Prefer std::string over std::wstring. (#179)

In the past, FTXUI switched from std::string to std::wstring to support
fullwidth characters. The reasons was that fullwidth characters can be
stored inside a single wchar_t.

Then FTXUI added support for combining characters. A single glygh
doesn't even fit a wchar_t. Instead, a glyph can be arbitrary large.

The usage of wstring doesn't really fit the new model and have several
drawbacks:
1. It doesn't simplify the implementation of FTXUI, because of combining
   characters.
2. It reduces drawing performance by 2x.
3. It increase Screen's memory allocation by 2x.

This patch converts FTXUI to use std::string internally. It now exposes
std::string based API. The std::wstring API remains, but is now
deprecated.

Tests and examples haven't been update to show the breakage is limited.
They will be updated in a second set of patches.

Bug: https://github.com/ArthurSonzogni/FTXUI/issues/153
Co-authored-by: Tushar Maheshwari <tushar27192@gmail.com>

src/ftxui/dom/text.cpp

@@ -1,13 +1,14 @@
-#include <algorithm>  // for max
-#include <memory>     // for make_shared
-#include <string>     // for wstring
+#include <memory>  // for make_shared
+#include <string>  // for string
+#include <vector>  // for vector
 
+#include "ftxui/dom/deprecated.hpp"   // for text, vtext
 #include "ftxui/dom/elements.hpp"     // for Element, text, vtext
 #include "ftxui/dom/node.hpp"         // for Node
 #include "ftxui/dom/requirement.hpp"  // for Requirement
 #include "ftxui/screen/box.hpp"       // for Box
-#include "ftxui/screen/screen.hpp"    // for Screen
-#include "ftxui/screen/string.hpp"    // for wchar_width, wstring_width
+#include "ftxui/screen/screen.hpp"    // for Pixel, Screen
+#include "ftxui/screen/string.hpp"  // for string_width, Utf8ToGlyphs, to_string
 
 namespace ftxui {
 
@@ -15,10 +16,10 @@ using ftxui::Screen;
 
 class Text : public Node {
  public:
-  Text(std::wstring text) : text_(text) {}
+  Text(std::string text) : text_(text) {}
 
   void ComputeRequirement() override {
-    requirement_.min_x = wstring_width(text_);
+    requirement_.min_x = string_width(text_);
     requirement_.min_y = 1;
   }
 
@@ -27,33 +28,26 @@ class Text : public Node {
     int y = box_.y_min;
     if (y > box_.y_max)
       return;
-    for (wchar_t c : text_) {
-      const int width = wchar_width(c);
-      if (width >= 1) {
-        if (x > box_.x_max)
-          return;
-        screen.PixelAt(x, y).character = c;
-      } else {
-        screen.PixelAt(x - 1, y).character += c;
-      }
-      x += std::max(width, 0);
+    for (const auto& cell : Utf8ToGlyphs(text_)) {
+      if (x > box_.x_max)
+        return;
+      screen.PixelAt(x, y).character = cell;
+      ++x;
     }
   }
 
  private:
-  std::wstring text_;
+  std::string text_;
 };
 
 class VText : public Node {
  public:
-  VText(std::wstring text) : text_(text) {
-    for (auto& c : text_)
-      width_ = std::max(width_, wchar_width(c));
-  }
+  VText(std::string text)
+      : text_(text), width_{std::min(string_width(text_), 1)} {}
 
   void ComputeRequirement() override {
     requirement_.min_x = width_;
-    requirement_.min_y = text_.size();
+    requirement_.min_y = string_width(text_);
   }
 
   void Render(Screen& screen) override {
@@ -61,20 +55,39 @@ class VText : public Node {
     int y = box_.y_min;
     if (x + width_ - 1 > box_.x_max)
       return;
-    for (wchar_t c : text_) {
+    for (const auto& it : Utf8ToGlyphs(text_)) {
       if (y > box_.y_max)
         return;
-      screen.at(x, y) = c;
+      screen.PixelAt(x, y).character = it;
       y += 1;
     }
   }
 
  private:
-  std::wstring text_;
+  std::string text_;
   int width_ = 1;
 };
 
-/// @brief Display a pieve of unicode text.
+/// @brief Display a piece of UTF8 encoded unicode text.
+/// @ingroup dom
+/// @see ftxui::to_wstring
+///
+/// ### Example
+///
+/// ```cpp
+/// Element document = text("Hello world!");
+/// ```
+///
+/// ### Output
+///
+/// ```bash
+/// Hello world!
+/// ```
+Element text(std::string text) {
+  return std::make_shared<Text>(text);
+}
+
+/// @brief Display a piece of UTF16 encoded unicode text.
 /// @ingroup dom
 /// @see ftxui::to_wstring
 ///
@@ -90,10 +103,40 @@ class VText : public Node {
 /// Hello world!
 /// ```
 Element text(std::wstring text) {
-  return std::make_shared<Text>(text);
+  return std::make_shared<Text>(to_string(text));
 }
 
-/// @brief Display a pieve of unicode text vertically.
+/// @brief Display a piece of unicode text vertically.
+/// @ingroup dom
+/// @see ftxui::to_wstring
+///
+/// ### Example
+///
+/// ```cpp
+/// Element document = vtext("Hello world!");
+/// ```
+///
+/// ### Output
+///
+/// ```bash
+/// H
+/// e
+/// l
+/// l
+/// o
+///
+/// w
+/// o
+/// r
+/// l
+/// d
+/// !
+/// ```
+Element vtext(std::string text) {
+  return std::make_shared<VText>(text);
+}
+
+/// @brief Display a piece of UTF16 encoded unicode text vertically.
 /// @ingroup dom
 /// @see ftxui::to_wstring
 ///
@@ -120,7 +163,7 @@ Element text(std::wstring text) {
 /// !
 /// ```
 Element vtext(std::wstring text) {
-  return std::make_shared<VText>(text);
+  return std::make_shared<VText>(to_string(text));
 }
 
 }  // namespace ftxui