Fixed incorrect transform from UTF-16 to US-ASCII

app/src/main/java/eu/faircode/email/ActivityEML.java

@@ -472,6 +472,7 @@ public class ActivityEML extends ActivityBase {
                             Charset cs = Charset.forName(charset);
                             Charset detected = CharsetHelper.detect(text, cs);
                             boolean isUtf8 = CharsetHelper.isUTF8(text.getBytes(cs));
+                            boolean isUtf16 = CharsetHelper.isUTF16(text.getBytes(cs));
                             boolean isW1252 = !Objects.equals(text, CharsetHelper.utf8toW1252(text));
 
                             for (int i = 0; i < level; i++)
@@ -480,6 +481,7 @@ public class ActivityEML extends ActivityBase {
                             ssb.append("Detected: ")
                                     .append(detected == null ? "?" : detected.toString())
                                     .append(" isUTF8=").append(Boolean.toString(isUtf8))
+                                    .append(" isUTF16=").append(Boolean.toString(isUtf16))
                                     .append(" isW1252=").append(Boolean.toString(isW1252))
                                     .append('\n');
                         }

app/src/main/java/eu/faircode/email/CharsetHelper.java

@@ -86,7 +86,21 @@ public class CharsetHelper {
         }
     }
 
+    static boolean isUTF16(byte[] octets) {
+        CharsetDecoder utf8Decoder = StandardCharsets.UTF_16.newDecoder()
+                .onMalformedInput(CodingErrorAction.REPORT)
+                .onUnmappableCharacter(CodingErrorAction.REPORT);
+        try {
+            utf8Decoder.decode(ByteBuffer.wrap(octets));
+            return true;
+        } catch (CharacterCodingException ex) {
+            Log.w(ex);
+            return false;
+        }
+    }
+
     static boolean isUTF8Alt(String text) {
+        // This doesn't check the characters and is therefore unreliable
         byte[] octets = text.getBytes(StandardCharsets.ISO_8859_1);
 
         int bytes;

app/src/main/java/eu/faircode/email/MessageHelper.java

@@ -3229,10 +3229,10 @@ public class MessageHelper {
                     try {
                         if (CHARSET16.contains(cs)) {
                             Charset detected = CharsetHelper.detect(result, cs);
+                            // UTF-16 can be detected as US-ASCII
                             if (!CHARSET16.contains(detected))
                                 Log.w(new Throwable("Charset=" + cs + " detected=" + detected));
-                            if (StandardCharsets.US_ASCII.equals(detected) ||
-                                    StandardCharsets.UTF_8.equals(detected)) {
+                            if (StandardCharsets.UTF_8.equals(detected)) {
                                 charset = null;
                                 result = new String(result.getBytes(cs), detected);
                             }