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https://github.com/propensive/camouflage

Caching data structures for Scala
https://github.com/propensive/camouflage

caching lfu-cache lru-cache scala

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Caching data structures for Scala

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# Camouflage



__An LRU cache implementation for Scala__



Caching provides a useful tradeoff between the costs of _time_ and _space_; that is, a cache can reduce the

amount of computation time at the expense of using more memory. _Camouflage_ provides one implementation of a

cache which will be appropriate for some (but not all) applications: a least-recently-used cache, or LRU cache.



## Features



- provides an implementation of a least-recently-used (LRU) cache

- cache requires only a _size_ parameter to initialize

- serves as a simple drop-in memoization wrapper around expensive method calls



## Availability



## Getting Started



All Camouflage terms and types are defined in the `camouflage` package:

```scala

import camouflage.*

```



### LRU Caches



An LRU cache is for storing a limited number of values which are expensive to compute. Values are added to the

cache when they are computed, and can be accessed in constant time by referencing them with a key. If the cache

already contains its limit of values, an existing value must be "evicted" to make space for the new value. The

value chosen is whichever has been in the cache the longest time without being accessed; in other words, the

_least-recently-used_ value.



### `LruCache`



To construct an LRU cache, specify a key and value type, and maximum size to the `LruCache` constructor:

```scala

import galilei.Path

import anticipation.Text



val cache = LruCache[Path, Text](100)

```



This `LruCache` will behave like a `Map[Path, Text]`: its values are `Text`s, indexed by `Path`s.



A single `apply` method is used to both retrieve and update a value in the cache, much like `getOrElseUpdate` in

a `Map`. For `LruCache`, the parameters are curried, since the second _ought_ to represent a costly

computation—since that is the purpose of caching.



Here is an example of using the `LruCache` to cache reading two files from disk:

```scala

import galilei.File

import galilei.filesystemOptions.{dereferenceSymlinks, doNotCreateNonexistent}

import contingency.strategies.throwUnsafely

import hieroglyph.charDecoders.utf8

import hieroglyph.badEncodingHandlers.skip

import serpentine.{%, p, Slash}

import serpentine.hierarchies.unix

import turbulence.readAs



val dataDir = % / p"home" / p"data"



def read(): Unit =

  cache(dataDir / p"intro.txt"):

    (dataDir / p"intro.txt").as[File].readAs[Text]



  cache(dataDir / p"chapter1.txt"):

    (dataDir / p"chapter1.txt").as[File].readAs[Text]

```



A subsequent call to,

```scala

def readAgain(): Text = cache(dataDir / p"chapter1.txt"):

  (dataDir / p"chapter1.txt").as[File].readAs[Text]

```

would retrieve the stored `Text` value from the cache, and the code which reads the file from disk would not be

executed.



This is true _unless_ the cache has filled up in the meantime and the text of `chapter1.txt` has been evicted.

And for an `LruCache` of size `100`, `chapter1.txt` would only be evicted if 100 distinct additions or accesses

of other keys were made since the addition or last access of `chapter1.txt`.



Here is a complete example of a tiny `LruCache`:

```scala

import turbulence.Out

import turbulence.stdioSources.virtualMachine

import gossamer.t



def numbers(): Unit =

  val cache: LruCache[Int, Text] = LruCache(4)



  Out.println(cache(1)(t"one"))

  Out.println(cache(2)(t"two"))

  Out.println(cache(3)(t"three"))

  Out.println(cache(4)(t"four"))

  Out.println(cache(1)(t"uno"))  // (1)

  Out.println(cache(5)(t"five")) // (2)

  Out.println(cache(1)(t"ein"))  // (3)

  Out.println(cache(2)(t"dos"))

```



The output would be: `one`, `two`, `three`, `four`, `one`, `five`, `one`, `dos`. Note that values retrieved on

the line marked _1_ is not `uno`, since at this point, the cache already contains the value `one` and the second

parameter (which produces `uno`) is not evaluated.



On the line marked _2_, a fifth value, `five`, is inserted. This would exceed the maximum size of the cache, so

one element must be evicted. That value is `two` because it is the least-recently-used value. Were it not for

the previous line (_1_), `one` would be the least-recently-used value, and the line marked _3_ would result in

the evaluation of `ein` and its insertion into the cache; but since `one` remains in the cache at the same

index, it is retrieved instead.



## Status



Camouflage is classified as __embryotic__. For reference, Soundness projects are

categorized into one of the following five stability levels:



- _embryonic_: for experimental or demonstrative purposes only, without any guarantees of longevity

- _fledgling_: of proven utility, seeking contributions, but liable to significant redesigns

- _maturescent_: major design decisions broady settled, seeking probatory adoption and refinement

- _dependable_: production-ready, subject to controlled ongoing maintenance and enhancement; tagged as version `1.0.0` or later

- _adamantine_: proven, reliable and production-ready, with no further breaking changes ever anticipated



Projects at any stability level, even _embryonic_ projects, can still be used,

as long as caution is taken to avoid a mismatch between the project's stability

level and the required stability and maintainability of your own project.



Camouflage is designed to be _small_. Its entire source code currently consists

of 94 lines of code.



## Building



Camouflage will ultimately be built by Fury, when it is published. In the

meantime, two possibilities are offered, however they are acknowledged to be

fragile, inadequately tested, and unsuitable for anything more than

experimentation. They are provided only for the necessity of providing _some_

answer to the question, "how can I try Camouflage?".



1. *Copy the sources into your own project*

   

   Read the `fury` file in the repository root to understand Camouflage's build

   structure, dependencies and source location; the file format should be short

   and quite intuitive. Copy the sources into a source directory in your own

   project, then repeat (recursively) for each of the dependencies.



   The sources are compiled against the latest nightly release of Scala 3.

   There should be no problem to compile the project together with all of its

   dependencies in a single compilation.



2. *Build with [Wrath](https://github.com/propensive/wrath/)*



   Wrath is a bootstrapping script for building Camouflage and other projects in

   the absence of a fully-featured build tool. It is designed to read the `fury`

   file in the project directory, and produce a collection of JAR files which can

   be added to a classpath, by compiling the project and all of its dependencies,

   including the Scala compiler itself.

   

   Download the latest version of

   [`wrath`](https://github.com/propensive/wrath/releases/latest), make it

   executable, and add it to your path, for example by copying it to

   `/usr/local/bin/`.



   Clone this repository inside an empty directory, so that the build can

   safely make clones of repositories it depends on as _peers_ of `camouflage`.

   Run `wrath -F` in the repository root. This will download and compile the

   latest version of Scala, as well as all of Camouflage's dependencies.



   If the build was successful, the compiled JAR files can be found in the

   `.wrath/dist` directory.



## Contributing



Contributors to Camouflage are welcome and encouraged. New contributors may like

to look for issues marked

[beginner](https://github.com/propensive/camouflage/labels/beginner).



We suggest that all contributors read the [Contributing

Guide](/contributing.md) to make the process of contributing to Camouflage

easier.



Please __do not__ contact project maintainers privately with questions unless

there is a good reason to keep them private. While it can be tempting to

repsond to such questions, private answers cannot be shared with a wider

audience, and it can result in duplication of effort.



## Author



Camouflage was designed and developed by Jon Pretty, and commercial support and

training on all aspects of Scala 3 is available from [Propensive

OÜ](https://propensive.com/).



## Name



Camouflage is used for hiding something, which is the original meaning of _caching_.



In general, Soundness project names are always chosen with some rationale,

however it is usually frivolous. Each name is chosen for more for its

_uniqueness_ and _intrigue_ than its concision or catchiness, and there is no

bias towards names with positive or "nice" meanings—since many of the libraries

perform some quite unpleasant tasks.



Names should be English words, though many are obscure or archaic, and it

should be noted how willingly English adopts foreign words. Names are generally

of Greek or Latin origin, and have often arrived in English via a romance

language.



## Logo



The logo shows a swatch of fabric in typical military camouflage colors.



## License



Camouflage is copyright © 2025 Jon Pretty & Propensive OÜ, and

is made available under the [Apache 2.0 License](/license.md).