Make Compress/Count Partition Vector Helper Generally Available

This function pair is useful also when loading a partition from
file to ensure that there are no gaps in block numbering.

opm/simulators/utils/compressPartition.cpp

@@ -0,0 +1,74 @@
+/*
+  Copyright 2023 Equinor ASA
+
+  This file is part of the Open Porous Media Project (OPM).
+
+  OPM is free software: you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation, either version 3 of the License, or
+  (at your option) any later version.
+
+  OPM is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with OPM.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#include <opm/simulators/utils/compressPartition.hpp>
+
+#include <algorithm>
+#include <tuple>
+#include <utility>
+#include <vector>
+
+namespace {
+
+    template <typename T>
+    std::pair<T, T> valueRange(const std::vector<T>& x)
+    {
+        auto mmPos = std::minmax_element(x.begin(), x.end());
+
+        return { *mmPos.first, *mmPos.second };
+    }
+
+} // Anonymous namespace
+
+std::pair<std::vector<int>, int>
+Opm::util::compressAndCountPartitionIDs(std::vector<int>&& parts0)
+{
+    auto parts = std::pair<std::vector<int>, int> { std::move(parts0), 0 };
+
+    auto& [partition, num_domains] = parts;
+
+    if (! partition.empty()) {
+        const auto& [low, high] = valueRange(partition);
+
+        auto seen = std::vector<bool>(high - low + 1, false);
+        for (const auto& domain : partition) {
+            seen[domain - low] = domain >= 0;
+        }
+
+        auto compressed = std::vector<int>(seen.size(), -1);
+        for (auto i = 0*compressed.size(); i < compressed.size(); ++i) {
+            if (seen[i]) {
+                compressed[i] = num_domains++;
+            }
+        }
+
+        for (auto& domain : partition) {
+            if (domain >= 0) {
+                domain = compressed[domain - low];
+            }
+        }
+    }
+
+    return parts;
+}
+
+std::vector<int> Opm::util::compressPartitionIDs(std::vector<int>&& parts0)
+{
+    return compressAndCountPartitionIDs(std::move(parts0)).first;
+}