FindFile: use std::lower_bound reduce the repeated code. (#4372)

Summary:
`FindFile()` and  `FindFileInRange()` actually works as the same
of `std::lower_bound()`. Use `std::lower_bound()` to reduce the
repeated code.

- change `FindFile()` and `FindFileInRange()` to use `std::lower_bound()`

Signed-off-by: JiYou <jiyou09@gmail.com>
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4372

Differential Revision: D9919677

Pulled By: ajkr

fbshipit-source-id: f74aaa30e2f80e410e299c5a5bca4eaf2a7a26de

db/compacted_db_impl.cc

@@ -25,22 +25,12 @@ CompactedDBImpl::~CompactedDBImpl() {
 }
 
 size_t CompactedDBImpl::FindFile(const Slice& key) {
-  size_t left = 0;
   size_t right = files_.num_files - 1;
-  while (left < right) {
-    size_t mid = (left + right) >> 1;
-    const FdWithKeyRange& f = files_.files[mid];
-    if (user_comparator_->Compare(ExtractUserKey(f.largest_key), key) < 0) {
-      // Key at "mid.largest" is < "target".  Therefore all
-      // files at or before "mid" are uninteresting.
-      left = mid + 1;
-    } else {
-      // Key at "mid.largest" is >= "target".  Therefore all files
-      // after "mid" are uninteresting.
-      right = mid;
-    }
-  }
-  return right;
+  auto cmp = [&](const FdWithKeyRange& f, const Slice& k) -> bool {
+    return user_comparator_->Compare(ExtractUserKey(f.largest_key), k) < 0;
+  };
+  return static_cast<size_t>(std::lower_bound(files_.files,
+                            files_.files + right, key, cmp) - files_.files);
 }
 
 Status CompactedDBImpl::Get(const ReadOptions& options, ColumnFamilyHandle*,

db/forward_iterator.cc

@@ -916,21 +916,13 @@ bool ForwardIterator::TEST_CheckDeletedIters(int* pdeleted_iters,
 uint32_t ForwardIterator::FindFileInRange(
     const std::vector<FileMetaData*>& files, const Slice& internal_key,
     uint32_t left, uint32_t right) {
-  while (left < right) {
-    uint32_t mid = (left + right) / 2;
-    const FileMetaData* f = files[mid];
-    if (cfd_->internal_comparator().InternalKeyComparator::Compare(
-          f->largest.Encode(), internal_key) < 0) {
-      // Key at "mid.largest" is < "target".  Therefore all
-      // files at or before "mid" are uninteresting.
-      left = mid + 1;
-    } else {
-      // Key at "mid.largest" is >= "target".  Therefore all files
-      // after "mid" are uninteresting.
-      right = mid;
-    }
-  }
-  return right;
+  auto cmp = [&](const FileMetaData* f, const Slice& key) -> bool {
+    return cfd_->internal_comparator().InternalKeyComparator::Compare(
+            f->largest.Encode(), key) < 0;
+  };
+  const auto &b = files.begin();
+  return static_cast<uint32_t>(std::lower_bound(b + left,
+                                 b + right, internal_key, cmp) - b);
 }
 
 void ForwardIterator::DeleteIterator(InternalIterator* iter, bool is_arena) {

db/version_set.cc

@@ -62,20 +62,12 @@ int FindFileInRange(const InternalKeyComparator& icmp,
     const Slice& key,
     uint32_t left,
     uint32_t right) {
-  while (left < right) {
-    uint32_t mid = (left + right) / 2;
-    const FdWithKeyRange& f = file_level.files[mid];
-    if (icmp.InternalKeyComparator::Compare(f.largest_key, key) < 0) {
-      // Key at "mid.largest" is < "target".  Therefore all
-      // files at or before "mid" are uninteresting.
-      left = mid + 1;
-    } else {
-      // Key at "mid.largest" is >= "target".  Therefore all files
-      // after "mid" are uninteresting.
-      right = mid;
-    }
-  }
-  return right;
+  auto cmp = [&](const FdWithKeyRange& f, const Slice& k) -> bool {
+    return icmp.InternalKeyComparator::Compare(f.largest_key, k) < 0;
+  };
+  const auto &b = file_level.files;
+  return static_cast<int>(std::lower_bound(b + left,
+                                           b + right, key, cmp) - b);
 }
 
 Status OverlapWithIterator(const Comparator* ucmp,