ah_comb_generators_test.cc

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/*
                          Aleph_w
 
  Data structures & Algorithms
  https://github.com/lrleon/Aleph-w
 
  This file is part of Aleph-w library
 
  Copyright (c) 2002-2026 Leandro Rabindranath Leon
 
  Permission is hereby granted, free of charge, to any person obtaining a copy
  of this software and associated documentation files (the "Software"), to deal
  in the Software without restriction, including without limitation the rights
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*/
 
#include <algorithm>
#include <bit>
#include <set>
#include <stdexcept>
#include <vector>
 
#include <gtest/gtest.h>
 
#include <ah-comb-generators.H>
 
using namespace Aleph;
 
namespace
{
template <typename T>
std::vector<T> vec_of(const Array<T> &a)
{
  return to_stdvector(a);
}
}  // namespace
 
TEST(CombGenerators, PermutationsOfEmptyAndSingleton)
{
  int count = 0;
  for (const Array<int> &p : lazy_permutations(Array<int>{}))
    { (void) p; ++count; }
  EXPECT_EQ(count, 1);  // the empty sequence has exactly one permutation
 
  std::vector<std::vector<int>> seen;
  for (const Array<int> &p : lazy_permutations(Array<int>{7}))
    seen.push_back(vec_of(p));
  EXPECT_EQ(seen, (std::vector<std::vector<int>>{{7}}));
}
 
TEST(CombGenerators, PermutationsMatchStdNextPermutationCount)
{
  std::vector<int> ref = {1, 2, 3, 4};
  size_t eager_count = 0;
  do
    ++eager_count;
  while (std::next_permutation(ref.begin(), ref.end()));
 
  size_t lazy_count = 0;
  for (const Array<int> &p : lazy_permutations(Array<int>{1, 2, 3, 4}))
    { (void) p; ++lazy_count; }
 
  EXPECT_EQ(eager_count, 24u);  // 4!
  EXPECT_EQ(lazy_count, eager_count);
}
 
TEST(CombGenerators, PermutationsMatchStdNextPermutationContent)
{
  std::vector<std::vector<int>> eager;
  std::vector<int> ref = {1, 2, 3};
  std::sort(ref.begin(), ref.end());
  do
    eager.push_back(ref);
  while (std::next_permutation(ref.begin(), ref.end()));
 
  std::vector<std::vector<int>> lazy;
  for (const Array<int> &p : lazy_permutations(Array<int>{3, 1, 2}))
    lazy.push_back(vec_of(p));  // must copy: the yielded array is reused
 
  EXPECT_EQ(lazy, eager);  // both lexicographically ordered from the sorted start
}
 
TEST(CombGenerators, PermutationsWithDuplicatesCollapseLikeStd)
{
  std::vector<int> ref = {1, 1, 2};
  std::sort(ref.begin(), ref.end());
  size_t eager_count = 0;
  do
    ++eager_count;
  while (std::next_permutation(ref.begin(), ref.end()));
 
  size_t lazy_count = 0;
  for (const Array<int> &p : lazy_permutations(Array<int>{2, 1, 1}))
    { (void) p; ++lazy_count; }
 
  EXPECT_EQ(eager_count, 3u);  // 3!/2! = 3, not 6
  EXPECT_EQ(lazy_count, eager_count);
}
 
TEST(CombGenerators, PermutationsAreDistinctAsMultiset)
{
  std::set<std::vector<int>> seen;
  for (const Array<int> &p : lazy_permutations(Array<int>{1, 2, 3, 4}))
    seen.insert(vec_of(p));  // copy before the buffer advances
  EXPECT_EQ(seen.size(), 24u);
}
 
TEST(CombGenerators, PermutationsEarlyBreakStopsWork)
{
  int count = 0;
  for (const Array<int> &p : lazy_permutations(Array<int>{1, 2, 3, 4, 5}))
    {
      (void) p;
      if (++count == 3)
        break;
    }
  EXPECT_EQ(count, 3);  // stopped well before all 120 permutations
}
 
TEST(CombGenerators, CombinationsKZeroYieldsOneEmptyCombination)
{
  std::vector<std::vector<int>> seen;
  for (const Array<int> &c : lazy_combinations(Array<int>{1, 2, 3}, 0))
    seen.push_back(vec_of(c));
  ASSERT_EQ(seen.size(), 1u);
  EXPECT_TRUE(seen[0].empty());
}
 
TEST(CombGenerators, CombinationsKEqualsNYieldsWholeSetOnce)
{
  std::vector<std::vector<int>> seen;
  for (const Array<int> &c : lazy_combinations(Array<int>{1, 2, 3}, 3))
    seen.push_back(vec_of(c));
  ASSERT_EQ(seen.size(), 1u);
  EXPECT_EQ(seen[0], (std::vector<int>{1, 2, 3}));
}
 
TEST(CombGenerators, CombinationsKGreaterThanNThrows)
{
  EXPECT_THROW(
    {
      for (const Array<int> &c : lazy_combinations(Array<int>{1, 2}, 3))
        (void) c;
    },
    std::domain_error);
}
 
TEST(CombGenerators, CombinationsCountMatchesCombinationCount)
{
  const Array<int> a{1, 2, 3, 4, 5, 6};
  for (size_t k = 0; k <= a.size(); ++k)
    {
      size_t count = 0;
      for (const Array<int> &c : lazy_combinations(a, k))
        { (void) c; ++count; }
      EXPECT_EQ(count, combination_count(a.size(), k)) << "k=" << k;
    }
}
 
TEST(CombGenerators, CombinationsMatchBruteForceBitmaskEnumeration)
{
  const Array<int> a{10, 20, 30, 40, 50};
  const size_t n = a.size();
  const size_t k = 3;
 
  // Brute-force reference: every n-bit mask with exactly k bits set,
  // in ascending numeric order, collects the values at the set positions
  // in index order — the same convention lazy_combinations documents.
  std::vector<std::vector<int>> expected;
  for (uint64_t mask = 0; mask < (1u << n); ++mask)
    {
      if (static_cast<size_t>(std::popcount(mask)) != k)
        continue;
      std::vector<int> combo;
      for (size_t i = 0; i < n; ++i)
        if (mask & (1u << i))
          combo.push_back(a[i]);
      expected.push_back(combo);
    }
  std::sort(expected.begin(), expected.end());
 
  std::vector<std::vector<int>> got;
  for (const Array<int> &c : lazy_combinations(a, k))
    got.push_back(vec_of(c));
  std::sort(got.begin(), got.end());
 
  EXPECT_EQ(got, expected);
}
 
TEST(CombGenerators, CombinationsAreDistinct)
{
  const Array<int> a{1, 2, 3, 4, 5};
  std::set<std::vector<int>> seen;
  for (const Array<int> &c : lazy_combinations(a, 2))
    seen.insert(vec_of(c));
  EXPECT_EQ(seen.size(), combination_count(5, 2));
}
 
TEST(CombGenerators, CombinationsPreserveRelativeOrderOfSource)
{
  const Array<char> a{'a', 'b', 'c'};
  std::vector<std::vector<char>> seen;
  for (const Array<char> &c : lazy_combinations(a, 2))
    seen.push_back(vec_of(c));
  EXPECT_EQ(seen, (std::vector<std::vector<char>>{{'a', 'b'}, {'a', 'c'}, {'b', 'c'}}));
}
 
TEST(CombGenerators, CombinationSnapshotsSurviveGeneratorAdvancement)
{
  Generator<Array<int>> gen = lazy_combinations(Array<int>{1, 2, 3}, 2);
 
  auto it = gen.begin();
  ASSERT_NE(it, gen.end());
  Array<int> first = *it;
 
  ++it;
  ASSERT_NE(it, gen.end());
  Array<int> second = *it;
 
  EXPECT_EQ(vec_of(first), (std::vector<int>{1, 2}));
  EXPECT_EQ(vec_of(second), (std::vector<int>{1, 3}));
}