Remove useless Scalerk0 and add necessary comments.

Author: LindaSummer

src/types/redis_tdigest.cc

@@ -46,7 +46,7 @@
 
 namespace redis {
 
-// It should be replaced by a iteration of the rocksdb iterator
+// TODO: It should be replaced by a iteration of the rocksdb iterator
 class DummyCentroids {
  public:
   DummyCentroids(const TDigestMetadata& meta_data, const std::vector<Centroid>& centroids)
@@ -68,6 +68,9 @@ class DummyCentroids {
       }
       return iter_ != centroids_.cend();
     }
+
+    // The Prev function can only be called for item is not cend,
+    // because we must gurantee the iterator to be inside the valid range before iteration.
     bool Prev() {
       if (Valid() && iter_ != centroids_.cbegin()) {
         std::advance(iter_, -1);
@@ -306,13 +309,25 @@ rocksdb::Status TDigest::decodeCentroidFromKeyValue(const rocksdb::Slice& key, c
   InternalKey ikey(key, storage_->IsSlotIdEncoded());
   auto subkey = ikey.GetSubKey();
   auto type_flg = static_cast<uint8_t>(SegmentType::kGuardFlag);
-  GetFixed8(&subkey, &type_flg);
+  if (!GetFixed8(&subkey, &type_flg)) {
+    LOG(ERROR) << "corrupted tdigest centroid key, extract type failed";
+    return rocksdb::Status::Corruption("corrupted tdigest centroid key");
+  }
   if (static_cast<SegmentType>(type_flg) != SegmentType::kCentroids) {
-    return rocksdb::Status::Corruption(fmt::format("corrupted tdigest centroid key type: {}", type_flg));
+    LOG(ERROR) << "corrupted tdigest centroid key type: " << type_flg << ", expect to be "
+               << static_cast<uint8_t>(SegmentType::kCentroids);
+    return rocksdb::Status::Corruption("corrupted tdigest centroid key type");
+  }
+  if (!GetDouble(&subkey, &centroid->mean)) {
+    LOG(ERROR) << "corrupted tdigest centroid key, extract mean failed";
+    return rocksdb::Status::Corruption("corrupted tdigest centroid key");
+  }
+
+  if (rocksdb::Slice value_slice = value;  // GetDouble needs a mutable pointer of slice
+      !GetDouble(&value_slice, &centroid->weight)) {
+    LOG(ERROR) << "corrupted tdigest centroid value, extract weight failed";
+    return rocksdb::Status::Corruption("corrupted tdigest centroid value");
   }
-  GetDouble(&subkey, &centroid->mean);
-  rocksdb::Slice value_slice = value;  // GetDouble needs a mutable pointer of slice
-  GetDouble(&value_slice, &centroid->weight);
   return rocksdb::Status::OK();
 }
 

src/types/tdigest.cc

@@ -35,16 +35,6 @@ refer to https://github.com/apache/arrow/blob/27bbd593625122a4a25d9471c8aaf5df54
 #include "common/status.h"
 
 namespace {
-// scale function K0: linear function, as baseline
-struct ScalerK0 {
-  explicit ScalerK0(uint32_t delta) : delta_norm(delta / 2.0) {}
-
-  double K(double q) const { return delta_norm * q; }
-  double Q(double k) const { return k / delta_norm; }
-
-  const double delta_norm;
-};
-
 // scale function K1
 struct ScalerK1 {
   explicit ScalerK1(uint32_t delta) : delta_norm(delta / (2.0 * M_PI)) {}