libtxt: support justification of RTL text (flutter/engine#7719)

Fixes https://github.com/flutter/flutter/issues/25567

engine/src/flutter/third_party/txt/src/txt/paragraph.cc

@@ -534,11 +534,12 @@ void Paragraph::Layout(double width, bool force) {
       }
     }
 
-    // Exclude trailing whitespace from right and center-justified lines so the
-    // last visible character in the line will be flush with the right margin.
+    // Exclude trailing whitespace from justified lines so the last visible
+    // character in the line will be flush with the right margin.
     size_t line_end_index =
         (paragraph_style_.effective_align() == TextAlign::right ||
-         paragraph_style_.effective_align() == TextAlign::center)
+         paragraph_style_.effective_align() == TextAlign::center ||
+         paragraph_style_.effective_align() == TextAlign::justify)
             ? line_range.end_excluding_whitespace
             : line_range.end;
 
@@ -552,6 +553,14 @@ void Paragraph::Layout(double width, bool force) {
                                bidi_run.direction(), bidi_run.style());
       }
     }
+    bool line_runs_all_rtl =
+        line_runs.size() &&
+        std::accumulate(
+            line_runs.begin(), line_runs.end(), true,
+            [](const bool a, const BidiRun& b) { return a && b.is_rtl(); });
+    if (line_runs_all_rtl) {
+      std::reverse(words.begin(), words.end());
+    }
 
     std::vector<GlyphPosition> line_glyph_positions;
     std::vector<CodeUnitRun> line_code_unit_runs;
@@ -720,15 +729,26 @@ void Paragraph::Layout(double width, bool force) {
                 grapheme_code_unit_counts[i]);
           }
 
-          if (word_index < words.size() &&
-              words[word_index].start == run.start() + glyph_code_units.start) {
+          bool at_word_start = false;
+          bool at_word_end = false;
+          if (word_index < words.size()) {
+            at_word_start =
+                words[word_index].start == run.start() + glyph_code_units.start;
+            at_word_end =
+                words[word_index].end == run.start() + glyph_code_units.end;
+            if (line_runs_all_rtl) {
+              std::swap(at_word_start, at_word_end);
+            }
+          }
+
+          if (at_word_start) {
             word_start_position = run_x_offset + glyph_x_offset;
           }
 
-          if (word_index < words.size() &&
-              words[word_index].end == run.start() + glyph_code_units.end) {
-            if (justify_line)
+          if (at_word_end) {
+            if (justify_line) {
               justify_x_offset += word_gap_width;
+            }
             word_index++;
 
             if (!isnan(word_start_position)) {

engine/src/flutter/third_party/txt/src/txt/paragraph.h

@@ -212,6 +212,7 @@ class Paragraph {
   FRIEND_TEST_WINDOWS_DISABLED(ParagraphTest, RightAlignParagraph);
   FRIEND_TEST_WINDOWS_DISABLED(ParagraphTest, CenterAlignParagraph);
   FRIEND_TEST_WINDOWS_DISABLED(ParagraphTest, JustifyAlignParagraph);
+  FRIEND_TEST_WINDOWS_DISABLED(ParagraphTest, JustifyRTL);
   FRIEND_TEST(ParagraphTest, DecorationsParagraph);
   FRIEND_TEST(ParagraphTest, ItalicsParagraph);
   FRIEND_TEST(ParagraphTest, ChineseParagraph);

engine/src/flutter/third_party/txt/tests/paragraph_unittests.cc

@@ -669,6 +669,53 @@ TEST_F(ParagraphTest, DISABLE_ON_WINDOWS(JustifyAlignParagraph)) {
   ASSERT_TRUE(Snapshot());
 }
 
+TEST_F(ParagraphTest, DISABLE_ON_WINDOWS(JustifyRTL)) {
+  const char* text =
+      "אאא בּבּבּבּ אאאא בּבּ אאא בּבּבּ אאאאא בּבּבּבּ אאאא בּבּבּבּבּ "
+      "אאאאא בּבּבּבּבּ אאאבּבּבּבּבּבּאאאאא בּבּבּבּבּבּאאאאאבּבּבּבּבּבּ אאאאא בּבּבּבּבּ "
+      "אאאאא בּבּבּבּבּבּ אאאאא בּבּבּבּבּבּ אאאאא בּבּבּבּבּבּ אאאאא בּבּבּבּבּבּ אאאאא בּבּבּבּבּבּ";
+
+  auto icu_text = icu::UnicodeString::fromUTF8(text);
+  std::u16string u16_text(icu_text.getBuffer(),
+                          icu_text.getBuffer() + icu_text.length());
+
+  txt::ParagraphStyle paragraph_style;
+  paragraph_style.max_lines = 14;
+  paragraph_style.text_align = TextAlign::justify;
+  txt::ParagraphBuilder builder(paragraph_style, GetTestFontCollection());
+
+  txt::TextStyle text_style;
+  text_style.font_families = std::vector<std::string>(1, "Ahem");
+  text_style.font_size = 26;
+  text_style.color = SK_ColorBLACK;
+  text_style.height = 1;
+  builder.PushStyle(text_style);
+
+  builder.AddText(u16_text);
+
+  builder.Pop();
+
+  auto paragraph = builder.Build();
+  size_t paragraph_width = GetTestCanvasWidth() - 100;
+  paragraph->Layout(paragraph_width);
+
+  paragraph->Paint(GetCanvas(), 0, 0);
+
+  ASSERT_TRUE(Snapshot());
+
+  auto glyph_line_width = [&paragraph](int index) {
+    return paragraph->glyph_lines_[index].positions.back().x_pos.end;
+  };
+
+  // All lines except the last should be justified to the width of the
+  // paragraph.
+  for (size_t i = 0; i < paragraph->glyph_lines_.size() - 1; ++i) {
+    ASSERT_EQ(glyph_line_width(i), paragraph_width);
+  }
+  ASSERT_NE(glyph_line_width(paragraph->glyph_lines_.size() - 1),
+            paragraph_width);
+}
+
 TEST_F(ParagraphTest, DecorationsParagraph) {
   txt::ParagraphStyle paragraph_style;
   paragraph_style.max_lines = 14;