODE_LITERAL => {
                let value = rnix::types::Value::cast(node).unwrap();
                self.compile_literal(value.to_value().expect("TODO"))
            }

            rnix::SyntaxKind::NODE_BIN_OP => {
                let op = rnix::types::BinOp::cast(node).expect("TODO (should not be possible)");
                self.compile_binop(op)
            }

            rnix::SyntaxKind::NODE_UNARY_OP => {
                let op = rnix::types::UnaryOp::cast(node).expect("TODO: (should not be possible)");
                self.compile_unary_op(op)
            }

            rnix::SyntaxKind::NODE_PAREN => {
                let node = rnix::types::Paren::cast(node).unwrap();
                self.compile(node.inner().unwrap())
            }

            rnix::SyntaxKind::NODE_IDENT => {
                let node = rnix::types::Ident::cast(node).unwrap();
                self.compile_ident(node)
            }

            rnix::SyntaxKind::NODE_ATTR_SET => {
                let node = rnix::types::AttrSet::cast(node).unwrap();
                self.compile_attr_set(node)
            }

            rnix::SyntaxKind::NODE_LIST => {
                let node = rnix::types::List::cast(node).unwrap();
                self.compile_list(node)
            }

            kind => {
                println!("visiting unsupported node: {:?}", kind);
                Ok(())
            }
        }
    }

    fn compile_literal(&mut self, value: rnix::value::Value) -> EvalResult<()> {
        match value {
            rnix::NixValue::Float(f) => {
                let idx = self.chunk.add_constant(Value::Float(f));
                self.chunk.add_op(OpCode::OpConstant(idx));
                Ok(())
            }

            rnix::NixValue::Integer(i) => {
                let idx = self.chunk.add_constant(Value::Integer(i));
                self.chunk.add_op(OpCode::OpConstant(idx));
                Ok(())
            }

            rnix::NixValue::String(_) => todo!(),
            rnix::NixValue::Path(_, _) => todo!(),
        }
    }

    fn compile_binop(&mut self, op: rnix::types::BinOp) -> EvalResult<()> {
        self.compile(op.lhs().unwrap())?;
        self.compile(op.rhs().unwrap())?;

        use rnix::types::BinOpKind;

        let opcode = match op.operator().unwrap() {
            BinOpKind::Add => OpCode::OpAdd,
            BinOpKind::Sub => OpCode::OpSub,
            BinOpKind::Mul => OpCode::OpMul,
            BinOpKind::Div => OpCode::OpDiv,
            BinOpKind::Equal => OpCode::OpEqual,
            _ => todo!(),
        };

        self.chunk.add_op(opcode);
        Ok(())
    }

    fn compile_unary_op(&mut self, op: rnix::types::UnaryOp) -> EvalResult<()> {
        self.compile(op.value().unwrap())?;

        use rnix::types::UnaryOpKind;
        let opcode = match op.operator() {
            UnaryOpKind::Invert => OpCode::OpInvert,
            UnaryOpKind::Negate => OpCode::OpNegate,
        };

        self.chunk.add_op(opcode);
        Ok(())
    }

    fn compile_ident(&mut self, node: rnix::types::Ident) -> EvalResult<()> {
        match node.as_str() {
            // TODO(tazjin): Nix technically allows code like
            //
            //   let null = 1; in null
            //   => 1
            //
            // which we do *not* want to check at runtime. Once
            // scoping is introduced, the compiler should carry some
            // optimised information about any "weird" stuff that's
            // happened to the scope (such as overrides of these
            // literals, or builtins).
            "true" => self.chunk.add_op(OpCode::OpTrue),
            "false" => self.chunk.add_op(OpCode::OpFalse),
            "null" => self.chunk.add_op(OpCode::OpNull),

            _ => todo!("identifier access"),
        };

        Ok(())
    }

    // Compile attribute set literals into equivalent bytecode.
    //
    // This is complicated by a number of features specific to Nix
    // attribute sets, most importantly:
    //
    // 1. Keys can be dynamically constructed through interpolation.
    // 2. Keys can refer to nested attribute sets.
    // 3. Attribute sets can (optionally) be recursive.
    fn compile_attr_set(&mut self, node: rnix::types::AttrSet) -> EvalResult<()> {
        let mut count = 0;

        for kv in node.entries() {
            count += 1;

            // Because attribute set literals can contain nested keys,
            // there is potentially more than one key fragment. If
            // this is the case, a special operation to construct a
            // runtime value representing the attribute path is
            // emitted.
            let mut key_count = 0;
            for fragment in kv.key().unwrap().path() {
                key_count += 1;

                match fragment.kind() {
                    rnix::SyntaxKind::NODE_IDENT => {
                        let ident = rnix::types::Ident::cast(fragment).unwrap();

                        // TODO(tazjin): intern!
                        let idx = self
                            .chunk
                            .add_constant(Value::String(NixString(ident.as_str().to_string())));
                        self.chunk.add_op(OpCode::OpConstant(idx));
                    }

                    // For all other expression types, we simply
                    // compile them as normal. The operation should
                    // result in a string value, which is checked at
                    // runtime on construction.
                    _ => self.compile(fragment)?,
                }
            }

            // We're done with the key if there was only one fragment,
            // otherwise we need to emit an instruction to construct
            // the attribute path.
            if key_count > 1 {
                todo!("emit OpAttrPath(n) instruction")
            }

            // The value is just compiled as normal so that its
            // resulting value is on the stack when the attribute set
            // is constructed at runtime.
            self.compile(kv.value().unwrap())?;
        }

        self.chunk.add_op(OpCode::OpAttrs(count));
        Ok(())
    }

    // Compile list literals into equivalent bytecode. List
    // construction is fairly simple, composing of pushing code for
    // each literal element and an instruction with the element count.
    //
    // The VM, after evaluating the code for each element, simply
    // constructs the list from the given number of elements.
    fn compile_list(&mut self, node: rnix::types::List) -> EvalResult<()> {
        let mut count = 0;

        for item in node.items() {
            count += 1;
            self.compile(item)?;
        }

        self.chunk.add_op(OpCode::OpList(count));
        Ok(())
    }
}

pub fn compile(ast: rnix::AST) -> EvalResult<Chunk> {
    let mut c = Compiler {
        chunk: Chunk::default(),
    };

    c.compile(ast.node())?;

    Ok(c.chunk)
}