function CPU(mem) {
  this.memory = mem;
  this.running = false;
  this.instructionPointer = 0;
  this.carryFlag =  0;
  this.acc = 0;

  this.instructions = {
    end:        ()     => {
      console.log('END');
      this.running = false
    },
    store_lit:  (lit)  => {
      console.log('STO lit#');
      this.memory[lit] = this.acc;
      log_memory();
      this.instructionPointer = this.instructionPointer += 1;
    },
    store_addr: (addr) => {
      console.log('STO addr');
      this.memory[this.memory[addr]] = this.acc;
      log_memory();
      this.instructionPointer = this.instructionPointer += 1;
    },
    load_lit:   (lit)  => {
      console.log('LDA lit#');
      this.acc = lit;
      this.instructionPointer = this.instructionPointer += 1;
    },
    load_addr:  (addr) => {
      console.log('LDA addr');
      console.log('mem at addr: ', this.memory[addr]);
      this.acc = this.memory[addr];
      this.instructionPointer = this.instructionPointer += 1;
    },
    add_lit: (lit) => {
      console.log("ADD lit");
      if ( (this.acc + lit) > 15 ) { this.carryFlag = 1; }
      this.acc = (this.acc + lit % 15);
      this.instructionPointer = this.instructionPointer += 1;
    },
    add_addr: (addr) => {
      console.log("ADD addr");
      if ( (this.acc + this.memory[addr]) > 15 ) { this.carryFlag = 1; }
      this.acc = (this.acc + this.memory[addr] % 15);
      this.instructionPointer = this.instructionPointer += 1;
    },
    sub_lit: (lit) => { // TODO: carry flag
      console.log("SUB lit");
      this.acc = this.acc - lit;
      this.instructionPointer = this.instructionPointer += 1;
    },
    sub_addr: (addr) => { // TODO: carry flag
      console.log("SUB addr");
      this.acc = this.acc - this.memory[addr];
      this.instructionPointer = this.instructionPointer += 1;
    },
  };

  log_debug_state = () => {
    console.log();
    console.group('CPU state');
    console.log( `IP: ${this.instructionPointer}   Acc: ${this.acc}   CF: ${this.carryFlag}   ${this.running ? "running" : "halted" }` );
    console.log();
    console.groupEnd('CPU state');
  };

  log_memory = () => {
    console.log();
    console.group('Memory');
    console.table(this.memory);
    console.groupEnd('Memory');
  }

  this.perform_operation = (opcode, arg) => {
    switch (opcode) {
      case 0:
        this.instructions.end(arg);
        break;

      case 1:
        this.instructions.store_lit(arg);
        break;

      case 2:
        this.instructions.store_addr(arg);
        break;

      case 3:
        this.instructions.load_lit(arg);
        break;

      case 4:
        this.instructions.load_addr(arg);
        break;

      case 5:
        this.instructions.add_lit(arg);
        break;

      case 6:
        this.instructions.add_addr(arg);
        break;

      case 7:
        this.instructions.sub_lit(arg);
        break;

      case 8:
        this.instructions.sub_addr(arg);
        break;
      
      default:
        console.error( `Invalid opcode: ${opcode} with argument ${arg}` );
    }
  };

  this.run_program = () => {
    console.log();
    console.log( "Running program..." );
    log_debug_state();

    this.running = true;

    for (let i = 1; i < 16; i++) {
      if ( this.running &&
            (this.instructionPointer < this.memory.length) ) {
        let op_arg_tuple = this.memory[this.instructionPointer];
        console.group("Proccessing instruction");
        console.log( op_arg_tuple );
        // console.log( `processing opcode ${op_arg_tuple[0]} with arg ${op_arg_tuple[1]}` );
        this.perform_operation(op_arg_tuple[0], op_arg_tuple[1]);
        log_debug_state();
        console.groupEnd("Processing instruction");
      }
    }
  }
};

// UNIMPLEMENTED
  let hop_lit = function(lit) { return; }
  let hop_addr = function(addr) { return; }
  let jump_lit = function(lit) { return; }
  let jump_addr = function(addr) { return; }
  let carry_toggle = function() { return; }
  let carry_hop = function() { return; }


// TEST

let halt_and_catch_fire = [
  [0, 0],
  [1, 0],
];

let test_lda_sto = [
  [3, 8], // LDA lit
  [1, 5], // STO lit
  [4, 5], // LDA addr
  [2, 6], // STO addr
  [0, 0], // END
];
// let comp = new CPU(test_lda_sto);
// comp.run_program();

let test_add_sub_nocarry = [
  [5, 6], // ADD lit  ... acc = 6
  [7, 1], // SUB lit  ... acc = 5
  [1, 8], // STO lit  ... mem[8] = 5
  [6, 8], // ADD addr ... acc = 10
  [8, 8], // SUB addr ... acc = 5
  [0, 0], // END
]

let test_add_sub = [
  [5, 26], // ADD lit
  [0, 0], // END
]

let comp = new CPU(test_add_sub);
comp.run_program();