1,11,211211111221, what's the next number?

1,11,211211111221, what's the next number?

The rule is: 111 --- indicates that the previous number "1" is a 1; 21 --- indicates that the previous number "11" is composed of two 1s; 1211 --- indicates that the previous number "21" is composed of one 2 and one 1; 111221 --- namely 11 12 21, indicates that the previous number "1211" is composed of one 1, one 2 and two 1s in turn; so the next one is 312211