From b416a67bb90ed4c0ac8129add7e01ca84ab6ff7d Mon Sep 17 00:00:00 2001
From: Ichthyostega <prg@ichthyostega.de>
Date: Sat, 23 Sep 2023 19:39:08 +0200
Subject: [PATCH] Library: extract summation of pipeline results

...first used as part of the test harness;
seemingly this is a generic and generally useful shortcut,
similar to algorithm::reduce (or some kind of fold-left operation)
---
 src/lib/iter-explorer.hpp            | 36 +++++++++++++++++++++++-----
 src/lib/sync-barrier.hpp             | 14 +++--------
 tests/library/iter-explorer-test.cpp | 19 +++++++++++++++
 3 files changed, 52 insertions(+), 17 deletions(-)

diff --git a/src/lib/iter-explorer.hpp b/src/lib/iter-explorer.hpp
index 422af1900..6b61fcabc 100644
--- a/src/lib/iter-explorer.hpp
+++ b/src/lib/iter-explorer.hpp
@@ -389,12 +389,13 @@ namespace lib {
      *   within the same processing step; and in addition, it allows the processing step to
      *   remain agnostic with respect to the adaptation and concrete type of the functor/lambda.
      * @tparam FUN either the signature, or something _"function-like"_ passed as functor to be bound
-     * @tparam SRC (optional) but need to specify the source iterator type to apply when passing
-     *             a generic lambda or template as FUN. Such a generic functor will be _instantiated_
-     *             passing the type `SRC&` as argument. This instantiation may fail (and abort compilation),
-     *             but when it succeeds, we can infer the result type `Res` from the generic lambda
+     * @tparam SRC source type to feed to the function to be adapted.
+     * @remark Especially need to specify the source iterator type to apply when passing a generic lambda
+     *         or template as FUN. Such a generic functor will be _instantiated_ passing the type `SRC&`
+     *         as argument. This instantiation may fail (and abort compilation), but when it succeeds,
+     *         the result type `Res` can be inferred from the generic lambda.
      */
-    template<class FUN, typename SRC =void>
+    template<class FUN, typename SRC>
     struct _FunTraits
       {
         /** handle all regular "function-like" entities */
@@ -1588,7 +1589,7 @@ namespace lib {
         }
       
       /**
-       * _terminal builder_ to invoke a functor for side effect on the complete pipeline. 
+       * _terminal builder_ to invoke a functor for side effect on the complete pipeline.
        * @note exhausts and discards the pipeline itself
        */
       template<class FUN>
@@ -1601,6 +1602,29 @@ namespace lib {
             consumeFun (pipeline);
         }
       
+      /**
+       *  _terminal builder_ to invoke sum up resulting number values from the pipeline.
+       * @return accumulation of all results from the pipeline, combined by `std::plus`
+       */
+      template<class FUN>
+      auto
+      resultSum (FUN&& accessor)
+        {
+          auto accessVal = iter_explorer::_FunTraits<FUN,SRC>::adaptFunctor (forward<FUN> (accessor));
+          value_type sum{};
+          SRC& pipeline = *this;
+          for ( ; pipeline; ++pipeline)
+            sum += accessVal (pipeline);
+          return sum;
+        }
+      
+      auto
+      resultSum()
+        {
+          return IterExplorer::resultSum ([](const reference val){ return val; });
+        }
+      
+      
       /** _terminal builder_ to pour and materialise all results from this Pipeline.
        * @tparam CON a STL compliant container to store generated values (defaults to `vector`)
        * @return new instance of the target container, filled with all values
diff --git a/src/lib/sync-barrier.hpp b/src/lib/sync-barrier.hpp
index 9782d6120..e3964f4ec 100644
--- a/src/lib/sync-barrier.hpp
+++ b/src/lib/sync-barrier.hpp
@@ -44,9 +44,6 @@
 
 #include "lib/error.hpp"
 #include "lib/nocopy.hpp"
-//#include "include/logging.h"
-//#include "lib/meta/function.hpp"
-//#include "lib/result.hpp"
 
 #include <thread>
 #include <atomic>
@@ -54,12 +51,6 @@
 
 namespace lib {
   
-//  using lib::Literal;
-//  namespace error = lumiera::error;
-//  using error::LERR_(STATE);
-//  using error::LERR_(EXTERNAL);
-  
-  
   
   /**
    * A one time N-fold mutual synchronisation barrier.
@@ -76,12 +67,13 @@ namespace lib {
       std::atomic_int latch_;
       
     public:
-      /** @param nFold the number of participants to sync */
+      /** @param nFold the number of participants to sync (min. 2)*/
       explicit
       SyncBarrier (uint nFold =2)
         : latch_{int(nFold)}
         {
           REQUIRE (nFold >= 2, "Pointless to sync less than two participants.");
+          ENSURE  (nFold < 100'000, "Danger territory.... sync 100k Threads??");
         }
       
       void
@@ -93,7 +85,7 @@ namespace lib {
             while (0 < latch_.load (std::memory_order_relaxed));
           else
             latch_.store (0, std::memory_order_relaxed);
-        }
+        }             //  prevent spurious calls from wrapping
     };
   
   
diff --git a/tests/library/iter-explorer-test.cpp b/tests/library/iter-explorer-test.cpp
index e4a6117d5..ff7a47ec6 100644
--- a/tests/library/iter-explorer-test.cpp
+++ b/tests/library/iter-explorer-test.cpp
@@ -286,6 +286,7 @@ namespace test{
           verify_FilterChanges();
           verify_asIterSource();
           verify_IterSource();
+          verify_resultSum();
           verify_effuse();
           
           verify_depthFirstExploration();
@@ -1069,6 +1070,24 @@ namespace test{
       
       
       
+      /** @test verify _terminal operation_ to sum up all values from the pipeline.
+       */
+      void
+      verify_resultSum()
+        {
+          auto accumulated = explore(CountDown{6})
+                               .transform([](int i){ return i-1; })
+                               .resultSum();
+          
+          using Res = decltype(accumulated);
+          CHECK (lib::test::showType<Res>()  == "int"_expect);
+          
+          auto expectedSum = [](auto N){ return N*(N+1) / 2; };
+          CHECK (accumulated == expectedSum(5));
+        }
+      
+      
+      
       /** @test verify _terminal operation_ to append all results into a container.
        */
       void