functional-php/tests/Collections/LazyTest.php

<?php
/**
 * Copyright 2026 Kacper Donat
 *
 * @author Kacper "Kadet" Donat <kacper@kadet.net>
 *
 * Full license available in separate LICENSE file
 */

use Kadet\Functional\Collections as c;
use Kadet\Functional\Collections\Lazy as lazy;

class LazyTest extends \PHPUnit\Framework\TestCase
{
    private static function gen(iterable $entries): \Generator
    {
        foreach ($entries as $key => $value) {
            yield $key => $value;
        }
    }

    private static function naturals(): \Generator
    {
        for ($i = 1; ; $i++) {
            yield $i;
        }
    }

    public function testLazyDoesNothingUntilConsumed()
    {
        $calls = 0;
        $spy   = static function ($value) use (&$calls) {
            $calls++;

            return $value;
        };

        $steps = [
            lazy\map($spy), lazy\filter($spy), lazy\reject($spy), lazy\uniqueBy($spy), lazy\each($spy), lazy\tap($spy),
            lazy\flatMap(static function ($value) use ($spy) { $spy($value); return [$value]; }),
        ];

        foreach ($steps as $step) {
            $result = $step(self::gen(['a' => 1, 'b' => 2]));
            $this->assertInstanceOf(\Generator::class, $result);
        }

        $this->assertSame(0, $calls);

        iterator_to_array(lazy\map($spy)(self::gen(['a' => 1, 'b' => 2])));
        $this->assertSame(2, $calls);
    }

    public function testLazyChainSingleWalk()
    {
        $walked = 0;

        $result = c\toArray()(lazy\take(3)(lazy\filter(static fn ($x) => $x % 2 === 0)(lazy\map(static function ($x) use (&$walked) {
            $walked++;

            return $x * 3;
        })(self::naturals()))));

        $this->assertSame([1 => 6, 3 => 12, 5 => 18], $result);
        $this->assertSame(6, $walked);
    }

    public function testLazyChainWorksWithPipe()
    {
        $result = self::naturals()
            |> lazy\map(static fn ($x) => $x * 2)
            |> lazy\skip(1)
            |> lazy\take(2)
            |> c\toArray();

        $this->assertSame([1 => 4, 2 => 6], $result);
    }

    public static function provideEagerEquivalents(): iterable
    {
        $double = static fn ($x) => $x * 2;
        $isTwo  = static fn ($x) => $x === 2;
        $inner  = static fn ($x, $k) => [$x, $k];

        yield 'map'          => [fn () => lazy\map($double), fn () => c\map($double)];
        yield 'filter'       => [fn () => lazy\filter($isTwo), fn () => c\filter($isTwo)];
        yield 'filter null'  => [fn () => lazy\filter(), fn () => c\filter()];
        yield 'reject'       => [fn () => lazy\reject($isTwo), fn () => c\reject($isTwo)];
        yield 'flatMap'      => [fn () => lazy\flatMap($inner), fn () => c\flatMap($inner)];
        yield 'values'       => [fn () => lazy\values(), fn () => c\values()];
        yield 'unique'       => [fn () => lazy\unique(), fn () => c\unique()];
        yield 'uniqueBy'     => [fn () => lazy\uniqueBy($double), fn () => c\uniqueBy($double)];
        yield 'slice'        => [fn () => lazy\slice(1, 1), fn () => c\slice(1, 1)];
        yield 'slice open'   => [fn () => lazy\slice(1), fn () => c\slice(1)];
        yield 'slice neg'    => [fn () => lazy\slice(-2, 1), fn () => c\slice(-2, 1)];
        yield 'slice negend' => [fn () => lazy\slice(0, -1), fn () => c\slice(0, -1)];
        yield 'take'         => [fn () => lazy\take(2), fn () => c\take(2)];
        yield 'skip'         => [fn () => lazy\skip(2), fn () => c\skip(2)];
        yield 'prepend'      => [fn () => lazy\prepend(7, 8), fn () => c\prepend(7, 8)];
        yield 'append'       => [fn () => lazy\append(7, 8), fn () => c\append(7, 8)];
        yield 'concat'       => [fn () => lazy\concat(['x' => 9]), fn () => c\concat(['x' => 9])];
        yield 'each'         => [fn () => lazy\each($double), fn () => c\each($double)];
        yield 'tap'          => [fn () => lazy\tap($double), fn () => c\tap($double)];
    }

    #[\PHPUnit\Framework\Attributes\DataProvider('provideEagerEquivalents')]
    public function testLazyMatchesEagerResults(\Closure $lazy, \Closure $eager)
    {
        $fixture = ['a' => 1, 'b' => 2, 'c' => 2];

        $this->assertSame($eager()($fixture), iterator_to_array($lazy()($fixture)));
    }

    public function testKeysMatchesEager()
    {
        $fixture = ['a' => 1, 'b' => 2];

        $this->assertSame(['a', 'b'], iterator_to_array(lazy\keys()($fixture)));
        $this->assertSame([0, 1], array_keys(iterator_to_array(lazy\keys()($fixture))));
    }

    public function testLazyUniqueStreams()
    {
        $walked = 0;
        $source = (static function () use (&$walked) {
            foreach ([1, 1, 2, 2, 3, 4] as $value) {
                $walked++;
                yield $value;
            }
        })();

        $generator = lazy\unique()($source);
        $this->assertSame(0, $walked);
        $this->assertSame(1, $generator->current());
        $this->assertSame(1, $walked);

        $generator->next();
        $this->assertSame(2, $generator->current());
        $this->assertSame(3, $walked);
    }

    public function testUniqueCanBeReusedAcrossRuns()
    {
        $step = lazy\unique();

        $this->assertSame([1, 2], array_values(iterator_to_array($step([1, 1, 2]))));
        $this->assertSame([1, 2], array_values(iterator_to_array($step([1, 2, 2]))));
    }

    public function testFlatMapRejectsNonIterable()
    {
        $this->expectException(\UnexpectedValueException::class);
        $this->expectExceptionMessage('flatMap(): the callback must return an iterable, got int.');

        iterator_to_array(lazy\flatMap(static fn ($x) => $x)([1]));
    }

    public function testTakeAndSkipRejectNegativeWhenBuilt()
    {
        try {
            lazy\take(-1);
            $this->fail('take(-1) should throw');
        } catch (\InvalidArgumentException $e) {
            $this->assertSame('take(): count must be >= 0, got -1', $e->getMessage());
        }

        $this->expectException(\InvalidArgumentException::class);
        $this->expectExceptionMessage('skip(): count must be >= 0, got -1');
        lazy\skip(-1);
    }

    public function testTakeStopsPullingTheSource()
    {
        $this->assertSame([1, 2], array_values(iterator_to_array(lazy\take(2)(self::naturals()))));
    }

    public function testEachYieldsEverythingThroughWithKeys()
    {
        $seen = [];
        $out  = iterator_to_array(lazy\each(static function ($v, $k) use (&$seen) { $seen[] = [$k, $v]; })(self::gen(['a' => 1, 'b' => 2])));

        $this->assertSame(['a' => 1, 'b' => 2], $out);
        $this->assertSame([['a', 1], ['b', 2]], $seen);
    }

    public function testCallbackIsResolvedOncePerStep()
    {
        $step = lazy\map(static fn ($x) => $x + 1);

        $this->assertSame([2, 3], iterator_to_array($step([1, 2])));
        $this->assertSame([4], iterator_to_array($step([3])));
    }
}