1//===-- llvm/ADT/CombinationGenerator.h ------------------------*- C++ -*--===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8///
9/// \file
10/// Combination generator.
11///
12/// Example: given input {{0, 1}, {2}, {3, 4}} it will produce the following
13/// combinations: {0, 2, 3}, {0, 2, 4}, {1, 2, 3}, {1, 2, 4}.
14///
15/// It is useful to think of input as vector-of-vectors, where the
16/// outer vector is the variable space, and inner vector is choice space.
17/// The number of choices for each variable can be different.
18///
19/// As for implementation, it is useful to think of this as a weird number,
20/// where each digit (==variable) may have different base (==number of choices).
21/// Thus modelling of 'produce next combination' is exactly analogous to the
22/// incrementing of an number - increment lowest digit (pick next choice for the
23/// variable), and if it wrapped to the beginning then increment next digit.
24///
25//===----------------------------------------------------------------------===//
26
27#ifndef LLVM_ADT_COMBINATIONGENERATOR_H
28#define LLVM_ADT_COMBINATIONGENERATOR_H
29
30#include "llvm/ADT/ArrayRef.h"
31#include "llvm/ADT/STLFunctionalExtras.h"
32#include "llvm/ADT/SmallVector.h"
33#include <cassert>
34#include <cstring>
35
36namespace llvm {
37
38template <typename choice_type, typename choices_storage_type,
39          int variable_smallsize>
40class CombinationGenerator {
41  template <typename T> struct WrappingIterator {
42    using value_type = T;
43
44    const ArrayRef<value_type> Range;
45    typename decltype(Range)::const_iterator Position;
46
47    // Rewind the tape, placing the position to again point at the beginning.
48    void rewind() { Position = Range.begin(); }
49
50    // Advance position forward, possibly wrapping to the beginning.
51    // Returns whether the wrap happened.
52    bool advance() {
53      ++Position;
54      bool Wrapped = Position == Range.end();
55      if (Wrapped)
56        rewind();
57      return Wrapped;
58    }
59
60    // Get the value at which we are currently pointing.
61    const value_type &operator*() const { return *Position; }
62
63    WrappingIterator(ArrayRef<value_type> Range_) : Range(Range_) {
64      assert(!Range.empty() && "The range must not be empty.");
65      rewind();
66    }
67  };
68
69  const ArrayRef<choices_storage_type> VariablesChoices;
70
71  void performGeneration(
72      const function_ref<bool(ArrayRef<choice_type>)> Callback) const {
73    SmallVector<WrappingIterator<choice_type>, variable_smallsize>
74        VariablesState;
75
76    // 'increment' of the whole VariablesState is defined identically to the
77    // increment of a number: starting from the least significant element,
78    // increment it, and if it wrapped, then propagate that carry by also
79    // incrementing next (more significant) element.
80    auto IncrementState =
81        [](MutableArrayRef<WrappingIterator<choice_type>> VariablesState)
82        -> bool {
83      for (WrappingIterator<choice_type> &Variable :
84           llvm::reverse(VariablesState)) {
85        bool Wrapped = Variable.advance();
86        if (!Wrapped)
87          return false; // There you go, next combination is ready.
88        // We have carry - increment more significant variable next..
89      }
90      return true; // MSB variable wrapped, no more unique combinations.
91    };
92
93    // Initialize the per-variable state to refer to the possible choices for
94    // that variable.
95    VariablesState.reserve(VariablesChoices.size());
96    for (ArrayRef<choice_type> VC : VariablesChoices)
97      VariablesState.emplace_back(VC);
98
99    // Temporary buffer to store each combination before performing Callback.
100    SmallVector<choice_type, variable_smallsize> CurrentCombination;
101    CurrentCombination.resize(VariablesState.size());
102
103    while (true) {
104      // Gather the currently-selected variable choices into a vector.
105      for (auto I : llvm::zip(VariablesState, CurrentCombination))
106        std::get<1>(I) = *std::get<0>(I);
107      // And pass the new combination into callback, as intended.
108      if (/*Abort=*/Callback(CurrentCombination))
109        return;
110      // And tick the state to next combination, which will be unique.
111      if (IncrementState(VariablesState))
112        return; // All combinations produced.
113    }
114  };
115
116public:
117  CombinationGenerator(ArrayRef<choices_storage_type> VariablesChoices_)
118      : VariablesChoices(VariablesChoices_) {
119#ifndef NDEBUG
120    assert(!VariablesChoices.empty() && "There should be some variables.");
121    llvm::for_each(VariablesChoices, [](ArrayRef<choice_type> VariableChoices) {
122      assert(!VariableChoices.empty() &&
123             "There must always be some choice, at least a placeholder one.");
124    });
125#endif
126  }
127
128  // How many combinations can we produce, max?
129  // This is at most how many times the callback will be called.
130  size_t numCombinations() const {
131    size_t NumVariants = 1;
132    for (ArrayRef<choice_type> VariableChoices : VariablesChoices)
133      NumVariants *= VariableChoices.size();
134    assert(NumVariants >= 1 &&
135           "We should always end up producing at least one combination");
136    return NumVariants;
137  }
138
139  // Actually perform exhaustive combination generation.
140  // Each result will be passed into the callback.
141  void generate(const function_ref<bool(ArrayRef<choice_type>)> Callback) {
142    performGeneration(Callback);
143  }
144};
145
146} // namespace llvm
147
148#endif
149