From ca6df3178106c0933c916cc2f5435c7804d53dd3 Mon Sep 17 00:00:00 2001
From: dpoeira <dpoeira@infinera.com>
Date: Wed, 28 Oct 2020 12:16:27 +0000
Subject: [PATCH] Fixed a bug where the first element of a Query was being
 iterated twice when the source was a list in conjunction with the reduce
 operation.

---
 .../java/org/jayield/advs/AdvancerList.java   |  7 +++++-
 .../java/org/jayield/QueryTraverseTest.java   | 24 +++++++++++++++++++
 2 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/src/main/java/org/jayield/advs/AdvancerList.java b/src/main/java/org/jayield/advs/AdvancerList.java
index 98d32f5..6983599 100644
--- a/src/main/java/org/jayield/advs/AdvancerList.java
+++ b/src/main/java/org/jayield/advs/AdvancerList.java
@@ -25,14 +25,17 @@
 public class AdvancerList<U> implements Advancer<U> {
     private final List<U> data;
     private final Iterator<U> current;
+    private int index;
 
     public AdvancerList(List<U> data) {
         this.data = data;
         this.current = data.iterator();
+        index = 0;
     }
 
     @Override
     public U next() {
+        index++;
         return current.next();
     }
 
@@ -43,6 +46,8 @@ public boolean hasNext() {
 
     @Override
     public void traverse(Yield<? super U> yield) {
-        data.forEach(yield::ret);
+        for (int i = index; i < data.size(); i++) {
+            yield.ret(data.get(i));
+        }
     }
 }
diff --git a/src/test/java/org/jayield/QueryTraverseTest.java b/src/test/java/org/jayield/QueryTraverseTest.java
index bfdffb1..3b9ba4e 100644
--- a/src/test/java/org/jayield/QueryTraverseTest.java
+++ b/src/test/java/org/jayield/QueryTraverseTest.java
@@ -17,6 +17,7 @@
 package org.jayield;
 
 import static java.util.Arrays.asList;
+import static org.jayield.Query.fromList;
 import static org.jayield.Query.fromStream;
 import static org.jayield.Query.iterate;
 import static org.jayield.Query.of;
@@ -293,6 +294,29 @@ public void testReduce() {
         assertEquals(actual, expected);
     }
 
+    @Test
+    public void testFlatMapAndReduce() {
+        List<Query<String>> input = new ArrayList<>();
+        input.add(Query.of("a"));
+        input.add(Query.of("b"));
+        input.add(Query.of("c"));
+        String expected = "abc";
+        String actual = fromList(input).flatMap(s -> s).reduce((p, c) -> p + c).orElseThrow();
+        assertEquals(actual, expected);
+    }
+
+
+    @Test
+    public void testFromListFlatMapAndReduce() {
+        List<Query<String>> input = new ArrayList<>();
+        input.add(fromList(List.of("a")));
+        input.add(fromList(List.of("b")));
+        input.add(fromList(List.of("c")));
+        String expected = "abc";
+        String actual = fromList(input).flatMap(s -> s).reduce((p, c) -> p + c).orElseThrow();
+        assertEquals(actual, expected);
+    }
+
     @Test
     public void testReduceOnEmpty() {
         String[] input = {};