Fix underline length of unkerned text

Author: camelpunch

lib/mudbrick/font.ex

@@ -82,16 +82,23 @@ defmodule Mudbrick.Font do
   end
 
   @doc false
-  def width(_font, _size, "") do
+  def width(_font, _size, "", _opts) do
     0
   end
 
-  def width(font, size, text) do
-    {_glyph_ids, positions} = OpenType.layout_text(font.parsed, text)
+  def width(font, size, text, opts) do
+    {glyph_ids, positions} = OpenType.layout_text(font.parsed, text)
 
-    for {_, _, _, width, _} <- positions, reduce: 0 do
-      acc -> acc + width / 1000 * size
-    end
+    widths =
+      if Keyword.get(opts, :auto_kern) do
+        Enum.map(positions, fn {_, _, _, width, _} -> width end)
+      else
+        Enum.map(glyph_ids, &Enum.at(font.parsed.glyphWidths, &1))
+      end
+
+    Enum.reduce(widths, 0, fn width, acc ->
+      acc + width / 1000 * size
+    end)
   end
 
   @doc false

lib/mudbrick/text_block/line.ex

@@ -30,7 +30,8 @@ defmodule Mudbrick.TextBlock.Line do
       Mudbrick.Font.width(
         part.font,
         part.font_size,
-        part.text
+        part.text,
+        auto_kern: part.auto_kern
       )
     end
 

test/mudbrick/text_block_test.exs

@@ -159,6 +159,22 @@ defmodule Mudbrick.TextBlockTest do
                }
              ] = block.lines
     end
+
+    test "length of line is different when kerned" do
+      line = fn opts ->
+        output(fn %{fonts: fonts} ->
+          TextBlock.new(Keyword.merge(opts, font: fonts.regular))
+          |> TextBlock.write("underlined", underline: [width: 1])
+        end)
+        |> operations()
+        |> Enum.find(fn op -> String.ends_with?(op, " l") end)
+      end
+
+      line_with_kerning = line.(auto_kern: true)
+      line_without_kerning = line.(auto_kern: false)
+
+      assert line_with_kerning != line_without_kerning
+    end
   end
 
   describe "leading" do