Fix an idempotency issue in Hsu and ETen26 layouts

Certain sequences should have the equivalent output, such as ai/ia with
Hsu (both should yield ㄘㄞ). While "ia" might be a mistake on the
user's part, since it is the equivalent of "ai", the layout code still
should honor it.

Author: lukhnos

Source/Engine/Mandarin/Mandarin.h

@@ -302,6 +302,16 @@ class BopomofoKeyboardLayout {
           syllable += head;
         } else if (syllable.maskType() < follow.maskType()) {
           syllable += follow;
+        } else if ((syllable.maskType() == follow.maskType()) &&
+                   syllable.maskType() == BPMF::VowelMask) {
+          // if the existing syllable contains only a vowel, and the
+          // next character has 2+ possibilities, and the second (follow)
+          // one is also a vowel, we know that the user may have typed the
+          // sequence in the wrong order (since the first or head is always
+          // a consonant); if we come all the way here, the only possibility
+          // for the second is an EI or E, but since we don't allow them to
+          // be standalone in Hsu or ETen26, we will pick the consonant.
+          syllable += head;
         } else {
           syllable += ending;
         }

Source/Engine/Mandarin/MandarinTest.cpp

@@ -101,6 +101,27 @@ TEST(MandarinTest, ETen26Layout) {
   ASSERT_EQ(buf.composedString(), "ㄗㄢˋ");
 }
 
+TEST(MandarinTest, ETen26LayoutIdempotentSequence) {
+  BopomofoReadingBuffer buf(BopomofoKeyboardLayout::ETen26Layout());
+  buf.combineKey('q');
+  buf.combineKey('a');
+  ASSERT_EQ(buf.composedString(), "ㄗㄚ");
+
+  buf.clear();
+  buf.combineKey('a');  // ㄚ first
+  buf.combineKey('q');  // even if q can be ㄟ, a vowel is already in place
+  ASSERT_EQ(buf.composedString(), "ㄗㄚ");
+
+  buf.clear();
+  buf.combineKey('q');
+  buf.combineKey('i');
+  ASSERT_EQ(buf.composedString(), "ㄗㄞ");
+
+  buf.clear();
+  buf.combineKey('i');
+  buf.combineKey('q');
+  ASSERT_EQ(buf.composedString(), "ㄗㄞ");
+}
 TEST(MandarinTest, HsuLayout) {
   BopomofoReadingBuffer buf(BopomofoKeyboardLayout::HsuLayout());
   buf.combineKey('f');
@@ -109,6 +130,28 @@ TEST(MandarinTest, HsuLayout) {
   ASSERT_EQ(buf.composedString(), "ㄈㄟˇ");
 }
 
+TEST(MandarinTest, HsuLayoutIdempotentSequence) {
+  BopomofoReadingBuffer buf(BopomofoKeyboardLayout::HsuLayout());
+  buf.combineKey('a');
+  buf.combineKey('i');
+  ASSERT_EQ(buf.composedString(), "ㄘㄞ");
+
+  buf.clear();
+  buf.combineKey('i');  // ㄞ first
+  buf.combineKey('a');  // even if a can be ㄟ, a vowel is already in place
+  ASSERT_EQ(buf.composedString(), "ㄘㄞ");
+
+  buf.clear();
+  buf.combineKey('a');
+  buf.combineKey('y');
+  ASSERT_EQ(buf.composedString(), "ㄘㄚ");
+
+  buf.clear();
+  buf.combineKey('y');
+  buf.combineKey('a');
+  ASSERT_EQ(buf.composedString(), "ㄘㄚ");
+}
+
 TEST(MandarinTest, IBMLayout) {
   BopomofoReadingBuffer buf(BopomofoKeyboardLayout::IBMLayout());
   buf.combineKey('9');