Arrow

Code kata to figure out efficient permutation generation and transformation (Colloquially, brute forcing 🕵️)

Goals

• Generate permutations of an ideograph (the english alphabet, numbers, emojis, whatever!)
• Non-recursive computation (space complexity of semi-infinite resources punishes the stack)
• Expose permutations as an infinite resource (A java.util Stream<String> in this case)
• Work on a continous flow of computed permutations in a chain of operations, allowing a "lifecycle" for the infinite resource
• Allow async computation
• Use JMH to measure results

Examples

Simply getting started

final CombinationsService combinationsService = CombinationsService.usingEnglishAlphabet();

combinationsService.computeNext();  // => a
combinationsService.computeNext();  // => b
combinationsService.computeNext();  // => c

Work on permutations as a Stream

final CombinationsService combinationsService = CombinationsService.usingEnglishAlphabet();

// Discard all elements which are shorter than 3 characters
combinationsService.computeCombinations().dropWhile(s -> s.length() < 3).forEach(System.out::println);  // => 'aaa' and onwards

Locking onto a target

final CombinationsService combinationsService = CombinationsService.usingEnglishAlphabet();

// Construct the (no lifecycle hooks) Arrow instance
final Arrow arrow = Arrow.using(combinationsService).build();

try (arrow)
{
    final Future<ArrowResult> result = arrow.tryFind("HELLO");
    final ArrowResult arrowResult = result.get(); 
    // => [result=HELLO, attempts=305223758, duration=15674 ms, performance=19473252.39 op/s]
}

We may add additional lifecycle hooks with "loggers" and "transformers". For example, tranform combinations to MD5 and print to System.out

final Arrow arrow = Arrow.using(combinationsService)
        .withLoggers(System.out::println)
        .withTransformers(HashingTransformers.md5())
        .build();

Of course, adding multiple steps (especially printing to standard out) severely slows down the Stream.

Using the previous setup and the following method call:

arrow.tryFind("02c425157ecd32f259548b33402ff6d3");  // => MD5("zzzz")

The runtime performance is about 0.8% compared to the previous result. If System.out is skipped, it becomes much faster.

[result=zzzz, attempts=3727490, duration=23101 ms, performance=161356.22 op/s]