FactorialBigWithCache.kt

package other

import java.math.BigInteger

/**
 *
 * Algorithm for finding the factorial of a positive number n
 *
 * optimization: caching previous factorial values
 *
 * also adding large numbers
 *
 * worst time: O(n)
 * the best time: O(1)
 * amount of memory: O(n)
 * problem: creating a huge number of BigInteger objects
 *
 */

class FactorialBigWithCache {

    private val cache = hashMapOf<Int, BigInteger>()

    fun compute(number: Int) : BigInteger {
        if (number <= 1) {
            return BigInteger.ONE
        }

        val cachedResult = cache[number]
        if (cachedResult != null) {
            return cachedResult
        }

        var result = BigInteger.ONE
        for (i in 2..number) {
            result = result.multiply(i.toBigInteger())
            cache[i] = result
        }
        return result
    }

}