Add Flux.unfold

[akuleshov7]

Summary

This PR introduces a new unfold operator to Flux, providing a functional approach for generating a sequence from an initial seed value. The unfold method offers a declarative alternative to the existing generate operator, inspired by similar functionality in Scala and Haskell.

Motivation

The unfold operator enhances the API by allowing users to construct sequences in a functional style, which may be more familiar to developers coming from FP-centric languages. While functionally similar to generate, unfold focuses on a clean, declarative approach to defining recursive sequences.

Functional approach

The current SynchronousSink API, while effective, operates at a low level and encourages side effects. In contrast, using a pure function S -> (T, S) eliminates these risks and offers a more reliable way to model state transitions.

The proposed unfold operator introduces a declarative approach to codata definition, where each step produces the next value and state. This leads to clearer, more maintainable code:

// evens [0, 2, 4, 6, ...]
Flux.unfold(0, s -> Optional.of(Tuples.of(s, s + 2)));

// fib [0, 1, 1, 2, 3, 5, 8, ...]
Flux.unfold(Tuples.of(0, 1), s -> {
    var a = s.getT1();
    var b = s.getT2();
    return Optional.of(Tuples.of(a, Tuples.of(b, a + b)));
});

// countdown [3, 2, 1]
Flux.unfold(3, s -> s == 0 ? Optional.empty() : Optional.of(Tuples.of(s, s - 1))); Asynchronous variant

State-of-the-art libraries like fs2 and zio-streams have set the pattern with unfoldEval and unfoldZIO. In our case, an API like this would be a logical extension:

abstract <A, S> Flux unfoldMono(S init, Function<S, Mono<Optional<Tuple2<A, S>>>> f); However, this is beyond the scope of this PR, as Flux.generate requires immediate state calculation, which doesn't align with asynchronous Mono.

Next steps

If the proposal is interesting, I’ll add documentation and tests. For a deeper theoretical foundation, I recommend Conal Elliott’s talk on folds and unfolds.

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[bissquit]

LGTM