From b55a0b6d033f90a75dec5132e380e8c05404d9e4 Mon Sep 17 00:00:00 2001
From: Ankit Srivastava <asrivast28@gmail.com>
Date: Thu, 6 Feb 2020 15:15:18 -0500
Subject: [PATCH 1/2] Fix in global_scan and global_scan_inplace. Updating the
 elements on the first processor after exscan may lead to erroneous results.

---
 include/mxx/reduction.hpp | 24 +++++++++++++-----------
 1 file changed, 13 insertions(+), 11 deletions(-)

diff --git a/include/mxx/reduction.hpp b/include/mxx/reduction.hpp
index a84b995..868f57a 100644
--- a/include/mxx/reduction.hpp
+++ b/include/mxx/reduction.hpp
@@ -741,14 +741,14 @@ void global_scan(InIterator begin, InIterator end, OutIterator out, Func func, c
         local_scan(begin, end, out, func);
         // mxx::scan
         typedef typename std::iterator_traits<OutIterator>::value_type T;
-        T sum = T();
-        if (n > 0)
-            sum = *(out+(n-1));
+        T sum = *(out+(n-1));
         T presum = exscan(sum, func, nonzero_comm, commutative);
-        // accumulate previous sum on all local elements
-        for (size_t i = 0; i < n; ++i) {
-            *o = func(presum, *o);
-            ++o;
+        if (nonzero_comm.rank() != 0) {
+            // accumulate previous sum on all local elements
+            for (size_t i = 0; i < n; ++i) {
+                *o = func(presum, *o);
+                ++o;
+            }
         }
     }
 }
@@ -773,10 +773,12 @@ inline void global_scan_inplace(Iterator begin, Iterator end, Func func, const b
         T sum = *(begin + (n-1));
         T presum = exscan(sum, func, nonzero_comm, commutative);
 
-        // accumulate previous sum on all local elements
-        for (size_t i = 0; i < n; ++i) {
-            *o = func(presum, *o);
-            ++o;
+        if (nonzero_comm.rank() != 0) {
+            // accumulate previous sum on all local elements
+            for (size_t i = 0; i < n; ++i) {
+                *o = func(presum, *o);
+                ++o;
+            }
         }
     }
 }

From 9e184c55cff4c9ef6b2045a3d47a6404bb4a259d Mon Sep 17 00:00:00 2001
From: Ankit Srivastava <asrivast28@gmail.com>
Date: Tue, 3 Mar 2020 12:13:30 -0500
Subject: [PATCH 2/2] Added a test that was failing before the fix in
 b55a0b6d033f90a75dec5132e380e8c05404d9e4.

---
 test/test_reductions.cpp | 27 ++++++++++++++++++++++++++-
 1 file changed, 26 insertions(+), 1 deletion(-)

diff --git a/test/test_reductions.cpp b/test/test_reductions.cpp
index bf6d113..c395401 100644
--- a/test/test_reductions.cpp
+++ b/test/test_reductions.cpp
@@ -247,7 +247,7 @@ TEST(MxxReduce, GlobalReduce) {
 
 TEST(MxxReduce, GlobalScan) {
     mxx::comm c;
-    // test reduce with zero elements for some processes
+    // test scan with zero elements for some processes
     size_t n = 0;
     int presize = 0;
     if (c.rank() % 2 == 0) {
@@ -273,6 +273,31 @@ TEST(MxxReduce, GlobalScan) {
     }
 }
 
+TEST(MxxReduce, GlobalScanMin) {
+    mxx::comm c;
+    // test scan with min and an array of positive
+    // elements sorted in ascending order
+    size_t n = c.size();
+    std::vector<int> local(n);
+    for (size_t i = 0; i < n; ++i) {
+        local[i] = (c.rank()*n)+i+1;
+    }
+    // test inplace scan
+    std::vector<int> local_cpy(local);
+    mxx::global_scan_inplace(local_cpy.begin(), local_cpy.end(), mxx::min<int>(), c);
+    for (size_t i = 0; i < n; ++i) {
+        // 1 is both the first as well as the minimum element
+        ASSERT_EQ(1, local_cpy[i]);
+    }
+    // test scan
+    std::vector<int> result = mxx::global_scan(local, mxx::min<int>(), c);
+    ASSERT_EQ(local.size(), result.size());
+    for (size_t i = 0; i < n; ++i) {
+        // 1 is both the first as well as the minimum element
+        ASSERT_EQ(1, result[i]);
+    }
+}
+
 TEST(MxxReduce, GlobalExScan) {
     mxx::comm c;
     // test reduce with zero elements for some processes