src/tint/utils/containers/filtered_iterator.h

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#ifndef SRC_TINT_UTILS_CONTAINERS_FILTERED_ITERATOR_H_
#define SRC_TINT_UTILS_CONTAINERS_FILTERED_ITERATOR_H_

#include <utility>

namespace tint {

/// FilteredIterator is an iterator that skip over elements where the predicate function returns
/// false.
/// @tparam PREDICATE the filter predicate function
/// @tparam ITERATOR the inner iterator type
template <typename PREDICATE, typename ITERATOR>
class FilteredIterator {
  public:
    /// @param current the current iterator.
    /// @param end the end iterator.
    FilteredIterator(ITERATOR current, ITERATOR end) : current_(current), end_(end) {
        // Skip to first element that passes the predicate
        while (current_ != end && !PREDICATE{}(*current_)) {
            ++current_;
        }
    }

    /// Copy constructor
    FilteredIterator(const FilteredIterator&) = default;

    /// Copy assignment operator
    FilteredIterator& operator=(const FilteredIterator&) = default;

    /// Move constructor
    FilteredIterator(FilteredIterator&&) = default;

    /// Move assignment operator
    FilteredIterator& operator=(FilteredIterator&&) = default;

    /// Increments the iterator until PREDICATE returns true, or the end is reached.
    /// @return this FilteredIterator
    FilteredIterator& operator++() {
        ++current_;
        while (current_ != end_ && !PREDICATE{}(*current_)) {
            ++current_;
        }
        return *this;
    }

    /// @returns the element at the current iterator
    auto operator*() const { return *current_; }

    /// @returns the element at the current iterator
    auto operator->() const { return current_.operator->(); }

    /// Equality operator
    /// @returns true if this iterator is equal to @p other
    bool operator==(const FilteredIterator& other) const { return current_ == other.current_; }

    /// Inequality operator
    /// @returns true if this iterator is not equal to @p other
    bool operator!=(const FilteredIterator& other) const { return current_ != other.current_; }

  private:
    /// The current iterator.
    ITERATOR current_;
    /// The end iterator.
    ITERATOR end_;
};

/// FilteredIterable is a wrapper around an object with begin() / end() methods, that returns
/// iterators that skip over elements where the predicate function returns false.
/// @tparam PREDICATE the filter predicate function
/// @tparam ITERABLE the inner iterable object type
template <typename PREDICATE, typename ITERABLE>
struct FilteredIterable {
    /// The iterator type
    using iterator = typename std::decay_t<ITERABLE>::iterator;
    /// The const iterator type
    using const_iterator = typename std::decay_t<ITERABLE>::const_iterator;

    /// The wrapped iterable object.
    ITERABLE iterable;

    /// @return the filtered start iterator
    FilteredIterator<PREDICATE, iterator> begin() { return {iterable.begin(), iterable.end()}; }

    /// @return the filtered end iterator
    FilteredIterator<PREDICATE, iterator> end() { return {iterable.end(), iterable.end()}; }

    /// @return the filtered const start iterator
    FilteredIterator<PREDICATE, const_iterator> begin() const {
        return {iterable.begin(), iterable.end()};
    }

    /// @return the filtered const end iterator
    FilteredIterator<PREDICATE, const_iterator> end() const {
        return {iterable.end(), iterable.end()};
    }
};

/// @returns a FilteredIterable from the filter function `PREDICATE`, wrapping @p iterable
/// @tparam PREDICATE the filter predicate function
/// @tparam ITERABLE the inner iterable object type
template <typename PREDICATE, typename ITERABLE>
FilteredIterable<PREDICATE, ITERABLE&> Filter(ITERABLE& iterable) {
    return {iterable};
}

}  // namespace tint

#endif  // SRC_TINT_UTILS_CONTAINERS_FILTERED_ITERATOR_H_