Merge branch 'GC-Master'

* GC-Master:
  Updated with README and stuff.
  Added the Mapbox files.
  Initial check-in.

.gitignore

@@ -0,0 +1,18 @@
+*#
+.#*
+*~
+*.orig
+npm-debug.log
+package.yml
+node_modules/*
+tmp/
+bin/_mocha
+bin/mocha
+bin/escodegen
+bin/esgenerate
+test-reports.xml
+LisperatorLanguage
+src/test.js
+src/test.coffee
+notes/
+build/

CMakeLists.txt

@@ -0,0 +1,6 @@
+project("Collector")
+
+list(APPEND CMAKE_CXX_FLAGS "${CXXMAKE_C_FLAGS} -std=c++1y -I../src/include/ -g")
+
+add_executable(collector src/collector.cpp)
+

README.md

@@ -0,0 +1,65 @@
+---
+**WHAT:**
+
+This is an example of Bob Nystrom's
+"[Baby's First Garbage Collector](http://journal.stuffwithstuff.com/2013/12/08/babys-first-garbage-collector/),"
+which I've been wanting to implement for a while in order to understand
+it better.  To make the problem harder (as I always do), I decided to
+write it in C++, and to make it even more fun, I've implemented it using
+the new Variant container from C++17.
+
+**WHY:**
+
+I've never written a garbage collector before.  Now I know what it is
+and how it works.
+
+**DETAILS:**
+
+The collector Nystrom wrote is a simple bi-color mark-and-sweep
+collector for a singly threaded process with distinct pauses based upon
+a straightforward memory usage heuristic.  That heuristic is simply, "Is
+the amount of memory currently in use twice as much as the last time we
+garbage collected?"  If the answer is yes, the collector runs and sweeps
+up the stack.
+
+Nystrom's code is highly readable, and I hope mine is as well.  Because
+I used Variant, my Object class has an internal Pair class, and then the
+Variant is just \<int, Pair\>, where "Pair" is a pair of pointers to
+other objects.  The entirety of the VM is basically a stack of
+singly-linked lists which either represents integers or collections of
+integers in a Lisp-like structure.
+
+The allocator creates two kinds of objects, then: pairs, and lists.  A
+pair is created by pushing two other objects onto the stack, then
+calling `push()`, which pops them off the stack and replaces them with a
+Pair object.  The VM class has two methods, both named `push()`, one of
+which pushes an integer, the other a pair.  Since a pair is built from
+objects on the stack, the Pair version takes no arguments, and since
+C++14 and beyond have move semantics that Variant honors,
+Variant\<Pair\> only constructs a single pair.  Pretty nice.  I was also
+able to use both lambda-style and constructor-style visitors in my
+Variant, which was a fun little bonus.
+
+**NOTE:**
+
+I have included the header files for the
+[Mapbox version of Variant](https://github.com/mapbox/variant) since the
+C++17 committee's standards haven't quite reached the general public and
+the Variant implementation is still a subject of some debate.  This
+implementation looks straightforward enough and is a header-only
+release.  It works with both GCC 4.8.5 and Clang 3.8, and that's good
+enough for me.
+
+The Mapbox variant is BSD licensed, and a copy of the license is
+included in the Include directory.
+
+**BUILDING:**
+
+From the base directory of the project:
+
+mkdir build
+cd build
+cmake ..
+make
+
+And you should be able to run the basic tests.  It's just one file.

src/collector.c

@@ -0,0 +1,238 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+#define MAX_STACK 256
+#define MAX_BARRIER 8
+
+typedef enum {
+  OBJ_INT,
+  OBJ_PAIR
+} ObjectType;
+
+typedef struct sObject {
+  unsigned char marked;
+  struct sObject* next;
+  ObjectType type;
+  union {
+    int value;
+    struct {
+      struct sObject* head;
+      struct sObject* tail;
+    };
+  };
+} Object;
+
+typedef struct {
+  Object* stack[MAX_STACK];
+  Object* root;
+  int stackSize;
+  int numObjects;
+  int maxObjects;
+} VM;
+
+void assert(int condition, const char* message) {
+  if (!condition) {
+    printf("%s\n", message);
+    exit(1);
+  }
+}
+
+VM* newVM() {
+  VM* vm = malloc(sizeof(VM));
+  vm->stackSize = 0;
+  vm->numObjects = 0;
+  vm->maxObjects = MAX_BARRIER;
+  return vm;
+}
+
+void push(VM* vm, Object* value) {
+  assert(vm->stackSize < MAX_STACK, "Stack overflow!");
+  vm->stack[vm->stackSize++] = value;
+}
+
+Object* pop(VM* vm) {
+  assert(vm->stackSize > 0, "Stack underflow!");
+  return vm->stack[--(vm->stackSize)];
+}
+
+void mark(Object* object) {
+  if (object->marked) {
+    return;
+  }
+  
+  object->marked = 1;
+  if (object->type == OBJ_PAIR) {
+    mark(object->head);
+    mark(object->tail);
+  }
+}
+
+void markAll(VM* vm) {
+  for(int i = 0; i < vm->stackSize; i++) {
+    mark(vm->stack[i]);
+  }
+}
+
+void sweep(VM* vm) {
+  Object** object = &vm->root;
+  while (*object) {
+    if (!(*object)->marked) {
+      Object* unreached = *object;
+      *object = unreached->next;
+      vm->numObjects--;
+      free(unreached);
+    } else {
+      (*object)->marked = 0;
+      object = &(*object)->next;
+    }
+  }
+}
+
+void gc(VM* vm) {
+  int numObjects = vm->numObjects;
+  markAll(vm);
+  sweep(vm);
+  vm->maxObjects = vm->numObjects * 2;
+  printf("Collected %d objects, %d remaining.\n", numObjects - vm->numObjects,
+         vm->numObjects);
+}
+
+Object *newObject(VM* vm, ObjectType type) {
+  if (vm->numObjects == vm->maxObjects) {
+    gc(vm);
+  }
+
+  Object* object = malloc(sizeof(Object));
+  object->marked = 0;
+  object->type = type;
+  object->next = vm->root;
+  vm->root = object;
+  vm->numObjects++;
+  return object;
+}
+
+Object* pushInt(VM* vm, int value) {
+  Object* object = newObject(vm, OBJ_INT);
+  object->value = value;
+  push(vm, object);
+  return object;
+}
+
+Object* pushPair(VM* vm) {
+  Object* object = newObject(vm, OBJ_PAIR);
+  object->head = pop(vm);
+  object->tail = pop(vm);
+  push(vm, object);
+  return object;
+}
+  
+void freeVM(VM *vm) {
+  vm->stackSize = 0;
+  gc(vm);
+  free(vm);
+}
+
+void objectPrint(Object* object) {
+  switch(object->type) {
+  case OBJ_INT:
+    printf("%d\n", object->value);
+    break;
+
+  case OBJ_PAIR:
+    printf("(");
+    objectPrint(object->head);
+    printf(", ");
+    objectPrint(object->tail);
+    printf(")");
+    break;
+  }
+}
+
+void test1() {
+  printf("Test 1: Objects on stack are preserved.\n");
+  VM* vm = newVM();
+  pushInt(vm, 1);
+  pushInt(vm, 2);
+
+  gc(vm);
+  assert(vm->numObjects == 2, "Should have preserved objects.");
+  freeVM(vm);
+}
+
+void test2() {
+  printf("Test 2: Unreached objects are collected.\n");
+  VM* vm = newVM();
+  pushInt(vm, 1);
+  pushInt(vm, 2);
+  pop(vm);
+  pop(vm);
+
+  gc(vm);
+  assert(vm->numObjects == 0, "Should have collected objects.");
+  freeVM(vm);
+}
+
+void test3() {
+  printf("Test 3: Reach nested objects.\n");
+  VM* vm = newVM();
+  pushInt(vm, 1);
+  pushInt(vm, 2);
+  pushPair(vm);
+  pushInt(vm, 3);
+  pushInt(vm, 4);
+  pushPair(vm);
+  pushPair(vm);
+
+  gc(vm);
+  assert(vm->numObjects == 7, "Should have reached objects.");
+  freeVM(vm);
+}
+
+void test4() {
+  printf("Test 4: Handle cycles.\n");
+  VM* vm = newVM();
+  pushInt(vm, 1);
+  pushInt(vm, 2);
+  Object* a = pushPair(vm);
+  pushInt(vm, 3);
+  pushInt(vm, 4);
+  Object* b = pushPair(vm);
+
+  /* Set up a cycle, and also make 2 and 4 unreachable and collectible. */
+  a->tail = b;
+  b->tail = a;
+
+  gc(vm);
+  assert(vm->numObjects == 4, "Should have collected objects.");
+  freeVM(vm);
+}
+
+void perfTest() {
+  printf("Performance Test.\n");
+  VM* vm = newVM();
+
+  for (int i = 0; i < 1000; i++) {
+    for (int j = 0; j < 20; j++) {
+      pushInt(vm, i);
+    }
+
+    for (int k = 0; k < 20; k++) {
+      pop(vm);
+    }
+  }
+  freeVM(vm);
+}
+
+int main(int argc, const char * argv[]) {
+  test1();
+  test2();
+  test3();
+  test4();
+  perfTest();
+
+  return 0;
+}
+
+
+
+    

src/collector.cpp

@@ -0,0 +1,252 @@
+#include <iostream>
+
+/* Requires the mapbox header-only variant found at
+   https://github.com/mapbox/variant
+
+   Compiles with: 
+   clang++ -std=c++14 -I./include/ -o collector collector.cpp
+   …  or
+   g++ -std=c++1y -I./include/ -g -o collector collector.cpp
+   … where 'include' has the variant headers.
+
+   clang version 3.9.1-svn288847-1~exp1 (branches/release_39)
+   g++ version (Ubuntu 4.8.5-2ubuntu1~14.04.1) 4.8.5
+
+*/
+#include <mapbox/variant.hpp>
+
+#define MAX_STACK 256
+#define MAX_BARRIER 8
+
+void my_assert(int condition, const char* message) {
+  if (!condition) {
+    std::cout << message << std::endl;
+    exit(1);
+  }
+}
+
+/* An implementation of Bob Nystrom's "Baby's First Garbage Collector"
+   http://journal.stuffwithstuff.com/2013/12/08/babys-first-garbage-collector/,
+   only in C++, and with some educational stuff along the way about
+   the new Variant (automagical discriminated unions) coming in
+   Libstdc++ version 4, part of the C++ 2017 standard.
+*/
+
+class Object {
+public:
+  unsigned char marked;
+  Object *next;
+  Object(int v): marked(0), value(v) {}
+  // Variant<Pair> uses move semantics; this doesn't result in Pair being built twice.
+  Object(Object* head, Object* tail): marked(0), value(Pair(head, tail)) {}
+
+  class Pair {
+  public:
+    Pair(Object* h, Object* t): head(h), tail(t) {};
+    Object* head;
+    Object* tail;
+  };
+
+  /* This is mostly an exploration of a discriminated union, and
+     making one work in the context of a primitive but functional
+     garbage collector. */
+  mapbox::util::variant<int, Pair> value;
+};
+
+class VM {
+public:
+  /* Imagine my surprise when I learned that clang doesn't bother to
+     zero out memory allocated on the threadstack. */
+  VM(): stackSize(0), numObjects(0), maxObjects(MAX_BARRIER), root(NULL) {};
+  
+  Object* pop() {
+    my_assert(stackSize > 0, "Stack underflow!");
+    return stack[--stackSize];
+  }
+
+  /* This is basically the interface for a very primitive reverse
+     polish notation calculator of some kind.  A garbage-collected
+     Forth interpreter, perhaps. */
+
+  Object* push(int v) {
+    return _push(insert(new Object(v)));
+  }
+  
+  Object* push() {
+    return _push(insert(new Object(pop(), pop())));
+  }
+
+  /* Lambda-style visitors, enabling descent. */
+  void mark(Object *o) {
+    auto marker = mapbox::util::make_visitor(
+        [this](int) {},
+        [this](Object::Pair p) {
+          this->mark(p.head);
+          this->mark(p.tail);
+    });
+    
+    if (o->marked) {
+      return;
+    }
+
+    o->marked = 1;
+    return mapbox::util::apply_visitor(marker, o->value);
+  }
+
+  /* So named because each scope resembles a collection of objects
+     leading horizontally from the vertical stack, creating a spine. */
+  void markSpine() {
+    for(auto i = 0; i < stackSize; i++) {
+      mark(stack[i]);
+    }
+  }
+
+  void collect() {
+    int num = numObjects;
+    markSpine();
+    sweep();
+    maxObjects = numObjects * 2;
+#ifdef DEBUG
+    std::cout << "Collected " << (num - numObjects) << " objects, "
+              << numObjects << " remain." << std::endl;
+#endif
+  }
+
+  /* The saddest fact: I went with using NULL as our end-of-stack
+     discriminator rather than something higher-level, like an
+     Optional or Either-variant, because to use those I'd have to use
+     recursion to sweep the interpreter's stack, which means I'm at
+     the mercy of the C stack, complete with the cost of the unwind at
+     the end.  Bummer. */
+
+  /* I look at this and ask, WWHSD?  What Would Herb Sutter Do? */
+  
+  void sweep() {
+    Object** o = &root;
+    while(*o) {
+      if (!(*o)->marked) {
+        Object* unreached = *o;
+        *o = unreached->next;
+        numObjects--;
+        delete unreached;
+      } else {
+        (*o)->marked = 0;
+        o = &(*o)->next;
+      }
+    }
+  }
+      
+  int numObjects;
+  
+private:
+
+  /* Heh.  Typo, "Stark overflow."  I'll just leave Tony right there anyway... */
+  Object* _push(Object *o) {
+    my_assert(stackSize < MAX_STACK, "Stark overflow");
+    stack[stackSize++] = o;
+    return o;
+  }
+  
+  Object* insert(Object *o) {
+    if (numObjects >= maxObjects) {
+      collect();
+    }
+    
+    o->marked = 0;
+    o->next = root;
+    root = o;
+    numObjects++;
+    return o;
+  }
+    
+  Object* stack[MAX_STACK];
+  Object* root;
+  int stackSize;
+  int maxObjects;
+};
+
+
+void test1() {
+  std::cout << "Test 1: Objects on stack are preserved." << std::endl;
+  VM vm;
+  vm.push(1);
+  vm.push(2);
+  vm.collect();
+  my_assert(vm.numObjects == 2, "Should have preserved objects.");
+}
+
+void test2() {
+  std::cout << "Test 2: Unreached objects are collected." << std::endl;
+  VM vm;
+  vm.push(1);
+  vm.push(2);
+  vm.pop();
+  vm.pop();
+  vm.collect();
+  my_assert(vm.numObjects == 0, "Should have collected objects.");
+
+}
+    
+void test3() {
+  std::cout << "Test 3: Reach nested objects." << std::endl;
+  VM vm;
+  vm.push(1);
+  vm.push(2);
+  vm.push();
+  vm.push(3);
+  vm.push(4);
+  vm.push();
+  vm.push();
+  vm.collect();
+  my_assert(vm.numObjects == 7, "Should have reached objects.");
+}
+
+void test4() {
+  std::cout << "Test 4: Handle cycles." << std::endl;
+  VM vm;
+  vm.push(1);
+  vm.push(2);
+  Object* a = vm.push();
+  vm.push(3);
+  vm.push(4);
+  Object* b = vm.push();
+
+  /* Constructor-based variant visitor. */
+  struct tail_setter {
+    Object* tail;
+    tail_setter(Object *t) : tail(t) {}
+    inline void operator()(int &i) {}
+    inline void operator()(Object::Pair &p) { p.tail = tail; }
+  };
+  
+  /* Set up a cycle, and also make 2 and 4 unreachable and collectible. */
+  mapbox::util::apply_visitor(tail_setter(b), a->value);
+  mapbox::util::apply_visitor(tail_setter(a), b->value);
+  vm.collect();
+  my_assert(vm.numObjects == 4, "Should have collected objects.");
+}
+
+void perfTest() {
+  std::cout << "Performance Test." << std::endl;
+  VM vm;
+
+  for (int i = 0; i < 1000; i++) {
+    for (int j = 0; j < 20; j++) {
+      vm.push(i);
+    }
+
+    for (int k = 0; k < 20; k++) {
+      vm.pop();
+    }
+  }
+}
+
+int main(int argc, const char * argv[]) {
+  test1();
+  test2();
+  test3();
+  test4();
+  perfTest();
+
+  return 0;
+}

src/include/mapbox/LICENSE

@@ -0,0 +1,25 @@
+Copyright (c) MapBox
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without modification,
+are permitted provided that the following conditions are met:
+
+- Redistributions of source code must retain the above copyright notice, this
+  list of conditions and the following disclaimer.
+- Redistributions in binary form must reproduce the above copyright notice, this
+  list of conditions and the following disclaimer in the documentation and/or
+  other materials provided with the distribution.
+- Neither the name "MapBox" nor the names of its contributors may be
+  used to endorse or promote products derived from this software without
+  specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR
+ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
+ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

src/include/mapbox/optional.hpp

@@ -0,0 +1,74 @@
+#ifndef MAPBOX_UTIL_OPTIONAL_HPP
+#define MAPBOX_UTIL_OPTIONAL_HPP
+
+#pragma message("This implementation of optional is deprecated. See https://github.com/mapbox/variant/issues/64.")
+
+#include <type_traits>
+#include <utility>
+
+#include <mapbox/variant.hpp>
+
+namespace mapbox {
+namespace util {
+
+template <typename T>
+class optional
+{
+    static_assert(!std::is_reference<T>::value, "optional doesn't support references");
+
+    struct none_type
+    {
+    };
+
+    variant<none_type, T> variant_;
+
+public:
+    optional() = default;
+
+    optional(optional const& rhs)
+    {
+        if (this != &rhs)
+        { // protect against invalid self-assignment
+            variant_ = rhs.variant_;
+        }
+    }
+
+    optional(T const& v) { variant_ = v; }
+
+    explicit operator bool() const noexcept { return variant_.template is<T>(); }
+
+    T const& get() const { return variant_.template get<T>(); }
+    T& get() { return variant_.template get<T>(); }
+
+    T const& operator*() const { return this->get(); }
+    T operator*() { return this->get(); }
+
+    optional& operator=(T const& v)
+    {
+        variant_ = v;
+        return *this;
+    }
+
+    optional& operator=(optional const& rhs)
+    {
+        if (this != &rhs)
+        {
+            variant_ = rhs.variant_;
+        }
+        return *this;
+    }
+
+    template <typename... Args>
+    void emplace(Args&&... args)
+    {
+        variant_ = T{std::forward<Args>(args)...};
+    }
+
+    void reset() { variant_ = none_type{}; }
+
+}; // class optional
+
+} // namespace util
+} // namespace mapbox
+
+#endif // MAPBOX_UTIL_OPTIONAL_HPP

src/include/mapbox/recursive_wrapper.hpp

@@ -0,0 +1,122 @@
+#ifndef MAPBOX_UTIL_RECURSIVE_WRAPPER_HPP
+#define MAPBOX_UTIL_RECURSIVE_WRAPPER_HPP
+
+// Based on variant/recursive_wrapper.hpp from boost.
+//
+// Original license:
+//
+// Copyright (c) 2002-2003
+// Eric Friedman, Itay Maman
+//
+// Distributed under the Boost Software License, Version 1.0. (See
+// accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+
+#include <cassert>
+#include <utility>
+
+namespace mapbox {
+namespace util {
+
+template <typename T>
+class recursive_wrapper
+{
+
+    T* p_;
+
+    void assign(T const& rhs)
+    {
+        this->get() = rhs;
+    }
+
+public:
+    using type = T;
+
+    /**
+     * Default constructor default initializes the internally stored value.
+     * For POD types this means nothing is done and the storage is
+     * uninitialized.
+     *
+     * @throws std::bad_alloc if there is insufficient memory for an object
+     *         of type T.
+     * @throws any exception thrown by the default constructur of T.
+     */
+    recursive_wrapper()
+        : p_(new T){}
+
+    ~recursive_wrapper() noexcept { delete p_; }
+
+    recursive_wrapper(recursive_wrapper const& operand)
+        : p_(new T(operand.get())) {}
+
+    recursive_wrapper(T const& operand)
+        : p_(new T(operand)) {}
+
+    recursive_wrapper(recursive_wrapper&& operand)
+        : p_(new T(std::move(operand.get()))) {}
+
+    recursive_wrapper(T&& operand)
+        : p_(new T(std::move(operand))) {}
+
+    inline recursive_wrapper& operator=(recursive_wrapper const& rhs)
+    {
+        assign(rhs.get());
+        return *this;
+    }
+
+    inline recursive_wrapper& operator=(T const& rhs)
+    {
+        assign(rhs);
+        return *this;
+    }
+
+    inline void swap(recursive_wrapper& operand) noexcept
+    {
+        T* temp = operand.p_;
+        operand.p_ = p_;
+        p_ = temp;
+    }
+
+    recursive_wrapper& operator=(recursive_wrapper&& rhs) noexcept
+    {
+        swap(rhs);
+        return *this;
+    }
+
+    recursive_wrapper& operator=(T&& rhs)
+    {
+        get() = std::move(rhs);
+        return *this;
+    }
+
+    T& get()
+    {
+        assert(p_);
+        return *get_pointer();
+    }
+
+    T const& get() const
+    {
+        assert(p_);
+        return *get_pointer();
+    }
+
+    T* get_pointer() { return p_; }
+
+    const T* get_pointer() const { return p_; }
+
+    operator T const&() const { return this->get(); }
+
+    operator T&() { return this->get(); }
+
+}; // class recursive_wrapper
+
+template <typename T>
+inline void swap(recursive_wrapper<T>& lhs, recursive_wrapper<T>& rhs) noexcept
+{
+    lhs.swap(rhs);
+}
+} // namespace util
+} // namespace mapbox
+
+#endif // MAPBOX_UTIL_RECURSIVE_WRAPPER_HPP

src/include/mapbox/variant_io.hpp

@@ -0,0 +1,45 @@
+#ifndef MAPBOX_UTIL_VARIANT_IO_HPP
+#define MAPBOX_UTIL_VARIANT_IO_HPP
+
+#include <iosfwd>
+
+#include <mapbox/variant.hpp>
+
+namespace mapbox {
+namespace util {
+
+namespace detail {
+// operator<< helper
+template <typename Out>
+class printer
+{
+public:
+    explicit printer(Out& out)
+        : out_(out) {}
+    printer& operator=(printer const&) = delete;
+
+    // visitor
+    template <typename T>
+    void operator()(T const& operand) const
+    {
+        out_ << operand;
+    }
+
+private:
+    Out& out_;
+};
+}
+
+// operator<<
+template <typename CharT, typename Traits, typename... Types>
+VARIANT_INLINE std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& out, variant<Types...> const& rhs)
+{
+    detail::printer<std::basic_ostream<CharT, Traits>> visitor(out);
+    apply_visitor(visitor, rhs);
+    return out;
+}
+} // namespace util
+} // namespace mapbox
+
+#endif // MAPBOX_UTIL_VARIANT_IO_HPP

src/include/mapbox/variant_visitor.hpp

@@ -0,0 +1,38 @@
+#ifndef MAPBOX_UTIL_VARIANT_VISITOR_HPP
+#define MAPBOX_UTIL_VARIANT_VISITOR_HPP
+
+namespace mapbox {
+namespace util {
+
+template <typename... Fns>
+struct visitor;
+
+template <typename Fn>
+struct visitor<Fn> : Fn
+{
+    using type = Fn;
+    using Fn::operator();
+
+    visitor(Fn fn) : Fn(fn) {}
+};
+
+template <typename Fn, typename... Fns>
+struct visitor<Fn, Fns...> : Fn, visitor<Fns...>
+{
+    using type = visitor;
+    using Fn::operator();
+    using visitor<Fns...>::operator();
+
+    visitor(Fn fn, Fns... fns) : Fn(fn), visitor<Fns...>(fns...) {}
+};
+
+template <typename... Fns>
+visitor<Fns...> make_visitor(Fns... fns)
+{
+    return visitor<Fns...>(fns...);
+}
+
+} // namespace util
+} // namespace mapbox
+
+#endif // MAPBOX_UTIL_VARIANT_VISITOR_HPP