From f2239016fe91e89a71ed9d3639bfaad081f98999 Mon Sep 17 00:00:00 2001
From: Conor Hoekstra <36027403+codereport@users.noreply.github.com>
Date: Thu, 9 Sep 2021 00:49:20 -0400
Subject: [PATCH] Minor C++17 cleanup of `groupby.cu`: structured bindings,
 more concise lambda, etc (#9193)

Due to the fact that my soul is slowly melting away due to my current internal PR I am working on, I set up this small cleanup in hopes of getting a dopamine or endorphin hit.

Authors:
  - Conor Hoekstra (https://github.com/codereport)

Approvers:
  - David Wendt (https://github.com/davidwendt)
  - Nghia Truong (https://github.com/ttnghia)
  - MithunR (https://github.com/mythrocks)
  - Jake Hemstad (https://github.com/jrhemstad)

URL: https://github.com/rapidsai/cudf/pull/9193
---
 cpp/src/groupby/hash/groupby.cu | 46 ++++++++++++++-------------------
 1 file changed, 19 insertions(+), 27 deletions(-)

diff --git a/cpp/src/groupby/hash/groupby.cu b/cpp/src/groupby/hash/groupby.cu
index 87f83c6edd6..694ca2afceb 100644
--- a/cpp/src/groupby/hash/groupby.cu
+++ b/cpp/src/groupby/hash/groupby.cu
@@ -537,27 +537,25 @@ void compute_single_pass_aggs(table_view const& keys,
  * `map`.
  */
 template <typename Map>
-std::pair<rmm::device_uvector<size_type>, size_type> extract_populated_keys(
-  Map map, size_type num_keys, rmm::cuda_stream_view stream)
+rmm::device_uvector<size_type> extract_populated_keys(Map map,
+                                                      size_type num_keys,
+                                                      rmm::cuda_stream_view stream)
 {
   rmm::device_uvector<size_type> populated_keys(num_keys, stream);
 
-  auto get_key = [] __device__(auto const& element) {
-    size_type key, value;
-    thrust::tie(key, value) = element;
-    return key;
-  };
+  auto get_key    = [] __device__(auto const& element) { return element.first; };  // first = key
+  auto get_key_it = thrust::make_transform_iterator(map.data(), get_key);
+  auto key_used   = [unused = map.get_unused_key()] __device__(auto key) { return key != unused; };
 
-  auto end_it = thrust::copy_if(
-    rmm::exec_policy(stream),
-    thrust::make_transform_iterator(map.data(), get_key),
-    thrust::make_transform_iterator(map.data() + map.capacity(), get_key),
-    populated_keys.begin(),
-    [unused_key = map.get_unused_key()] __device__(size_type key) { return key != unused_key; });
+  auto end_it = thrust::copy_if(rmm::exec_policy(stream),
+                                get_key_it,
+                                get_key_it + map.capacity(),
+                                populated_keys.begin(),
+                                key_used);
 
-  size_type map_size = end_it - populated_keys.begin();
+  populated_keys.resize(std::distance(populated_keys.begin(), end_it), stream);
 
-  return std::make_pair(std::move(populated_keys), map_size);
+  return populated_keys;
 }
 
 /**
@@ -594,8 +592,8 @@ std::unique_ptr<table> groupby_null_templated(table_view const& keys,
                                               rmm::cuda_stream_view stream,
                                               rmm::mr::device_memory_resource* mr)
 {
-  auto d_keys = table_device_view::create(keys, stream);
-  auto map    = create_hash_map<keys_have_nulls>(*d_keys, include_null_keys, stream);
+  auto d_keys_ptr = table_device_view::create(keys, stream);
+  auto map        = create_hash_map<keys_have_nulls>(*d_keys_ptr, include_null_keys, stream);
 
   // Cache of sparse results where the location of aggregate value in each
   // column is indexed by the hash map
@@ -607,9 +605,7 @@ std::unique_ptr<table> groupby_null_templated(table_view const& keys,
 
   // Extract the populated indices from the hash map and create a gather map.
   // Gathering using this map from sparse results will give dense results.
-  auto map_and_size = extract_populated_keys(*map, keys.num_rows(), stream);
-  rmm::device_uvector<size_type> gather_map{std::move(map_and_size.first)};
-  size_type const map_size = map_and_size.second;
+  auto gather_map = extract_populated_keys(*map, keys.num_rows(), stream);
 
   // Compact all results from sparse_results and insert into cache
   sparse_to_dense_results(keys,
@@ -617,19 +613,15 @@ std::unique_ptr<table> groupby_null_templated(table_view const& keys,
                           &sparse_results,
                           cache,
                           gather_map,
-                          map_size,
+                          gather_map.size(),
                           *map,
                           keys_have_nulls,
                           include_null_keys,
                           stream,
                           mr);
 
-  return cudf::detail::gather(keys,
-                              gather_map.begin(),
-                              gather_map.begin() + map_size,
-                              out_of_bounds_policy::DONT_CHECK,
-                              stream,
-                              mr);
+  return cudf::detail::gather(
+    keys, gather_map.begin(), gather_map.end(), out_of_bounds_policy::DONT_CHECK, stream, mr);
 }
 
 }  // namespace