snowplow-incubator / scala-lru-map

Pure LRU map implementation

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Scala LruMap

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A pure least recently used hash map based on java.util.LinkedHashMap.

Example Usage

This library is useful if you're trying to memoize expensive methods.

import cats.effect.IO

class Fibonacci(lru: LruMap[IO, BigInt, BigInt]) {
  def calc(n: BigInt): IO[BigInt] = {
    if (n <= 0)
      IO.pure(0)
    else if (n == 1)
      IO.pure(1)
    else
      lru.get(n).flatMap(r => r match {
        case Some(r) => IO.pure(r)
        case None => for {
          m1 <- calc(n-1)
          m2 <- calc(n-2)
          _  <- lru.put(n, m1+m2)
        } yield m1+m2
      })
  }
}

val result = (for {
  // When the size of the map exceeds 500 the least recently used element is
  // removed
  lru    <- LruMap.create[IO, BigInt, BigInt](500)
  result <- (new Fibonacci(lru)).calc(100)
} yield result).unsafeRunSync()

// Prints 354224848179261915075
println(result)

All impure methods and constructors are wrapped in cats.effect.Sync. When you want to use the result in an impure environment you can use unsafeRunSync as shown above.

Note on Thread Safety

Calls to lruMap.get and LruMap.set are not inherently thread-safe, so concurrency concerns are left up to the choice of Sync.

Copyright and license

Copyright 2012-2018 Snowplow Analytics Ltd.

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this software except in compliance with the License.

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.