//! 构造共享 VM 调用序列与同帧循环驱动的字节码。
//! 用托管函数串联步骤，避免为每个布局节点反复创建独立解释器调用；重复驱动会保留同一个执行帧和预算域。
//!
//! Validated call sequences share the current VM budget and explicit frames.
use endor_core::{
    CodeLayout, EndorCode, EndorCodeObject, Instruction, NativeContext, ObjectRef, Opcode, Value,
    VmError,
};

/// 将零参数调用列表编成一个托管驱动，按输入顺序执行，返回最后一次调用的值。
/// 空列表返回 Nil。所有调用都在调用者的 VM 预算内运行，避免宿主逐个调用时重置预算。
/// name 用于诊断位置；desktop 用来给生成函数挂接桌面 Function 类型。
pub fn build(
    cx: &mut NativeContext<'_>,
    calls: Vec<ObjectRef>,
    name: &'static str,
    desktop: ObjectRef,
) -> Result<ObjectRef, VmError> {
    // 每个函数放入常量池。LoadConst/PushNull/Call 组成零参数调用，
    // 中间结果由 PopTop 丢弃；最后结果留在栈顶交给 ReturnValue。
    let mut instructions = Vec::with_capacity(calls.len() * 4 + 2);
    let mut constants: Vec<Value> = calls.into_iter().map(Value::Object).collect();
    for index in 0..constants.len() {
        instructions.push(Instruction::new(Opcode::LoadConst, index as u32));
        instructions.push(Instruction::simple(Opcode::PushNull));
        instructions.push(Instruction::new(Opcode::Call, 0));
        if index + 1 != constants.len() {
            instructions.push(Instruction::simple(Opcode::PopTop));
        }
    }
    // 没有调用时也必须给 ReturnValue 提供合法栈顶，不能返回一个空栈。
    if constants.is_empty() {
        constants.push(Value::Nil);
        instructions.push(Instruction::new(Opcode::LoadConst, 0));
    }
    instructions.push(Instruction::simple(Opcode::ReturnValue));
    let code = EndorCode::from_instructions(&instructions, constants, CodeLayout::default())?
        .with_location(name, "<tessera>", 1);
    let code = cx.allocate(EndorCodeObject::new(code))?;
    let globals = cx.new_dict()?;
    let driver = cx.new_function(name, code, globals)?;
    crate::ui_bridge::attach_desktop_type(cx, driver, "Function", desktop)?;
    Ok(driver)
}

/// A fixed-size driver. Each callback returns whether another occurrence is
/// needed; delegated source calls and all iterations share this VM budget.
/// 构造重复调用同一托管函数的驱动，保持共享参数与引用生命周期。
pub fn repeat_call(
    cx: &mut NativeContext<'_>,
    callback: ObjectRef,
    name: &'static str,
    desktop: ObjectRef,
) -> Result<ObjectRef, VmError> {
    // 回调返回 Bool；repeat_while 消费这个条件并决定是否跳回本帧开头。
    // 退出后显式返回 Nil，回调的继续条件不会泄露为驱动返回值。
    let mut instructions = vec![
        Instruction::new(Opcode::LoadConst, 0),
        Instruction::simple(Opcode::PushNull),
        Instruction::new(Opcode::Call, 0),
    ];
    repeat_while(&mut instructions)?;
    instructions.extend([
        Instruction::new(Opcode::LoadConst, 1),
        Instruction::simple(Opcode::ReturnValue),
    ]);
    driver(
        cx,
        instructions,
        vec![Value::Object(callback), Value::Nil],
        name,
        desktop,
    )
}

/// Prepare only the current occurrence, collect its result, then continue the
/// outer driver. The helper frame and its managed captures die before the next.
/// 将一步调用结果收集到驱动状态，再安排下一步调用。
pub fn collect_and_continue(
    cx: &mut NativeContext<'_>,
    collector: ObjectRef,
    function: ObjectRef,
    arguments: Vec<Value>,
    name: &'static str,
    desktop: ObjectRef,
) -> Result<ObjectRef, VmError> {
    // 常量布局固定为 collector、function、各实参、true。
    // 先把 collector 的调用前缀压栈，再执行 function，使其返回值直接成为 collector 的唯一实参。
    let count = arguments.len();
    let mut constants = vec![Value::Object(collector), Value::Object(function)];
    constants.extend(arguments);
    constants.push(Value::Bool(true));
    let mut instructions = vec![
        Instruction::new(Opcode::LoadConst, 0),
        Instruction::simple(Opcode::PushNull),
        Instruction::new(Opcode::LoadConst, 1),
        Instruction::simple(Opcode::PushNull),
    ];
    for index in 0..count {
        instructions.push(Instruction::new(Opcode::LoadConst, index as u32 + 2));
    }
    // 收集器返回值不参与外层调度；丢弃后返回 true，通知重复驱动继续取下一项。
    instructions.extend([
        Instruction::new(Opcode::Call, count as u32),
        Instruction::new(Opcode::Call, 1),
        Instruction::simple(Opcode::PopTop),
        Instruction::new(Opcode::LoadConst, count as u32 + 2),
        Instruction::simple(Opcode::ReturnValue),
    ]);
    driver(cx, instructions, constants, name, desktop)
}

/// 建立承载整个步骤序列的托管函数，使续体可以在同一个 VM 帧中恢复。
fn driver(
    cx: &mut NativeContext<'_>,
    instructions: Vec<Instruction>,
    constants: Vec<Value>,
    name: &'static str,
    desktop: ObjectRef,
) -> Result<ObjectRef, VmError> {
    let code = EndorCode::from_instructions(&instructions, constants, CodeLayout::default())?
        .with_location(name, "<tessera>", 1);
    let code = cx.allocate(EndorCodeObject::new(code))?;
    let globals = cx.new_dict()?;
    let function = cx.new_function(name, code, globals)?;
    crate::ui_bridge::attach_desktop_type(cx, function, "Function", desktop)?;
    Ok(function)
}

/// Consume the condition on the stack: true repeats this same frame from zero,
/// false falls through. Operand widths include EXTENDED_ARG code units.
/// 构造由继续条件决定是否再执行一轮的同帧循环。
pub fn repeat_while(instructions: &mut Vec<Instruction>) -> Result<(), VmError> {
    // 跳转单位是编码后的双字节 code unit，不是 Instruction 数量。
    // 大操作数会生成 EXTENDED_ARG，因此必须编码后计算实际长度。
    let body = Instruction::encode_all(instructions)?.len() / 2;
    let mut back = Instruction::new(Opcode::JumpBackward, 0);
    // 回跳距离包含自身宽度；而距离变大又可能扩大自身编码宽度。
    // 迭代到宽度/距离一致后，false 跳过回跳指令，true 回到整个驱动的起点。
    loop {
        let units = Instruction::encode_all(&[back])?.len() / 2;
        let distance = u32::try_from(body + 1 + units)
            .map_err(|_| VmError::ExecutionLimit("layout bytecode"))?;
        if back.argument == distance {
            instructions.push(Instruction::new(Opcode::PopJumpIfFalse, units as u32));
            instructions.push(back);
            return Ok(());
        }
        back.argument = distance;
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use endor_core::{ExecutionLimits, Vm};
    #[test]
    /// 生成超过单字节跳距的循环，false 条件正常返回，true 条件只耗尽指令预算而非一帧预算；释放后对象数归零。
    fn repeated_bytecode_handles_extended_operands_and_keeps_one_frame() {
        // 300 个 Nop 强制跨越单字节跳转距离：false 应正常退出；true 应耗尽指令预算，
        // 而非耗尽帧预算，从而验证循环确实复用同一个执行帧。
        for condition in [false, true] {
            let mut instructions = vec![Instruction::simple(Opcode::Nop); 300];
            instructions.push(Instruction::new(Opcode::LoadConst, 0));
            repeat_while(&mut instructions).unwrap();
            instructions.extend([
                Instruction::new(Opcode::LoadConst, 1),
                Instruction::simple(Opcode::ReturnValue),
            ]);
            let code = EndorCode::from_instructions(
                &instructions,
                vec![Value::Bool(condition), Value::Nil],
                CodeLayout::default(),
            )
            .unwrap();
            let mut vm = Vm::new().unwrap();
            let code = vm.new_code(code).unwrap();
            let globals = vm.new_dict().unwrap();
            let result = vm.execute_with_limits(
                code,
                globals,
                ExecutionLimits {
                    max_frames: 1,
                    max_instructions: 1000,
                },
            );
            if condition {
                assert!(matches!(
                    result,
                    Err(VmError::ExecutionLimit("instructions"))
                ));
            } else {
                assert_eq!(result.unwrap(), Value::Nil);
            }
            vm.release(code).unwrap();
            vm.release(globals).unwrap();
            // 释放代码与全局字典后执行循环回收，确认生成的常量/函数没有遗留托管对象。
            vm.collect_cycles();
            assert_eq!(vm.object_count(), 0);
        }
    }
}