chore: add parallelFor

Author: vinniefalco

include/mrdox/Support/ParallelFor.hpp

@@ -0,0 +1,77 @@
+//
+// This is a derivative work. originally part of the LLVM Project.
+// Licensed under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// Copyright (c) 2023 Vinnie Falco ([email protected])
+//
+// Official repository: https://github.com/cppalliance/mrdox
+//
+
+#ifndef MRDOX_SUPPORT_PARALLELFOR_HPP
+#define MRDOX_SUPPORT_PARALLELFOR_HPP
+
+#include <mrdox/Platform.hpp>
+#include <mrdox/Support/Error.hpp>
+#include <iterator>
+#include <mutex>
+#include <thread>
+#include <vector>
+
+namespace clang {
+namespace mrdox {
+
+/** Visit all elements of a range concurrently.
+*/
+template<
+    class Range,
+    class Worker,
+    class... Args>
+Error
+parallelFor(
+    Range&& range,
+    std::vector<Worker>& workers,
+    Args&&... args)
+{
+    std::mutex m;
+    bool cancel = false;
+    auto it = std::begin(range);
+    auto const end = std::end(range);
+    auto const do_work =
+        [&](Worker& worker)
+        {
+            std::unique_lock<std::mutex> lock(m);
+            if(it == end || cancel)
+                return false;
+            auto it0 = it;
+            ++it;
+            lock.unlock();
+            bool cancel_ = worker(*it0,
+                std::forward<Args>(args)...);
+            if(! cancel_)
+                return true;
+            cancel = true;
+            return false;
+        };
+    std::vector<std::thread> threads;
+    threads.reserve(workers.size());
+    for(std::size_t i = 0; i < workers.size(); ++i)
+        threads.emplace_back(std::thread(
+            [&](std::size_t i_)
+            {
+                for(;;)
+                    if(! do_work(workers[i]))
+                        break;
+            }, i));
+    for(auto& t : threads)
+        t.join();
+    if(cancel)
+        return Error("canceled");
+    return Error::success();
+}
+
+} // mrdox
+} // clang
+
+#endif