iter-explorer-test.cpp

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/*
  IterExplorer(Test)  -  verify tree expanding and backtracking iterator

  Copyright (C)         Lumiera.org
    2017,               Hermann Vosseler <Ichthyostega@web.de>

  This program is free software; you can redistribute it and/or
  modify it under the terms of the GNU General Public License as
  published by the Free Software Foundation; either version 2 of
  the License, or (at your option) any later version.

  This program is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.

  You should have received a copy of the GNU General Public License
  along with this program; if not, write to the Free Software
  Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.

* *****************************************************/

#include "lib/test/run.hpp"
#include "lib/test/test-helper.hpp"
#include "lib/iter-adapter-stl.hpp"
#include "lib/format-string.hpp"
#include "lib/format-cout.hpp"
#include "lib/format-util.hpp"
#include "lib/itertools.hpp"
#include "lib/util.hpp"

#include "lib/iter-explorer.hpp"
#include "lib/meta/trait.hpp"

#include <utility>
#include <vector>
#include <limits>
#include <string>
#include <tuple>
#include <cmath>


namespace lib {
namespace test{
  
  using ::Test;
  using util::_Fmt;
  using util::isnil;
  using util::isSameObject;
  using lib::iter_stl::eachElm;
  using LERR_(ITER_EXHAUST);
  using std::vector;
  using std::string;
  
  
  namespace { // test substrate: simple number sequence iterator
    
    struct CountDown
      {
        uint p,e;
        
        CountDown(uint start =0, uint end =0)
          : p(start)
          , e(end)
          { }
        
        bool
        checkPoint ()  const
          {
            return p > e;
          }
        
        uint&
        yield ()  const
          {
            return util::unConst (checkPoint()? p : e);
          }
        
        void
        iterNext ()
          {
            if (not checkPoint()) return;
            --p;
          }
        
        bool
        operator== (CountDown const& o)  const
          {
            return e == o.e
               and p == o.p;
          }
      };
    
    
    class NumberSequence
      : public IterStateWrapper<uint, CountDown>
      {
        
      public:
        explicit
        NumberSequence(uint start = 0)
          : IterStateWrapper<uint,CountDown> (CountDown{start})
          { }
        NumberSequence(uint start, uint end)
          : IterStateWrapper<uint,CountDown> (CountDown(start,end))
          { }
      };
    
    
    
    class RandomSeq
      {
        size_t lim_;
        size_t cnt_;
        char letter_;
        
        static char
        rndLetter()
          {
            return 'A' + rand() % 26;
          }
        
      public:
        RandomSeq(int len  =0)
          : lim_{len>=0? len : std::numeric_limits<size_t>::max()}
          , cnt_{0}
          , letter_{rndLetter()}
          { }
        
        bool
        checkPoint ()  const
          {
            return cnt_ < lim_;
          }
        
        char&
        yield ()  const
          {
            return unConst(this)->letter_;
          }
        
        void
        iterNext ()
          {
            ASSERT (checkPoint());
            ++cnt_;
            letter_ = rndLetter();
          }
      };
    
    
    template<class II>
    inline string
    materialise (II&& ii)
    {
      return util::join (std::forward<II> (ii), "-");
    }
    
    template<class II>
    inline void
    pullOut (II & ii)
    {
      while (ii)
        {
          cout << *ii;
          if (++ii) cout << "-";
        }
      cout << endl;
    }
    
  } // (END) test helpers
  
  
  
  
  
  
  
  /*******************************************************************/
  class IterExplorer_test : public Test
    {
      
      virtual void
      run (Arg)
        {
          verify_wrappedState();
          verify_wrappedIterator();
          
          verify_expandOperation();
          verify_expand_rootCurrent();
          verify_transformOperation();
          verify_elementGroupingOperation();
          verify_aggregatingGroupItration();
          verify_combinedExpandTransform();
          verify_customProcessingLayer();
          verify_scheduledExpansion();
          verify_untilStopTrigger();
          verify_FilterIterator();
          verify_FilterChanges();
          verify_asIterSource();
          verify_IterSource();
          verify_reduceVal();
          verify_effuse();
          
          verify_depthFirstExploration();
          demonstrate_LayeredEvaluation();
        }
      
      
      
      void
      verify_wrappedState()
        {
          auto ii = explore (CountDown{5,0});
          CHECK (!isnil (ii));
          CHECK (5 == *ii);
          ++ii;
          CHECK (4 == *ii);
          pullOut(ii);
          CHECK ( isnil (ii));
          CHECK (!ii);
          
          VERIFY_ERROR (ITER_EXHAUST, *ii );
          VERIFY_ERROR (ITER_EXHAUST, ++ii );
          
          ii = explore (CountDown{5});
          CHECK (materialise(ii) == "5-4-3-2-1");
          ii = explore (CountDown{7,4});
          CHECK (materialise(ii) == "7-6-5");
          ii = explore (CountDown{});
          CHECK ( isnil (ii));
          CHECK (!ii);
        }
      
      
      void
      verify_wrappedIterator()
        {
          vector<int> numz{1,-2,3,-5,8,-13};
          auto ii = eachElm(numz);
          CHECK (!isnil (ii));
          CHECK (1 == *ii);
          ++ii;
          CHECK (-2 == *ii);
          
          auto jj = explore(ii);
          CHECK (!isnil (jj));
          CHECK (-2 == *jj);
          ++jj;
          CHECK (3 == *jj);
          
          // we passed a LValue-Ref, thus a copy was made
          CHECK (-2 == *ii);

          CHECK (materialise(ii) == "-2-3--5-8--13");
          CHECK (materialise(jj) ==    "3--5-8--13");
          
          // can even adapt STL container automatically
          auto kk = explore(numz);
          CHECK (!isnil (kk));
          CHECK (1 == *kk);
          CHECK (materialise(kk) == "1--2-3--5-8--13");
        }
      
      
      
      void
      verify_expandOperation()
        {
          /* == "monadic flatMap" == */
          
          verify_treeExpandingIterator(
                    explore(CountDown{5})
                      .expand([](uint j){ return CountDown{j-1}; })                                   // expand-functor: Val > StateCore
                      );
          
          verify_treeExpandingIterator(
                    explore(CountDown{5})
                      .expand([](uint j){ return NumberSequence{j-1}; })                              // expand-functor: Val > Iter
                      );                // NOTE: different Iterator type than the source!
          
          // lambda with side-effect and return type different from source iter
          vector<vector<uint>> childBuffer;
          auto expandIntoChildBuffer = [&](uint j) -> vector<uint>&
                                          {
                                            childBuffer.emplace_back();
                                            vector<uint>& childNumbz = childBuffer.back();
                                            for (size_t i=0; i<j-1; ++i)
                                              childNumbz.push_back(j-1 - i);
                                            return childNumbz;
                                          };
          
          verify_treeExpandingIterator(
                    explore(CountDown{5})
                      .expand(expandIntoChildBuffer)                                                  // expand-functor: Val > STL-container&
                      );
          
          // test routine called the expansion functor five times
          CHECK (5 == childBuffer.size());
          
          
          
          /* == "state manipulation" use cases == */
          
          verify_treeExpandingIterator(
                    explore(CountDown{5})
                      .expand([](CountDown const& core){ return CountDown{ core.yield() - 1}; })      // expand-functor: StateCore const& -> StateCore
                      );
          
          verify_treeExpandingIterator(
                    explore(CountDown{5})
                      .expand([](CountDown core){ return NumberSequence{ core.yield() - 1}; })        // expand-functor: StateCore -> Iter
                      );
          
          verify_treeExpandingIterator(
                    explore(CountDown{5})
                      .expand([](auto & it){ return CountDown{ *it - 1}; })                           // generic Lambda: Iter& -> StateCore
                      );
          
          verify_treeExpandingIterator(
                    explore(CountDown{5})
                      .expand([](auto it){ return decltype(it){ *it - 1}; })                          // generic Lambda: Iter -> Iter
                      );
        }
      
      
      template<class EXP>
      void
      verify_treeExpandingIterator (EXP ii)
        {
          CHECK (!isnil (ii));
          CHECK (5 == *ii);
          ++ii;
          CHECK (4 == *ii);
          
          CHECK (0 == ii.depth());
          ii.expandChildren();
          CHECK (3 == *ii);
          CHECK (1 == ii.depth());
          ++ii;
          CHECK (2 == *ii);
          CHECK (1 == ii.depth());
          ii.expandChildren();
          CHECK (1 == *ii);
          CHECK (2 == ii.depth());
          ++ii;
          CHECK (1 == *ii);
          CHECK (1 == ii.depth());
          ++ii;
          CHECK (3 == *ii);
          CHECK (0 == ii.depth());
          CHECK (materialise(ii) == "3-2-1");
          ii.expandChildren();
          CHECK (1 == ii.depth());
          CHECK (materialise(ii) == "2-1-2-1");
          ++++ii;
          CHECK (0 == ii.depth());
          CHECK (materialise(ii) == "2-1");
          ii.expandChildren();
          CHECK (1 == ii.depth());
          CHECK (materialise(ii) == "1-1");
          ++ii;
          CHECK (0 == ii.depth());
          CHECK (1 == *ii);
          CHECK (materialise(ii) == "1");
          ii.expandChildren();
          CHECK (isnil (ii));
          VERIFY_ERROR (ITER_EXHAUST, *ii );
          VERIFY_ERROR (ITER_EXHAUST, ++ii );
        }
      
      
      void
      verify_expand_rootCurrent()
        {
          auto tree = explore(CountDown{25})
                        .expand([](uint j){ return CountDown{j-1}; });
          
          CHECK (materialise(tree) == "25-24-23-22-21-20-19-18-17-16-15-14-13-12-11-10-9-8-7-6-5-4-3-2-1");
          
          CHECK (0 == tree.depth());
          CHECK (25 == *tree);
          ++tree;
          ++tree;
          ++tree;
          ++tree;
          CHECK (21 == *tree);
          tree.expandChildren();
          CHECK (1 == tree.depth());
          ++tree;
          ++tree;
          ++tree;
          ++tree;
          ++tree;
          CHECK (15 == *tree);
          tree.expandChildren();
          ++tree;
          ++tree;
          CHECK (2 == tree.depth());
          CHECK (materialise(tree) == "12-11-10-9-8-7-6-5-4-3-2-1-"                          // this is the level-2 child sequence
                                      "14-13-12-11-10-9-8-7-6-5-4-3-2-1-"                    // ...returning to the rest of the level-1 sequence
                                      "20-19-18-17-16-15-14-13-12-11-10-9-8-7-6-5-4-3-2-1"); // ...followed by the rest of the original root sequence
          CHECK (12 == *tree);
          
          tree.rootCurrent();
          CHECK (12 == *tree);
          CHECK (materialise(tree) == "12-11-10-9-8-7-6-5-4-3-2-1");  // note: level-2 continues unaltered, but level-1 and the original root are gone.
          CHECK (0 == tree.depth());
        }
      
      
      
      void
      verify_transformOperation()
        {
          auto multiply  = [](int v){ return 2*v; };                             // functional map: value -> value
          
          _Fmt embrace{"≺%s≻"};
          auto formatify = [&](auto it){ return string{embrace % *it}; };        // generic lambda: assumed to take an Iterator&
          
          
          auto ii = explore(CountDown{7,4})
                      .transform(multiply)
                      ;
          
          CHECK (14 == *ii);
          CHECK (14 == *ii);
          ++ii;
          CHECK (12 == *ii);
          ++ii;
          CHECK (10 == *ii);
          ++ii;
          CHECK (isnil (ii));
          VERIFY_ERROR (ITER_EXHAUST, *ii );
          VERIFY_ERROR (ITER_EXHAUST, ++ii );
          
          
          
          // demonstrate chaining of several transformation layers
          vector<int64_t> numz{1,-2,3,-5,8,-13};
          
          CHECK ("≺1≻-≺-2≻-≺3≻-≺-5≻-≺8≻-≺-13≻"                == materialise (explore(numz)
                                                                                .transform(formatify)) );
          
          CHECK ("≺2≻-≺-4≻-≺6≻-≺-10≻-≺16≻-≺-26≻"              == materialise (explore(numz)
                                                                                .transform(multiply)
                                                                                .transform(formatify)) );
          
          CHECK ("≺≺4≻≻-≺≺-8≻≻-≺≺12≻≻-≺≺-20≻≻-≺≺32≻≻-≺≺-52≻≻" == materialise (explore(numz)
                                                                                .transform(multiply)
                                                                                .transform(multiply)
                                                                                .transform(formatify)
                                                                                .transform(formatify)) );
          
          
          // demonstrate the functor is evaluated only once per step
          int fact = 3;
          
          auto jj = explore (CountDown{4})
                      .transform([&](int v)
                                    {
                                      v *=fact;
                                      fact *= -2;
                                      return v;
                                    });
          CHECK (3*4 == *jj);
          CHECK (fact == -2*3);
          
          CHECK (3*4 == *jj);
          CHECK (3*4 == *jj);
          
          ++jj;
          CHECK (fact == -2*3);    // NOTE : functor is evaluated on first demand
          CHECK (-2*3*3 == *jj);   //        ...which happens on yield (access the iterator value)
          CHECK (fact == 2*2*3);   //        and this also causes the side-effect
          CHECK (-2*3*3 == *jj);
          CHECK (-2*3*3 == *jj);
          CHECK (fact == 2*2*3);   //        no further evaluation and thus no further side-effect
          
          ++jj;
          CHECK (2*2*3*2 == *jj);
          CHECK (fact == -2*2*2*3);
          
          fact = -23;
          CHECK (2*2*3*2 == *jj);
          
          ++jj;
          CHECK (fact == -23);
          CHECK (-23*1 == *jj);
          CHECK (fact == 2*23);
          
          ++jj;
          CHECK (isnil (jj));
          CHECK (fact == 2*23);
          
          VERIFY_ERROR (ITER_EXHAUST, *ii );
          CHECK (fact == 2*23);    // exhaustion detected on source and thus no further evaluation
          
          
          
          // demonstrate a transformer accessing the source state core...
          // should not be relevant in practice, but works due to the generic adapters
          auto kk = explore (CountDown{9,4})
                      .transform([](CountDown& core)
                                   {
                                     uint delta = core.p - core.e;
                                     if (delta % 2 == 0)
                                       --core.p;     // EVIL EVIL
                                     return delta;
                                   });
          
          CHECK (5 == *kk); // the delta between 9 (start) and 4 (end)
          ++kk;
          CHECK (4 == *kk); // Core manipulated by SIDE-EFFECT at this point...
          CHECK (4 == *kk); // ...but not yet obvious, since the result is cached
          ++kk;
          CHECK (2 == *kk); // Surprise -- someone ate my numberz...
          ++kk;
          CHECK (isnil (kk));
        }
      
      
      
      void
      verify_elementGroupingOperation()
        {
          auto showGroup = [](auto it){ return "["+util::join(*it)+"]"; };
          CHECK (materialise (
                    explore(CountDown{10})
                      .grouped<3>()
                      .transform(showGroup)
                    )
                 == "[10, 9, 8]-[7, 6, 5]-[4, 3, 2]"_expect);
          
          
          auto ii = explore(CountDown{23})
                      .grouped<5>();
          CHECK(ii);
          CHECK(ii.getGroupedElms());
          CHECK(not ii.getRestElms());
          CHECK (materialise(ii.getGroupedElms()) == "23-22-21-20-19"_expect);
          
          CHECK ( test::showType<decltype(*ii)>()== "array<unsigned int, 5ul>&"_expect);
          
          uint s = *(ii.getGroupedElms());
          for ( ; ii; ++ii)
            {
              auto grp = *ii;
              CHECK (5 == grp.size());
              auto& [a,b,c,d,e] = grp;
              CHECK (a == s);
              CHECK (b == a-1);
              CHECK (c == a-2);
              CHECK (d == a-3);
              CHECK (e == a-4);
              CHECK (not ii.getRestElms());
              s -= 5;
            }
          CHECK (s < 5);
          CHECK (s == 3);
          
          CHECK (not ii);
          CHECK(ii.getGroupedElms());
          CHECK(ii.getRestElms());
          CHECK (materialise(ii.getGroupedElms()) == "3-2-1"_expect);
          CHECK (materialise(ii.getRestElms())    == "3-2-1"_expect);
          
          
          auto iii = explore(CountDown{4})
                      .grouped<5>();
          CHECK (not iii);
          CHECK (materialise(iii.getRestElms())    == "4-3-2-1"_expect);
        }
      
      
      void
      verify_aggregatingGroupItration()
        {
          CHECK (materialise (
                    explore(CountDown{10})
                      .groupedBy(std::ilogbf)
                    )
                 == "27-22-5-1"_expect);       // 10+9+8|7+6+5+4|3+2|1

          CHECK (materialise (
                    explore(CountDown{10})
                      .transform(util::toString<uint>)
                      .groupedBy([](auto& it) { return std::ilogbf (it.p); })    // note trickery: takes not the value, rather the iterator and
                    )                                                           //  accesses internals of CountDown, bypassing the transform layer above
                 == "1098-7654-32-1"_expect);  // `+` does string concatenation
          
          
          auto showGroup = [](auto it){ return "["+util::join(*it)+"]"; };
          // elaborate form with custom aggregation...
          CHECK (materialise (
                    explore(CountDown{10})
                      .groupedBy(std::ilogbf
                                ,[](vector<uint>& accum, uint val)
                                    {
                                      accum.push_back (val);
                                    })
                      .transform(showGroup)
                    )
                 == "[10, 9, 8]-[7, 6, 5, 4]-[3, 2]-[1]"_expect);
        }
      
      
      void
      verify_combinedExpandTransform()
        {
          auto ii = explore(CountDown{5})
                      .expand([](uint j){ return CountDown{j-1}; })
                      .transform([](int v){ return 2*v; })
                      ;
          
          CHECK ("int" == meta::typeStr(*ii));  // result type is what the last transformer yields
          CHECK (10 == *ii);
          ++ii;
          CHECK (8 == *ii);
          ii.expandChildren();
          CHECK ("6-4-2-6-4-2" == materialise(ii) );
          
          
          // the following contrived example demonstrates
          // how intermediary processing steps may interact
          
          CHECK (materialise (
                    explore(CountDown{5})
                      .expand([](uint j){ return CountDown{j-1}; })
                      .transform([](int v){ return 2*v; })
                      .transform([](auto& it)
                                   {
                                     auto elm = *it;
                                     if (elm == 6)
                                       {
                                         it.expandChildren();         // NOTE at that point we're forced to decide if
                                         elm = *it * 10;              //      we want to return the parent or the 1st child
                                       }
                                     return elm;
                                   })
                      .transform([](float f){ return 0.055 + f/2; })
                 )
                 == "5.055-4.055-20.055-1.055-2.055-1.055" );
        }
      
      
      template<class SRC>
      struct MagicTestRubbish
        : public SRC
        {
          using SRC::SRC;
          
          void
          iterNext()
            {
              ++(*this);
              if (*this)
                ++(*this);
            }
        };
      
      void
      verify_customProcessingLayer()
        {
          CHECK (materialise(
                    explore(CountDown{7})
                      .processingLayer<MagicTestRubbish>()
                )
                == "7-5-3-1");
          
          CHECK (materialise(
                    explore(CountDown{7})
                      .transform([](uint v){ return 2*v; })
                      .processingLayer<MagicTestRubbish>()
                      .filter([](int v){ return v % 3; })
                )
                == "14-10-2");
        }
      
      
      
      void
      verify_scheduledExpansion()
        {
          auto ii = explore(CountDown{6})
                      .expand([](uint j){ return CountDown{j-2}; })
                      .expandOnIteration();
          
          CHECK (!isnil (ii));
          CHECK (6 == *ii);
          ++ii;
          CHECK (5 == *ii);
          CHECK (ii.depth() == 0);
          
          ii.expandChildren();
          CHECK (5 == *ii);
          CHECK (ii.depth() == 0);
          ++ii;
          CHECK (3 == *ii);
          CHECK (ii.depth() == 1);
          
          ii.expandChildren();
          ii.expandChildren();
          CHECK (ii.depth() == 1);
          CHECK (3 == *ii);
          ++ii;
          CHECK (1 == *ii);
          CHECK (ii.depth() == 2);
          ++ii;
          CHECK (2 == *ii);
          CHECK (ii.depth() == 1);
          
          ii.expandChildren();
          ++ii;
          CHECK (1 == *ii);
          CHECK (ii.depth() == 1);
          ++ii;
          CHECK (4 == *ii);
          CHECK (ii.depth() == 0);
          ++ii;
          CHECK (3 == *ii);
          ++ii;
          CHECK (2 == *ii);
          ++ii;
          CHECK (1 == *ii);
          ++ii;
          CHECK (isnil (ii));
        }
      
      
      
      void
      verify_untilStopTrigger()
        {
          CHECK (materialise (
                    explore (CountDown{10})
                      .iterUntil([](uint j){ return j < 5; })
                    )
                 == "10-9-8-7-6-5"_expect);
          
          CHECK (materialise (
                    explore (CountDown{10})
                      .iterWhile([](uint j){ return j > 5; })
                    )
                 == "10-9-8-7-6"_expect);
          
          CHECK (materialise (
                    explore (CountDown{10})
                      .iterWhile([](int j){ return j > -5; })
                    )
                 == "10-9-8-7-6-5-4-3-2-1"_expect);
          
          CHECK (materialise (
                    explore (CountDown{10})
                      .iterWhile([](uint j){ return j > 25; })
                    )
                 == ""_expect);
        }
      
      
      
      void
      verify_FilterIterator()
        {
          // canonical example, using a clean side-effect free predicate based on element values
          CHECK (materialise (
                    explore(CountDown{10})
                      .filter([](uint j){ return j % 2; })
                    )
                 == "9-7-5-3-1"_expect);
          
          
          // Filter may lead to consuming util exhaustion...
          auto ii = explore(CountDown{10})
                      .filter([](int j){ return j > 9; });
          
          CHECK (not isnil (ii));
          CHECK (10 == *ii);
          ++ ii;
          CHECK (isnil (ii));
          VERIFY_ERROR (ITER_EXHAUST, ++ii );
          
          
          // none of the source elements can be approved here...
          auto jj = explore(CountDown{5})
                      .filter([](int j){ return j > 9; });
          
          CHECK (isnil (jj));
          
          
          
          // a tricky example, where the predicate takes the source core as argument;
          // since the source core is embedded as baseclass, it can thus "undermine"
          // and bypass the layers configured in between; here the transformer changes
          // uint to float, but the filter interacts directly with the core and thus
          // judges based on the original values
          CHECK (materialise (
                    explore(CountDown{10,4})
                      .transform([](float f){ return 0.55 + 2*f; })
                      .filter([](CountDown& core){ return core.p % 2; })
                    )
                 == "18.55-14.55-10.55"_expect);
          
          
          
          // contrived example to verify interplay of filtering and child expansion;
          // especially note that the filter is re-evaluated after expansion happened.
          CHECK (materialise (
                    explore(CountDown{10})
                      .expand([](uint i){ return CountDown{i%4==0? i-1 : 0}; })      // generate subtree at 8 and 4 ==> 10-9-8-7-6-5-4-3-2-1-3-2-1-7-6-5-4-3-2-1-3-2-1
                      .filter([](uint i){ return i%2 == 0; })
                      .expandAll()                                                   // Note: sends the expandChildren down through the filter
                    )
                 == "10-8-6-4-2-2-6-4-2-2"_expect);
          
          
          
          // another convoluted example to demonstrate
          // - a filter predicate with side-effect
          // - and moreover the predicate is a generic lambda
          // - accepting the iterator to trigger child expansion
          // - which also causes re-evaluation of the preceding transformer
          bool toggle = false;
          auto kk = explore(CountDown{10,5})
                      .expand([](uint j){ return CountDown{j-1}; })
                      .transform([](int v){ return 2*v; })
                      .filter([&](auto& it)
                                   {
                                     if (*it == 16)
                                       {
                                         it.expandChildren();
                                         toggle = true;
                                       }
                                     return toggle;
                                   });
          
          CHECK (materialise(kk)
                 == "14-12-10-8-6-4-2-14-12"_expect);
                 // Explanation:
                 // The source starts at 10, but since the toggle is false,
                 // none of the initial values makes it though to the result.
                 // The interspersed transformer doubles the source values, and
                 // thus at source == 8 the trigger value (16) is hit. Thus the
                 // filter now flips the context-bound toggle (side-effect) and
                 // then expands children, which consumes current source value 8
                 // to replace it with the sequence 7,6,5,4,3,2,1, followed by
                 // the rest of the original sequence, 7,6 (which stops above 5).
          
          CHECK (materialise(kk.filter([](long i){ return i % 7; }))
                 == "12-10-8-6-4-2-12"_expect);
                 // Explanation:
                 // Since the original IterExplorer was assigned to variable kk,
                 // the materialise()-Function got a lvalue-ref and thus made a copy
                 // of the whole compound. For that reason, the original state within
                 // kk still rests at 7 -- because the filter evaluates eagerly, the
                 // source was pulled right at construction until we reached the first
                 // value to yield, which is the first child (7,....) within the
                 // expanded sequence. But now, in the second call to materialise(),
                 // we don't just copy, rather we add another filter layer on top,
                 // which happens to filter away this first result (== 2*7), and
                 // also the first element of the original sequence after the
                 // expanded children
          
          // WARNING: kk is now defunct, since we moved it into the builder expression
          //          and then moved the resulting extended iterator into materialise!
        }
      
      
      
      void
      verify_FilterChanges()
        {
          auto seq = explore(CountDown{20})
                       .mutableFilter();
          
          auto takeEve = [](uint i){ return i%2 == 0; };
          auto takeTrd = [](uint i){ return i%3 == 0; };
          
          CHECK (20 == *seq);
          ++seq;
          CHECK (19 == *seq);
          CHECK (19 == *seq);
          
          seq.andFilter (takeEve);
          CHECK (18 == *seq);
          ++seq;
          CHECK (16 == *seq);
          
          seq.andFilter (takeTrd);
          CHECK (12 == *seq);   //  is divisible (by 2 AND by 3)
          
          seq.flipFilter();
          CHECK (11 == *seq);   // not divisible (by 2 AND by 3)
          ++seq;
          CHECK (10 == *seq);
          
          seq.setNewFilter (takeTrd);
          CHECK ( 9 == *seq);
          ++seq;
          CHECK ( 6 == *seq);
          
          seq.orNotFilter (takeEve);
          CHECK ( 6 == *seq);
          ++seq;
          CHECK ( 5 == *seq);   // disjunctive condition actually weakens the filter
          ++seq;
          CHECK ( 3 == *seq);
          
          // NOTE: arbitrary functors can be used/combined,
          //       since they are adapted individually.
          // To demonstrate this, we use a functor accessing and
          // manipulating the state core by side effect...
          string buff{"."};
          seq.andNotFilter ([&](CountDown& core)
                              {
                                buff += util::toString(core.p) + ".";
                                --core.p;                              // manipulate state core
                                return core.p % 2;                     // return a number, not bool
                              });
          
          CHECK ( 2 == *seq);                   // value in the core has been manipulated
          CHECK (".3." == buff);                // the filter has been invoked once, and saw core == 3
          
          ++seq;                                // core == 2 is filtered by the existing other filter (== not take even)
          CHECK (".3.1." == buff);              // the filter has been invoked again, and saw core == 1
          CHECK (0 == seq.p);                   // ...which he manipulated, so that core == 0
          CHECK (isnil (seq));                  // .....and thus iteration end is detected
          VERIFY_ERROR (ITER_EXHAUST, *seq );
          
          
          // verify enabling and disabling...
          seq = explore(CountDown{10})
                  .mutableFilter(takeTrd);
          
          CHECK (9 == *seq);
          seq.disableFilter();
          CHECK (9 == *seq);
          ++seq;
          CHECK (8 == *seq);
          seq.andNotFilter (takeEve);
          CHECK (7 == *seq);
          ++seq;
          CHECK (5 == *seq);
          seq.disableFilter();
          CHECK (5 == *seq);
          ++seq;
          CHECK (4 == *seq);
          ++seq;
          CHECK (3 == *seq);
          seq.flipFilter();  // everything rejected
          CHECK (isnil (seq));
        }
      
      
      
      
      void
      verify_reduceVal()
        {
          auto accumulated = explore(CountDown{30})
                               .transform([](int i){ return i-1; })              // note: implicitly converts uint -> int
                               .resultSum();
          
          using Res = decltype(accumulated);
          CHECK (lib::test::showType<Res>()  == "int"_expect);
          
          auto expectedSum = [](auto N){ return N*(N+1) / 2; };
          CHECK (accumulated == expectedSum(29));
          
          // In the general case an accessor and a junctor can be given...
          CHECK (explore(CountDown{10})
                   .reduce([](int i){ return i - 0.5; }                          // accessor: produce a double
                          ,[](string accu, float val)
                                    {
                                      return accu+">"+util::toString(val);       // junctor:  convert to String and combine with separator char
                                    }
                          , string{">-"}                                         // seedVal:  starting point for the reduction; also defines result type
                          )
                 == ">->9.5>8.5>7.5>6.5>5.5>4.5>3.5>2.5>1.5>0.5"_expect);
          
          // If only the accessor is given, values are combined by std::plus...
          CHECK (explore(CountDown{9})
                   .reduce([](auto it) -> string
                             {
                               return _Fmt{"○%s●"} % *it;                        // accessor: format into a string
                             })
                 == "○9●○8●○7●○6●○5●○4●○3●○2●○1●"_expect);
          
          // a predefined IDENTITY accessor takes values from the pipeline as-is
          CHECK (explore(CountDown{9})
                   .reduce(iter_explorer::IDENTITY, std::minus<int>(), expectedSum(9))
                 == 0);
        }
      
      
      
      void
      verify_effuse()
        {
          auto solidified = explore(CountDown{20})
                               .filter   ([](uint i){ return i % 2; })
                               .transform([](uint i){ return 0.5*i; })
                               .effuse();
          
          using Res = decltype(solidified);
          CHECK (lib::test::showType<Res>()  == "vector<double>"_expect);
          CHECK (util::join(solidified, "|") == "9.5|8.5|7.5|6.5|5.5|4.5|3.5|2.5|1.5|0.5"_expect);
        }
      
      
      
      
      void
      verify_asIterSource()
        {
          IterSource<uint>::iterator sequence;     // note `sequence` is polymorphic
          CHECK (isnil (sequence));
          
          sequence = explore(CountDown{20,10})
                        .filter([](uint i){ return i % 2; })
                        .asIterSource();           // note this terminal builder function
                                                   // moves the whole pipeline onto the heap
          CHECK (not isnil (sequence));
          CHECK (19 == *sequence);
          
          
          // use one sequence as source to build another one
          sequence = explore(sequence)
                        .transform([](uint i){ return i*2; })
                        .asIterSource();
          
          CHECK (38 == *sequence);
          CHECK ("38-34-30-26-22" == materialise(sequence));
          
          // WARNING pitfall: `sequence` is a copyable iterator front-end
          //                  but holds onto the actual pipeline by shared-ptr
          //                  Thus, even while materialise() creates a copy,
          //                  the iteration state gets shared....
          CHECK (22 == *sequence);
          ++sequence;         // ...and even worse, iteration end is only detected after increment
          CHECK (isnil (sequence));
          
          
          // extended API to invoke child expansion opaquely
          IterExploreSource<char> exploreIter;
          CHECK (isnil (exploreIter));
          
          exploreIter = explore(CountDown{20,10})
                          .filter([](uint i){ return i % 2; })
                          .transform([](uint i){ return i*2; })
                          .filter([](int i){ return i>25; })
                          .expand([](uint i){ return CountDown{i-10, 20}; })
                          .transform([](uint u) -> char { return '@'+u-20; })
                          .asIterSource();
          
          
          CHECK ('R' == *exploreIter);   // 38-20 + '@'
          ++exploreIter;
          CHECK ('N' == *exploreIter);   // 34-20 + '@'
          
          exploreIter.expandChildren();  // expand consumes the current element (34)
                                         // and injects the sequence (24...20[ instead
          CHECK ('D' == *exploreIter);   // 34-10 == 24 and 'D' ==  24-20 + '@'
          
          CHECK ("D-C-B-A-J-F" == materialise(exploreIter));
        }                                // note how the remainder of the original sequence is picked up with 'J'...
      
      
      
      
      void
      verify_IterSource()
        {
          class PrivateSource
            : public IterSource<uint>
            {
            public:
              virtual PrivateSource* expandChildren()  const  =0;
            };
          
          class VerySpecificIter
            : public WrappedLumieraIter<NumberSequence
            ,        PrivateSource     >
            {
            public:
              VerySpecificIter(uint start)
                : WrappedLumieraIter(NumberSequence{start})
                { }
              
              virtual PrivateSource*
              expandChildren()  const override
                {
                  return new VerySpecificIter{*wrappedIter() - 2};
                }
              
              uint
              currentVal()  const
                {
                  return *wrappedIter();
                }
            };
          
          
          // simple standard case: create a new heap allocated IterSource implementation.
          // IterExplorer will take ownership (by smart-ptr) and build a Lumiera Iterator front-End
          CHECK ("7-6-5-4-3-2-1" == materialise (
                                      explore (new VerySpecificIter{7})));
          
          
          // missing source detected
          PrivateSource* niente = nullptr;
          CHECK (isnil (explore (niente)));
          
          
          // attach to an IterSource living here in local scope...
          VerySpecificIter vsit{5};
          
          // ...and build a child expansion on top, which calls through the PrivateSource-API
          // Effectively this means we do not know the concrete type of the "expanded children" iterator,
          // only that it adheres to the same IterSource sub-interface as used on the base iterator.
          auto ii = explore(vsit)
                      .expand ([](PrivateSource& source){ return source.expandChildren(); });
          
          CHECK (not isnil (ii));
          CHECK (5 == *ii);
          CHECK (5 == vsit.currentVal());
          ++ii;
          CHECK (4 == *ii);
          CHECK (4 == vsit.currentVal());
          
          CHECK (0 == ii.depth());
          ii.expandChildren();             // note: calls through source's VTable to invoke VerySpecificIter::expandChildren()
          CHECK (1 == ii.depth());
          
          CHECK (2 == *ii);
          ++ii;
          CHECK (1 == *ii);
          
          CHECK (4 == vsit.currentVal());  // note: as long as expanded children are alive, the source pipeline is not pulled further
          CHECK (1 == ii.depth());
          ++ii;
          CHECK (0 == ii.depth());         //   ... but now the children were exhausted and thus also the source advanced
          CHECK (3 == *ii);
          CHECK (3 == vsit.currentVal());
          ++ii;
          CHECK (2 == *ii);
          CHECK (2 == vsit.currentVal());
          ++ii;
          CHECK (1 == *ii);
          CHECK (1 == vsit.currentVal());
          ++ii;
          CHECK (isnil (ii));
        }
      
      
      
      void
      verify_depthFirstExploration()
        {
          CHECK (materialise(
                    explore(CountDown{4})
                      .expand([](uint j){ return CountDown{j-1}; })
                      .expandAll()
                      .transform([](int i){ return i*10; })
                    )
                 == "40-30-20-10-10-20-10-10-30-20-10-10-20-10-10");
          
          
          using std::get;
          using Tu2 = std::tuple<uint, uint>;
          auto summingExpander = [](Tu2 const& tup)
                                  {
                                    uint val = get<0>(tup);
                                    uint sum = get<1>(tup);
                                    return val? singleValIterator (Tu2{val-1, sum+val})
                                              : SingleValIter<Tu2>();
                                  };
          
          CHECK (materialise(
                    explore(CountDown{4})
                      .transform([](uint i){ return Tu2{i,0}; })
                      .expand(summingExpander)
                      .expandAll()
                      .transform([](Tu2 res){ return get<1>(res); })
                    )
                 == "0-4-7-9-10-0-3-5-6-0-2-3-0-1");
        }
      
      
      
      void
      demonstrate_LayeredEvaluation()
        {
          // Layer-1: the search space with "hidden" implementation
          using DataSrc = IterExploreSource<char>;
          DataSrc searchSpace = explore(RandomSeq{-1})
                                  .expand([](char){ return RandomSeq{15}; })
                                  .asIterSource();
          
          // Layer-2: State for search algorithm
          struct State
            {
              DataSrc& src;
              string& toFind;
              vector<uint> protocol;
              
              State(DataSrc& s, string& t)
                : src{s}
                , toFind{t}
                , protocol{0}
                { }
              
              bool
              checkPoint()  const
                {
                  return bool{src};
                }
              
              State&
              yield()  const
                {
                  return *unConst(this);
                }
              
              void
              iterNext()
                {
                  ++src;
                  protocol.resize (1+src.depth());
                  ++protocol.back();
                }
              
              void
              expandChildren()
                {
                  src.expandChildren();
                  protocol.resize (1+src.depth());
                }
              
              bool
              isMatch()  const
                {
                  ASSERT (src.depth() < toFind.size());
                  return *src == toFind[src.depth()];
                }
            };
          
          
          // Layer-3: Evaluation pipeline to drive search
          string toFind = util::join (explore (RandomSeq{5}), "");
          cout << "Search in random tree: toFind = "<<toFind<<endl;
          
          auto theSearch = explore (State{searchSpace, toFind})
                             .filter([](auto& it)
                                       {
                                         while (it->src.depth() < it->toFind.size() - 1
                                                and it->isMatch())
                                           it->expandChildren();
                                         
                                         return it->isMatch();
                                       });
          
          
          // perform the search over a random tree...
          CHECK (not isnil(theSearch));
          cout << "Protocol of the search: " << materialise(theSearch->protocol) <<endl;
        }
    };
  
  
  
  LAUNCHER (IterExplorer_test, "unit common");
  
  
}} // namespace lib::test