ENH: add list operation findMinMax to return label pair.

- when both min and max are required, this is more efficient than
  traversing the list twice.

src/OpenFOAM/containers/Lists/ListOps/ListOps.H

@@ -43,6 +43,7 @@ SourceFiles
 #define ListOps_H
 
 #include "FlatOutput.H"
+#include "labelPair.H"
 #include "labelList.H"
 #include "HashSet.H"
 #include "Map.H"
@@ -360,7 +361,21 @@ labelList findIndices
     label start=0
 );
 
-//- Linear search for the index of the max element.
+
+//- Linear search for the index of the min element,
+//- similar to std::min_element but for lists and returns the index.
+//
+//  \tparam ListType  The input list type
+//
+//  \param input The list to search
+//  \param start The start index in the list (default: 0)
+//
+//  \return The min index or -1 on error.
+template<class ListType>
+label findMin(const ListType& input, label start=0);
+
+//- Linear search for the index of the max element,
+//- similar to std::max_element but for lists and returns the index.
 //
 //  \tparam ListType  The input list type
 //
@@ -372,16 +387,18 @@ template<class ListType>
 label findMax(const ListType& input, label start=0);
 
 
-//- Linear search for the index of the min element.
+//- Linear search for the index of the min/max element,
+//- similar to std::minmax_element but for lists and returns the index.
 //
 //  \tparam ListType  The input list type
 //
 //  \param input The list to search
 //  \param start The start index in the list (default: 0)
 //
-//  \return The min index or -1 on error.
+//  \return The min/max indices as a Pair (min is first max is second)
+//      or (-1,-1) on error.
 template<class ListType>
-label findMin(const ListType& input, label start=0);
+labelPair findMinMax(const ListType& input, label start=0);
 
 
 //- Binary search to find the index of the last element in a sorted list

src/OpenFOAM/containers/Lists/ListOps/ListOpsTemplates.C

@@ -671,6 +671,32 @@ Foam::labelList Foam::findIndices
 }
 
 
+template<class ListType>
+Foam::label Foam::findMin
+(
+    const ListType& input,
+    label start
+)
+{
+    const label len = input.size();
+
+    if (start < 0 || start >= len)
+    {
+        return -1;
+    }
+
+    for (label i = start+1; i < len; ++i)
+    {
+        if (input[i] < input[start])
+        {
+            start = i;
+        }
+    }
+
+    return start;
+}
+
+
 template<class ListType>
 Foam::label Foam::findMax
 (
@@ -698,7 +724,7 @@ Foam::label Foam::findMax
 
 
 template<class ListType>
-Foam::label Foam::findMin
+Foam::labelPair Foam::findMinMax
 (
     const ListType& input,
     label start
@@ -708,18 +734,25 @@ Foam::label Foam::findMin
 
     if (start < 0 || start >= len)
     {
-        return -1;
+        return labelPair(-1,-1);
     }
 
+    label minIdx = start;
+    label maxIdx = start;
+
     for (label i = start+1; i < len; ++i)
     {
-        if (input[i] < input[start])
+        if (input[i] < input[minIdx])
         {
-            start = i;
+            minIdx = i;
+        }
+        if (input[maxIdx] < input[i])
+        {
+            maxIdx = i;
         }
     }
 
-    return start;
+    return labelPair(minIdx, maxIdx);
 }