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diff --git a/src/include/lib/hyperloglog.h b/src/include/lib/hyperloglog.h
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+/*
+ * hyperloglog.h
+ *
+ * A simple HyperLogLog cardinality estimator implementation
+ *
+ * Portions Copyright (c) 2014-2020, PostgreSQL Global Development Group
+ *
+ * Based on Hideaki Ohno's C++ implementation.  The copyright terms of Ohno's
+ * original version (the MIT license) follow.
+ *
+ * src/include/lib/hyperloglog.h
+ */
+
+/*
+ * Copyright (c) 2013 Hideaki Ohno <hide.o.j55{at}gmail.com>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the 'Software'), to
+ * deal in the Software without restriction, including without limitation the
+ * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
+ * sell copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
+ * IN THE SOFTWARE.
+ */
+
+#ifndef HYPERLOGLOG_H
+#define HYPERLOGLOG_H
+
+/*
+ * HyperLogLog is an approximate technique for computing the number of distinct
+ * entries in a set.  Importantly, it does this by using a fixed amount of
+ * memory.  See the 2007 paper "HyperLogLog: the analysis of a near-optimal
+ * cardinality estimation algorithm" for more.
+ *
+ * hyperLogLogState
+ *
+ *		registerWidth		register width, in bits ("k")
+ *		nRegisters			number of registers
+ *		alphaMM				alpha * m ^ 2 (see initHyperLogLog())
+ *		hashesArr			array of hashes
+ *		arrSize				size of hashesArr
+ */
+typedef struct hyperLogLogState
+{
+	uint8		registerWidth;
+	Size		nRegisters;
+	double		alphaMM;
+	uint8	   *hashesArr;
+	Size		arrSize;
+} hyperLogLogState;
+
+extern void initHyperLogLog(hyperLogLogState *cState, uint8 bwidth);
+extern void initHyperLogLogError(hyperLogLogState *cState, double error);
+extern void addHyperLogLog(hyperLogLogState *cState, uint32 hash);
+extern double estimateHyperLogLog(hyperLogLogState *cState);
+extern void freeHyperLogLog(hyperLogLogState *cState);
+
+#endif							/* HYPERLOGLOG_H */