cpp-lazy-stream/include/stream/algorithm/fold.h

//
//  accumulate.h
//  stream
//
//  Created by Sam Jaffe on 3/29/23.
//

#pragma once

#include <optional>

#include <stream/detail/identity.h>
#include <stream/detail/traits.h>

#define FWD(x) std::forward<decltype(x)>(x)

namespace stream::ranges {
template <typename It, typename S, typename T, typename F>
auto fold_left(It it, S end, T init, F reduce) {
  for (; it != end; ++it) {
    init = std::invoke(reduce, init, *it);
  }
  return init;
}

template <typename Stream, typename T, typename F>
auto fold_left(Stream && stream, T init, F reduce) {
  for (auto && elem : FWD(stream)) {
    init = std::invoke(reduce, init, FWD(elem));
  }
  return init;
}

template <typename Stream, typename F>
auto fold_left_with_first(Stream && stream, F reduce) {
  return fold_left(++stream.begin(), stream.end(), *stream.begin(),
                   std::ref(reduce));
}
}

#undef FWD