sub method

Big sub(dynamic y)

Return a new Big whose value is the value of this Big minus the value of Big y.

Implementation

Big sub(dynamic y) {
  var yBig = Big(y);
  int i, j;
  List<int> t;
  var x = this, a = x.s, b = yBig.s;
  bool isXLtY;
  // Signs differ?
  if (a != b) {
    yBig.s = -b;
    return x.add(yBig);
  }

  var xc = [...x.c], xe = x.e, yc = yBig.c, ye = yBig.e;

  // Either zero?
  if (!xc.isElementIsValid(0) || !yc.isElementIsValid(0)) {
    if (yc.isElementIsValid(0)) {
      yBig.s = -b;
    } else if (xc.isElementIsValid(0)) {
      yBig = Big(x);
    } else {
      yBig.s = 1;
    }
    return yBig;
  }

  a = xe - ye;
  // Determine which is the bigger number. Prepend zeros to equalise exponents.
  if (a != 0) {
    if (isXLtY = a < 0) {
      a = -a;
      t = xc;
    } else {
      ye = xe;
      t = yc;
    }

    t.reverse();
    for (b = a; b > 0;) {
      b--;
      t.add(0);
    }
    t.reverse();
  } else {
    // Exponents equal. Check digit by digit.
    j = ((isXLtY = xc.length < yc.length) ? xc : yc).length;

    for (a = b = 0; b < j; b++) {
      if (xc[b] != yc[b]) {
        isXLtY = xc[b] < yc[b];
        break;
      }
    }
  }

  // x < y? Point xc to the array of the bigger number.
  if (isXLtY) {
    t = xc;
    xc = yc;
    yc = t;
    yBig.s = -yBig.s;
  }

  /// Append zeros to xc if shorter. No need to add zeros to yc if shorter as subtraction only
  /// needs to start at yc.length.

  if ((b = (j = yc.length) - (i = xc.length)) > 0) {
    for (; b > 0;) {
      b--;
      i++;
      xc.add(0);
    }
  }

  // Subtract yc from xc.
  for (b = i; j > a;) {
    if (xc[--j] < yc[j]) {
      for (i = j; i > 0 && !xc.isElementIsValid(--i);) {
        xc[i] = 9;
      }
      --xc[i];
      xc[j] += 10;
    }

    xc[j] -= yc[j];
  }

  // Remove trailing zeros.
  for (; xc[--b] == 0;) {
    xc.removeLast();
    if (b == 0) {
      break;
    }
  }
  // Remove leading zeros and adjust exponent accordingly.
  for (; xc.isNotEmpty && xc[0] == 0;) {
    xc.removeAt(0);
    --ye;
  }

  if (!xc.isElementIsValid(0)) {
    // n - n = +0
    yBig.s = 1;

    // Result must be zero.
    xc = [ye = 0];
  }

  yBig.c = xc;
  yBig.e = ye;

  return yBig;
}