commit 49aeaa49c5f23e0aa09d97bc4f6fa1f1a0f4dfea
Author: Arthur Sonzogni <sonzogniarthur@gmail.com>
Date:   Tue Sep 18 08:48:40 2018 +0200

    Initial prototype

diff --git a/.clang-format b/.clang-format
new file mode 100644
index 0000000..6fdf1dc
--- /dev/null
+++ b/.clang-format
@@ -0,0 +1,8 @@
+# Defines the Chromium style for automatic reformatting.
+# http://clang.llvm.org/docs/ClangFormatStyleOptions.html
+BasedOnStyle: Chromium
+# This defaults to 'Auto'. Explicitly set it for a while, so that
+# 'vector<vector<int> >' in existing files gets formatted to
+# 'vector<vector<int>>'. ('Auto' means that clang-format will only use
+# 'int>>' if the file already contains at least one such instance.)
+Standard: Cpp11
diff --git a/CMakeLists.txt b/CMakeLists.txt
new file mode 100644
index 0000000..e56a391
--- /dev/null
+++ b/CMakeLists.txt
@@ -0,0 +1,5 @@
+cmake_minimum_required(VERSION 3.0)
+enable_testing()
+
+add_subdirectory(ftxui)
+add_subdirectory(examples)
diff --git a/examples/CMakeLists.txt b/examples/CMakeLists.txt
new file mode 100644
index 0000000..f0ca2d7
--- /dev/null
+++ b/examples/CMakeLists.txt
@@ -0,0 +1 @@
+add_subdirectory(text)
diff --git a/examples/text/CMakeLists.txt b/examples/text/CMakeLists.txt
new file mode 100644
index 0000000..bd70946
--- /dev/null
+++ b/examples/text/CMakeLists.txt
@@ -0,0 +1,4 @@
+add_executable(main
+  main.cpp
+)
+target_link_libraries(main PRIVATE ftxui)
diff --git a/examples/text/main.cpp b/examples/text/main.cpp
new file mode 100644
index 0000000..59b1e7f
--- /dev/null
+++ b/examples/text/main.cpp
@@ -0,0 +1,38 @@
+#include "ftxui/core/screen.hpp"
+#include "ftxui/core/dom/elements.hpp"
+#include <iostream>
+
+int main(int argc, const char *argv[])
+{
+  using namespace ftxui::dom;
+  auto root =
+    vbox(
+      hbox(
+        text(L"north-west"),
+        flex(),
+        text(L"north-east")
+      ),
+      flex(),
+      hbox(
+        hbox(
+          flex(),
+          text(L"center"),
+          flex()
+        )
+      ),
+      flex(),
+      hbox(
+        text(L"south-west"),
+        flex(),
+        text(L"south-east")
+      )
+    );
+  auto screen = ftxui::Screen::WholeTerminal();
+  Render(screen, root.get());
+
+  std::cout << screen.ToString();
+
+  getchar();
+
+  return 0;
+}
diff --git a/ftxui/CMakeLists.txt b/ftxui/CMakeLists.txt
new file mode 100644
index 0000000..18e128a
--- /dev/null
+++ b/ftxui/CMakeLists.txt
@@ -0,0 +1,52 @@
+cmake_minimum_required(VERSION 3.0)
+project(ftxui)
+
+add_library(ftxui
+  src/ftxui/core/component.cpp
+  src/ftxui/core/dom/flex.cpp
+  src/ftxui/core/dom/hbox.cpp
+  src/ftxui/core/dom/node.cpp
+  src/ftxui/core/dom/text.cpp
+  src/ftxui/core/dom/vbox.cpp
+  src/ftxui/core/screen.cpp
+  src/ftxui/core/terminal.cpp
+  src/ftxui/util/string.cpp
+)
+
+
+target_include_directories(ftxui
+  PUBLIC include
+  PRIVATE src
+)
+
+target_compile_features(ftxui PUBLIC cxx_std_17)
+target_compile_options(ftxui PRIVATE -Wall)
+
+# Note: For gtest, please follow:
+# https://stackoverflow.com/questions/24295876/cmake-cannot-find-a-googletest-required-library
+find_package(GTest)
+find_package(Threads)
+if (GTEST_FOUND AND THREADS_FOUND)
+  function(add_new_test test_name test_files)
+    add_executable(${ARGV})
+    target_link_libraries(${test_name}
+      PRIVATE
+        ftxui
+        Threads::Threads
+        ${GTEST_BOTH_LIBRARIES}
+    )
+    target_include_directories(ftxui
+      PRIVATE
+        ${GTest_INCLUDE_DIRS}
+        ftxui
+    )
+    gtest_discover_tests(${test_name})
+    add_test(${test_name} ${test_name})
+  endfunction(add_new_test)
+
+  add_new_test(dom_tests
+    src/ftxui/core/dom/text_test.cpp
+    src/ftxui/core/dom/hbox_test.cpp
+    src/ftxui/core/dom/vbox_test.cpp
+  )
+endif()
diff --git a/ftxui/include/ftxui/README.md b/ftxui/include/ftxui/README.md
new file mode 100644
index 0000000..5b0dc90
--- /dev/null
+++ b/ftxui/include/ftxui/README.md
@@ -0,0 +1 @@
+State => Components => Document => Text.
diff --git a/ftxui/include/ftxui/core/box.hpp b/ftxui/include/ftxui/core/box.hpp
new file mode 100644
index 0000000..021a594
--- /dev/null
+++ b/ftxui/include/ftxui/core/box.hpp
@@ -0,0 +1,11 @@
+#ifndef FTX_UI_CORE_BOX
+#define FTX_UI_CORE_BOX
+
+struct Box {
+  int left;
+  int right;
+  int top;
+  int bottom;
+};
+
+#endif /* end of include guard: FTX_UI_CORE_BOX */
diff --git a/ftxui/include/ftxui/core/component.hpp b/ftxui/include/ftxui/core/component.hpp
new file mode 100644
index 0000000..fb0c4f0
--- /dev/null
+++ b/ftxui/include/ftxui/core/component.hpp
@@ -0,0 +1,23 @@
+#ifndef FTXUI_STATE_HPP
+#define FTXUI_STATE_HPP
+
+#include "ftxui/core/requirement.hpp"
+#include "ftxui/core/document.hpp"
+
+namespace ftxui {
+
+class Component {
+ public:
+  virtual Document Render() = 0;
+
+  // Requirement -------------------------------------------------------------
+  virtual void ComputeRequirement();
+  Requirement requirement() { return requirement_; }
+
+ private:
+  Requirement requirement_;
+};
+
+};  // namespace ftxui
+
+#endif /* end of include guard: FTXUI_STATE_HPP */
diff --git a/ftxui/include/ftxui/core/document.hpp b/ftxui/include/ftxui/core/document.hpp
new file mode 100644
index 0000000..51034f9
--- /dev/null
+++ b/ftxui/include/ftxui/core/document.hpp
@@ -0,0 +1,12 @@
+#ifndef FTXUI_DOCUMENT_HPP
+#define FTXUI_DOCUMENT_HPP
+
+namespace ftxui {
+
+class Document {
+   
+};
+
+};
+
+#endif /* end of include guard: FTXUI_DOCUMENT_HPP */
diff --git a/ftxui/include/ftxui/core/dom/elements.hpp b/ftxui/include/ftxui/core/dom/elements.hpp
new file mode 100644
index 0000000..2c2e5dd
--- /dev/null
+++ b/ftxui/include/ftxui/core/dom/elements.hpp
@@ -0,0 +1,38 @@
+#ifndef FTXUI_CORE_DOM_ELEMENTS_HPP
+#define FTXUI_CORE_DOM_ELEMENTS_HPP
+
+#include "ftxui/core/dom/node.hpp"
+#include <initializer_list>
+
+namespace ftxui {
+namespace dom {
+
+using Child = std::unique_ptr<Node>;
+using Children = std::vector<std::unique_ptr<Node>>;
+
+std::unique_ptr<Node> vbox(Children);
+std::unique_ptr<Node> hbox(Children);
+std::unique_ptr<Node> text(std::wstring text);
+std::unique_ptr<Node> flex();
+
+template <class... Args>
+std::vector<Child> unpack(Args... args) {
+  std::vector<Child> vec;
+  (vec.push_back(std::forward<Args>(args)), ...);
+  return vec;
+}
+
+template <class... Args>
+std::unique_ptr<Node> vbox(Args... children) {
+  return vbox(unpack(std::forward<Args>(children)...));
+}
+
+template <class... Args>
+std::unique_ptr<Node> hbox(Args... children) {
+  return hbox(unpack(std::forward<Args>(children)...));
+}
+
+};  // namespace dom
+};  // namespace ftxui
+
+#endif /* end of include guard: FTXUI_CORE_DOM_ELEMENTS_HPP */
diff --git a/ftxui/include/ftxui/core/dom/node.hpp b/ftxui/include/ftxui/core/dom/node.hpp
new file mode 100644
index 0000000..f843228
--- /dev/null
+++ b/ftxui/include/ftxui/core/dom/node.hpp
@@ -0,0 +1,44 @@
+#ifndef CORE_DOM_NODE_HPP
+#define CORE_DOM_NODE_HPP
+
+#include <memory>
+#include <vector>
+
+#include "ftxui/core/requirement.hpp"
+#include "ftxui/core/screen.hpp"
+#include "ftxui/core/box.hpp"
+
+namespace ftxui {
+namespace dom {
+
+class Node {
+ public:
+  Node();
+  Node(std::vector<std::unique_ptr<Node>> children);
+  virtual ~Node();
+
+  // Step 1: Direction parent <= children.
+  //         Compute layout requirement. Tell parent what dimensions this
+  //         element wants to be.
+  virtual void ComputeRequirement();
+  Requirement requirement() { return requirement_; }
+
+  // Step 2: Direction parent => children.
+  //         Assign this element its final dimensions.
+  virtual void SetBox(Box box);
+
+  // Step 3: Draw this element.
+  virtual void Render(Screen& screen);
+
+  std::vector<std::unique_ptr<Node>> children;
+ protected:
+  Requirement requirement_;
+  Box box_;
+};
+
+void Render(Screen& screen, Node* node);
+
+};  // namespace dom
+};  // namespace ftxui
+
+#endif /* end of include guard: CORE_DOM_NODE_HPP */
diff --git a/ftxui/include/ftxui/core/requirement.hpp b/ftxui/include/ftxui/core/requirement.hpp
new file mode 100644
index 0000000..74089b4
--- /dev/null
+++ b/ftxui/include/ftxui/core/requirement.hpp
@@ -0,0 +1,28 @@
+#ifndef FTXUI_LAYOUT_REQUIREMENT_HPP
+#define FTXUI_LAYOUT_REQUIREMENT_HPP
+
+namespace ftxui {
+
+struct Requirement {
+  // Minimal dimensions.
+  struct {
+    int x = 0;
+    int y = 0;
+  } min;
+
+  // Maximal dimensions.
+  struct {
+    int x = -1;
+    int y = -1;
+  } max;
+
+  // Flex.
+  struct {
+    int x = 0;
+    int y = 0;
+  } flex;
+};
+
+};  // namespace ftxui
+
+#endif /* end of include guard: FTXUI_LAYOUT_REQUIREMENT_HPP */
diff --git a/ftxui/include/ftxui/core/screen.hpp b/ftxui/include/ftxui/core/screen.hpp
new file mode 100644
index 0000000..c02e1d8
--- /dev/null
+++ b/ftxui/include/ftxui/core/screen.hpp
@@ -0,0 +1,28 @@
+#ifndef FTXUI_CORE_SCREEN
+#define FTXUI_CORE_SCREEN
+
+#include <string>
+#include <vector>
+
+namespace ftxui {
+
+class Screen {
+ public:
+  Screen(size_t dimx, size_t dimy);
+  wchar_t& at(size_t x, size_t y);
+  std::string ToString();
+
+  size_t dimx() { return dimx_;}
+  size_t dimy() { return dimy_;}
+
+  static Screen WholeTerminal();
+
+ private:
+  size_t dimx_;
+  size_t dimy_;
+  std::vector<std::wstring> lines_;
+};
+
+};  // namespace ftxui
+
+#endif /* end of include guard: FTXUI_CORE_SCREEN */
diff --git a/ftxui/include/ftxui/util/string.hpp b/ftxui/include/ftxui/util/string.hpp
new file mode 100644
index 0000000..d2383e7
--- /dev/null
+++ b/ftxui/include/ftxui/util/string.hpp
@@ -0,0 +1,4 @@
+#include <string>
+
+std::string to_string(const std::wstring& s);
+std::wstring to_wstring(const std::string& s);
diff --git a/ftxui/src/ftxui/core/component.cpp b/ftxui/src/ftxui/core/component.cpp
new file mode 100644
index 0000000..cd28bc8
--- /dev/null
+++ b/ftxui/src/ftxui/core/component.cpp
@@ -0,0 +1,5 @@
+#include "ftxui/core/component.hpp"
+
+namespace ftxui {
+  void Component::ComputeRequirement() {}
+} // namespace ftxui.
diff --git a/ftxui/src/ftxui/core/dom/flex.cpp b/ftxui/src/ftxui/core/dom/flex.cpp
new file mode 100644
index 0000000..1643ccd
--- /dev/null
+++ b/ftxui/src/ftxui/core/dom/flex.cpp
@@ -0,0 +1,23 @@
+#include "ftxui/core/dom/node.hpp"
+
+namespace ftxui {
+namespace dom {
+
+class Flex : public Node {
+ public:
+  Flex() {}
+  ~Flex() override {}
+  void ComputeRequirement() {
+    requirement_.min.x = 0;
+    requirement_.min.y = 0;
+    requirement_.flex.x = 1;
+    requirement_.flex.y = 1;
+  }
+};
+
+std::unique_ptr<Node> flex() {
+  return std::make_unique<Flex>();
+}
+
+};  // namespace dom
+};  // namespace ftxui
diff --git a/ftxui/src/ftxui/core/dom/hbox.cpp b/ftxui/src/ftxui/core/dom/hbox.cpp
new file mode 100644
index 0000000..dfd66d7
--- /dev/null
+++ b/ftxui/src/ftxui/core/dom/hbox.cpp
@@ -0,0 +1,68 @@
+#include "ftxui/core/dom/node.hpp"
+#include "ftxui/core/dom/elements.hpp"
+
+namespace ftxui {
+namespace dom {
+
+class HBox : public Node {
+ public:
+  HBox(Children children) : Node(std::move(children)) {}
+  ~HBox() {}
+
+  void ComputeRequirement() override {
+    requirement_.min.x = 0;
+    requirement_.min.y = 0;
+    requirement_.flex.x = 1;
+    requirement_.flex.y = 0;
+    for (auto& child : children) {
+      child->ComputeRequirement();
+      requirement_.min.x += child->requirement().min.x;
+      requirement_.min.y =
+          std::max(requirement_.min.y, child->requirement().min.y);
+    }
+  }
+
+  void SetBox(Box box) override {
+    Node::SetBox(box);
+
+    int flex_sum = 0;
+    for (auto& child : children)
+      flex_sum += child->requirement().flex.x;
+
+    int space = box.right - box.left + 1;
+    int extra_space = space - requirement_.min.x;
+
+    int remaining_flex = flex_sum;
+    int remaining_extra_space = extra_space;
+
+    int x = box.left;
+    for (auto& child : children) {
+      if (x > box.right)
+        break;
+
+      Box child_box = box;
+      child_box.left = x;
+
+      child_box.right = x + child->requirement().min.x - 1;
+
+      if (child->requirement().flex.x && remaining_extra_space > 0) {
+        int added_space = remaining_extra_space * child->requirement().flex.x /
+                          remaining_flex;
+        remaining_extra_space -= added_space;
+        remaining_flex -= child->requirement().flex.x;
+        child_box.right += added_space;
+      }
+      child_box.right = std::min(child_box.right, box.right);
+
+      child->SetBox(child_box);
+      x = child_box.right + 1;
+    }
+  }
+};
+
+std::unique_ptr<Node> hbox(Children children) {
+  return std::make_unique<HBox>(std::move(children));
+}
+
+};  // namespace dom
+};  // namespace ftxui
diff --git a/ftxui/src/ftxui/core/dom/hbox_test.cpp b/ftxui/src/ftxui/core/dom/hbox_test.cpp
new file mode 100644
index 0000000..fb05538
--- /dev/null
+++ b/ftxui/src/ftxui/core/dom/hbox_test.cpp
@@ -0,0 +1,123 @@
+#include "ftxui/core/dom/elements.hpp"
+#include "ftxui/core/screen.hpp"
+#include "gtest/gtest.h"
+
+namespace ftxui {
+namespace dom {
+
+TEST(HBoxTest, ScreenSmaller1) {
+  auto root = hbox(
+    text(L"text_1"),
+    text(L"text_2")
+  );
+  Screen screen(11,1);
+  Render(screen, root.get());
+
+  EXPECT_EQ("text_1text_", screen.ToString());
+}
+
+TEST(HBoxTest, ScreenSmaller2) {
+  auto root = hbox(
+    text(L"text_1"),
+    text(L"text_2")
+  );
+  Screen screen(10,1);
+  Render(screen, root.get());
+
+  EXPECT_EQ("text_1text", screen.ToString());
+}
+
+TEST(HBoxTest, ScreenFit) {
+  auto root = hbox(
+    text(L"text_1"),
+    text(L"text_2")
+  );
+  Screen screen(12,1);
+  Render(screen, root.get());
+
+  EXPECT_EQ("text_1text_2", screen.ToString());
+}
+
+TEST(HBoxTest, ScreenBigger1) {
+  auto root = hbox(
+    text(L"text_1"),
+    text(L"text_2")
+  );
+  Screen screen(13,1);
+  Render(screen, root.get());
+
+  EXPECT_EQ("text_1text_2 ", screen.ToString());
+}
+TEST(HBoxTest, ScreenBigger2) {
+  auto root = hbox(
+    text(L"text_1"),
+    text(L"text_2")
+  );
+  Screen screen(14,1);
+  Render(screen, root.get());
+
+  EXPECT_EQ("text_1text_2  ", screen.ToString());
+}
+
+TEST(HBoxTest, ScreenSmaller1Flex) {
+  auto root = hbox(
+    text(L"text_1"),
+    flex(),
+    text(L"text_2")
+  );
+  Screen screen(11,1);
+  Render(screen, root.get());
+
+  EXPECT_EQ("text_1text_", screen.ToString());
+}
+
+TEST(HBoxTest, ScreenSmaller2Flex) {
+  auto root = hbox(
+    text(L"text_1"),
+    flex(),
+    text(L"text_2")
+  );
+  Screen screen(10,1);
+  Render(screen, root.get());
+
+  EXPECT_EQ("text_1text", screen.ToString());
+}
+
+TEST(HBoxTest, ScreenFitFlex) {
+  auto root = hbox(
+    text(L"text_1"),
+    flex(),
+    text(L"text_2")
+  );
+  Screen screen(12,1);
+  Render(screen, root.get());
+
+  EXPECT_EQ("text_1text_2", screen.ToString());
+}
+
+TEST(HBoxTest, ScreenBigger1Flex) {
+  auto root = hbox(
+    text(L"text_1"),
+    flex(),
+    text(L"text_2")
+  );
+  Screen screen(13,1);
+  Render(screen, root.get());
+
+  EXPECT_EQ("text_1 text_2", screen.ToString());
+}
+
+TEST(HBoxTest, ScreenBigger2Flex) {
+  auto root = hbox(
+    text(L"text_1"),
+    flex(),
+    text(L"text_2")
+  );
+  Screen screen(14,1);
+  Render(screen, root.get());
+
+  EXPECT_EQ("text_1  text_2", screen.ToString());
+}
+
+};  // namespace dom
+};  // namespace ftxui
diff --git a/ftxui/src/ftxui/core/dom/node.cpp b/ftxui/src/ftxui/core/dom/node.cpp
new file mode 100644
index 0000000..db6b266
--- /dev/null
+++ b/ftxui/src/ftxui/core/dom/node.cpp
@@ -0,0 +1,38 @@
+#include "ftxui/core/dom/node.hpp"
+
+namespace ftxui {
+namespace dom {
+
+Node::Node() {}
+Node::Node(std::vector<std::unique_ptr<Node>> children)
+    : children(std::move(children)) {}
+Node::~Node() {}
+
+void Node::ComputeRequirement() {}
+void Node::SetBox(Box box) {
+  box_ = box;
+}
+void Node::Render(Screen& screen) {
+  for(auto& child : children)
+    child->Render(screen);
+}
+
+void Render(Screen& screen, Node* node) {
+  // Step 1: Find what dimension this elements wants to be.
+  node->ComputeRequirement();
+  
+  Box box;
+  box.left = 0;
+  box.top = 0;
+  box.right = screen.dimx() - 1;
+  box.bottom = screen.dimy() - 1;
+  
+  // Step 2: Assign a dimension to the element.
+  node->SetBox(box);
+
+  // Step 3: Draw the element.
+  node->Render(screen);
+}
+
+};  // namespace dom
+};  // namespace ftxui
diff --git a/ftxui/src/ftxui/core/dom/text.cpp b/ftxui/src/ftxui/core/dom/text.cpp
new file mode 100644
index 0000000..7b604fb
--- /dev/null
+++ b/ftxui/src/ftxui/core/dom/text.cpp
@@ -0,0 +1,37 @@
+#include "ftxui/core/dom/node.hpp"
+
+namespace ftxui {
+namespace dom {
+
+class Text : public Node {
+ public:
+  Text(std::wstring text) : Node(), text_(text) {}
+  ~Text() {}
+
+  void ComputeRequirement() override {
+    requirement_.min.x = text_.size();
+    requirement_.min.y = 1;
+  }
+
+  void Render(Screen& screen) override {
+    int x = box_.left;
+    int y = box_.top;
+    if (y > box_.bottom)
+      return;
+    for (wchar_t c : text_) {
+      if (x > box_.right)
+        return;
+      screen.at(x++, y) = c;
+    }
+  }
+
+ private:
+  std::wstring text_;
+};
+
+std::unique_ptr<Node> text(std::wstring text) {
+  return std::make_unique<Text>(text);
+}
+
+};  // namespace dom
+};  // namespace ftxui
diff --git a/ftxui/src/ftxui/core/dom/text_test.cpp b/ftxui/src/ftxui/core/dom/text_test.cpp
new file mode 100644
index 0000000..72775ac
--- /dev/null
+++ b/ftxui/src/ftxui/core/dom/text_test.cpp
@@ -0,0 +1,49 @@
+#include "ftxui/core/dom/elements.hpp"
+#include "ftxui/core/screen.hpp"
+#include "gtest/gtest.h"
+
+namespace ftxui {
+namespace dom {
+
+TEST(TextTest, ScreenHeightSmaller) {
+  auto element = text(L"test");
+  Screen screen(2, 0);
+  Render(screen, element.get());
+
+  EXPECT_EQ("", screen.ToString());
+}
+
+TEST(TextTest, ScreenSmaller) {
+  auto element = text(L"test");
+  Screen screen(2, 1);
+  Render(screen, element.get());
+
+  EXPECT_EQ("te", screen.ToString());
+}
+
+TEST(TextTest, ScreenFit) {
+  auto element = text(L"test");
+  Screen screen(4, 1);
+  Render(screen, element.get());
+
+  EXPECT_EQ("test", screen.ToString());
+}
+
+TEST(TextTest, ScreenBigger) {
+  auto element = text(L"test");
+  Screen screen(6, 1);
+  Render(screen, element.get());
+
+  EXPECT_EQ("test  ", screen.ToString());
+}
+
+TEST(TextTest, ScreenBigger2) {
+  auto element = text(L"test");
+  Screen screen(6, 2);
+  Render(screen, element.get());
+
+  EXPECT_EQ("test  \n      ", screen.ToString());
+}
+
+};  // namespace dom
+};  // namespace ftxui
diff --git a/ftxui/src/ftxui/core/dom/vbox.cpp b/ftxui/src/ftxui/core/dom/vbox.cpp
new file mode 100644
index 0000000..a2bd4d7
--- /dev/null
+++ b/ftxui/src/ftxui/core/dom/vbox.cpp
@@ -0,0 +1,68 @@
+#include "ftxui/core/dom/node.hpp"
+#include "ftxui/core/dom/elements.hpp"
+
+namespace ftxui {
+namespace dom {
+
+class VBox : public Node {
+ public:
+  VBox(Children children) : Node(std::move(children)) {}
+  ~VBox() {}
+
+  void ComputeRequirement() {
+    requirement_.min.x = 0;
+    requirement_.min.y = 0;
+    requirement_.flex.x = 0;
+    requirement_.flex.y = 1;
+    for (auto& child : children) {
+      child->ComputeRequirement();
+      requirement_.min.y += child->requirement().min.y;
+      requirement_.min.x =
+          std::max(requirement_.min.x, child->requirement().min.x);
+    }
+  }
+
+  void SetBox(Box box) {
+    Node::SetBox(box);
+
+    int flex_sum = 0;
+    for (auto& child : children)
+      flex_sum += child->requirement().flex.y;
+
+    int space = box.bottom - box.top + 1;
+    int extra_space = space - requirement_.min.y;
+
+    int remaining_flex = flex_sum;
+    int remaining_extra_space = extra_space;
+
+    int y = box.left;
+    for (auto& child : children) {
+      if (y > box.right)
+        break;
+
+      Box child_box = box;
+      child_box.top = y;
+
+      child_box.bottom = y + child->requirement().min.y - 1;
+
+      if (child->requirement().flex.y && remaining_extra_space > 0) {
+        int added_space = remaining_extra_space * child->requirement().flex.y /
+                          remaining_flex;
+        remaining_extra_space -= added_space;
+        remaining_flex -= child->requirement().flex.y;
+        child_box.bottom += added_space;
+      }
+      child_box.bottom = std::min(child_box.bottom, box.bottom);
+
+      child->SetBox(child_box);
+      y = child_box.bottom + 1;
+    }
+  }
+};
+
+std::unique_ptr<Node> vbox(Children children) {
+  return std::make_unique<VBox>(std::move(children));
+}
+
+};  // namespace dom
+};  // namespace ftxui
diff --git a/ftxui/src/ftxui/core/dom/vbox_test.cpp b/ftxui/src/ftxui/core/dom/vbox_test.cpp
new file mode 100644
index 0000000..f9005a6
--- /dev/null
+++ b/ftxui/src/ftxui/core/dom/vbox_test.cpp
@@ -0,0 +1,71 @@
+#include "ftxui/core/dom/elements.hpp"
+#include "ftxui/core/screen.hpp"
+#include "gtest/gtest.h"
+
+namespace ftxui {
+namespace dom {
+
+TEST(VBoxTest, ScreenSmaller1) {
+  auto root = vbox(text(L"text_1"), text(L"text_2"));
+  Screen screen(6, 1);
+  Render(screen, root.get());
+
+  EXPECT_EQ("text_1", screen.ToString());
+}
+
+TEST(VBoxTest, ScreenFit) {
+  auto root = vbox(text(L"text_1"), text(L"text_2"));
+  Screen screen(6, 2);
+  Render(screen, root.get());
+
+  EXPECT_EQ("text_1\ntext_2", screen.ToString());
+}
+
+TEST(VBoxTest, ScreenBigger1) {
+  auto root = vbox(text(L"text_1"), text(L"text_2"));
+  Screen screen(6, 3);
+  Render(screen, root.get());
+
+  EXPECT_EQ("text_1\ntext_2\n      ", screen.ToString());
+}
+TEST(VBoxTest, ScreenBigger2) {
+  auto root = vbox(text(L"text_1"), text(L"text_2"));
+  Screen screen(6, 4);
+  Render(screen, root.get());
+
+  EXPECT_EQ("text_1\ntext_2\n      \n      ", screen.ToString());
+}
+
+TEST(VBoxTest, ScreenSmaller1Flex) {
+  auto root = vbox(text(L"text_1"), flex(), text(L"text_2"));
+  Screen screen(6, 1);
+  Render(screen, root.get());
+
+  EXPECT_EQ("text_1", screen.ToString());
+}
+
+TEST(VBoxTest, ScreenFitFlex) {
+  auto root = vbox(text(L"text_1"), flex(), text(L"text_2"));
+  Screen screen(6, 2);
+  Render(screen, root.get());
+
+  EXPECT_EQ("text_1\ntext_2", screen.ToString());
+}
+
+TEST(VBoxTest, ScreenBigger1Flex) {
+  auto root = vbox(text(L"text_1"), flex(), text(L"text_2"));
+  Screen screen(6, 3);
+  Render(screen, root.get());
+
+  EXPECT_EQ("text_1\n      \ntext_2", screen.ToString());
+}
+TEST(VBoxTest, ScreenBigger2Flex) {
+  auto root = vbox(text(L"text_1"), flex(), text(L"text_2"));
+  Screen screen(6, 4);
+  Render(screen, root.get());
+
+  EXPECT_EQ("text_1\n      \n      \ntext_2", screen.ToString());
+}
+
+};  // namespace dom
+};  // namespace ftxui
diff --git a/ftxui/src/ftxui/core/screen.cpp b/ftxui/src/ftxui/core/screen.cpp
new file mode 100644
index 0000000..0411e6b
--- /dev/null
+++ b/ftxui/src/ftxui/core/screen.cpp
@@ -0,0 +1,31 @@
+#include "ftxui/core/screen.hpp"
+#include "ftxui/core/terminal.hpp"
+#include "ftxui/util/string.hpp"
+
+#include <sstream>
+
+namespace ftxui {
+
+Screen::Screen(size_t dimx, size_t dimy)
+    : dimx_(dimx), dimy_(dimy), lines_(dimy, std::wstring(dimx, U' ')) {}
+
+std::string Screen::ToString() {
+  std::stringstream ss;
+  for (size_t y = 0; y < dimy_; ++y) {
+    ss << to_string(lines_[y]);
+    if (y + 1 < dimy_)
+      ss << '\n';
+  }
+  return ss.str();
+}
+
+wchar_t& Screen::at(size_t x, size_t y) {
+  return lines_[y][x];
+}
+
+Screen Screen::WholeTerminal() {
+  Terminal::Dimensions size = Terminal::Size();
+  return Screen(size.dimx, size.dimy);
+}
+
+};  // namespace ftxui
diff --git a/ftxui/src/ftxui/core/terminal.cpp b/ftxui/src/ftxui/core/terminal.cpp
new file mode 100644
index 0000000..f8bf7c8
--- /dev/null
+++ b/ftxui/src/ftxui/core/terminal.cpp
@@ -0,0 +1,15 @@
+#include <sys/ioctl.h>
+#include <stdio.h>
+#include <unistd.h>
+
+#include "ftxui/core/terminal.hpp"
+
+namespace ftxui {
+
+Terminal::Dimensions Terminal::Size() {
+  winsize w;
+  ioctl(STDOUT_FILENO, TIOCGWINSZ, &w);
+  return Dimensions{w.ws_col, w.ws_row};
+}
+
+} // namespace ftxui
diff --git a/ftxui/src/ftxui/core/terminal.hpp b/ftxui/src/ftxui/core/terminal.hpp
new file mode 100644
index 0000000..2310bac
--- /dev/null
+++ b/ftxui/src/ftxui/core/terminal.hpp
@@ -0,0 +1,18 @@
+#ifndef FTXUI_CORE_TERMINAL_HPP
+#define FTXUI_CORE_TERMINAL_HPP
+
+namespace ftxui {
+
+class Terminal {
+ public:
+  struct Dimensions {
+    int dimx;
+    int dimy;
+  };
+
+  static Dimensions Size();
+};
+
+}  // namespace ftxui
+
+#endif /* end of include guard: FTXUI_CORE_TERMINAL_HPP */
diff --git a/ftxui/src/ftxui/util/string.cpp b/ftxui/src/ftxui/util/string.cpp
new file mode 100644
index 0000000..8569495
--- /dev/null
+++ b/ftxui/src/ftxui/util/string.cpp
@@ -0,0 +1,14 @@
+#include "ftxui/util/string.hpp"
+
+#include <codecvt>
+#include <locale>
+
+std::string to_string(const std::wstring& s) {
+  std::wstring_convert<std::codecvt_utf8_utf16<wchar_t>> converter;
+  return converter.to_bytes(s);
+}
+
+std::wstring to_wstring(const std::string& s) {
+  std::wstring_convert<std::codecvt_utf8_utf16<wchar_t>> converter;
+  return converter.from_bytes(s);
+}