diff --git a/src/main/java/com/onthegomap/flatmap/collections/LongLongMultimap.java b/src/main/java/com/onthegomap/flatmap/collections/LongLongMultimap.java
index a3885614..a7fce845 100644
--- a/src/main/java/com/onthegomap/flatmap/collections/LongLongMultimap.java
+++ b/src/main/java/com/onthegomap/flatmap/collections/LongLongMultimap.java
@@ -1,22 +1,171 @@
 package com.onthegomap.flatmap.collections;
 
+import com.carrotsearch.hppc.LongArrayList;
+import com.carrotsearch.hppc.LongIntHashMap;
+import com.graphhopper.util.StopWatch;
+import java.util.Arrays;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 public interface LongLongMultimap {
 
   void put(long key, long value);
 
-  class FewUnorderedBinarySearchMultimap implements LongLongMultimap {
+  LongArrayList get(long key);
 
-    @Override
-    public void put(long key, long value) {
+  long estimatedMemoryUsageBytes();
 
+  default void putAll(long key, LongArrayList vals) {
+    for (int i = 0; i < vals.size(); i++) {
+      put(key, vals.get(i));
     }
   }
 
-  class ManyOrderedBinarySearchMultimap implements LongLongMultimap {
+  static LongLongMultimap newDensedOrderedMultimap() {
+    return new DenseOrderedHppcMultimap();
+  }
+
+  static LongLongMultimap newSparseUnorderedMultimap() {
+    return new SparseUnorderedBinarySearchMultimap();
+  }
+
+  class SparseUnorderedBinarySearchMultimap implements LongLongMultimap {
+
+    private static final Logger LOGGER = LoggerFactory.getLogger(SparseUnorderedBinarySearchMultimap.class);
+
+    private static final LongArrayList EMPTY_LIST = new LongArrayList();
+    private final LongArrayList keys = new LongArrayList();
+    private final LongArrayList values = new LongArrayList();
+    private volatile boolean prepared = false;
+    private static final ThreadLocal<LongArrayList> resultHolder = ThreadLocal.withInitial(LongArrayList::new);
+
+    protected LongArrayList getResultHolder() {
+      LongArrayList res = resultHolder.get();
+      res.elementsCount = 0;
+      return res;
+    }
 
     @Override
-    public void put(long key, long value) {
+    public void put(long key, long val) {
+      if (val <= 0) {
+        throw new IllegalArgumentException("Invalid value: " + val + " must be >0");
+      }
+      if (prepared) {
+        throw new IllegalArgumentException("Cannot insert after preparing");
+      }
+      keys.add(key);
+      values.add(val);
+    }
 
+    private void prepare() {
+      if (!prepared) {
+        synchronized (this) {
+          if (!prepared) {
+            doPrepare();
+            prepared = true;
+          }
+        }
+      }
+    }
+
+    private void doPrepare() {
+      StopWatch watch = new StopWatch().start();
+
+      LOGGER.info("Sorting long long multimap keys...");
+      long[] sortedKeys = keys.toArray();
+      Arrays.parallelSort(sortedKeys);
+
+      LOGGER.info("Sorted keys, now values...");
+      long[] sortedValues = new long[sortedKeys.length];
+      int from = 0;
+      while (from < keys.size()) {
+        long key = keys.get(from);
+        int to = Arrays.binarySearch(sortedKeys, key);
+        if (to < 0) {
+          throw new IllegalStateException("Key not found: " + key);
+        }
+        // skip back to the first entry for this key
+        while (to >= 0 && sortedKeys[to] == key) {
+          to--;
+        }
+        // skip ahead past values we've already added for this key
+        do {
+          to++;
+        } while (sortedValues[to] != 0);
+        while (from < keys.size() && keys.get(from) == key) {
+          sortedValues[to++] = values.get(from++);
+        }
+      }
+      keys.buffer = sortedKeys;
+      values.buffer = sortedValues;
+      LOGGER.info("Sorted long long multimap " + watch.stop());
+    }
+
+    @Override
+    public LongArrayList get(long key) {
+      prepare();
+      if (keys.isEmpty()) {
+        return EMPTY_LIST;
+      }
+      int size = keys.size();
+      int index = Arrays.binarySearch(keys.buffer, 0, size, key);
+      LongArrayList result = getResultHolder();
+      if (index >= 0) {
+        for (int i = index; i < size && keys.get(i) == key; i++) {
+          result.add(values.get(i));
+        }
+        for (int i = index - 1; i >= 0 && keys.get(i) == key; i--) {
+          result.add(values.get(i));
+        }
+      }
+      return result;
+    }
+
+    @Override
+    public long estimatedMemoryUsageBytes() {
+      return 24L + 8L * keys.buffer.length + 24L + 8L * values.buffer.length;
+    }
+  }
+
+  class DenseOrderedHppcMultimap implements LongLongMultimap {
+
+    private static final LongArrayList EMPTY_LIST = new LongArrayList();
+    private final LongIntHashMap keys = new LongIntHashMap();
+    private final LongArrayList values = new LongArrayList();
+
+    @Override
+    public void putAll(long key, LongArrayList others) {
+      if (others.isEmpty()) {
+        return;
+      }
+      keys.put(key, values.size());
+      values.add(others.size());
+      values.add(others.buffer, 0, others.size());
+    }
+
+    @Override
+    public void put(long key, long val) {
+      putAll(key, LongArrayList.from(val));
+    }
+
+    @Override
+    public LongArrayList get(long key) {
+      int index = keys.getOrDefault(key, -1);
+      if (index >= 0) {
+        LongArrayList result = new LongArrayList();
+        int num = (int) values.get(index);
+        result.add(values.buffer, index + 1, num);
+        return result;
+      } else {
+        return EMPTY_LIST;
+      }
+    }
+
+    @Override
+    public long estimatedMemoryUsageBytes() {
+      return 24L + 8L * keys.keys.length +
+        24L + 4L * keys.values.length +
+        24L + 8L * values.buffer.length;
     }
   }
 }
diff --git a/src/main/java/com/onthegomap/flatmap/read/OpenStreetMapReader.java b/src/main/java/com/onthegomap/flatmap/read/OpenStreetMapReader.java
index 1c7be447..071d74ba 100644
--- a/src/main/java/com/onthegomap/flatmap/read/OpenStreetMapReader.java
+++ b/src/main/java/com/onthegomap/flatmap/read/OpenStreetMapReader.java
@@ -47,12 +47,12 @@ public class OpenStreetMapReader implements Closeable {
   private GHLongObjectHashMap<RelationInfo> relationInfo = new GHLongObjectHashMap<>();
   private final AtomicLong relationInfoSizes = new AtomicLong(0);
   // ~800mb, ~1.6GB when sorting
-  private LongLongMultimap wayToRelations = new LongLongMultimap.FewUnorderedBinarySearchMultimap();
+  private LongLongMultimap wayToRelations = LongLongMultimap.newSparseUnorderedMultimap();
   // for multipolygons need to store way info (20m ways, 800m nodes) to use when processing relations (4.5m)
   // ~300mb
   private LongHashSet waysInMultipolygon = new GHLongHashSet();
   // ~7GB
-  private LongLongMultimap multipolygonWayGeometries = new LongLongMultimap.ManyOrderedBinarySearchMultimap();
+  private LongLongMultimap multipolygonWayGeometries = LongLongMultimap.newDensedOrderedMultimap();
 
   public OpenStreetMapReader(OsmInputFile osmInputFile, LongLongMap nodeDb, Profile profile, Stats stats) {
     this.osmInputFile = osmInputFile;
diff --git a/src/test/java/com/onthegomap/flatmap/collections/LongLongMultimapTest.java b/src/test/java/com/onthegomap/flatmap/collections/LongLongMultimapTest.java
new file mode 100644
index 00000000..837a8a1d
--- /dev/null
+++ b/src/test/java/com/onthegomap/flatmap/collections/LongLongMultimapTest.java
@@ -0,0 +1,164 @@
+package com.onthegomap.flatmap.collections;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import com.carrotsearch.hppc.LongArrayList;
+import java.util.Arrays;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+public abstract class LongLongMultimapTest {
+
+  protected LongLongMultimap map;
+  protected boolean retainInputOrder = false;
+
+  @Test
+  public void missingValue() {
+    assertTrue(map.get(0).isEmpty());
+  }
+
+  @Test
+  public void oneValue() {
+    map.put(1, 1);
+    assertResultLists(LongArrayList.from(), map.get(0));
+    assertResultLists(LongArrayList.from(1), map.get(1));
+    assertResultLists(LongArrayList.from(), map.get(2));
+  }
+
+  @Test
+  public void twoConsecutiveValues() {
+    map.put(1, 1);
+    map.put(2, 2);
+    assertResultLists(LongArrayList.from(), map.get(0));
+    assertResultLists(LongArrayList.from(1), map.get(1));
+    assertResultLists(LongArrayList.from(2), map.get(2));
+    assertResultLists(LongArrayList.from(), map.get(3));
+  }
+
+  @Test
+  public void twoNonconsecutiveValues() {
+    map.put(1, 1);
+    map.put(3, 3);
+    assertResultLists(LongArrayList.from(), map.get(0));
+    assertResultLists(LongArrayList.from(1), map.get(1));
+    assertResultLists(LongArrayList.from(), map.get(2));
+    assertResultLists(LongArrayList.from(3), map.get(3));
+    assertResultLists(LongArrayList.from(), map.get(4));
+  }
+
+  @Test
+  public void returnToFirstKey() {
+    if (retainInputOrder) {
+      return;
+    }
+    map.put(3, 31);
+    map.put(2, 21);
+    map.put(1, 11);
+    map.put(1, 12);
+    map.put(2, 22);
+    map.put(3, 32);
+    map.put(3, 33);
+    map.put(2, 23);
+    map.put(1, 13);
+    assertResultLists(LongArrayList.from(11, 12, 13), map.get(1));
+    assertResultLists(LongArrayList.from(21, 22, 23), map.get(2));
+    assertResultLists(LongArrayList.from(31, 32, 33), map.get(3));
+    assertResultLists(LongArrayList.from(), map.get(4));
+  }
+
+  @Test
+  public void manyInsertsOrdered() {
+    long[] toInsert = new long[10];
+    for (int i = 0; i < 100; i++) {
+      for (int j = 0; j < 10; j++) {
+        toInsert[j] = i * 10 + j + 1;
+      }
+      map.putAll(i, LongArrayList.from(toInsert));
+    }
+    for (int i = 0; i < 100; i++) {
+      assertResultLists(LongArrayList.from(
+        i * 10 + 1,
+        i * 10 + 2,
+        i * 10 + 3,
+        i * 10 + 4,
+        i * 10 + 5,
+        i * 10 + 6,
+        i * 10 + 7,
+        i * 10 + 8,
+        i * 10 + 9,
+        i * 10 + 10
+      ), map.get(i));
+    }
+  }
+
+  private void assertResultLists(LongArrayList expected, LongArrayList actual) {
+    if (!retainInputOrder) {
+      if (!expected.isEmpty()) {
+        Arrays.sort(expected.buffer, 0, expected.size());
+      }
+      if (!actual.isEmpty()) {
+        Arrays.sort(actual.buffer, 0, actual.size());
+      }
+    }
+    assertEquals(expected, actual);
+  }
+
+  @Test
+  public void manyInsertsUnordered() {
+    for (long i = 99; i >= 0; i--) {
+      map.putAll(i, LongArrayList.from(
+        i * 10 + 10,
+        i * 10 + 9,
+        i * 10 + 8,
+        i * 10 + 7,
+        i * 10 + 6,
+        i * 10 + 5,
+        i * 10 + 4,
+        i * 10 + 3,
+        i * 10 + 2,
+        i * 10 + 1
+      ));
+    }
+    for (int i = 0; i < 100; i++) {
+      assertResultLists(LongArrayList.from(
+        i * 10 + 10,
+        i * 10 + 9,
+        i * 10 + 8,
+        i * 10 + 7,
+        i * 10 + 6,
+        i * 10 + 5,
+        i * 10 + 4,
+        i * 10 + 3,
+        i * 10 + 2,
+        i * 10 + 1
+      ), map.get(i));
+    }
+  }
+
+  @Test
+  public void multiInsert() {
+    map.putAll(1, LongArrayList.from(1, 2, 3));
+    map.put(0, 3);
+    assertResultLists(LongArrayList.from(3), map.get(0));
+    assertResultLists(LongArrayList.from(1, 2, 3), map.get(1));
+    assertResultLists(LongArrayList.from(), map.get(2));
+  }
+
+  public static class SparseUnorderedTest extends LongLongMultimapTest {
+
+    @BeforeEach
+    public void setup() {
+      this.map = LongLongMultimap.newSparseUnorderedMultimap();
+    }
+  }
+
+  public static class DenseOrderedTest extends LongLongMultimapTest {
+
+    @BeforeEach
+    public void setup() {
+      retainInputOrder = true;
+      this.map = LongLongMultimap.newDensedOrderedMultimap();
+    }
+  }
+}