# UNDF: UNDF-2026-000000015
--- a/src/broad_phase.h
+++ b/src/broad_phase.h
@@ -30,8 +30,9 @@ typedef struct b2BroadPhase
 	// The move set and array are used to track shapes that have moved significantly
 	// and need a pair query for new contacts. The array has a deterministic order.
-	// todo perhaps just a move set?
-	// todo implement a 32bit hash set for faster lookup
-	// todo moveSet can grow quite large on the first time step and remain large
 	b2HashSet moveSet;
 	b2IntArray moveArray;
+	// Maps proxyKey+1 → index into moveArray for O(1) removal.
+	// Updated on every push and on every RemoveSwap (displaced element index fix-up).
+	b2HashTable moveIndex; // key: proxyKey+1, value: array index (stored in value field)

--- a/src/broad_phase.c
+++ b/src/broad_phase.c
@@ -35,6 +35,7 @@ void b2CreateBroadPhase( b2BroadPhase* bp )
 	bp->moveSet = b2CreateSet( 16 );
 	bp->moveArray = b2IntArray_Create( 16 );
+	bp->moveIndex = b2CreateTable( 16 );

 	bp->pairSet = b2CreateSet( 16 );
 }
@@ -55,6 +56,7 @@ void b2DestroyBroadPhase( b2BroadPhase* bp )
 	b2DestroySet( &bp->moveSet );
 	b2IntArray_Destroy( &bp->moveArray );
+	b2DestroyTable( &bp->moveIndex );

 	b2DestroySet( &bp->pairSet );

@@ -63,25 +64,29 @@ void b2DestroyBroadPhase( b2BroadPhase* bp )
 // in b2BufferMove — add both to moveSet (O(1)) and moveArray, recording the
 // new array index into moveIndex (O(1)).
 static inline void b2BufferMove( b2BroadPhase* bp, int queryProxy )
 {
 	// Adding 1 because 0 is the sentinel
 	bool alreadyAdded = b2AddKey( &bp->moveSet, queryProxy + 1 );
 	if ( alreadyAdded == false )
 	{
+		int idx = bp->moveArray.count;
 		b2IntArray_Push( &bp->moveArray, queryProxy );
+		b2TableSet( &bp->moveIndex, (uint32_t)( queryProxy + 1 ), (uint32_t)idx );
 	}
 }

 static inline void b2UnBufferMove( b2BroadPhase* bp, int proxyKey )
 {
 	bool found = b2RemoveKey( &bp->moveSet, proxyKey + 1 );

 	if ( found )
 	{
-		// Purge from move buffer. Linear search.
-		// todo if I can iterate the move set then I don't need the moveArray
-		int count = bp->moveArray.count;
-		for ( int i = 0; i < count; ++i )
-		{
-			if ( bp->moveArray.data[i] == proxyKey )
-			{
-				b2IntArray_RemoveSwap( &bp->moveArray, i );
-				break;
-			}
-		}
+		// O(1) index lookup, then RemoveSwap with displaced-element fix-up.
+		uint32_t idx = b2TableGet( &bp->moveIndex, (uint32_t)( proxyKey + 1 ) );
+		b2TableRemove( &bp->moveIndex, (uint32_t)( proxyKey + 1 ) );
+		int last = bp->moveArray.count - 1;
+		if ( (int)idx != last )
+		{
+			// RemoveSwap moves the last element to position idx.
+			int displaced = bp->moveArray.data[last];
+			bp->moveArray.data[idx] = displaced;
+			bp->moveArray.count = last;
+			// Update the displaced element's index in the map.
+			b2TableSet( &bp->moveIndex, (uint32_t)( displaced + 1 ), idx );
+		}
+		else
+		{
+			bp->moveArray.count = last;
+		}
 	}
 }