// cpu.c
// SplitBit CPU Emulator Core
// Written by Anachronaut
// 10/16/2024

#include "cpu.h"
#include "io.h"

uint16_t shiftRegister;

void initializeCPU(CPURegisters *cpu, uint8_t *programMemory, uint8_t *dataMemory) {
    cpu->A = 0;
    cpu->B = 0;
    cpu->Q = 0;
    cpu->Status = 0;
    cpu->ProgramCounter = 0x0000;
    // Every Data Pointer starts at the bottom of Data Memory.
    for (int i = 0; i < DATA_POINTERS; i++) {
        cpu->DataPointer[i] = 0x0000;
    }
    cpu->StackPointer = 0xFFFF;
    cpu->Program = programMemory;
    cpu->Data = dataMemory;
}

void genericBranch(CPURegisters *cpu){
    // Load the next two bytes from program memory into the Program Counter.
    // Byte order is imporant. Most Significant first, then Least Significant.
    cpu->ProgramCounter++; // Move to the next byte. (MSB)
    uint16_t DestinationAddress;
    DestinationAddress = (uint16_t)cpu->Program[cpu->ProgramCounter] << 8; // Cast the 8 bit value to a 16 bit value and shifts it up to the high byte.
    cpu->ProgramCounter++; // Move to the next byte. (LSB)
    DestinationAddress = DestinationAddress | (uint16_t)cpu->Program[cpu->ProgramCounter]; // Cast the 8 bit value to a 16 bit value and or it to add it to the desination.
    cpu->ProgramCounter = DestinationAddress-1;
}

void genericCall(CPURegisters *cpu){
    // Order, low byte, high byte
    cpu->Data[cpu->StackPointer] = cpu->ProgramCounter & 0xFF;
    cpu->StackPointer--;
    cpu->Data[cpu->StackPointer] = (cpu->ProgramCounter >> 8) & 0xFF;
    cpu->StackPointer--;
    // Push the preserved Data Pointers to the Stack, lowest numbered first.
    // Order within each one, low byte, high byte.
    // The pointers above PRESERVED_DATA_POINTERS are deliberately left alone, so a
    // subroutine can use one to hand an address back to whoever called it.
    for (int i = 0; i < PRESERVED_DATA_POINTERS; i++) {
        cpu->Data[cpu->StackPointer] = cpu->DataPointer[i] & 0xFF;
        cpu->StackPointer--;
        cpu->Data[cpu->StackPointer] = (cpu->DataPointer[i] >> 8) & 0xFF;
        cpu->StackPointer--;
    }
    // Push B to the Stack.
    cpu->Data[cpu->StackPointer] = cpu->B;
    cpu->StackPointer--;
    // Push A to the Stack.
    cpu->Data[cpu->StackPointer] = cpu->A;
    cpu->StackPointer--;
    // Perform a Generic Branch to the Address.
    genericBranch(cpu);
}

uint16_t *selectDataPointer(CPURegisters *cpu) {
    // Every instruction that works through a Data Pointer names which one in the
    // byte immediately after the opcode. Out of range selectors are masked down
    // rather than rejected, the way a narrow field in hardware would be. It is the
    // assembler's job to refuse to emit one in the first place.
    cpu->ProgramCounter++;
    return &cpu->DataPointer[cpu->Program[cpu->ProgramCounter] & (DATA_POINTERS - 1)];
}

uint8_t executeOperation(uint8_t Instruction, CPURegisters *cpu) {
    switch(Instruction) {
        // 0x - Arithmetic and Logic Operations.
        case 0x00:
            // ADD - A + B + Carry -> Q
            uint16_t result = (uint16_t)cpu->A + (uint16_t)cpu->B + (cpu->Status & 0x01);
            if (result > 255) {
                cpu->Status |= 0x01;
            } else {
                cpu->Status &= ~0x01;
            }
            cpu->Q = result & 0xFF;
        break;
        case 0x01:
            // SUB - A - B - Carry -> Q
            result = (uint16_t)cpu->A - (uint16_t)cpu->B - (cpu->Status & 0x01);
            if (result > 255) {
                cpu->Status |= 0x01;
            } else {
                cpu->Status &= ~0x01;
            }
            cpu->Q = result & 0xFF;
        break;
        case 0x02:
            // AND - A and B -> Q
            cpu->Q = cpu->A&cpu->B;
        break;
        case 0x03:
            // OR - A or B -> Q
            cpu->Q = cpu->A|cpu->B;
        break;
        case 0x04:
            // XOR - A xor B -> Q
            cpu->Q = cpu->A^cpu->B;
        break;
        case 0x05:
            // NOTA - not A -> Q
            cpu->Q = ~cpu->A;
        break;
        case 0x06:
            // NOTB - not B -> Q
            cpu->Q = ~cpu->B;
        break;
        case 0x07:
            // SHL - Shift AB left.
            shiftRegister = ((uint16_t)cpu->A << 8) | cpu->B;
            shiftRegister = (shiftRegister << 1) | (shiftRegister >> 15);
            cpu->A = shiftRegister >> 8;
            cpu->B = shiftRegister & 0xFF;
        break;
        case 0x08:
            // SHR - Shift AB right.
            shiftRegister = ((uint16_t)cpu->A << 8) | cpu->B;
            shiftRegister = (shiftRegister >> 1) | (shiftRegister << 15);
            cpu->A = shiftRegister >> 8;
            cpu->B = shiftRegister & 0xFF;
        break;
        //
        // 1x - Branch Operations:
        //
        case 0x10:
            // BRI - Branch Immediately
            genericBranch(cpu);
        break;
        case 0x11:
            // BRQ - Branch if Q = 0
            if(cpu->Q == 0) {
                genericBranch(cpu);
            } else {

                cpu->ProgramCounter+=2;
            }
        break;
        case 0x12:
            // BRA - Branch if A = 0
            if(cpu->A == 0) {
                genericBranch(cpu);

            } else {
                cpu->ProgramCounter+=2;
            }
        break;
        case 0x13:
            // BRB -  if B = 0
            if(cpu->B == 0) {

                genericBranch(cpu);
            } else {
                cpu->ProgramCounter+=2;
            }
        break;
        case 0x14:
            // BRC - Do an immediate branch if the Carry Flag is set.
            if (cpu->Status & 0x01) {
                genericBranch(cpu);
            } else {
                cpu->ProgramCounter+=2;
            }
        break;
        case 0x17:
            // CALL - Push the Program Counter to the Stack, and perform an immediate branch.
            genericCall(cpu);
        break;
        case 0x1F:
            // RET - Return from subroutine, restore the registers and set the Program Counter to the Return Address.
            // Pop A from the Stack.
            cpu->StackPointer++;
            cpu->A = cpu->Data[cpu->StackPointer];
            // Pop B from the Stack.
            cpu->StackPointer++;
            cpu->B = cpu->Data[cpu->StackPointer];
            // Pop the preserved Data Pointers from the Stack. This walks the pointers
            // in the opposite order to genericCall, and takes the high byte before the
            // low byte, so that it exactly mirrors the way they were pushed.
            for (int i = PRESERVED_DATA_POINTERS - 1; i >= 0; i--) {
                cpu->StackPointer++;
                cpu->DataPointer[i] = (uint16_t)cpu->Data[cpu->StackPointer] << 8;
                cpu->StackPointer++;
                cpu->DataPointer[i] |= (uint16_t)cpu->Data[cpu->StackPointer];
            }
            // Pop the Return Address from the Stack.
            cpu->StackPointer++;
            cpu->ProgramCounter = (uint16_t)cpu->Data[cpu->StackPointer] << 8;
            cpu->StackPointer++;
            cpu->ProgramCounter = cpu->ProgramCounter | (uint16_t)cpu->Data[cpu->StackPointer];
            // Add 2 to the Program Counter to skip over the address when it returns.
            cpu->ProgramCounter += 2;

        break;
        //
        // 2x - Register Operations:
        //
        case 0x20:
            // RSTA - Reset A to 0.
            cpu->A = 0;
        break;
        case 0x21:
            // RSTB - Reset B to 0.
            cpu->B = 0;
        break;
        case 0x22:
            // INCA - Add 1 to A.
            // Set the Carry Flag if the register overflows.
            if (cpu->A == 0xFF) {
                cpu->Status |= 0x01;
            } else {
                cpu->Status &= ~0x01;
            }
            cpu->A++;
        break;
        case 0x23:
            // INCB - Add 1 to B.
            // Set the Carry Flag if the register overflows.
            if (cpu->B == 0xFF) {
                cpu->Status |= 0x01;
            } else {
                cpu->Status &= ~0x01;
            }
            cpu->B++;
        break;
        case 0x24:
            // DECA - Subtract 1 from A.
            // Set the Carry Flag if the register underflows.
            if (cpu->A == 0x00) {
                cpu->Status |= 0x01;
            } else {
                cpu->Status &= ~0x01;
            }
            cpu->A--;
        break;
        case 0x25:
            // DECB - Subtract 1 from B.
            // Set the Carry Flag if the register underflows.
            if (cpu->B == 0x00) {
                cpu->Status |= 0x01;
            } else {
                cpu->Status &= ~0x01;
            }
            cpu->B--;
        break;
        case 0x26:
            // INIA - Initialize A Immediately from Program Memory.
            cpu->ProgramCounter++;
            cpu->A = cpu->Program[cpu->ProgramCounter];
        break;
        case 0x27:
            // INIB - Initialize A Immediately from Program Memory.
            cpu->ProgramCounter++;
            cpu->B = cpu->Program[cpu->ProgramCounter];
        break;
        case 0x28:
            // CCF - Clear the Carry Flag.
            cpu->Status &= ~0x01;
        break;
        //
        // 3x - Stack Operations:
        //
        case 0x30:
            // PSHQ - Push Q to the Stack.
            cpu->Data[cpu->StackPointer] = cpu->Q;
            cpu->StackPointer--;
        break;
        case 0x31:
            // PSHA - Push A to the Stack.
            cpu->Data[cpu->StackPointer] = cpu->A;
            cpu->StackPointer--;
        break;
        case 0x32:
            // PSHB - Push B to the Stack.
            cpu->Data[cpu->StackPointer] = cpu->B;
            cpu->StackPointer--;
        break;
        case 0x33: {
            // PSHD - Push the selected Data Pointer Address to the Stack.
            // Order, high byte, low byte
            // This ordering makes it easier to add offsets with register math.
            uint16_t pushed = *selectDataPointer(cpu);
            cpu->Data[cpu->StackPointer] = (pushed >> 8) & 0xFF;
            cpu->StackPointer--;
            cpu->Data[cpu->StackPointer] = pushed & 0xFF;
            cpu->StackPointer--;
        }
        break;
        case 0x34:
            // POPA - Pop A from the Stack.
            cpu->StackPointer++;
            cpu->A = cpu->Data[cpu->StackPointer];

        break;
        case 0x35:
            // POPB - Pop B from the Stack.
            cpu->StackPointer++;
            cpu->B = cpu->Data[cpu->StackPointer];
        break;
        case 0x36: {
            // POPD - Pop a Data Address from the Stack into the selected Data Pointer.
            uint16_t *popped = selectDataPointer(cpu);
            cpu->StackPointer++;
            *popped = (uint16_t)cpu->Data[cpu->StackPointer];
            cpu->StackPointer++;
            *popped |= (uint16_t)cpu->Data[cpu->StackPointer] << 8;
        }
        break;
        //
        // 4x - Data Operations:
        //
        case 0x40:
            // INCD - Increment the selected Data Pointer.
            (*selectDataPointer(cpu))++;
        break;
        case 0x41:
            // DECD - Decrement the selected Data Pointer.
            (*selectDataPointer(cpu))--;
        break;
        case 0x42:
            // LDA - Load A from Data.
            cpu->A = cpu->Data[*selectDataPointer(cpu)];
        break;
        case 0x43:
            // LDB - Load B from Data.
            cpu->B = cpu->Data[*selectDataPointer(cpu)];
        break;
        case 0x44:
            // STQ - Store Q into Data.
            cpu->Data[*selectDataPointer(cpu)] = cpu->Q;
        break;
        case 0x45:
            // STA - Store A into Data.
            cpu->Data[*selectDataPointer(cpu)] = cpu->A;
        break;
        case 0x46:
            // STB - Store B into Data.
            cpu->Data[*selectDataPointer(cpu)] = cpu->B;
        break;
        case 0x47: {
            // SETD - Set the selected Data Pointer.
            uint16_t *destination = selectDataPointer(cpu);
            cpu->ProgramCounter++;
            uint16_t Address;
            Address = (uint16_t)cpu->Program[cpu->ProgramCounter] << 8; // Cast the 8 bits to a 16 bit value and shift them to the high byte.
            cpu->ProgramCounter++;
            Address |= (uint16_t)cpu->Program[cpu->ProgramCounter];
            *destination = Address;
        }
        break;
        case 0x48: {
            // DPUP - Offset the selected Data Pointer up by the value of the next byte of Program Memory.
            uint16_t *target = selectDataPointer(cpu);
            cpu->ProgramCounter++;
            *target += cpu->Program[cpu->ProgramCounter];
        }
        break;
        case 0x49: {
            // DPDN - Offset the selected Data Pointer down by the value of the next byte of Program Memory.
            uint16_t *target = selectDataPointer(cpu);
            cpu->ProgramCounter++;
            *target -= cpu->Program[cpu->ProgramCounter];
        }
        break;
        //
        // Dx - Output Operations:
        //
        case 0xD0:
            // OUTQ - Write the value of Q to an output port.
            cpu->ProgramCounter++;
            OutputHandler(cpu->Q, cpu->Program[cpu->ProgramCounter]);
        break;
        case 0xD1:
            // OUTA - Write the value of A to an output port.
            cpu->ProgramCounter++;
            OutputHandler(cpu->A, cpu->Program[cpu->ProgramCounter]);
        break;
        case 0xD2:
            // OUTB - Write the value of B to an output port.
            cpu->ProgramCounter++;
            OutputHandler(cpu->B, cpu->Program[cpu->ProgramCounter]);
        break;
        //
        // Ex - Input Operations:
        //
        case 0xE0:
            // INA - Read an Input to A.
            cpu->ProgramCounter++;
            cpu->A = InputHandler(cpu->Program[cpu->ProgramCounter]);
        break;
        case 0xE1:
            // INB - Read an Input to B.
            cpu->ProgramCounter++;
            cpu->B = InputHandler(cpu->Program[cpu->ProgramCounter]);
        break;
        //
        // Fx - Special Operations:
        //
        case 0xF0:
            // NOP - Do nothing.
        break;
        case 0xFF:
            // HALT - Set the Halt Bit of the Status Register.
            cpu->Status |= 0x80;
        break;
        default:
            // Unknown Instruction.
            return 1;
    }
    return 0;
}

void stepCPU(CPURegisters *cpu) {
    if (!(cpu->Status & 0x80)) {
        // The CPU is not halted, so do a cycle.
        executeOperation(cpu->Program[cpu->ProgramCounter], cpu);
        cpu->ProgramCounter++;
    }
}