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import java.util.Arrays;
import java.util.List;
import java.util.stream.Gatherers;
import java.util.stream.Stream;

import org.junit.jupiter.api.Test;
import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.MethodSource;
import org.junit.jupiter.params.provider.ValueSource;
import static org.junit.jupiter.api.Assertions.*;
import static org.junit.jupiter.api.Assumptions.*;

/**
 * @test
 * @summary Tests the API and contract of Gatherers.windowSliding
 * @run junit GatherersWindowSlidingTest
 */

public class GatherersWindowSlidingTest {

    record Config(int streamSize, boolean parallel) {
        Stream<Integer> stream() {
            var stream = Stream.iterate(1, i -> i + 1).limit(streamSize);
            stream = parallel ? stream.parallel() : stream.sequential();
            return stream;
        }
    }

    static final Stream<Integer> sizes(){
        return Stream.of(0,1,10,33,99,9999);
    }

    static final Stream<Config> sequentialAndParallel(int size) {
        return Stream.of(false, true)
                .map(parallel ->
                        new Config(size, parallel));
    }

    static final Stream<Config> configurations() {
        return sizes().flatMap(i -> sequentialAndParallel(i));
    }

    static final Stream<Config> nonempty_configurations() {
        return sizes().filter(i -> i > 0).flatMap(i -> sequentialAndParallel(i));
    }

    @ParameterizedTest
    @ValueSource(ints = { Integer.MIN_VALUE, -999, -1, 0})
    public void throwsIAEWhenWindowSizeIsSmallerThanOne(int windowSize) {
        assertThrows(IllegalArgumentException.class, () -> Gatherers.windowSliding(windowSize));
    }

    @ParameterizedTest
    @MethodSource("nonempty_configurations")
    public void behavesAsExpectedWhenWindowSizeIsLargerThanStreamSize(Config config) {
        final var streamSize = config.streamSize();
        final var result = config.stream()
                .gather(Gatherers.windowSliding(streamSize + 1))
                .toList();
        assertEquals(1, result.size());
        assertEquals(config.stream().toList(), result.get(0));
    }

    @Test
    public void toleratesNullElements() {
        assertEquals(
                List.of(
                        Arrays.asList(null, null),
                        Arrays.asList(null, null)
                ),
                Stream.of(null, null, null)
                        .gather(Gatherers.windowSliding(2))
                        .toList());
    }

    @Test
    public void throwsUOEWhenWindowsAreAttemptedToBeModified() {
        var window = Stream.of(1)
                .gather(Gatherers.windowSliding(1))
                .findFirst()
                .get();
        assertThrows(UnsupportedOperationException.class,
                () -> window.add(2));
    }

    @ParameterizedTest
    @MethodSource("configurations")
    public void behavesAsExpected(Config config) {
        final var streamSize = config.streamSize();
        // Tests that the layout of the returned data is as expected
        for (var windowSize : List.of(1, 2, 3, 10)) {
            final var expectLastWindowSize = streamSize < windowSize ? streamSize : windowSize;
            final var expectedNumberOfWindows = streamSize == 0 ? 0 : Math.max(1, 1 + streamSize - windowSize);

            int expectedElement = 0;
            int currentWindow = 0;

            final var result = config.stream()
                    .gather(Gatherers.windowSliding(windowSize))
                    .toList();

            for (var window : result) {
                ++currentWindow;
                assertEquals(currentWindow < expectedNumberOfWindows ? windowSize : expectLastWindowSize, window.size());
                for (var element : window) {
                    assertEquals(++expectedElement, element.intValue());
                }
                // rewind for the sliding motion
                expectedElement -= (window.size() - 1);
            }

            assertEquals(expectedNumberOfWindows, currentWindow);
        }
    }
}