Hands-On Design Patterns with Kotlin

Memoization

If our function always returns the same output for the same input, we could easily map between previous input and output, and use it as a cache. That technique is called memoization:

class Summarizer {    private val resultsCache = mutableMapOf<List<Int>, Double>()    fun summarize(numbers: List<Int>): Double {        return resultsCache.computeIfAbsent(numbers, ::sum)    }    private fun sum(numbers: List<Int>): Double {        return numbers.sumByDouble { it.toDouble() }    }}

We use a method reference operator, ::, to tell computeIfAbsent to use the sum() method in the event that input wasn't cached yet.

Note that sum() is a pure function, while summarize() is not. The latter will behave differently for the same input. But that's exactly what we want ...