Lazy Lists in C++

list.cpp

/*
Copyright(C) Freddy A Cubas "Superstar64"
Boost Software License - Version 1.0 - August 17th, 2003

Permission is hereby granted, free of charge, to any person or organization
obtaining a copy of the software and accompanying documentation covered by
this license (the "Software") to use, reproduce, display, distribute,
execute, and transmit the Software, and to prepare derivative works of the
Software, and to permit third-parties to whom the Software is furnished to
do so, all subject to the following:

The copyright notices in the Software and this entire statement, including
the above license grant, this restriction and the following disclaimer,
must be included in all copies of the Software, in whole or in part, and
all derivative works of the Software, unless such copies or derivative
works are solely in the form of machine-executable object code generated by
a source language processor.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT
SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
FOR ANY DAMAGES OR OTHER LIABILITY, WHETHER IN CONTRACT, TORT OR OTHERWISE,
ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
DEALINGS IN THE SOFTWARE.
*/
#include <cstdio>

#include <functional>
#include <memory>
#include <optional>
#include <type_traits>
#include <utility>

// this is a data structure that can pull new elements from a stream
// and uses reference counting to remember old elements when needed

template <class Function,
          class Element = std::decay_t<decltype(std::declval<Function>()())>>
struct lazy_list;

template <class Function, class Element> struct lazy_list_element {
  Element head;
  lazy_list<Function, Element> tail;

  lazy_list_element(decltype(head) head, decltype(tail) tail)
      : head(head), tail(tail) {}
};

template <class Function, class Element> struct lazy_list {
  Function function;
  std::shared_ptr<lazy_list_element<Function, Element>> future;

  // create a lazy list from a function object that takes no arguments
  lazy_list(Function function)
      : function(function),
        future() {}

  lazy_list_element<Function, Element> next() {
    if (!future) {
      Element head = function();
      lazy_list<Function, Element> tail = lazy_list(std::move(function));
      future = std::make_shared<lazy_list_element<Function, Element>>(head, tail);
    }
    return *future;
  }
};

// create a function object from a lazy list
template <class Function, class Element>
auto stream(lazy_list<Function, Element> list) {
  return [list]() mutable {
    auto next = list.next();
    list = next.tail;
    return next.head;
  };
}

// wrapper for getchar
std::optional<char> in() {
  char next = getchar();
  if (next == '\n') {
    return in();
  } else if (next != -1) {
    return next;
  } else {
    return {};
  }
}

int main() {
  auto stdin = lazy_list{in};
  auto main = stream(stdin);
  auto secondary = stream(stdin);

  while (true) {

    // print 10 characters while advancing the stream

    for (int i = 0; i < 10; i++) {
      auto next = main();
      if (!next) {
        break;
      }
      printf("1(%c) ", *next);
    }

    // then remember 10 older characters

    for (int i = 0; i < 10; i++) {
      auto next = secondary();
      if (!next) {
        return 0;
      }
      printf("2(%c) ", *next);
    }
  }
}

nice. 👍