diff --git a/ConflictSet.cpp b/ConflictSet.cpp
index c2c5558..4773963 100644
--- a/ConflictSet.cpp
+++ b/ConflictSet.cpp
@@ -12,6 +12,8 @@
 #include <utility>
 #include <vector>
 
+#define DEBUG 0
+
 __attribute__((always_inline)) void *safe_malloc(size_t s) {
   if (void *p = malloc(s)) {
     return p;
@@ -714,6 +716,48 @@ int getChildGeq(Node *self, int child) {
   return -1;
 }
 
+int getChildLeq(Node *self, int child) {
+  if (self->type == Type::Node4) {
+    auto *self4 = static_cast<Node4 *>(self);
+    for (int i = self->numChildren - 1; i >= 0; --i) {
+      if (i > 0) {
+        assert(self4->index[i - 1] < self4->index[i]);
+      }
+      if (self4->index[i] <= child) {
+        return self4->index[i];
+      }
+    }
+  } else if (self->type == Type::Node16) {
+    auto *self16 = static_cast<Node16 *>(self);
+    for (int i = self->numChildren - 1; i >= 0; --i) {
+      if (i > 0) {
+        assert(self16->index[i - 1] < self16->index[i]);
+      }
+      if (self16->index[i] <= child) {
+        return self16->index[i];
+      }
+    }
+  } else if (self->type == Type::Node48) {
+    auto *self48 = static_cast<Node48 *>(self);
+    // TODO simd
+    for (int i = child; i >= 0; --i) {
+      if (self48->index[i] >= 0) {
+        assert(self48->children[self48->index[i]] != nullptr);
+        return i;
+      }
+    }
+  } else {
+    auto *self256 = static_cast<Node256 *>(self);
+    // TODO simd?
+    for (int i = child; i >= 0; --i) {
+      if (self256->children[i]) {
+        return i;
+      }
+    }
+  }
+  return -1;
+}
+
 static void setChildrenParents(Node *node) {
   for (int i = getChildGeq(node, 0); i >= 0; i = getChildGeq(node, i + 1)) {
     getChildExists(node, i)->parent = node;
@@ -905,15 +949,95 @@ void debugPrintDot(FILE *file, Node *node) {
   fprintf(file, "}\n");
 }
 
-static void insert(Node **self_, std::span<const uint8_t> key,
-                   int64_t writeVersion) {
+void printSearchPath(Node *n) {
+  Arena arena;
+  auto result = vector<char>(arena);
+  for (; n->parent != nullptr; n = n->parent) {
+    result.push_back(n->parentsIndex);
+  }
+  std::reverse(result.begin(), result.end());
+  result.push_back(0);
+  printf("Search path: %s\n", result.data());
+}
+
+Node *prevPhysical(Node *node) {
+  // Move up until there's a node at a lower index than the current search path
+  int selfIndex = 256;
+  for (;;) {
+    if (node->parent == nullptr) {
+      return nullptr;
+    }
+    auto prevChild = getChildLeq(node->parent, node->parentsIndex - 1);
+    if (prevChild >= 0) {
+      node = getChildExists(node->parent, prevChild);
+      break;
+    } else {
+      node = node->parent;
+      selfIndex = prevChild;
+      if (node->entryPresent) {
+        break;
+      }
+    }
+  }
+  // Move down the right spine
+  for (;;) {
+    auto rightMostChild = getChildLeq(node, selfIndex - 1);
+    if (rightMostChild >= 0) {
+      node = getChildExists(node, rightMostChild);
+    } else {
+      break;
+    }
+  }
+  return node;
+}
+
+struct Iterator {
+  Node *n;
+  int cmp;
+};
+
+Iterator lastLeq(Node *n, std::span<const uint8_t> key) {
+  for (;;) {
+    if (key.size() == 0) {
+      if (n->entryPresent) {
+        return {n, 0};
+      } else {
+        break;
+      }
+    } else {
+      int c = getChildLeq(n, key[0]);
+      if (c == key[0]) {
+        n = getChildExists(n, c);
+        key = key.subspan(1, key.size() - 1);
+      } else if (c >= 0) {
+        n = getChildExists(n, c);
+        break;
+      } else {
+        break;
+      }
+    }
+  }
+  for (;;) {
+    if (n->entryPresent) {
+      break;
+    }
+    n = prevPhysical(n);
+    assert(n != nullptr);
+  }
+  return {n, -1};
+}
+
+void insert(Node **self_, std::span<const uint8_t> key, int64_t writeVersion) {
   for (;;) {
     auto &self = *self_;
     self->maxVersion = writeVersion;
     if (key.size() == 0) {
+      auto l = lastLeq(self, key);
       self->entryPresent = true;
       self->entry.pointVersion = writeVersion;
-      // TODO set correct rangeVersion
+      assert(l.n != nullptr);
+      assert(l.n->entryPresent);
+      self->entry.rangeVersion = l.n->entry.rangeVersion;
       return;
     }
     auto &child = getOrCreateChild(self, key.front());
@@ -928,7 +1052,25 @@ static void insert(Node **self_, std::span<const uint8_t> key,
 }
 
 struct __attribute__((visibility("hidden"))) ConflictSet::Impl {
-  void check(const ReadRange *, Result *, int) const {}
+  void check(const ReadRange *reads, Result *result, int count) const {
+    for (int i = 0; i < count; ++i) {
+      const auto &r = reads[i];
+      if (r.readVersion < oldestVersion) {
+        result[i] = TooOld;
+        continue;
+      }
+      // TODO support non-point reads
+      assert(r.end.len == 0);
+      auto [l, c] =
+          lastLeq(root, std::span<const uint8_t>(r.begin.p, r.begin.len));
+      assert(l != nullptr);
+      assert(l->entryPresent);
+      result[i] = (c == 0 ? l->entry.pointVersion : l->entry.rangeVersion) >
+                          r.readVersion
+                      ? Conflict
+                      : Commit;
+    }
+  }
   void addWrites(const WriteRange *writes, int count) {
     for (int i = 0; i < count; ++i) {
       const auto &w = writes[i];
@@ -1092,6 +1234,10 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         writes[i].begin.len = iter->size();
         writes[i].end.len = 0;
         writes[i].writeVersion = v;
+#if DEBUG
+        printf("Write: {%.*s} -> %d\n", writes[i].begin.len, writes[i].begin.p,
+               int(writes[i].writeVersion));
+#endif
       }
       cs.addWrites(writes, numWrites);
       refImpl.addWrites(writes, numWrites);
@@ -1121,6 +1267,10 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         reads[i].begin.len = iter->size();
         reads[i].end.len = 0;
         reads[i].readVersion = v;
+#if DEBUG
+        printf("Read: {%.*s} at %d\n", reads[i].begin.len, reads[i].begin.p,
+               int(reads[i].readVersion));
+#endif
       }
       auto *results1 = new (arena) ConflictSet::Result[numReads];
       auto *results2 = new (arena) ConflictSet::Result[numReads];