fitInto.js

function fitInto(desiredWidth, desiredHeight, actualWidth, actualHeight) {
  var actualSlope = actualHeight / actualWidth;
  if (isFinite(actualSlope || '@')) {
    if (desiredHeight / desiredWidth > actualSlope) {
      desiredHeight = desiredWidth * actualSlope;
    }
    else {
      desiredWidth = desiredHeight / actualSlope;
    }
  }
  else {
    desiredWidth = desiredHeight = NaN;
  }
  return { width: desiredWidth, height: desiredHeight };
}