cpp: map, multimap, move in from cpp-cheat

userland/cpp/multimap.cpp

@@ -0,0 +1,100 @@
+// https://cirosantilli.com/linux-kernel-module-cheat#cpp
+//
+// Map where a key can have multiple values.
+//
+// Simple to implement with map of set or vector:
+//
+// - http://stackoverflow.com/questions/8602068/whats-the-difference-between-stdmultimapkey-value-and-stdmapkey-stds
+// - http://stackoverflow.com/questions/4437862/whats-the-advantage-of-multimap-over-map-of-vectors
+
+#include <cassert> // map, multimap
+#include <map> // map, multimap
+#include <sstream> // stringstream
+#include <vector> // stringstream
+
+int main() {
+    // equal_range iterates over all key value pairs with a given key.
+    {
+        std::multimap<int, int> m{
+            {1, 2},
+            {1, 3},
+            {2, 4}
+        };
+        auto range = m.equal_range(1);
+        auto it = range.first;
+        assert(it->second == 2);
+        it++;
+        assert(it->second == 3);
+        it++;
+        assert(it == range.second);
+    }
+
+    // Iteration over map iterates all pairs.
+    {
+        std::multimap<int, int> m{
+            {1, 2},
+            {1, 3},
+            {2, 4}
+        };
+
+        auto it = m.begin();
+        assert(it->first == 1);
+        assert(it->second == 2);
+
+        it++;
+        assert(it->first == 1);
+        assert(it->second == 3);
+
+        it++;
+        assert(it->first == 2);
+        assert(it->second == 4);
+
+        it++;
+        assert(it == m.end());
+    }
+
+    // # Iterate each key only once
+    //
+    // # Group by key
+    //
+    // - https://stackoverflow.com/questions/9371236/is-there-an-iterator-across-unique-keys-in-a-stdmultimap/41523639#41523639
+    // - http://stackoverflow.com/questions/247818/stlmultimap-how-do-i-get-groups-of-data
+    {
+        std::multimap<int, int> m{
+            {1, 2},
+            {1, 3},
+            {2, 4}
+        };
+        std::vector<int> out;
+
+        //for (auto it = m.begin(), end = m.end(); it != end;) {
+            //auto key = it->first;
+            //out.push_back(key);
+            //do {
+                //it++;
+            //} while (it != end && it->first == key);
+        //}
+
+        auto it = m.begin();
+        auto end = m.end();
+        auto pair = *it;
+        auto key = pair.first;
+        while (true) {
+            // Operate on key.
+            out.push_back(key);
+            decltype(key) next_key;
+            do {
+                // Operate on value.
+                it++;
+                if (it == end) {
+                    goto end;
+                }
+                next_key = it->first;
+            } while (next_key == key);
+            key = next_key;
+        }
+        end:
+
+        assert(out == std::vector<int>({1, 2}));
+    }
+}