SERVER-75471 Rename t-digest related files to better reflect type names

1 files changed, 109 insertions, 0 deletions

diff --git a/src/mongo/db/pipeline/percentile_algo_tdigest_distributed.cpp b/src/mongo/db/pipeline/percentile_algo_tdigest_distributed.cpp
new file mode 100644
index 00000000000..42849e93763
--- /dev/null
+++ b/src/mongo/db/pipeline/percentile_algo_tdigest_distributed.cpp
@@ -0,0 +1,109 @@
+/**
+ *    Copyright (C) 2023-present MongoDB, Inc.
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the Server Side Public License, version 1,
+ *    as published by MongoDB, Inc.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    Server Side Public License for more details.
+ *
+ *    You should have received a copy of the Server Side Public License
+ *    along with this program. If not, see
+ *    <http://www.mongodb.com/licensing/server-side-public-license>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the Server Side Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+#include <algorithm>
+#include <iostream>
+
+#include "mongo/db/pipeline/percentile_algo_tdigest.h"
+
+#include "mongo/db/exec/document_value/value.h"
+#include "mongo/db/query/query_knobs_gen.h"
+#include "mongo/util/assert_util.h"
+
+namespace mongo {
+
+class TDigestDistributedClassic : public TDigest, public PartialPercentile<Value> {
+    constexpr static size_t kNegInfCountOffset = 0;
+    constexpr static size_t kPosInfCountOffset = 1;
+    constexpr static size_t kMinOffset = 2;
+    constexpr static size_t kMaxOffset = 3;
+    constexpr static size_t kCentroidsOffset = 4;
+
+public:
+    TDigestDistributedClassic(ScalingFunction k_limit, int delta) : TDigest(k_limit, delta) {}
+
+    /**
+     * Serialize the digest's state into a flat array of doubles:
+     * [negInfCount, posInfCount, min, max, c0.weight, c0.mean, c1.weight, c1.weight, ...]
+     */
+    Value serialize() final {
+        flushBuffer();
+
+        std::vector<double> flattened(kCentroidsOffset, 0);
+        flattened[kNegInfCountOffset] = _negInfCount;
+        flattened[kPosInfCountOffset] = _posInfCount;
+        flattened[kMinOffset] = _min;
+        flattened[kMaxOffset] = _max;
+
+        flattened.reserve(kCentroidsOffset + 2 * _centroids.size());
+        for (const Centroid& c : _centroids) {
+            flattened.push_back(c.weight);
+            flattened.push_back(c.mean);
+        }
+        return Value(std::vector<Value>(flattened.begin(), flattened.end()));
+    }
+
+    /*
+     * 'partial' is expected to be an array, created by the 'serialize()' above.
+     */
+    void combine(const Value& partial) final {
+        tassert(7492700,
+                "TDigest should have been serialized into an array of even size",
+                partial.isArray() && (partial.getArrayLength() % 2 == 0));
+
+        if (partial.getArrayLength() == 0) {
+            return;  // the other t-digest was empty, nothing to do here
+        }
+
+        tassert(7492701,
+                "Serialized array of non-empty TDigest must contain the min of required elements",
+                partial.getArrayLength() >= kCentroidsOffset);
+
+        const auto& other = partial.getArray();
+        const int64_t negInfCount = other[kNegInfCountOffset].coerceToLong();
+        const int64_t posInfCount = other[kPosInfCountOffset].coerceToLong();
+        const double min = other[kMinOffset].coerceToDouble();
+        const double max = other[kMaxOffset].coerceToDouble();
+
+        std::vector<Centroid> centroids;
+        centroids.reserve((other.size() - kCentroidsOffset) / 2);
+        for (size_t i = kCentroidsOffset; i < other.size(); i += 2) {
+            centroids.push_back({other[i].coerceToDouble(), other[i + 1].coerceToDouble()});
+        }
+
+        merge(TDigest{negInfCount, posInfCount, min, max, centroids, _k_limit, _delta});
+    }
+};
+
+std::unique_ptr<PercentileAlgorithm> createTDigestDistributedClassic() {
+    return std::make_unique<TDigestDistributedClassic>(TDigest::k2_limit,
+                                                       internalQueryTdigestDelta.load());
+}
+
+}  // namespace mongo