Rand() function added

Reviewers: mislav.bradac, buda

Reviewed By: mislav.bradac

Subscribers: pullbot

Differential Revision: https://phabricator.memgraph.io/D690

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Major Features and Improvements
 
 * CASE construct (without aggregations)
+* rand() function added
 
 ### Bug Fixes and Other Changes
 

docs/user_technical/open-cypher.md

@@ -440,6 +440,7 @@ functions.
  `sign`       | Applies the signum function to a given number and returns the result. The signum of positive numbers is 1, of negative -1 and for 0 returns 0.
  `e`          | Returns the base of the natural logarithm.
  `pi`         | Returns the constant *pi*.
+ `rand`       | Returns a random floating point number between 0 (inclusive) and 1 (exclusive).
  `startsWith` | Check if the first argument starts with the second.
  `endsWith`   | Check if the first argument ends with the second.
  `contains`   | Check if the first argument has an element which is equal to the second argument.

src/database/graph_db_accessor.cpp

@@ -9,7 +9,9 @@
 #include "utils/on_scope_exit.hpp"
 
 GraphDbAccessor::GraphDbAccessor(GraphDb &db)
-    : db_(db), transaction_(db.tx_engine_.Begin()) {}
+    : db_(db), transaction_(db.tx_engine_.Begin()) {
+  pseudo_rand_gen_.seed(std::random_device()());
+}
 
 GraphDbAccessor::~GraphDbAccessor() {
   if (!commited_ && !aborted_) {
@@ -332,3 +334,5 @@ const std::string &GraphDbAccessor::PropertyName(
   debug_assert(!commited_ && !aborted_, "Accessor committed or aborted");
   return *property;
 }
+
+double GraphDbAccessor::Rand() { return rand_dist_(pseudo_rand_gen_); }

src/database/graph_db_accessor.hpp

@@ -6,6 +6,7 @@
 #pragma once
 
 #include <experimental/optional>
+#include <random>
 
 #include "cppitertools/filter.hpp"
 #include "cppitertools/imap.hpp"
@@ -552,6 +553,11 @@ class GraphDbAccessor {
     if (!accessor.new_) accessor.new_ = accessor.vlist_->update(*transaction_);
   }
 
+  /**
+   * Returns a uniformly random-generated number from the [0, 1) interval.
+   */
+  double Rand();
+
  private:
   /**
    * Insert this vertex into corresponding label and label+property (if it
@@ -584,6 +590,7 @@ class GraphDbAccessor {
   void UpdatePropertyIndex(const GraphDbTypes::Property &property,
                            const RecordAccessor<Vertex> &record_accessor,
                            const Vertex *const vertex);
+
   GraphDb &db_;
 
   /** The current transaction */
@@ -591,4 +598,8 @@ class GraphDbAccessor {
 
   bool commited_{false};
   bool aborted_{false};
+
+  // Random number generation stuff.
+  std::mt19937 pseudo_rand_gen_;
+  std::uniform_real_distribution<> rand_dist_{0, 1};
 };

src/query/interpret/awesome_memgraph_functions.cpp

@@ -453,6 +453,13 @@ TypedValue Pi(const std::vector<TypedValue> &args, GraphDbAccessor &) {
   return M_PI;
 }
 
+TypedValue Rand(const std::vector<TypedValue> &args, GraphDbAccessor &dba) {
+  if (args.size() != 0U) {
+    throw QueryRuntimeException("rand shouldn't be called with arguments");
+  }
+  return dba.Rand();
+}
+
 template <bool (*Predicate)(const std::string &s1, const std::string &s2)>
 TypedValue StringMatchOperator(const std::vector<TypedValue> &args,
                                GraphDbAccessor &) {
@@ -534,6 +541,7 @@ NameToFunction(const std::string &function_name) {
   if (function_name == "SIGN") return Sign;
   if (function_name == "E") return E;
   if (function_name == "PI") return Pi;
+  if (function_name == "RAND") return Rand;
   if (function_name == kStartsWith) return StartsWith;
   if (function_name == kEndsWith) return EndsWith;
   if (function_name == kContains) return Contains;

tests/qa/tck_engine/tests/memgraph_V1/features/functions.feature

@@ -616,6 +616,15 @@ Feature: Functions
             | n                   |
             | 3.141592653589793   |
 
+    Scenario: Rand test:
+        When executing query:
+            """
+            WITH rand() as r RETURN r >= 0.0 AND r < 1.0 as result
+            """
+        Then the result should be:
+            | result |
+            | true   |
+
     Scenario: All test 01:
         When executing query:
             """

tests/unit/graph_db_accessor.cpp

@@ -352,6 +352,19 @@ TEST(GraphDbAccessorTest, Transfer) {
   EXPECT_EQ(dba3->Transfer(e12)->PropsAt(prop).Value<int64_t>(), 12);
 }
 
+TEST(GraphDbAccessorTest, Rand) {
+  Dbms dbms;
+  auto dba = dbms.active();
+
+  double a = dba->Rand();
+  EXPECT_GE(a, 0.0);
+  EXPECT_LT(a, 1.0);
+  double b = dba->Rand();
+  EXPECT_GE(b, 0.0);
+  EXPECT_LT(b, 1.0);
+  EXPECT_NE(a, b);
+}
+
 int main(int argc, char **argv) {
   ::testing::InitGoogleTest(&argc, argv);
   //  ::testing::GTEST_FLAG(filter) = "*.DetachRemoveVertex";

tests/unit/query_expression_evaluator.cpp

@@ -1047,6 +1047,12 @@ TEST(ExpressionEvaluator, FunctionPi) {
   ASSERT_DOUBLE_EQ(EvaluateFunction("PI", {}).Value<double>(), M_PI);
 }
 
+TEST(ExpressionEvaluator, FunctionRand) {
+  ASSERT_THROW(EvaluateFunction("RAND", {1}), QueryRuntimeException);
+  ASSERT_GE(EvaluateFunction("RAND", {}).Value<double>(), 0.0);
+  ASSERT_LT(EvaluateFunction("RAND", {}).Value<double>(), 1.0);
+}
+
 TEST(ExpressionEvaluator, FunctionStartsWith) {
   EXPECT_THROW(EvaluateFunction(kStartsWith, {}), QueryRuntimeException);
   EXPECT_TRUE(EvaluateFunction(kStartsWith, {"a", TypedValue::Null}).IsNull());