Add more CHECKs and use .at() to access a value in the map

Bug: 787460
Change-Id: Ic2666e71531b6f1d593fb8a8fd500d1b42b1f7ce
Reviewed-on: https://chromium-review.googlesource.com/850539
Commit-Queue: Varun Khaneja <vakh@chromium.org>
Reviewed-by: Nathan Parker <nparker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#527151}

components/safe_browsing/db/v4_store.cc

@@ -494,9 +494,12 @@ bool V4Store::GetNextSmallestUnmergedPrefix(
 
   for (const auto& iterator_pair : iterator_map) {
     PrefixSize prefix_size = iterator_pair.first;
+    CHECK_GE(prefix_size, 4u);  // Convert to DCHECK after fixing 787460.
     HashPrefixes::const_iterator start = iterator_pair.second;
 
-    CHECK(hash_prefix_map.end() != hash_prefix_map.find(prefix_size));
+    CHECK(hash_prefix_map.end() !=
+          hash_prefix_map.find(
+              prefix_size));  // Convert to DCHECK after fixing 787460.
     const HashPrefixes& hash_prefixes = hash_prefix_map.at(prefix_size);
 
     PrefixSize distance = std::distance(start, hash_prefixes.end());
@@ -600,12 +603,14 @@ ApplyUpdateResult V4Store::MergeUpdate(const HashPrefixMap& old_prefixes_map,
 
       // Update the iterator map, which means that we have merged one hash
       // prefix of size |next_smallest_prefix_size| from the old store.
-      old_iterator_map[next_smallest_prefix_size] += next_smallest_prefix_size;
+      old_iterator_map.at(next_smallest_prefix_size) +=
+          next_smallest_prefix_size;
 
       if (!raw_removals || removals_iter == raw_removals->end() ||
           *removals_iter != total_picked_from_old) {
         // Append the smallest hash to the appropriate list.
-        hash_prefix_map_[next_smallest_prefix_size] += next_smallest_prefix_old;
+        hash_prefix_map_.at(next_smallest_prefix_size) +=
+            next_smallest_prefix_old;
 
         if (calculate_checksum) {
           checksum_ctx->Update(next_smallest_prefix_old.data(),
@@ -625,7 +630,7 @@ ApplyUpdateResult V4Store::MergeUpdate(const HashPrefixMap& old_prefixes_map,
       next_smallest_prefix_size = next_smallest_prefix_additions.size();
 
       // Append the smallest hash to the appropriate list.
-      hash_prefix_map_[next_smallest_prefix_size] +=
+      hash_prefix_map_.at(next_smallest_prefix_size) +=
           next_smallest_prefix_additions;
 
       if (calculate_checksum) {
@@ -635,7 +640,7 @@ ApplyUpdateResult V4Store::MergeUpdate(const HashPrefixMap& old_prefixes_map,
 
       // Update the iterator map, which means that we have merged one hash
       // prefix of size |next_smallest_prefix_size| from the update.
-      additions_iterator_map[next_smallest_prefix_size] +=
+      additions_iterator_map.at(next_smallest_prefix_size) +=
           next_smallest_prefix_size;
 
       // Find the next smallest unmerged element in the additions map.
@@ -821,6 +826,7 @@ bool V4Store::VerifyChecksum() {
   IteratorMap iterator_map;
   HashPrefix next_smallest_prefix;
   InitializeIteratorMap(hash_prefix_map_, &iterator_map);
+  CHECK_EQ(hash_prefix_map_.size(), iterator_map.size());
   bool has_unmerged = GetNextSmallestUnmergedPrefix(
       hash_prefix_map_, iterator_map, &next_smallest_prefix);
 
@@ -832,7 +838,7 @@ bool V4Store::VerifyChecksum() {
 
     // Update the iterator map, which means that we have read one hash
     // prefix of size |next_smallest_prefix_size| from hash_prefix_map_.
-    iterator_map[next_smallest_prefix_size] += next_smallest_prefix_size;
+    iterator_map.at(next_smallest_prefix_size) += next_smallest_prefix_size;
 
     checksum_ctx->Update(next_smallest_prefix.data(),
                          next_smallest_prefix_size);