kotlin-simple-cache

Cache.kt

package cache

import java.time.Duration

/*
 Usage:
 data class Something(val whatever: String)
 internal val myAwesomeCache by lazy { Cache<Something>(expire = Duration.ofDays(10)) }
 myAwesomeCache.pushOrFetch("identifierForMySomething") { someMethodThatReturnsSomething() }
*/

internal class CacheElement<T>(val element: T, val seen: Long = System.currentTimeMillis())
internal class CacheStats(var expired: Int = 0, var appended: Int = 0)

private fun <T> safe(default: T? = null, f: () -> T?) = try {
  f()
} catch (e: Exception) {
  default
}

private fun <T> T.asCacheElement() = CacheElement(this)

internal class Cache<T>(private val cache: HashMap<String, CacheElement<T>> = HashMap(), private val expire: Duration) {

  private val expireInMillis by lazy { expire.toMillis() }
  private val cacheStats = CacheStats()

  fun pushOrFetch(identifier: String, f: () -> T? = { null }): T? = cache[identifier]
    ?.filterNotExpired(identifier)
    ?.element
    ?: safe { f() }
      ?.also { cache[identifier] = it.asCacheElement() }
      ?.also { cacheStats.appended.inc() }

  fun clear() = cache.clear()

  fun remove(identifier: String) = cache.remove(identifier).also { cacheStats.expired.inc() }

  fun entries() = cache.size

  private fun CacheElement<T>.filterNotExpired(identifier: String) =
    if (System.currentTimeMillis() - seen > expireInMillis) {
      remove(identifier)
      null
    } else this
}

This could most likely be created with inline fun <T> ... with an extension function too - but, for now, it's an object.