(Please note: the Big Integer Calculator, also by this developer, includes this prime factorization algorithm, along with other features.)
The app uses Pollard's Rho algorithm to find factors of large integers, in combination with trial division optimized for speed when possible by interpreting sub-factors as long integers.
A menu button allows factorization calculations to be extended for faster devices. Pollard's Rho calculations are iterated more times when the depth in increased, which results in factorizing 'harder' numbers, but takes longer. The depth can also be reduced.
The app will accept arbitrarily large integers, and will find the prime factors of powers of 10 or other numbers with small factors very quickly. For example, if you calculate 3000! using some big integer calculator, and paste this number into the prime factor app, it will find the complete factorization in around ten seconds (Galaxy S4).
Integers with prime factors of seven or more digits will take longer to factorize, depending on the device the app is running on.
Progress is shown by listing factors as they are found. If a sub-factor which is known to be non-prime is taking a long time to factorize, the display will show "working ...". Calculation can be cancelled by pressing the back button on the device. However, if the app cannot find all the factors, it will terminate and list any factors it has found, rather than hanging or freezing.