rabbit hole: refactor dirAdd to find gaps in "full" directories.

- This would potentially be very expensive to implement, so don't.
- However, it might be a good start for a ledger fix option.

include/xrpl/ledger/helpers/NFTokenHelpers.h

@@ -20,6 +20,10 @@ removeTokenOffersWithLimit(
     Keylet const& directory,
     std::size_t maxDeletableOffers);
 
+/** Returns tesSUCCESS if NFToken has few enough offers that it can be burned */
+TER
+notTooManyOffers(ReadView const& view, uint256 const& nftokenID);
+
 /** Finds the specified token in the owner's token directory. */
 std::optional<STObject>
 findToken(ReadView const& view, AccountID const& owner, uint256 const& nftokenID);

include/xrpl/protocol/detail/features.macro

@@ -15,6 +15,7 @@
 // Add new amendments to the top of this list.
 // Keep it sorted in reverse chronological order.
 
+XRPL_FEATURE(DefragDirectories,          Supported::no,  VoteBehavior::DefaultNo)
 XRPL_FEATURE(MPTokensV2,                  Supported::no, VoteBehavior::DefaultNo)
 XRPL_FIX    (Security3_1_3,               Supported::no, VoteBehavior::DefaultNo)
 XRPL_FIX    (PermissionedDomainInvariant, Supported::yes, VoteBehavior::DefaultNo)

src/libxrpl/ledger/ApplyView.cpp

@@ -10,6 +10,14 @@ namespace xrpl {
 
 namespace directory {
 
+struct Gap
+{
+    uint64_t const page;
+    SLE::pointer node;
+    uint64_t const nextPage;
+    SLE::pointer next;
+};
+
 std::uint64_t
 createRoot(
     ApplyView& view,
@@ -111,7 +119,9 @@ insertPage(
     if (page == 0)
         return std::nullopt;
     if (!view.rules().enabled(fixDirectoryLimit) && page >= dirNodeMaxPages)  // Old pages limit
+    {
         return std::nullopt;
+    }
 
     // We are about to create a new node; we'll link it to
     // the chain first:
@@ -133,12 +143,8 @@ insertPage(
     // it's the default.
     if (page != 1)
         node->setFieldU64(sfIndexPrevious, page - 1);
-    XRPL_ASSERT_PARTS(!nextPage, "xrpl::directory::insertPage", "nextPage has default value");
-    /* Reserved for future use when directory pages may be inserted in
-     * between two other pages instead of only at the end of the chain.
     if (nextPage)
         node->setFieldU64(sfIndexNext, nextPage);
-    */
     describe(node);
     view.insert(node);
 
@@ -154,7 +160,7 @@ ApplyView::dirAdd(
     uint256 const& key,
     std::function<void(std::shared_ptr<SLE> const&)> const& describe)
 {
-    auto root = peek(directory);
+    auto const root = peek(directory);
 
     if (!root)
     {
@@ -164,6 +170,43 @@ ApplyView::dirAdd(
 
     auto [page, node, indexes] = directory::findPreviousPage(*this, directory, root);
 
+    if (rules().enabled(featureDefragDirectories))
+    {
+        // If there are more nodes than just the root, and there's no space in
+        // the last one, walk backwards to find one with space, or to find one
+        // missing.
+        std::optional<directory::Gap> gapPages;
+        while (page && indexes.size() >= dirNodeMaxEntries)
+        {
+            // Find a page with space, or a gap in pages.
+            auto [prevPage, prevNode, prevIndexes] =
+                directory::findPreviousPage(*this, directory, node);
+            if (!gapPages && prevPage != page - 1)
+                gapPages.emplace(prevPage, prevNode, page, node);
+            page = prevPage;
+            node = prevNode;
+            indexes = prevIndexes;
+        }
+        // We looped through all the pages back to the root.
+        if (!page)
+        {
+            // If we found a gap, use it.
+            if (gapPages)
+            {
+                return directory::insertPage(
+                    *this,
+                    gapPages->page,
+                    gapPages->node,
+                    gapPages->nextPage,
+                    gapPages->next,
+                    key,
+                    directory,
+                    describe);
+            }
+            std::tie(page, node, indexes) = directory::findPreviousPage(*this, directory, root);
+        }
+    }
+
     // If there's space, we use it:
     if (indexes.size() < dirNodeMaxEntries)
     {

src/libxrpl/ledger/helpers/NFTokenHelpers.cpp

@@ -607,6 +607,33 @@ removeTokenOffersWithLimit(ApplyView& view, Keylet const& directory, std::size_t
     return deletedOffersCount;
 }
 
+TER
+notTooManyOffers(ReadView const& view, uint256 const& nftokenID)
+{
+    std::size_t totalOffers = 0;
+
+    {
+        Dir const buys(view, keylet::nft_buys(nftokenID));
+        for (auto iter = buys.begin(); iter != buys.end(); iter.next_page())
+        {
+            totalOffers += iter.page_size();
+            if (totalOffers > maxDeletableTokenOfferEntries)
+                return tefTOO_BIG;
+        }
+    }
+
+    {
+        Dir const sells(view, keylet::nft_sells(nftokenID));
+        for (auto iter = sells.begin(); iter != sells.end(); iter.next_page())
+        {
+            totalOffers += iter.page_size();
+            if (totalOffers > maxDeletableTokenOfferEntries)
+                return tefTOO_BIG;
+        }
+    }
+    return tesSUCCESS;
+}
+
 bool
 deleteTokenOffer(ApplyView& view, std::shared_ptr<SLE> const& offer)
 {