differentiate subchapters denoted by an alpha prefix

app/src/main/java/eu/kanade/tachiyomi/util/ChapterRecognition.java

@@ -12,6 +12,7 @@ public class ChapterRecognition {
 
     private static final Pattern cleanWithToken = Pattern.compile("ch[^0-9]?\\s*(\\d+[\\.,]?\\d+)($|\\b)");
     private static final Pattern uncleanWithToken = Pattern.compile("ch[^0-9]?\\s*(\\d+[\\.,]?\\d*)");
+    private static final Pattern withAlphaPostfix = Pattern.compile("(\\d+[\\.,]?\\d*\\s*)([a-z])($|\\b)");
     private static final Pattern cleanNumber = Pattern.compile("(\\d+[\\.,]?\\d+)($|\\b)");
     private static final Pattern uncleanNumber = Pattern.compile("(\\d+[\\.,]?\\d*)");
     private static final Pattern withColon = Pattern.compile("(\\d+[\\.,]?\\d*\\s*:)");
@@ -33,6 +34,13 @@ public class ChapterRecognition {
             return;
         }
 
+        // a number with a single alpha prefix is parsed as sub-chapter
+        matcher = withAlphaPostfix.matcher(name);
+        if (matcher.find()) {
+            chapter.chapter_number = Float.parseFloat(matcher.group(1)) + parseAlphaPostFix(matcher.group(2));
+            return;
+        }
+
         // the chapter has a token prepended and something at the end of the number
         matcher = uncleanWithToken.matcher(name);
         if (matcher.find()) {
@@ -88,6 +96,14 @@ public class ChapterRecognition {
 
     }
 
+    /**
+     * x.a -> x.1, x.b -> x.2, etc
+     */
+    private static float parseAlphaPostFix(String postfix) {
+        char alpha = postfix.charAt(0);
+        return Float.parseFloat("0." + Integer.toString((int)alpha - 96));
+    }
+
     public static List<Float> getAllOccurrences(Matcher matcher) {
         List<Float> occurences = new ArrayList<>();
         while (matcher.find()) {

app/src/test/java/eu/kanade/tachiyomi/ChapterRecognitionTest.java

@@ -148,4 +148,14 @@ public class ChapterRecognitionTest {
         ChapterRecognition.parseChapterNumber(c, randomManga);
         assertThat(c.chapter_number).isEqualTo(99f);
     }
+
+    @Test
+    public void testAlphaSubChapters() {
+        Chapter c = createChapter("Asu No Yoichi 19a");
+        ChapterRecognition.parseChapterNumber(c, randomManga);
+        assertThat(c.chapter_number).isEqualTo(19.1f);
+        c = createChapter("Asu No Yoichi 19b");
+        ChapterRecognition.parseChapterNumber(c, randomManga);
+        assertThat(c.chapter_number).isEqualTo(19.2f);
+    }
 }