const { INITIAL_IP_ADDRESS, CYCLE_LIMIT } = require('./machine.config');
const { num2hex, num2bin_4bit } = require('./logging.js');
const display = require('./display.js');

// STATE
const CPU = {
  running: false,
  IP: INITIAL_IP_ADDRESS,
  FLAGS: 0, // A bit field! 0000 = NZOC
  Acc: 0,
  memory: null,

  currentInstruction: {
    opcode: null,
    operand: null,
    mnemonic: null,
  },

  /** @param {Uint8Array} data */
  loadMemory: (data) => {
      if (data.length > 256) { throw new Error("Out of memory error (program too long)"); }
      CPU.memory = data;
  },

  setFlagNegative: () => { CPU.FLAGS |= 8 },
  setFlagZero:     () => { CPU.FLAGS |= 4 },
  setFlagOverflow: () => { CPU.FLAGS |= 2 },
  setFlagCarry:    () => { CPU.FLAGS |= 1 },
  unsetFlagNegative: () => { CPU.FLAGS &= ~8 },
  unsetFlagZero:     () => { CPU.FLAGS &= ~4 },
  unsetFlagOverflow: () => { CPU.FLAGS &= ~2 },
  unsetFlagCarry:    () => { CPU.FLAGS &= ~1 },

  updateFlagZero: () => {
    if (CPU.Acc === 0) {
      CPU.setFlagZero();
    } else {
      CPU.unsetFlagZero();
    }
  },

  updateFlagNegative: () => {
    CPU.Acc & 128 ? CPU.setFlagNegative : CPU.unsetFlagNegative },
}


// FUNCTIONS THAT MODIFY STATE

const Instructions = {
  end: () => {
    CPU.currentInstruction.mnemonic = 'END';
    CPU.running = false;
  },

  store_lit: (lit) => {
    CPU.currentInstruction.mnemonic = 'STO lit';
    CPU.memory[lit] = CPU.Acc;
    CPU.IP = CPU.IP += 2;
  },

  store_addr: (addr) => {
    CPU.currentInstruction.mnemonic = 'STO addr';
    CPU.memory[CPU.memory[addr]] = CPU.Acc;
    CPU.IP = CPU.IP += 2;
  },

  load_lit: (lit) => {
    CPU.currentInstruction.mnemonic = 'LDA lit';
    CPU.Acc = lit;
    CPU.updateFlagNegative();
    CPU.updateFlagZero();
    CPU.IP = CPU.IP += 2;
  },

  load_addr: (addr) => {
    CPU.currentInstruction.mnemonic = `LDA addr; @ addr: ${num2hex(CPU.memory[addr])}`;
    CPU.Acc = CPU.memory[addr];
    CPU.updateFlagNegative();
    CPU.updateFlagZero();
    CPU.IP = CPU.IP += 2;
  },

  add_lit: (lit) => {
    CPU.currentInstruction.mnemonic = 'ADD lit';
    // Calculate sum
    let sum = CPU.Acc + lit;
    if (sum > 255) {
      CPU.setFlagCarry();
      sum = (sum % 255) - 1;
    } else {
      CPU.unsetFlagCarry();
    }
    // Calculate overflow flag status
    let bitSixCarry = 0;
    if ((CPU.Acc & 64) && (lit & 64)) { bitSixCarry = 1; }
    let overflow = bitSixCarry ^ (CPU.FLAGS & 1)
    if (overflow) {
      CPU.setFlagOverflow();
    } else {
      CPU.unsetFlagOverflow();
    }
    CPU.Acc = sum;
    CPU.updateFlagNegative();
    CPU.updateFlagZero();
    CPU.IP = CPU.IP += 2;
  },

  add_addr: (addr) => {
    CPU.currentInstruction.mnemonic = 'ADD addr';
    // Calculate sum
    let sum = CPU.Acc + CPU.memory[addr];
    if (sum > 255) {
      CPU.setFlagCarry();
      sum = (sum % 255) - 1;
    } else {
      CPU.unsetFlagCarry();
    }
    // Calculate overflow flag status
    let bitSixCarry = 0;
    if ((CPU.Acc & 64) && (addr & 64)) { bitSixCarry = 1; }
    let overflow = bitSixCarry ^ (CPU.FLAGS & 1)
    if (overflow) {
      CPU.setFlagOverflow();
    } else {
      CPU.unsetFlagOverflow();
    }
    CPU.Acc = sum;
    CPU.updateFlagNegative();
    CPU.updateFlagZero();
    CPU.IP = CPU.IP += 2;
  },

  sub_lit: (lit) => {
    CPU.currentInstruction.mnemonic = 'SUB lit';
    // Calculate sum
    let sum = CPU.Acc - lit;
    if (sum < 0) {
      CPU.setFlagCarry();
      sum = sum + 256;
    } else {
      CPU.unsetFlagCarry();
    }
    // Calculate overflow flag status
    let bitSixCarry = 0;
    if ((CPU.Acc & 64) && (lit & 64)) { bitSixCarry = 1; }
    let overflow = bitSixCarry ^ (CPU.FLAGS & 1)
    if (overflow) {
      CPU.setFlagOverflow();
    } else {
      CPU.unsetFlagOverflow();
    }
    CPU.Acc = sum;
    CPU.updateFlagNegative();
    CPU.updateFlagZero();
    CPU.IP = CPU.IP += 2;
  },

  sub_addr: (addr) => {
    CPU.currentInstruction.mnemonic = 'SUB addr';
    // Calculate sum
    let sum = CPU.Acc - CPU.memory[addr];
    if (sum < 0) {
      CPU.setFlagCarry();
      sum = sum + 256;
    } else {
      CPU.unsetFlagCarry();
    }
    // Calculate overflow flag status
    let bitSixCarry = 0;
    if ((CPU.Acc & 64) && (addr & 64)) { bitSixCarry = 1; }
    let overflow = bitSixCarry ^ (CPU.FLAGS & 1)
    if (overflow) {
      CPU.setFlagOverflow();
    } else {
      CPU.unsetFlagOverflow();
    }
    CPU.Acc = sum;
    CPU.updateFlagNegative();
    CPU.updateFlagZero();
    CPU.IP = CPU.IP += 2;
  },

  hop_lit: (lit) => {
    CPU.currentInstruction.mnemonic = `HOP lit;  IP+2: ${CPU.memory[CPU.IP+2]}, IP+3: ${CPU.memory[CPU.IP+3]}`;
    if (CPU.Acc === lit) {
      CPU.IP += 4;
    } else {
      CPU.IP += 2;
    }
  },

  hop_addr: (addr) => {
    CPU.currentInstruction.mnemonic = 'HOP addr';
    if (CPU.Acc === CPU.memory[addr]) {
      CPU.IP += 4;
    } else {
      CPU.IP += 2;
    }
  },

  jump_lit: (lit) => {
    CPU.currentInstruction.mnemonic = 'JMP lit';
    CPU.IP = lit;
  },

  jump_addr: (addr) => {
    CPU.currentInstruction.mnemonic = 'JMP addr';
    CPU.IP = CPU.memory[addr];
  },

  flag_toggle: (flagNum) => {
    CPU.currentInstruction.mnemonic = 'FTG';
    let mask = null;
    if (flagNum === 0) { mask = 1; }
    if (flagNum === 1) { mask = 2; }
    if (flagNum === 2) { mask = 4; }
    if (flagNum === 3) { mask = 8; }
    if (mask === null) { throw new Error('Invalid flag number'); }
    CPU.FLAGS = CPU.FLAGS ^= mask;
    CPU.IP += 2;
  },

  flag_hop: (flagNum) => {
    CPU.currentInstruction.mnemonic = `FHP; IP+2: ${CPU.memory[CPU.IP+2]}, IP+3: ${CPU.memory[CPU.IP+3]}`;
    let mask = null;
    if (flagNum === 0) { mask = 1; }
    if (flagNum === 1) { mask = 2; }
    if (flagNum === 2) { mask = 4; }
    if (flagNum === 3) { mask = 8; }
    if (mask === null) { throw new Error('Invalid flag number'); }
    if (CPU.FLAGS & mask) {
      CPU.IP += 4;
    } else {
      CPU.IP += 2;
    }
  },

  no_op: () => {
    CPU.currentInstruction.mnemonic = `NOP`;
    CPU.IP += 2;
  },
}

const opcodes2mnemonics = {
  0: (operand) => Instructions.end(),
  1: (operand) => Instructions.store_lit(operand),
  2: (operand) => Instructions.store_addr(operand),
  3: (operand) => Instructions.load_lit(operand),
  4: (operand) => Instructions.load_addr(operand),
  5: (operand) => Instructions.add_lit(operand),
  6: (operand) => Instructions.add_addr(operand),
  7: (operand) => Instructions.sub_lit(operand),
  8: (operand) => Instructions.sub_addr(operand),
  9: (operand) => Instructions.hop_lit(operand),
  10: (operand) => Instructions.hop_addr(operand),
  11: (operand) => Instructions.jump_lit(operand),
  12: (operand) => Instructions.jump_addr(operand),
  13: (operand) => Instructions.flag_toggle(operand),
  14: (operand) => Instructions.flag_hop(operand),
  15: (operand) => Instructions.no_op(),
};

/**
 * Load code into memory and set CPU state to "running"
 * @param {Uint8Array} code - Machine code to load
 **/
function startCPU(code) {
  CPU.loadMemory(code);
  CPU.running = true;
}

/**
 * Execute just the next instruction in memory
 **/
function stepCPU() {
  CPU.currentInstruction.opcode = CPU.memory[CPU.IP];
  CPU.currentInstruction.operand = CPU.memory[CPU.IP+1];
  let executeInstruction = opcodes2mnemonics[CPU.currentInstruction.opcode];
  executeInstruction(CPU.currentInstruction.operand);
}

/**
 * @param {Uint8Array} code - Machine code to run
 * @param {Boolean} [debug] - Enable/disable debugging printouts
 **/
exports.runProgram = async (code, debug = false) => {
  startCPU(code);
  let step = 0;
  while (true) {
    step = step + 1;
    if (CYCLE_LIMIT && (step > CYCLE_LIMIT)) { break; } // Temporary limit as a lazy way to halt infinite loops:
    if (!CPU.running) break;
    if (CPU.IP >= CPU.memory.length) break;
    stepCPU();
    await logCPUState(debug);
  };
}


// FUNCTIONS THAT PULL INFO FROM STATE TO DISPLAY

/**
 * @param {Boolean} [debug] - Enable/disable debugging printouts
 **/
async function logCPUState(debug = false) {
  console.group(`Step`);
    if (!debug) console.clear();
    if (debug) {
      display.printDisplay(CPU.memory);
    } else {
      display.prettyPrintDisplay(CPU.memory);
    }
    console.log();
    console.log('Mnemonic:', CPU.currentInstruction.mnemonic);
    console.log(`Machine:  $${num2hex(CPU.currentInstruction.opcode)} $${num2hex(CPU.currentInstruction.operand)}`);
    console.log();
    console.log(`IP: $${num2hex(CPU.IP)}  Acc: $${num2hex(CPU.Acc)}  NVOC: ${num2bin_4bit(CPU.FLAGS)}  ${CPU.running ? "running" : "halted" }`);
    console.log();
    // Pause to show animated display:
    if (!debug) await new Promise(resolve => setTimeout(resolve, 75));
  console.groupEnd();
};