implementation of map ranking algorithm on gpu (#5129)

* - implementation of map ranking algorithm
  - also effected necessary suggestions mentioned in the earlier ranking pr's
  - made some performance improvements to the ndcg algo as well

tests/cpp/common/test_device_helpers.cu

@@ -84,3 +84,81 @@ void TestAllocator() {
 TEST(bulkAllocator, Test) {
   TestAllocator();
 }
+
+template <typename T, typename Comp = thrust::less<T>>
+void TestUpperBoundImpl(const std::vector<T> &vec, T val_to_find,
+                        const Comp &comp = Comp()) {
+  EXPECT_EQ(dh::UpperBound(vec.data(), vec.size(), val_to_find, comp),
+            std::upper_bound(vec.begin(), vec.end(), val_to_find, comp) - vec.begin());
+}
+
+template <typename T, typename Comp = thrust::less<T>>
+void TestLowerBoundImpl(const std::vector<T> &vec, T val_to_find,
+                        const Comp &comp = Comp()) {
+  EXPECT_EQ(dh::LowerBound(vec.data(), vec.size(), val_to_find, comp),
+            std::lower_bound(vec.begin(), vec.end(), val_to_find, comp) - vec.begin());
+}
+
+TEST(UpperBound, DataAscending) {
+  std::vector<int> hvec{0, 3, 5, 5, 7, 8, 9, 10, 10};
+
+  // Test boundary conditions
+  TestUpperBoundImpl(hvec, hvec.front());  // Result 1
+  TestUpperBoundImpl(hvec, hvec.front() - 1);  // Result 0
+  TestUpperBoundImpl(hvec, hvec.back() + 1);  // Result hvec.size()
+  TestUpperBoundImpl(hvec, hvec.back());  // Result hvec.size()
+
+  // Test other values - both missing and present
+  TestUpperBoundImpl(hvec, 3);  // Result 2
+  TestUpperBoundImpl(hvec, 4);  // Result 2
+  TestUpperBoundImpl(hvec, 5);  // Result 4
+}
+
+TEST(UpperBound, DataDescending) {
+  std::vector<int> hvec{10, 10, 9, 8, 7, 5, 5, 3, 0, 0};
+  const auto &comparator = thrust::greater<int>();
+
+  // Test boundary conditions
+  TestUpperBoundImpl(hvec, hvec.front(), comparator);  // Result 2
+  TestUpperBoundImpl(hvec, hvec.front() + 1, comparator);  // Result 0
+  TestUpperBoundImpl(hvec, hvec.back(), comparator);  // Result hvec.size()
+  TestUpperBoundImpl(hvec, hvec.back() - 1, comparator);  // Result hvec.size()
+
+  // Test other values - both missing and present
+  TestUpperBoundImpl(hvec, 9, comparator);  // Result 3
+  TestUpperBoundImpl(hvec, 7, comparator);  // Result 5
+  TestUpperBoundImpl(hvec, 4, comparator);  // Result 7
+  TestUpperBoundImpl(hvec, 8, comparator);  // Result 4
+}
+
+TEST(LowerBound, DataAscending) {
+  std::vector<int> hvec{0, 3, 5, 5, 7, 8, 9, 10, 10};
+
+  // Test boundary conditions
+  TestLowerBoundImpl(hvec, hvec.front());  // Result 0
+  TestLowerBoundImpl(hvec, hvec.front() - 1);  // Result 0
+  TestLowerBoundImpl(hvec, hvec.back());  // Result 7
+  TestLowerBoundImpl(hvec, hvec.back() + 1);  // Result hvec.size()
+
+  // Test other values - both missing and present
+  TestLowerBoundImpl(hvec, 3);  // Result 1
+  TestLowerBoundImpl(hvec, 4);  // Result 2
+  TestLowerBoundImpl(hvec, 5);  // Result 2
+}
+
+TEST(LowerBound, DataDescending) {
+  std::vector<int> hvec{10, 10, 9, 8, 7, 5, 5, 3, 0, 0};
+  const auto &comparator = thrust::greater<int>();
+
+  // Test boundary conditions
+  TestLowerBoundImpl(hvec, hvec.front(), comparator);  // Result 0
+  TestLowerBoundImpl(hvec, hvec.front() + 1, comparator);  // Result 0
+  TestLowerBoundImpl(hvec, hvec.back(), comparator);  // Result 8
+  TestLowerBoundImpl(hvec, hvec.back() - 1, comparator);  // Result hvec.size()
+
+  // Test other values - both missing and present
+  TestLowerBoundImpl(hvec, 9, comparator);  // Result 2
+  TestLowerBoundImpl(hvec, 7, comparator);  // Result 4
+  TestLowerBoundImpl(hvec, 4, comparator);  // Result 7
+  TestLowerBoundImpl(hvec, 8, comparator);  // Result 3
+}