Fast integer inverse root mapping

This came up in a /sci thread
(1-10 ->)
1,4,9,(1)6,(2)5,(3)6,(4)9,(6)4,(8)1,(10)0

How difficult is reverse mapping a square this way? (it runs least significant digit first lol)

>>15
Only if its somehow faster than https://en.wikipedia.org/wiki/General_number_field_sieve
In general you can check first million primes in seconds with brute force divisibility tests..
The difficulty of factoring (prime*prime) is that prime chosen is huge and random(not in the first Nth billion of primes), and this method doesn't scale well to large numbers.