/// Represents a single instruction in the Prometeu Virtual Machine.
///
/// Each OpCode is encoded as a 16-bit unsigned integer (u16) in the bytecode.
/// The PVM is a stack-based machine, meaning most instructions take their
/// operands from the top of the stack and push their results back onto it.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[repr(u16)]
pub enum OpCode {
    // --- 6.1 Execution Control ---
    /// No operation. Does nothing for 1 cycle.
    Nop = 0x00,
    /// Stops the Virtual Machine execution immediately.
    Halt = 0x01,
    /// Unconditional jump to a specific PC (Program Counter) address.
    /// Operand: addr (u32)
    Jmp = 0x02,
    /// Jumps to `addr` if the value at the top of the stack is `false`.
    /// Operand: addr (u32)
    /// Stack: [bool] -> []
    JmpIfFalse = 0x03,
    /// Jumps to `addr` if the value at the top of the stack is `true`.
    /// Operand: addr (u32)
    /// Stack: [bool] -> []
    JmpIfTrue = 0x04,
    /// Triggers a software breakpoint. Used for debugging.
    Trap = 0x05,

    // --- 6.2 Stack Manipulation ---
    /// Loads a constant from the Constant Pool into the stack.
    /// Operand: index (u32)
    /// Stack: [] -> [value]
    PushConst = 0x10,
    /// Removes the top value from the stack.
    /// Stack: [val] -> []
    Pop = 0x11,
    /// Duplicates the top value of the stack.
    /// Stack: [val] -> [val, val]
    Dup = 0x12,
    /// Swaps the two top values of the stack.
    /// Stack: [a, b] -> [b, a]
    Swap = 0x13,
    /// Pushes a 64-bit integer literal onto the stack.
    /// Operand: value (i64)
    PushI64 = 0x14,
    /// Pushes a 64-bit float literal onto the stack.
    /// Operand: value (f64)
    PushF64 = 0x15,
    /// Pushes a boolean literal onto the stack (0=false, 1=true).
    /// Operand: value (u8)
    PushBool = 0x16,
    /// Pushes a 32-bit integer literal onto the stack.
    /// Operand: value (i32)
    PushI32 = 0x17,
    /// Removes `n` values from the stack.
    /// Operand: n (u32)
    PopN = 0x18,
    // --- 6.3 Arithmetic ---
    /// Adds the two top values (a + b).
    /// Stack: [a, b] -> [result]
    Add = 0x20,
    /// Subtracts the top value from the second one (a - b).
    /// Stack: [a, b] -> [result]
    Sub = 0x21,
    /// Multiplies the two top values (a * b).
    /// Stack: [a, b] -> [result]
    Mul = 0x22,
    /// Divides the second top value by the top one (a / b).
    /// Stack: [a, b] -> [result]
    Div = 0x23,
    /// Remainder of the division of the second top value by the top one (a % b).
    /// Stack: [a, b] -> [result]
    Mod = 0x24,
    // --- 6.4 Comparison and Logic ---
    /// Checks if a equals b.
    /// Stack: [a, b] -> [bool]
    Eq = 0x30,
    /// Checks if a is not equal to b.
    /// Stack: [a, b] -> [bool]
    Neq = 0x31,
    /// Checks if a is less than b.
    /// Stack: [a, b] -> [bool]
    Lt = 0x32,
    /// Checks if a is greater than b.
    /// Stack: [a, b] -> [bool]
    Gt = 0x33,
    /// Logical AND.
    /// Stack: [bool, bool] -> [bool]
    And = 0x34,
    /// Logical OR.
    /// Stack: [bool, bool] -> [bool]
    Or = 0x35,
    /// Logical NOT.
    /// Stack: [bool] -> [bool]
    Not = 0x36,
    /// Bitwise AND.
    /// Stack: [int, int] -> [int]
    BitAnd = 0x37,
    /// Bitwise OR.
    /// Stack: [int, int] -> [int]
    BitOr = 0x38,
    /// Bitwise XOR.
    /// Stack: [int, int] -> [int]
    BitXor = 0x39,
    /// Bitwise Shift Left.
    /// Stack: [int, count] -> [int]
    Shl = 0x3A,
    /// Bitwise Shift Right.
    /// Stack: [int, count] -> [int]
    Shr = 0x3B,
    /// Checks if a is less than or equal to b.
    /// Stack: [a, b] -> [bool]
    Lte = 0x3C,
    /// Checks if a is greater than or equal to b.
    /// Stack: [a, b] -> [bool]
    Gte = 0x3D,
    /// Negates a number (-a).
    /// Stack: [num] -> [num]
    Neg = 0x3E,

    // --- 6.5 Variables ---
    /// Loads a value from a global variable slot.
    /// Operand: slot_index (u32)
    /// Stack: [] -> [value]
    GetGlobal = 0x40,
    /// Stores the top value into a global variable slot.
    /// Operand: slot_index (u32)
    /// Stack: [value] -> []
    SetGlobal = 0x41,
    /// Loads a value from a local variable slot in the current frame.
    /// Operand: slot_index (u32)
    /// Stack: [] -> [value]
    GetLocal = 0x42,
    /// Stores the top value into a local variable slot in the current frame.
    /// Operand: slot_index (u32)
    /// Stack: [value] -> []
    SetLocal = 0x43,

    // --- 6.6 Functions ---
    /// Calls a function by its index in the function table.
    /// Operand: func_id (u32)
    /// Stack: [arg0, arg1, ...] -> [return_slots...]
    Call = 0x50,
    /// Returns from the current function.
    /// Stack: [return_val] -> [return_val]
    Ret = 0x51,
    /// Creates a closure capturing values from the operand stack (Model B).
    /// Operands: fn_id (u32), capture_count (u32)
    /// Stack before: [..., captured_N, ..., captured_1]
    /// Pops capture_count values (top-first), preserves order as [captured_1..captured_N]
    /// and stores them inside the closure environment. Pushes a HeapRef to the closure.
    MakeClosure = 0x52,
    /// Calls a closure value with hidden arg0 semantics (Model B).
    /// Operand: arg_count (u32) — number of user-supplied args (excludes hidden arg0)
    /// Stack before: [..., argN, ..., arg1, closure_ref]
    /// Behavior:
    ///   - Pops `closure_ref` and validates it is a Closure.
    ///   - Pops `arg_count` user args.
    ///   - Fetches `fn_id` from the closure and creates a new call frame.
    ///   - Injects hidden arg0 = closure_ref, followed by user args as arg1..argN.
    CallClosure = 0x53,

    // --- 7.x Concurrency / Coroutines ---
    /// Spawns a new coroutine to run a function with arguments.
    /// Operands: fn_id (u32), arg_count (u32)
    /// Semantics:
    ///   - Pops `arg_count` arguments from the current operand stack (top-first),
    ///     preserving user order as arg1..argN for the callee.
    ///   - Allocates a new Coroutine object with its own stack and a single entry frame
    ///     pointing at `fn_id`.
    ///   - Enqueues the coroutine into the scheduler ready queue.
    ///   - Does NOT switch execution immediately; current coroutine continues.
    Spawn = 0x54,

    /// Cooperatively yields the current coroutine. Execution continues
    /// until the next VM safepoint (FRAME_SYNC), where the scheduler
    /// may switch to another ready coroutine.
    Yield = 0x55,

    /// Suspends the current coroutine for a number of logical ticks.
    /// Operand: duration_ticks (u32)
    /// Semantics:
    ///   - Set the coroutine wake tick to `current_tick + duration_ticks`.
    ///   - End the current logical frame (as if reaching FRAME_SYNC).
    ///   - The coroutine will resume execution on or after the wake tick.
    Sleep = 0x56,

    // --- 6.8 Peripherals and System ---
    /// Pre-load host binding call by `SYSC` table index.
    /// Operand: sysc_index (u32)
    /// This opcode is valid only in PBX artifact form and must be patched by the loader
    /// into a final numeric `SYSCALL <id>` before verification or execution.
    Hostcall = 0x71,
    /// Invokes a final numeric system function (Firmware/OS).
    /// Raw `SYSCALL` is valid only after loader patching and is rejected in PBX pre-load artifacts.
    /// Operand: syscall_id (u32)
    /// Stack: [args...] -> [results...] (depends on syscall)
    Syscall = 0x70,
    /// Invokes a VM-owned intrinsic by final numeric id.
    /// Operand: intrinsic_id (u32)
    /// Stack: [args...] -> [results...] (depends on intrinsic metadata)
    Intrinsic = 0x72,
    /// Synchronizes the VM with the hardware frame (usually 60Hz).
    /// Execution pauses until the next VSync.
    FrameSync = 0x80,
}

impl TryFrom<u16> for OpCode {
    type Error = String;

    fn try_from(value: u16) -> Result<Self, Self::Error> {
        match value {
            0x00 => Ok(OpCode::Nop),
            0x01 => Ok(OpCode::Halt),
            0x02 => Ok(OpCode::Jmp),
            0x03 => Ok(OpCode::JmpIfFalse),
            0x04 => Ok(OpCode::JmpIfTrue),
            0x05 => Ok(OpCode::Trap),
            0x10 => Ok(OpCode::PushConst),
            0x11 => Ok(OpCode::Pop),
            0x12 => Ok(OpCode::Dup),
            0x13 => Ok(OpCode::Swap),
            0x14 => Ok(OpCode::PushI64),
            0x15 => Ok(OpCode::PushF64),
            0x16 => Ok(OpCode::PushBool),
            0x17 => Ok(OpCode::PushI32),
            0x18 => Ok(OpCode::PopN),
            0x20 => Ok(OpCode::Add),
            0x21 => Ok(OpCode::Sub),
            0x22 => Ok(OpCode::Mul),
            0x23 => Ok(OpCode::Div),
            0x24 => Ok(OpCode::Mod),
            0x30 => Ok(OpCode::Eq),
            0x31 => Ok(OpCode::Neq),
            0x32 => Ok(OpCode::Lt),
            0x33 => Ok(OpCode::Gt),
            0x34 => Ok(OpCode::And),
            0x35 => Ok(OpCode::Or),
            0x36 => Ok(OpCode::Not),
            0x37 => Ok(OpCode::BitAnd),
            0x38 => Ok(OpCode::BitOr),
            0x39 => Ok(OpCode::BitXor),
            0x3A => Ok(OpCode::Shl),
            0x3B => Ok(OpCode::Shr),
            0x3C => Ok(OpCode::Lte),
            0x3D => Ok(OpCode::Gte),
            0x3E => Ok(OpCode::Neg),
            0x40 => Ok(OpCode::GetGlobal),
            0x41 => Ok(OpCode::SetGlobal),
            0x42 => Ok(OpCode::GetLocal),
            0x43 => Ok(OpCode::SetLocal),
            0x50 => Ok(OpCode::Call),
            0x51 => Ok(OpCode::Ret),
            0x52 => Ok(OpCode::MakeClosure),
            0x53 => Ok(OpCode::CallClosure),
            0x54 => Ok(OpCode::Spawn),
            0x55 => Ok(OpCode::Yield),
            0x56 => Ok(OpCode::Sleep),
            0x70 => Ok(OpCode::Syscall),
            0x71 => Ok(OpCode::Hostcall),
            0x72 => Ok(OpCode::Intrinsic),
            0x80 => Ok(OpCode::FrameSync),
            _ => Err(format!("Invalid OpCode: 0x{:04X}", value)),
        }
    }
}

impl OpCode {
    /// Returns the cost of the instruction in VM cycles.
    /// This is used for performance monitoring and resource limiting (Certification).
    pub fn cycles(&self) -> u64 {
        match self {
            OpCode::Nop => 1,
            OpCode::Halt => 1,
            OpCode::Jmp => 2,
            OpCode::JmpIfFalse => 3,
            OpCode::JmpIfTrue => 3,
            OpCode::Trap => 1,
            OpCode::PushConst => 2,
            OpCode::Pop => 1,
            OpCode::PopN => 2,
            OpCode::Dup => 1,
            OpCode::Swap => 1,
            OpCode::PushI64 => 2,
            OpCode::PushF64 => 2,
            OpCode::PushBool => 2,
            OpCode::PushI32 => 2,
            OpCode::Add => 2,
            OpCode::Sub => 2,
            OpCode::Mul => 4,
            OpCode::Div => 6,
            OpCode::Mod => 6,
            OpCode::Eq => 2,
            OpCode::Neq => 2,
            OpCode::Lt => 2,
            OpCode::Gt => 2,
            OpCode::And => 2,
            OpCode::Or => 2,
            OpCode::Not => 1,
            OpCode::BitAnd => 2,
            OpCode::BitOr => 2,
            OpCode::BitXor => 2,
            OpCode::Shl => 2,
            OpCode::Shr => 2,
            OpCode::Lte => 2,
            OpCode::Gte => 2,
            OpCode::Neg => 1,
            OpCode::GetGlobal => 3,
            OpCode::SetGlobal => 3,
            OpCode::GetLocal => 2,
            OpCode::SetLocal => 2,
            OpCode::Call => 5,
            OpCode::Ret => 4,
            OpCode::MakeClosure => 8,
            OpCode::CallClosure => 6,
            OpCode::Spawn => 6,
            OpCode::Yield => 1,
            OpCode::Sleep => 1,
            OpCode::Syscall => 1,
            OpCode::Hostcall => 1,
            OpCode::Intrinsic => 1,
            OpCode::FrameSync => 1,
        }
    }
}