test(diamond): add comprehensive BTT coverage

src/diamond/DiamondInspectFacet.sol

@@ -114,7 +114,7 @@ contract DiamondInspectFacet {
     function facetFunctionSelectors(address _facet) external view returns (bytes4[] memory facetSelectors) {
         DiamondStorage storage s = getStorage();
         facetSelectors = unpackSelectors(IFacet(_facet).exportSelectors());
-        if (facetSelectors.length == 0 || s.facetNodes[facetSelectors[0]].facet == address(0)) {
+        if (facetSelectors.length == 0 || s.facetNodes[facetSelectors[0]].facet != _facet) {
             facetSelectors = new bytes4[](0);
         }
     }

src/diamond/DiamondMod.sol

@@ -183,6 +183,33 @@ function at(bytes memory selectors, uint256 index) pure returns (bytes4 selector
     }
 }
 
+/*
+ * Add all selectors of a facet to the diamond, except the first.
+ *
+ * The first selector (index 0) is the facet's linked-list node identifier.
+ * It carries prev/next pointers and must be stored by the caller after
+ * resolving the linked-list position. This function stores the remaining
+ * selectors as leaf nodes with zero prev/next links.
+ *
+ * Reverts if any selector already belongs to a different facet.
+ * Returns the number of selectors processed (excluding the first).
+ */
+function addFacetSelectors(address _facet, bytes memory _selectors) returns (uint256 selectorsLength) {
+    DiamondStorage storage s = getDiamondStorage();
+    /*
+     * Shift right by 2 is the same as dividing by 4, but cheaper.
+     * We do this to get the number of selectors.
+     */
+    selectorsLength = _selectors.length >> 2;
+    for (uint256 selectorIndex = 1; selectorIndex < selectorsLength; selectorIndex++) {
+        bytes4 selector = at(_selectors, selectorIndex);
+        if (s.facetNodes[selector].facet != address(0)) {
+            revert CannotAddFunctionToDiamondThatAlreadyExists(selector);
+        }
+        s.facetNodes[selector] = FacetNode(_facet, bytes4(0), bytes4(0));
+    }
+}
+
 function addFacets(address[] memory _facets) {
     DiamondStorage storage s = getDiamondStorage();
     uint256 facetLength = _facets.length;
@@ -235,23 +262,13 @@ function addFacets(address[] memory _facets) {
          */
         s.facetNodes[prevFacetNodeId].nextFacetNodeId = currentFacetNodeId;
     }
-    /*
-     * Shift right by 2 is the same as dividing by 4, but cheaper.
-     * We do this to get the number of selectors
-     */
-    uint256 selectorsLength = selectors.length >> 2;
-    unchecked {
-        facetList.selectorCount += uint32(selectorsLength);
-    }
     /*
      * Add all selectors, except the first, to the diamond.
+     * The first selector was already extracted as currentFacetNodeId above.
+     * The returned count is used to update selectorCount.
      */
-    for (uint256 selectorIndex = 1; selectorIndex < selectorsLength; selectorIndex++) {
-        bytes4 selector = at(selectors, selectorIndex);
-        if (s.facetNodes[selector].facet != address(0)) {
-            revert CannotAddFunctionToDiamondThatAlreadyExists(selector);
-        }
-        s.facetNodes[selector] = FacetNode(facet, bytes4(0), bytes4(0));
+    unchecked {
+        facetList.selectorCount += uint32(addFacetSelectors(facet, selectors));
     }
     /*
      * Reset memory for the main loop.
@@ -296,23 +313,13 @@ function addFacets(address[] memory _facets) {
         facet = nextFacet;
         prevFacetNodeId = currentFacetNodeId;
         currentFacetNodeId = nextFacetNodeId;
-        /*
-         * Shift right by 2 is the same as dividing by 4, but cheaper.
-         * We do this to get the number of selectors.
-         */
-        selectorsLength = selectors.length >> 2;
         /*
          * Add all the selectors of the facet to the diamond, except the first selector.
+         * The first selector (currentFacetNodeId) is the pending linked-list node
+         * that will be stored in the next iteration once nextFacetNodeId is known.
          */
-        for (uint256 selectorIndex = 1; selectorIndex < selectorsLength; selectorIndex++) {
-            bytes4 selector = at(selectors, selectorIndex);
-            if (s.facetNodes[selector].facet != address(0)) {
-                revert CannotAddFunctionToDiamondThatAlreadyExists(selector);
-            }
-            s.facetNodes[selector] = FacetNode(facet, bytes4(0), bytes4(0));
-        }
         unchecked {
-            facetList.selectorCount += uint32(selectorsLength);
+            facetList.selectorCount += uint32(addFacetSelectors(facet, selectors));
         }
         /*
          * Restore Free Memory Pointer to reuse memory from packedSelectors() calls.

src/diamond/DiamondUpgradeFacet.sol

@@ -283,6 +283,33 @@ contract DiamondUpgradeFacet {
         }
     }
 
+    /*
+     * Add all selectors of a facet to the diamond, except the first.
+     *
+     * The first selector (index 0) is the facet's linked-list node identifier.
+     * It carries prev/next pointers and must be stored by the caller after
+     * resolving the linked-list position. This function stores the remaining
+     * selectors as leaf nodes with zero prev/next links.
+     *
+     * Reverts if any selector already belongs to a different facet.
+     * Returns the number of selectors processed (excluding the first).
+     */
+    function addFacetSelectors(address _facet, bytes memory _selectors) internal returns (uint256 selectorsLength) {
+        DiamondStorage storage s = getDiamondStorage();
+        /*
+         * Shift right by 2 is the same as dividing by 4, but cheaper.
+         * We do this to get the number of selectors.
+         */
+        selectorsLength = _selectors.length >> 2;
+        for (uint256 selectorIndex = 1; selectorIndex < selectorsLength; selectorIndex++) {
+            bytes4 selector = at(_selectors, selectorIndex);
+            if (s.facetNodes[selector].facet != address(0)) {
+                revert CannotAddFunctionToDiamondThatAlreadyExists(selector);
+            }
+            s.facetNodes[selector] = FacetNode(_facet, bytes4(0), bytes4(0));
+        }
+    }
+
     function addFacets(address[] calldata _facets) internal {
         DiamondStorage storage s = getDiamondStorage();
         uint256 facetLength = _facets.length;
@@ -335,23 +362,13 @@ contract DiamondUpgradeFacet {
              */
             s.facetNodes[prevFacetNodeId].nextFacetNodeId = currentFacetNodeId;
         }
-        /*
-         * Shift right by 2 is the same as dividing by 4, but cheaper.
-         * We do this to get the number of selectors
-         */
-        uint256 selectorsLength = selectors.length >> 2;
-        unchecked {
-            facetList.selectorCount += uint32(selectorsLength);
-        }
         /*
          * Add all selectors, except the first, to the diamond.
+         * The first selector was already extracted as currentFacetNodeId above.
+         * The returned count is used to update selectorCount.
          */
-        for (uint256 selectorIndex = 1; selectorIndex < selectorsLength; selectorIndex++) {
-            bytes4 selector = at(selectors, selectorIndex);
-            if (s.facetNodes[selector].facet != address(0)) {
-                revert CannotAddFunctionToDiamondThatAlreadyExists(selector);
-            }
-            s.facetNodes[selector] = FacetNode(facet, bytes4(0), bytes4(0));
+        unchecked {
+            facetList.selectorCount += uint32(addFacetSelectors(facet, selectors));
         }
         /*
          * Reset memory for the main loop.
@@ -396,23 +413,13 @@ contract DiamondUpgradeFacet {
             facet = nextFacet;
             prevFacetNodeId = currentFacetNodeId;
             currentFacetNodeId = nextFacetNodeId;
-            /*
-             * Shift right by 2 is the same as dividing by 4, but cheaper.
-             * We do this to get the number of selectors.
-             */
-            selectorsLength = selectors.length >> 2;
             /*
              * Add all the selectors of the facet to the diamond, except the first selector.
+             * The first selector (currentFacetNodeId) is the pending linked-list node
+             * that will be stored in the next iteration once nextFacetNodeId is known.
              */
-            for (uint256 selectorIndex = 1; selectorIndex < selectorsLength; selectorIndex++) {
-                bytes4 selector = at(selectors, selectorIndex);
-                if (s.facetNodes[selector].facet != address(0)) {
-                    revert CannotAddFunctionToDiamondThatAlreadyExists(selector);
-                }
-                s.facetNodes[selector] = FacetNode(facet, bytes4(0), bytes4(0));
-            }
             unchecked {
-                facetList.selectorCount += uint32(selectorsLength);
+                facetList.selectorCount += uint32(addFacetSelectors(facet, selectors));
             }
             /*
              * Restore Free Memory Pointer to reuse memory from packedSelectors() calls.

src/diamond/DiamondUpgradeMod.sol

@@ -256,6 +256,33 @@ function at(bytes memory selectors, uint256 index) pure returns (bytes4 selector
     }
 }
 
+/*
+ * Add all selectors of a facet to the diamond, except the first.
+ *
+ * The first selector (index 0) is the facet's linked-list node identifier.
+ * It carries prev/next pointers and must be stored by the caller after
+ * resolving the linked-list position. This function stores the remaining
+ * selectors as leaf nodes with zero prev/next links.
+ *
+ * Reverts if any selector already belongs to a different facet.
+ * Returns the number of selectors processed (excluding the first).
+ */
+function addFacetSelectors(address _facet, bytes memory _selectors) returns (uint256 selectorsLength) {
+    DiamondStorage storage s = getDiamondStorage();
+    /*
+     * Shift right by 2 is the same as dividing by 4, but cheaper.
+     * We do this to get the number of selectors.
+     */
+    selectorsLength = _selectors.length >> 2;
+    for (uint256 selectorIndex = 1; selectorIndex < selectorsLength; selectorIndex++) {
+        bytes4 selector = at(_selectors, selectorIndex);
+        if (s.facetNodes[selector].facet != address(0)) {
+            revert CannotAddFunctionToDiamondThatAlreadyExists(selector);
+        }
+        s.facetNodes[selector] = FacetNode(_facet, bytes4(0), bytes4(0));
+    }
+}
+
 function addFacets(address[] calldata _facets) {
     DiamondStorage storage s = getDiamondStorage();
     uint256 facetLength = _facets.length;
@@ -308,23 +335,13 @@ function addFacets(address[] calldata _facets) {
          */
         s.facetNodes[prevFacetNodeId].nextFacetNodeId = currentFacetNodeId;
     }
-    /*
-     * Shift right by 2 is the same as dividing by 4, but cheaper.
-     * We do this to get the number of selectors
-     */
-    uint256 selectorsLength = selectors.length >> 2;
-    unchecked {
-        facetList.selectorCount += uint32(selectorsLength);
-    }
     /*
      * Add all selectors, except the first, to the diamond.
+     * The first selector was already extracted as currentFacetNodeId above.
+     * The returned count is used to update selectorCount.
      */
-    for (uint256 selectorIndex = 1; selectorIndex < selectorsLength; selectorIndex++) {
-        bytes4 selector = at(selectors, selectorIndex);
-        if (s.facetNodes[selector].facet != address(0)) {
-            revert CannotAddFunctionToDiamondThatAlreadyExists(selector);
-        }
-        s.facetNodes[selector] = FacetNode(facet, bytes4(0), bytes4(0));
+    unchecked {
+        facetList.selectorCount += uint32(addFacetSelectors(facet, selectors));
     }
     /*
      * Reset memory for the main loop.
@@ -369,23 +386,13 @@ function addFacets(address[] calldata _facets) {
         facet = nextFacet;
         prevFacetNodeId = currentFacetNodeId;
         currentFacetNodeId = nextFacetNodeId;
-        /*
-         * Shift right by 2 is the same as dividing by 4, but cheaper.
-         * We do this to get the number of selectors.
-         */
-        selectorsLength = selectors.length >> 2;
         /*
          * Add all the selectors of the facet to the diamond, except the first selector.
+         * The first selector (currentFacetNodeId) is the pending linked-list node
+         * that will be stored in the next iteration once nextFacetNodeId is known.
          */
-        for (uint256 selectorIndex = 1; selectorIndex < selectorsLength; selectorIndex++) {
-            bytes4 selector = at(selectors, selectorIndex);
-            if (s.facetNodes[selector].facet != address(0)) {
-                revert CannotAddFunctionToDiamondThatAlreadyExists(selector);
-            }
-            s.facetNodes[selector] = FacetNode(facet, bytes4(0), bytes4(0));
-        }
         unchecked {
-            facetList.selectorCount += uint32(selectorsLength);
+            facetList.selectorCount += uint32(addFacetSelectors(facet, selectors));
         }
         /*
          * Restore Free Memory Pointer to reuse memory from packedSelectors() calls.