//! # Bytecode Emitter
//!
//! This module is responsible for the final stage of the compilation process:
//! converting the Intermediate Representation (IR) into the binary Prometeu ByteCode (PBC) format.
//!
//! It performs two main tasks:
//! 1. **Instruction Lowering**: Translates `ir_vm::Instruction` into `prometeu_bytecode::asm::Asm` ops.
//! 2. **DebugSymbol Mapping**: Associates bytecode offsets (Program Counter) with source code locations.

use crate::ir_core::ConstantValue;
use crate::ir_vm;
use crate::ir_vm::instr::InstrKind;
use anyhow::{anyhow, Result};
use prometeu_bytecode::abi::SourceSpan;
use prometeu_bytecode::asm::{update_pc_by_operand, Asm, Operand};
use prometeu_bytecode::opcode::OpCode;
use prometeu_bytecode::{BytecodeModule, ConstantPoolEntry, DebugInfo, FunctionMeta};

/// The final output of the code generation phase.
pub struct EmitResult {
    /// The serialized binary data of the PBC file.
    pub rom: Vec<u8>,
}

pub struct EmitFragments {
    pub const_pool: Vec<ConstantPoolEntry>,
    pub functions: Vec<FunctionMeta>,
    pub code: Vec<u8>,
    pub debug_info: Option<DebugInfo>,
    pub unresolved_labels: std::collections::HashMap<String, Vec<u32>>,
}

/// Entry point for emitting a bytecode module from the IR.
pub fn emit_module(module: &ir_vm::Module) -> Result<EmitResult> {
    let fragments = emit_fragments(module)?;
    
    let exports: Vec<_> = module.functions.iter().enumerate().map(|(i, f)| {
        prometeu_bytecode::Export {
            symbol: f.name.clone(),
            func_idx: i as u32,
        }
    }).collect();

    let bytecode_module = BytecodeModule {
        version: 0,
        const_pool: fragments.const_pool,
        functions: fragments.functions,
        code: fragments.code,
        debug_info: fragments.debug_info,
        exports,
    };

    Ok(EmitResult {
        rom: bytecode_module.serialize(),
    })
}

pub fn emit_fragments(module: &ir_vm::Module) -> Result<EmitFragments> {
    let mut emitter = BytecodeEmitter::new();
    
    let mut mapped_const_ids = Vec::with_capacity(module.const_pool.constants.len());
    for val in &module.const_pool.constants {
        mapped_const_ids.push(emitter.add_ir_constant(val));
    }

    let mut asm_instrs = Vec::new();
    let mut ir_instr_map = Vec::new();
    let function_ranges = emitter.lower_instrs(module, &mut asm_instrs, &mut ir_instr_map, &mapped_const_ids)?;

    let pcs = BytecodeEmitter::calculate_pcs(&asm_instrs);
    let assemble_res = prometeu_bytecode::asm::assemble_with_unresolved(&asm_instrs).map_err(|e| anyhow!(e))?;
    let bytecode = assemble_res.code;

    let mut functions = Vec::new();
    let mut function_names = Vec::new();
    for (i, function) in module.functions.iter().enumerate() {
        let (start_idx, end_idx) = function_ranges[i];
        let start_pc = pcs[start_idx];
        let end_pc = if end_idx < pcs.len() { pcs[end_idx] } else { bytecode.len() as u32 };
        
        functions.push(FunctionMeta {
            code_offset: start_pc,
            code_len: end_pc - start_pc,
            param_slots: function.param_slots,
            local_slots: function.local_slots,
            return_slots: function.return_slots,
            max_stack_slots: 0, // Will be filled by verifier
        });
        function_names.push((i as u32, function.name.clone()));
    }

    let mut pc_to_span = Vec::new();
    for (i, instr_opt) in ir_instr_map.iter().enumerate() {
        let current_pc = pcs[i];
        if let Some(instr) = instr_opt {
            if let Some(span) = &instr.span {
                pc_to_span.push((current_pc, SourceSpan {
                    file_id: span.file_id as u32,
                    start: span.start,
                    end: span.end,
                }));
            }
        }
    }
    pc_to_span.sort_by_key(|(pc, _)| *pc);
    pc_to_span.dedup_by_key(|(pc, _)| *pc);

    Ok(EmitFragments {
        const_pool: emitter.constant_pool,
        functions,
        code: bytecode,
        debug_info: Some(DebugInfo {
            pc_to_span,
            function_names,
        }),
        unresolved_labels: assemble_res.unresolved_labels,
    })
}

/// Internal helper for managing the bytecode emission state.
struct BytecodeEmitter {
    /// Stores constant values (like strings) that are referenced by instructions.
    constant_pool: Vec<ConstantPoolEntry>,
}

impl BytecodeEmitter {
    fn new() -> Self {
        Self {
            // Index 0 is traditionally reserved for Null in many VMs
            constant_pool: vec![ConstantPoolEntry::Null],
        }
    }

    /// Adds a value to the constant pool if it doesn't exist, returning its unique index.
    fn add_constant(&mut self, entry: ConstantPoolEntry) -> u32 {
        if let Some(pos) = self.constant_pool.iter().position(|e| e == &entry) {
            pos as u32
        } else {
            let id = self.constant_pool.len() as u32;
            self.constant_pool.push(entry);
            id
        }
    }

    fn add_ir_constant(&mut self, val: &ConstantValue) -> u32 {
        let entry = match val {
            ConstantValue::Int(v) => ConstantPoolEntry::Int64(*v),
            ConstantValue::Float(v) => ConstantPoolEntry::Float64(*v),
            ConstantValue::String(s) => ConstantPoolEntry::String(s.clone()),
        };
        self.add_constant(entry)
    }

    fn lower_instrs<'b>(
        &mut self, 
        module: &'b ir_vm::Module, 
        asm_instrs: &mut Vec<Asm>, 
        ir_instr_map: &mut Vec<Option<&'b ir_vm::Instruction>>,
        mapped_const_ids: &[u32]
    ) -> Result<Vec<(usize, usize)>> {
        let mut func_names = std::collections::HashMap::new();
        for func in &module.functions {
            func_names.insert(func.id, func.name.clone());
        }

        let mut ranges = Vec::new();

        for function in &module.functions {
            let start_idx = asm_instrs.len();
            // Each function starts with a label for its entry point.
            asm_instrs.push(Asm::Label(function.name.clone()));
            ir_instr_map.push(None);
            
            for instr in &function.body {
                let op_start_idx = asm_instrs.len();
                
                // Translate each IR instruction to its equivalent Bytecode OpCode.
                match &instr.kind {
                    InstrKind::Nop => asm_instrs.push(Asm::Op(OpCode::Nop, vec![])),
                    InstrKind::Halt => asm_instrs.push(Asm::Op(OpCode::Halt, vec![])),
                    InstrKind::PushConst(id) => {
                        let mapped_id = mapped_const_ids[id.0 as usize];
                        asm_instrs.push(Asm::Op(OpCode::PushConst, vec![Operand::U32(mapped_id)]));
                    }
                    InstrKind::PushBounded(val) => {
                        asm_instrs.push(Asm::Op(OpCode::PushBounded, vec![Operand::U32(*val)]));
                    }
                    InstrKind::PushBool(v) => {
                        asm_instrs.push(Asm::Op(OpCode::PushBool, vec![Operand::Bool(*v)]));
                    }
                    InstrKind::PushNull => {
                        asm_instrs.push(Asm::Op(OpCode::PushConst, vec![Operand::U32(0)]));
                    }
                    InstrKind::Pop => asm_instrs.push(Asm::Op(OpCode::Pop, vec![])),
                    InstrKind::Dup => asm_instrs.push(Asm::Op(OpCode::Dup, vec![])),
                    InstrKind::Swap => asm_instrs.push(Asm::Op(OpCode::Swap, vec![])),
                    InstrKind::Add => asm_instrs.push(Asm::Op(OpCode::Add, vec![])),
                    InstrKind::Sub => asm_instrs.push(Asm::Op(OpCode::Sub, vec![])),
                    InstrKind::Mul => asm_instrs.push(Asm::Op(OpCode::Mul, vec![])),
                    InstrKind::Div => asm_instrs.push(Asm::Op(OpCode::Div, vec![])),
                    InstrKind::Neg => asm_instrs.push(Asm::Op(OpCode::Neg, vec![])),
                    InstrKind::Eq => asm_instrs.push(Asm::Op(OpCode::Eq, vec![])),
                    InstrKind::Neq => asm_instrs.push(Asm::Op(OpCode::Neq, vec![])),
                    InstrKind::Lt => asm_instrs.push(Asm::Op(OpCode::Lt, vec![])),
                    InstrKind::Gt => asm_instrs.push(Asm::Op(OpCode::Gt, vec![])),
                    InstrKind::Lte => asm_instrs.push(Asm::Op(OpCode::Lte, vec![])),
                    InstrKind::Gte => asm_instrs.push(Asm::Op(OpCode::Gte, vec![])),
                    InstrKind::And => asm_instrs.push(Asm::Op(OpCode::And, vec![])),
                    InstrKind::Or => asm_instrs.push(Asm::Op(OpCode::Or, vec![])),
                    InstrKind::Not => asm_instrs.push(Asm::Op(OpCode::Not, vec![])),
                    InstrKind::BitAnd => asm_instrs.push(Asm::Op(OpCode::BitAnd, vec![])),
                    InstrKind::BitOr => asm_instrs.push(Asm::Op(OpCode::BitOr, vec![])),
                    InstrKind::BitXor => asm_instrs.push(Asm::Op(OpCode::BitXor, vec![])),
                    InstrKind::Shl => asm_instrs.push(Asm::Op(OpCode::Shl, vec![])),
                    InstrKind::Shr => asm_instrs.push(Asm::Op(OpCode::Shr, vec![])),
                    InstrKind::LocalLoad { slot } => {
                        asm_instrs.push(Asm::Op(OpCode::GetLocal, vec![Operand::U32(*slot)]));
                    }
                    InstrKind::LocalStore { slot } => {
                        asm_instrs.push(Asm::Op(OpCode::SetLocal, vec![Operand::U32(*slot)]));
                    }
                    InstrKind::GetGlobal(slot) => {
                        asm_instrs.push(Asm::Op(OpCode::GetGlobal, vec![Operand::U32(*slot)]));
                    }
                    InstrKind::SetGlobal(slot) => {
                        asm_instrs.push(Asm::Op(OpCode::SetGlobal, vec![Operand::U32(*slot)]));
                    }
                    InstrKind::Jmp(label) => {
                        asm_instrs.push(Asm::Op(OpCode::Jmp, vec![Operand::RelLabel(label.0.clone(), function.name.clone())]));
                    }
                    InstrKind::JmpIfFalse(label) => {
                        asm_instrs.push(Asm::Op(OpCode::JmpIfFalse, vec![Operand::RelLabel(label.0.clone(), function.name.clone())]));
                    }
                    InstrKind::Label(label) => {
                        asm_instrs.push(Asm::Label(label.0.clone()));
                    }
                    InstrKind::Call { func_id, .. } => {
                        let name = func_names.get(func_id).ok_or_else(|| anyhow!("Undefined function ID: {:?}", func_id))?;
                        asm_instrs.push(Asm::Op(OpCode::Call, vec![Operand::Label(name.clone())]));
                    }
                    InstrKind::ImportCall { dep_alias, module_path, symbol_name, .. } => {
                        let label = format!("@{}::{}:{}", dep_alias, module_path, symbol_name);
                        asm_instrs.push(Asm::Op(OpCode::Call, vec![Operand::Label(label)]));
                    }
                    InstrKind::Ret => asm_instrs.push(Asm::Op(OpCode::Ret, vec![])),
                    InstrKind::Syscall(id) => {
                        asm_instrs.push(Asm::Op(OpCode::Syscall, vec![Operand::U32(*id)]));
                    }
                    InstrKind::FrameSync => asm_instrs.push(Asm::Op(OpCode::FrameSync, vec![])),
                    InstrKind::Alloc { type_id, slots } => {
                        asm_instrs.push(Asm::Op(OpCode::Alloc, vec![Operand::U32(type_id.0), Operand::U32(*slots)]));
                    }
                    InstrKind::GateLoad { offset } => {
                        asm_instrs.push(Asm::Op(OpCode::GateLoad, vec![Operand::U32(*offset)]));
                    }
                    InstrKind::GateStore { offset } => {
                        asm_instrs.push(Asm::Op(OpCode::GateStore, vec![Operand::U32(*offset)]));
                    }
                    InstrKind::GateBeginPeek => asm_instrs.push(Asm::Op(OpCode::GateBeginPeek, vec![])),
                    InstrKind::GateEndPeek => asm_instrs.push(Asm::Op(OpCode::GateEndPeek, vec![])),
                    InstrKind::GateBeginBorrow => asm_instrs.push(Asm::Op(OpCode::GateBeginBorrow, vec![])),
                    InstrKind::GateEndBorrow => asm_instrs.push(Asm::Op(OpCode::GateEndBorrow, vec![])),
                    InstrKind::GateBeginMutate => asm_instrs.push(Asm::Op(OpCode::GateBeginMutate, vec![])),
                    InstrKind::GateEndMutate => asm_instrs.push(Asm::Op(OpCode::GateEndMutate, vec![])),
                    InstrKind::GateRetain => asm_instrs.push(Asm::Op(OpCode::GateRetain, vec![])),
                    InstrKind::GateRelease => asm_instrs.push(Asm::Op(OpCode::GateRelease, vec![])),
                }
                
                let op_end_idx = asm_instrs.len();
                for _ in op_start_idx..op_end_idx {
                    ir_instr_map.push(Some(instr));
                }
            }
            let end_idx = asm_instrs.len();
            ranges.push((start_idx, end_idx));
        }
        Ok(ranges)
    }

    fn calculate_pcs(asm_instrs: &[Asm]) -> Vec<u32> {
        let mut pcs = Vec::with_capacity(asm_instrs.len());
        let mut current_pc = 0u32;
        for instr in asm_instrs {
            pcs.push(current_pc);
            match instr {
                Asm::Label(_) => {}
                Asm::Op(_opcode, operands) => {
                    current_pc += 2;
                    current_pc = update_pc_by_operand(current_pc, operands);
                }
            }
        }
        pcs
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::ir_core::const_pool::ConstantValue;
    use crate::ir_core::ids::FunctionId;
    use crate::ir_vm::instr::{InstrKind, Instruction};
    use crate::ir_vm::module::{Function, Module};
    use crate::ir_vm::types::Type;
    use prometeu_bytecode::{BytecodeLoader, ConstantPoolEntry};

    #[test]
    fn test_emit_module_with_const_pool() {
        let mut module = Module::new("test".to_string());
        
        let id_int = module.const_pool.insert(ConstantValue::Int(12345));
        let id_str = module.const_pool.insert(ConstantValue::String("hello".to_string()));
        
        let function = Function {
            id: FunctionId(0),
            name: "main".to_string(),
            params: vec![],
            return_type: Type::Void,
            body: vec![
                Instruction::new(InstrKind::PushConst(ir_vm::ConstId(id_int.0)), None),
                Instruction::new(InstrKind::PushConst(ir_vm::ConstId(id_str.0)), None),
                Instruction::new(InstrKind::Ret, None),
            ],
            param_slots: 0,
            local_slots: 0,
            return_slots: 0,
        };
        
        module.functions.push(function);
        
        let result = emit_module(&module).expect("Failed to emit module");
        
        let pbc = BytecodeLoader::load(&result.rom).expect("Failed to parse emitted PBC");
        
        assert_eq!(pbc.const_pool.len(), 3);
        assert_eq!(pbc.const_pool[0], ConstantPoolEntry::Null);
        assert_eq!(pbc.const_pool[1], ConstantPoolEntry::Int64(12345));
        assert_eq!(pbc.const_pool[2], ConstantPoolEntry::String("hello".to_string()));
    }
}