From c6a2d5b3262ca358d357435fcaa2ff191d39b314 Mon Sep 17 00:00:00 2001
From: Johannes Link <business@johanneslink.net>
Date: Wed, 15 Nov 2023 10:56:44 +0100
Subject: [PATCH] Fixed out of memory error in shrinking. See
 https://github.com/jqwik-team/jqwik/issues/525

---
 .../jqwik/engine/support/Combinatorics.java   | 48 ++++++++++++++++---
 .../engine/support/CombinatoricsTests.java    | 19 ++++++++
 2 files changed, 60 insertions(+), 7 deletions(-)

diff --git a/engine/src/main/java/net/jqwik/engine/support/Combinatorics.java b/engine/src/main/java/net/jqwik/engine/support/Combinatorics.java
index d5d0d2fd7..19a55c413 100644
--- a/engine/src/main/java/net/jqwik/engine/support/Combinatorics.java
+++ b/engine/src/main/java/net/jqwik/engine/support/Combinatorics.java
@@ -2,6 +2,7 @@
 
 import java.util.ArrayList;
 import java.util.*;
+import java.util.function.*;
 import java.util.stream.*;
 
 import net.jqwik.api.*;
@@ -81,15 +82,48 @@ public static Stream<Tuple2<Integer, Integer>> distinctPairs(int maxExclusive) {
 		if (maxExclusive < 2) {
 			return Stream.empty();
 		}
-		List<Tuple2<Integer, Integer>> pairs = new ArrayList<>();
-		for (int i = 0; i < maxExclusive; i++) {
-			for (int j = i + 1; j < maxExclusive; j++) {
-				if (i != j) {
-					pairs.add(Tuple.of(i, j));
-				}
+		return StreamSupport.stream(new PairSpliterator(maxExclusive), false);
+	}
+
+	private static class PairSpliterator implements Spliterator<Tuple2<Integer, Integer>> {
+		private final int maxExclusive;
+
+		private int i = 0;
+		private int j = 1;
+
+		public PairSpliterator(int maxExclusive) {
+			this.maxExclusive = maxExclusive;
+		}
+
+		@Override
+		public boolean tryAdvance(Consumer<? super Tuple2<Integer, Integer>> action) {
+			if (j >= maxExclusive) {
+				return false;
+			}
+			action.accept(Tuple.of(i, j));
+			j += 1;
+			if (j >= maxExclusive) {
+				i += 1;
+				j = i + 1;
 			}
+			return true;
+		}
+
+		@Override
+		public Spliterator<Tuple2<Integer, Integer>> trySplit() {
+			return null;
+		}
+
+		@Override
+		public long estimateSize() {
+			return (long) maxExclusive * (maxExclusive - 1) / 2;
+		}
+
+		@Override
+		public int characteristics() {
+			return Spliterator.DISTINCT | Spliterator.ORDERED | Spliterator.SIZED | Spliterator.NONNULL | Spliterator.IMMUTABLE;
 		}
-		return pairs.stream();
 	}
+
 }
 
diff --git a/engine/src/test/java/net/jqwik/engine/support/CombinatoricsTests.java b/engine/src/test/java/net/jqwik/engine/support/CombinatoricsTests.java
index 282f68033..f5d15e89d 100644
--- a/engine/src/test/java/net/jqwik/engine/support/CombinatoricsTests.java
+++ b/engine/src/test/java/net/jqwik/engine/support/CombinatoricsTests.java
@@ -4,6 +4,7 @@
 import java.util.stream.*;
 
 import net.jqwik.api.*;
+import net.jqwik.api.Tuple.*;
 import net.jqwik.api.constraints.*;
 
 import static java.util.Arrays.*;
@@ -27,6 +28,24 @@ <T> void concatenatingIterables(@ForAll @Size(max = 10) List<@Size(max = 10) Lis
 		assertThat(iterator.hasNext()).isFalse();
 	}
 
+	@Example
+	void distinctPairs() {
+		assertThat(Combinatorics.distinctPairs(0)).isEmpty();
+		assertThat(Combinatorics.distinctPairs(1)).isEmpty();
+		assertThat(Combinatorics.distinctPairs(2)).containsExactly(
+			Tuple.of(0, 1)
+		);
+		assertThat(Combinatorics.distinctPairs(3)).containsExactly(
+			Tuple.of(0, 1),
+			Tuple.of(0, 2),
+			Tuple.of(1, 2)
+		);
+		// (n * n-1) / 2:
+		assertThat(Combinatorics.distinctPairs(10)).hasSize(45);
+		assertThat(Combinatorics.distinctPairs(100)).hasSize(4950);
+		assertThat(Combinatorics.distinctPairs(1000)).hasSize(499500);
+	}
+
 	@Group
 	@Label("listCombinations")
 	class CombineList {