Add custom HTML tag handler to be used when converting HTML to Text.

Hides code in STYLE and SCRIPT tags and adds text-equivalent of HR.

src/com/fsck/k9/helper/HtmlToTextTagHandler.java

@@ -0,0 +1,85 @@
+package com.fsck.k9.helper;
+
+import android.text.Annotation;
+import android.text.Editable;
+import android.text.Html;
+import android.text.Spannable;
+import org.xml.sax.XMLReader;
+
+/**
+ * Custom tag handler to use when converting HTML messages to text. It currently handles text
+ * representations of HTML tags that Android's built-in parser doesn't understand and hides code
+ * contained in STYLE and SCRIPT blocks.
+ */
+public class HtmlToTextTagHandler implements Html.TagHandler
+{
+    @Override
+    public void handleTag(boolean opening, String tag, Editable output, XMLReader xmlReader)
+    {
+        if (tag.equalsIgnoreCase("hr") && opening)
+        {
+            // In the case of an <hr>, replace it with a bunch of underscores.  This is roughly the behaviour of
+            // Outlook in Rich Text mode.
+            output.append("_____________________________________________\n");
+        }
+        else if (tag.equalsIgnoreCase("style") || tag.equalsIgnoreCase("script"))
+        {
+            handleIgnoredTag(opening, output);
+        }
+    }
+
+    private static final String IGNORED_ANNOTATION_KEY = "K9_ANNOTATION";
+    private static final String IGNORED_ANNOTATION_VALUE = "hiddenSpan";
+
+    /**
+     * When we come upon an ignored tag, we mark it with an Annotation object with a specific key
+     * and value as above. We don't really need to be checking these values since Html.fromHtml()
+     * doesn't use Annotation spans, but we should do it now to be safe in case they do start using
+     * it in the future.
+     * @param opening If this is an opening tag or not.
+     * @param output Spannable string that we're working with.
+     */
+    private void handleIgnoredTag(boolean opening, Editable output)
+    {
+        int len = output.length();
+        if (opening)
+        {
+            output.setSpan(new Annotation(IGNORED_ANNOTATION_KEY, IGNORED_ANNOTATION_VALUE), len,
+                    len, Spannable.SPAN_MARK_MARK);
+        }
+        else
+        {
+            Object start = getOpeningAnnotation(output);
+            if (start != null)
+            {
+                int where = output.getSpanStart(start);
+                // Remove the temporary Annotation span.
+                output.removeSpan(start);
+                // Delete everything between the start of the Annotation and the end of the string
+                // (what we've generated so far).
+                output.delete(where, len);
+            }
+        }
+    }
+
+    /**
+     * Fetch the matching opening Annotation object and verify that it's the one added by K9.
+     * @param output Spannable string we're working with.
+     * @return Starting Annotation object.
+     */
+    private Object getOpeningAnnotation(Editable output)
+    {
+        Object[] objs = output.getSpans(0, output.length(), Annotation.class);
+        for (int i = objs.length - 1; i >= 0; i--)
+        {
+            Annotation span = (Annotation) objs[i];
+            if (output.getSpanFlags(objs[i]) == Spannable.SPAN_MARK_MARK
+                    && span.getKey().equals(IGNORED_ANNOTATION_KEY)
+                    && span.getValue().equals(IGNORED_ANNOTATION_VALUE))
+            {
+                return objs[i];
+            }
+        }
+        return null;
+    }
+}

src/com/fsck/k9/mail/store/LocalStore.java

@@ -15,6 +15,7 @@ import java.util.Set;
 import java.util.UUID;
 import java.util.regex.Matcher;
 
+import com.fsck.k9.helper.HtmlToTextTagHandler;
 import org.apache.commons.io.IOUtils;
 
 import android.app.Application;
@@ -2369,7 +2370,7 @@ public class LocalStore extends Store implements Serializable
                                 // If we couldn't generate a reasonable preview from the text part, try doing it with the HTML part.
                                 if (preview == null || preview.length() == 0)
                                 {
-                                    preview = calculateContentPreview(Html.fromHtml(html).toString().replace(PREVIEW_OBJECT_CHARACTER, PREVIEW_OBJECT_REPLACEMENT));
+                                    preview = calculateContentPreview(Html.fromHtml(html, null, new HtmlToTextTagHandler()).toString().replace(PREVIEW_OBJECT_CHARACTER, PREVIEW_OBJECT_REPLACEMENT));
                                 }
 
                                 try
@@ -2497,7 +2498,7 @@ public class LocalStore extends Store implements Serializable
                             // If we couldn't generate a reasonable preview from the text part, try doing it with the HTML part.
                             if (preview == null || preview.length() == 0)
                             {
-                                preview = calculateContentPreview(Html.fromHtml(html).toString().replace(PREVIEW_OBJECT_CHARACTER, PREVIEW_OBJECT_REPLACEMENT));
+                                preview = calculateContentPreview(Html.fromHtml(html, null, new HtmlToTextTagHandler()).toString().replace(PREVIEW_OBJECT_CHARACTER, PREVIEW_OBJECT_REPLACEMENT));
                             }
                             try
                             {