// Syntax:
// ADD $01   ; comments follow a `;`
// ADD $FF   ; this is direct addressing
// ADD ($CC) ; this is indirect addressing
// END       ; END, CFC, and CHP don't require arguments
//           ; (a default value of 0 will be used as their operand)

const mnemonicsWithOptionalArgs = ['end', 'cfc', 'chp'];
const mnemonics2opcodes = {
  end: { direct: 0, indirect: 0 },
  sto: { direct: 1, indirect: 2 },
  lda: { direct: 3, indirect: 4 },
  add: { direct: 5, indirect: 6 },
  sub: { direct: 7, indirect: 8 },
  hop: { direct: 9, indirect: 10 },
  jmp: { direct: 11, indirect: 12 },
  cfc: { direct: 13, indirect: 13 },
  chp: { direct: 14, indirect: 14 },
};

function decodeMultipleInstructions(str) {
  let lines = str.split(/\n/); // returns an array of lines
  let output = [];
  lines.forEach( (l) => {
    let decoded = decodeInstruction(l);
    if (decoded) {
      output.push(decoded.op);
      output.push(decoded.arg);
    }
  });
  return new Uint8Array(output);
}

/**
 * @param {string} line - A line of assembly code
 * @returns {(object|false)} Either {op: machineOp, arg: 0}, or false if the line was blank
 */
function decodeInstruction(line) {
  line = stripWhitespaceFromEnds(stripComments(line));
  let op_arg_array = line.split(" "); // split line into an array of [op, arg]
  let opName = op_arg_array[0].toLowerCase();
  let addressingMode = 'direct'; // Must be "direct" or "indirect"

  // Handle blank lines and mnemonics without arguments (eg END)
  if (op_arg_array.length < 2) { // No argument
    // handle blank lines, or lines that just contain a comment:
    if (line.length === 0) { return false; }

    // handle mnemonics that aren't paired with an argument:
    if (mnemonicsWithOptionalArgs.indexOf(opName) < 0) {
      console.error(`Missing opcode: ${line}`);
      throw new Error("Missing opcode");
    }
    let machineOp = mnemonics2opcodes[opName][addressingMode];
    return { op: machineOp, arg: 0 };
  }

  // Handle mnemonics with arguments (eg ADD $FF)
  let arg_str = op_arg_array[1];
  if (arg_str.startsWith("(")) {
    addressingMode = "indirect";
    arg_str = arg_str.replace("(", "");
    arg_str = arg_str.replace(")", "");
  }
  if (arg_str.startsWith("$")) {
    arg_str = arg_str.replace("$", "");
    arg_num = hex2num(arg_str);
  } else {
    arg_num = parseInt(arg_str);
  }
  let machineOp = mnemonics2opcodes[opName][addressingMode];
  return { op: machineOp, arg: arg_num };
}

function stripComments(line) {
  return line.replace(/;.+/,"");
}

function stripWhitespaceFromEnds(line) {
  line = line.replace(/^\s+/,"");
  line = line.replace(/\s+$/,"");
  return line;
}

function hex2num(hex) { return parseInt(hex, 16) };


// RUN IT

exports.assemble = (str) => {
  return decodeMultipleInstructions(str);
}