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//===-- llvm/ADT/CombinationGenerator.h ------------------------*- C++ -*--===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
/// \file
/// Combination generator.
/// Example: given input {{0, 1}, {2}, {3, 4}} it will produce the following
/// combinations: {0, 2, 3}, {0, 2, 4}, {1, 2, 3}, {1, 2, 4}.
/// It is useful to think of input as vector-of-vectors, where the
/// outer vector is the variable space, and inner vector is choice space.
/// The number of choices for each variable can be different.
/// As for implementation, it is useful to think of this as a weird number,
/// where each digit (==variable) may have different base (==number of choices).
/// Thus modelling of 'produce next combination' is exactly analogous to the
/// incrementing of an number - increment lowest digit (pick next choice for the
/// variable), and if it wrapped to the beginning then increment next digit.
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallVector.h"
#include <cassert>
#include <cstring>
namespace llvm {
template <typename choice_type, typename choices_storage_type,
int variable_smallsize>
class CombinationGenerator {
template <typename T> struct WrappingIterator {
using value_type = T;
const ArrayRef<value_type> Range;
typename decltype(Range)::const_iterator Position;
// Rewind the tape, placing the position to again point at the beginning.
void rewind() { Position = Range.begin(); }
// Advance position forward, possibly wrapping to the beginning.
// Returns whether the wrap happened.
bool advance() {
bool Wrapped = Position == Range.end();
if (Wrapped)
return Wrapped;
// Get the value at which we are currently pointing.
const value_type &operator*() const { return *Position; }
WrappingIterator(ArrayRef<value_type> Range_) : Range(Range_) {
assert(!Range.empty() && "The range must not be empty.");
const ArrayRef<choices_storage_type> VariablesChoices;
void performGeneration(
const function_ref<bool(ArrayRef<choice_type>)> Callback) const {
SmallVector<WrappingIterator<choice_type>, variable_smallsize>
// 'increment' of the the whole VariablesState is defined identically to the
// increment of a number: starting from the least significant element,
// increment it, and if it wrapped, then propagate that carry by also
// incrementing next (more significant) element.
auto IncrementState =
[](MutableArrayRef<WrappingIterator<choice_type>> VariablesState)
-> bool {
for (WrappingIterator<choice_type> &Variable :
llvm::reverse(VariablesState)) {
bool Wrapped = Variable.advance();
if (!Wrapped)
return false; // There you go, next combination is ready.
// We have carry - increment more significant variable next..
return true; // MSB variable wrapped, no more unique combinations.
// Initialize the per-variable state to refer to the possible choices for
// that variable.
for (ArrayRef<choice_type> VC : VariablesChoices)
// Temporary buffer to store each combination before performing Callback.
SmallVector<choice_type, variable_smallsize> CurrentCombination;
while (true) {
// Gather the currently-selected variable choices into a vector.
for (auto I : llvm::zip(VariablesState, CurrentCombination))
std::get<1>(I) = *std::get<0>(I);
// And pass the new combination into callback, as intended.
if (/*Abort=*/Callback(CurrentCombination))
// And tick the state to next combination, which will be unique.
if (IncrementState(VariablesState))
return; // All combinations produced.
CombinationGenerator(ArrayRef<choices_storage_type> VariablesChoices_)
: VariablesChoices(VariablesChoices_) {
#ifndef NDEBUG
assert(!VariablesChoices.empty() && "There should be some variables.");
llvm::for_each(VariablesChoices, [](ArrayRef<choice_type> VariableChoices) {
assert(!VariableChoices.empty() &&
"There must always be some choice, at least a placeholder one.");
// How many combinations can we produce, max?
// This is at most how many times the callback will be called.
size_t numCombinations() const {
size_t NumVariants = 1;
for (ArrayRef<choice_type> VariableChoices : VariablesChoices)
NumVariants *= VariableChoices.size();
assert(NumVariants >= 1 &&
"We should always end up producing at least one combination");
return NumVariants;
// Actually perform exhaustive combination generation.
// Each result will be passed into the callback.
void generate(const function_ref<bool(ArrayRef<choice_type>)> Callback) {
} // namespace llvm