swift中双值的奇怪结果

Question

为什么我在swift中获得双重值这个奇怪的结果

let x = 1000000000000000000000.0;
let y = 1000000000000000000001.0;

print(x-y);    //0      but should be -1
print(y-x);    //0      but should be 1
print(x==y);  //true    but should be false

我正在使用 swift 3