/// 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,

    // --- 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 at a specific address.
    /// Operands: addr (u32), args_count (u32)
    /// Stack: [arg0, arg1, ...] -> [return_value]
    Call = 0x50,
    /// Returns from the current function.
    /// Stack: [return_val] -> [return_val]
    Ret = 0x51,
    /// Starts a new local scope (for blocks/loops).
    PushScope = 0x52,
    /// Ends the current local scope, discarding its local variables.
    PopScope = 0x53,

    // --- 6.7 HIP (Heap Interface Protocol) ---
    
    /// Allocates `slots` slots on the heap with the given `type_id`.
    /// Operands: type_id (u32), slots (u32)
    /// Stack: [] -> [gate]
    Alloc = 0x60,
    /// Reads a value from the heap at `gate + offset`.
    /// Operand: offset (u32)
    /// Stack: [gate] -> [value]
    GateLoad = 0x61,
    /// Writes a value to the heap at `gate + offset`.
    /// Operand: offset (u32)
    /// Stack: [gate, value] -> []
    GateStore = 0x62,

    /// Marks the beginning of a Peek scope for a gate.
    /// Stack: [gate] -> [gate]
    GateBeginPeek = 0x63,
    /// Marks the end of a Peek scope for a gate.
    /// Stack: [gate] -> [gate]
    GateEndPeek = 0x64,
    /// Marks the beginning of a Borrow scope for a gate.
    /// Stack: [gate] -> [gate]
    GateBeginBorrow = 0x65,
    /// Marks the end of a Borrow scope for a gate.
    /// Stack: [gate] -> [gate]
    GateEndBorrow = 0x66,
    /// Marks the beginning of a Mutate scope for a gate.
    /// Stack: [gate] -> [gate]
    GateBeginMutate = 0x67,
    /// Marks the end of a Mutate scope for a gate.
    /// Stack: [gate] -> [gate]
    GateEndMutate = 0x68,

    /// Increments the reference count of a gate.
    /// Stack: [gate] -> [gate]
    GateRetain = 0x69,
    /// Decrements the reference count of a gate.
    /// Stack: [gate] -> []
    GateRelease = 0x6A,

    // --- 6.8 Peripherals and System ---
    
    /// Invokes a system function (Firmware/OS).
    /// Operand: syscall_id (u32)
    /// Stack: [args...] -> [results...] (depends on syscall)
    Syscall = 0x70,
    /// 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),
            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::PushScope),
            0x53 => Ok(OpCode::PopScope),
            0x60 => Ok(OpCode::Alloc),
            0x61 => Ok(OpCode::GateLoad),
            0x62 => Ok(OpCode::GateStore),
            0x63 => Ok(OpCode::GateBeginPeek),
            0x64 => Ok(OpCode::GateEndPeek),
            0x65 => Ok(OpCode::GateBeginBorrow),
            0x66 => Ok(OpCode::GateEndBorrow),
            0x67 => Ok(OpCode::GateBeginMutate),
            0x68 => Ok(OpCode::GateEndMutate),
            0x69 => Ok(OpCode::GateRetain),
            0x6A => Ok(OpCode::GateRelease),
            0x70 => Ok(OpCode::Syscall),
            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::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::PushScope => 3,
            OpCode::PopScope => 3,
            OpCode::Alloc => 10,
            OpCode::GateLoad => 3,
            OpCode::GateStore => 3,
            OpCode::GateBeginPeek => 1,
            OpCode::GateEndPeek => 1,
            OpCode::GateBeginBorrow => 1,
            OpCode::GateEndBorrow => 1,
            OpCode::GateBeginMutate => 1,
            OpCode::GateEndMutate => 1,
            OpCode::GateRetain => 1,
            OpCode::GateRelease => 1,
            OpCode::Syscall => 1,
            OpCode::FrameSync => 1,
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::asm::{assemble, Asm, Operand};

    #[test]
    fn test_opcode_stability() {
        // Normative test: ensures opcode numeric values are frozen.
        assert_eq!(OpCode::Nop as u16, 0x00);
        assert_eq!(OpCode::PushConst as u16, 0x10);
        assert_eq!(OpCode::Alloc as u16, 0x60);
        assert_eq!(OpCode::GateLoad as u16, 0x61);
        assert_eq!(OpCode::GateStore as u16, 0x62);
        assert_eq!(OpCode::GateBeginPeek as u16, 0x63);
        assert_eq!(OpCode::GateEndPeek as u16, 0x64);
        assert_eq!(OpCode::GateBeginBorrow as u16, 0x65);
        assert_eq!(OpCode::GateEndBorrow as u16, 0x66);
        assert_eq!(OpCode::GateBeginMutate as u16, 0x67);
        assert_eq!(OpCode::GateEndMutate as u16, 0x68);
        assert_eq!(OpCode::GateRetain as u16, 0x69);
        assert_eq!(OpCode::GateRelease as u16, 0x6A);
        assert_eq!(OpCode::FrameSync as u16, 0x80);
    }

    #[test]
    fn test_hip_bytecode_golden() {
        // Golden test for HIP opcodes and their encodings.
        // Rule: All multi-byte operands are little-endian.
        
        let instructions = vec![
            Asm::Op(OpCode::Alloc, vec![Operand::U32(0x11223344), Operand::U32(0x55667788)]),
            Asm::Op(OpCode::GateLoad, vec![Operand::U32(0xAABBCCDD)]),
            Asm::Op(OpCode::GateStore, vec![Operand::U32(0x11223344)]),
            Asm::Op(OpCode::GateBeginPeek, vec![]),
            Asm::Op(OpCode::GateRetain, vec![]),
            Asm::Op(OpCode::GateRelease, vec![]),
        ];

        let bytes = assemble(&instructions).unwrap();
        
        let mut expected = Vec::new();
        // Alloc (0x60, 0x00) + type_id (44 33 22 11) + slots (88 77 66 55)
        expected.extend_from_slice(&[0x60, 0x00, 0x44, 0x33, 0x22, 0x11, 0x88, 0x77, 0x66, 0x55]);
        // GateLoad (0x61, 0x00) + offset (DD CC BB AA)
        expected.extend_from_slice(&[0x61, 0x00, 0xDD, 0xCC, 0xBB, 0xAA]);
        // GateStore (0x62, 0x00) + offset (44 33 22 11)
        expected.extend_from_slice(&[0x62, 0x00, 0x44, 0x33, 0x22, 0x11]);
        // GateBeginPeek (0x63, 0x00)
        expected.extend_from_slice(&[0x63, 0x00]);
        // GateRetain (0x69, 0x00)
        expected.extend_from_slice(&[0x69, 0x00]);
        // GateRelease (0x6A, 0x00)
        expected.extend_from_slice(&[0x6A, 0x00]);

        assert_eq!(bytes, expected);
    }
}