Optimize inserts in vector and refactor const reference objects (#659)

Signed-off-by: German Semenov <GermanAizek@yandex.ru>
Co-authored-by: ArthurSonzogni <sonzogniarthur@gmail.com>

src/ftxui/dom/flexbox.cpp

@@ -68,6 +68,7 @@ class Flexbox : public Node {
 
   void Layout(flexbox_helper::Global& global,
               bool compute_requirement = false) {
+    global.blocks.reserve(children_.size());
     for (auto& child : children_) {
       flexbox_helper::Block block;
       block.min_size_x = child->requirement().min_x;

src/ftxui/dom/flexbox_helper.cpp

@@ -88,6 +88,7 @@ struct Line {
 void SetX(Global& global, std::vector<Line> lines) {
   for (auto& line : lines) {
     std::vector<box_helper::Element> elements;
+    elements.reserve(line.blocks.size());
     for (auto* block : line.blocks) {
       box_helper::Element element;
       element.min_size = block->min_size_x;
@@ -117,6 +118,7 @@ void SetX(Global& global, std::vector<Line> lines) {
 // NOLINTNEXTLINE(readability-function-cognitive-complexity)
 void SetY(Global& g, std::vector<Line> lines) {
   std::vector<box_helper::Element> elements;
+  elements.reserve(lines.size());
   for (auto& line : lines) {
     box_helper::Element element;
     element.flex_shrink = line.blocks.front()->flex_shrink_y;
@@ -317,6 +319,7 @@ void Compute3(Global& global) {
   {
     Line line;
     int x = 0;
+    line.blocks.reserve(global.blocks.size());
     for (auto& block : global.blocks) {
       // Does it fit the end of the row?
       // No? Then we need to start a new one:

src/ftxui/dom/gridbox.cpp

@@ -37,6 +37,8 @@ class GridBox : public Node {
     for (const auto& line : lines_) {
       x_size = std::max(x_size, int(line.size()));
     }
+
+    // Fill in empty cells, in case the user did not used the API correctly:
     for (auto& line : lines_) {
       while (line.size() < size_t(x_size)) {
         line.push_back(filler());

src/ftxui/dom/linear_gradient.cpp

@@ -78,7 +78,7 @@ LinearGradientNormalized Normalize(LinearGradient gradient) {
   LinearGradientNormalized normalized;
   // NOLINTNEXTLINE
   normalized.angle = std::fmod(std::fmod(gradient.angle, 360.f) + 360.f, 360.f);
-  for (auto& stop : gradient.stops) {
+  for (const auto& stop : gradient.stops) {
     normalized.colors.push_back(stop.color);
     normalized.positions.push_back(stop.position.value());  // NOLINT
   }

src/ftxui/dom/table.cpp

@@ -44,9 +44,11 @@ Table::Table() {
 
 Table::Table(std::vector<std::vector<std::string>> input) {
   std::vector<std::vector<Element>> output;
+  output.reserve(input.size());
   for (auto& row : input) {
     output.emplace_back();
     auto& output_row = output.back();
+    output_row.reserve(row.size());
     for (auto& cell : row) {
       output_row.push_back(text(std::move(cell)));
     }

src/ftxui/dom/util.cpp

@@ -46,6 +46,7 @@ Decorator operator|(Decorator a, Decorator b) {
 /// @ingroup dom
 Elements operator|(Elements elements, Decorator decorator) {  // NOLINT
   Elements output;
+  output.reserve(elements.size());
   for (auto& it : elements) {
     output.push_back(std::move(it) | decorator);
   }