From 92b7277b5a32b9762b70b853aeb233b82beeaad0 Mon Sep 17 00:00:00 2001
From: Adam Lugowski <alugowski@gmail.com>
Date: Wed, 15 Nov 2023 20:14:06 -0800
Subject: [PATCH] Add std::copy_n

---
 README.md                 | 16 ++++++++--------
 include/poolstl/algorithm | 15 +++++++++++++++
 tests/poolstl_test.cpp    | 19 +++++++++++++++++++
 3 files changed, 42 insertions(+), 8 deletions(-)

diff --git a/README.md b/README.md
index 24bc12e..3601c3a 100644
--- a/README.md
+++ b/README.md
@@ -16,12 +16,12 @@ It is not meant as a full implementation, only the basics are expected to be cov
 Supports C++11 and higher, C++17 preferred.
 Tested in CI on GCC 7+, Clang/LLVM 5+, Apple Clang, MSVC.
 
-## Implemented algorithms
+## Implemented Algorithms
 Algorithms are added on an as-needed basis. If you need one [open an issue](https://github.com/alugowski/poolSTL/issues) or contribute a PR.
 
 ### `<algorithm>`
 * [all_of](https://en.cppreference.com/w/cpp/algorithm/all_of), [any_of](https://en.cppreference.com/w/cpp/algorithm/any_of), [none_of](https://en.cppreference.com/w/cpp/algorithm/none_of)
-* [copy](https://en.cppreference.com/w/cpp/algorithm/copy)
+* [copy](https://en.cppreference.com/w/cpp/algorithm/copy), [copy_n](https://en.cppreference.com/w/cpp/algorithm/copy_n)
 * [fill](https://en.cppreference.com/w/cpp/algorithm/fill), [fill_n](https://en.cppreference.com/w/cpp/algorithm/fill_n)
 * [find](https://en.cppreference.com/w/cpp/algorithm/find), [find_if](https://en.cppreference.com/w/cpp/algorithm/find_if), [find_if_not](https://en.cppreference.com/w/cpp/algorithm/find_if_not)
 * [for_each](https://en.cppreference.com/w/cpp/algorithm/for_each), [for_each_n](https://en.cppreference.com/w/cpp/algorithm/for_each_n)
@@ -49,21 +49,21 @@ int main() {
     std::vector<int> v = {0, 1, 2, 3, 4, 5};
     auto sum = std::reduce(poolstl::par, v.cbegin(), v.cend());
     //                     ^^^^^^^^^^^^
-    //                     Just add this to make your code parallel.
+    //                     Add this to make your code parallel.
     std::cout << "Sum=" << sum << std::endl;
     return 0;
 }
 ```
 
-### Pool control
+### Controlling Thread Pool Size
 
 The thread pool used by `poolstl::par` is managed internally by poolSTL. It is started on first use.
 
-Full control over thread count, startup/shutdown, etc. with your own [thread pool](https://github.com/alugowski/task-thread-pool)
-and `poolstl::par_pool`:
+Use your own [thread pool](https://github.com/alugowski/task-thread-pool)
+with `poolstl::par_pool` for full control over thread count, startup/shutdown, etc.:
 
 ```c++
-task_thread_pool::task_thread_pool pool;
+task_thread_pool::task_thread_pool pool{4};  // 4 threads
 
 std::reduce(poolstl::par_pool(pool), v.cbegin(), v.cbegin());
 ```
@@ -127,7 +127,7 @@ reduce(poolstl::par)/real_time                                     4.21 ms
 reduce(std::execution::par)/real_time                              3.55 ms         3.09 ms          199
 ```
 
-# poolSTL as `std::execution::par` substitute
+# poolSTL as `std::execution::par` Substitute
 **USE AT YOUR OWN RISK!**
 
 Two-line hack for missing compiler support. A no-op on compilers with support.
diff --git a/include/poolstl/algorithm b/include/poolstl/algorithm
index 9973eb8..48ac082 100644
--- a/include/poolstl/algorithm
+++ b/include/poolstl/algorithm
@@ -25,6 +25,21 @@ namespace std {
         poolstl::internal::get_futures(futures);
     }
 
+    /**
+     * NOTE: Iterators are expected to be random access.
+     * See std::copy_n https://en.cppreference.com/w/cpp/algorithm/copy_n
+     */
+    template <class ExecPolicy, class RandIt1, class Size, class RandIt2>
+    poolstl::internal::enable_if_poolstl_execution_policy<ExecPolicy, RandIt2>
+    copy_n(ExecPolicy &&policy, RandIt1 first, Size n, RandIt2 dest) {
+        if (n <= 0) {
+            return dest;
+        }
+        RandIt1 last = poolstl::internal::advanced(first, n);
+        std::copy(std::forward<ExecPolicy>(policy), first, last, dest);
+        return poolstl::internal::advanced(dest, n);
+    }
+
     /**
      * NOTE: Iterators are expected to be random access.
      * See std::fill https://en.cppreference.com/w/cpp/algorithm/fill
diff --git a/tests/poolstl_test.cpp b/tests/poolstl_test.cpp
index e28acee..5642326 100644
--- a/tests/poolstl_test.cpp
+++ b/tests/poolstl_test.cpp
@@ -72,6 +72,25 @@ TEST_CASE("copy", "[alg][algorithm]") {
     }
 }
 
+TEST_CASE("copy_n", "[alg][algorithm]") {
+    for (auto num_threads : test_thread_counts) {
+        ttp::task_thread_pool pool(num_threads);
+
+        auto vec_size = *std::max_element(test_arr_sizes.cbegin(), test_arr_sizes.cend());
+        for (auto num_iters : test_arr_sizes) {
+            auto source = iota_vector(num_iters);
+            std::vector<int> dest1(vec_size);
+            std::vector<int> dest2(vec_size);
+
+            std::copy_n(source.cbegin(), num_iters, dest1.begin());
+            std::copy_n(poolstl::par_pool(pool), source.cbegin(), num_iters, dest2.begin());
+
+            REQUIRE(dest1 == dest2);
+        }
+    }
+}
+
+
 TEST_CASE("fill", "[alg][algorithm]") {
     for (auto num_threads : test_thread_counts) {
         ttp::task_thread_pool pool(num_threads);