Handle FastScroller behavior in Honeycomb and above

I'm not exactly sure what it's trying to accomplish.. but it was
messing up the scrollbar before since getSectionForPosition was
not implemented.

src/org/kreed/vanilla/MediaAdapter.java

@@ -95,6 +95,10 @@ public class MediaAdapter extends CursorAdapter implements SectionIndexer, Libra
 	 * The section indexer, for the letter pop-up when scrolling.
 	 */
 	private final MusicAlphabetIndexer mIndexer;
+	/**
+	 * The sections used by the indexer.
+	 */
+	private Object[] mSections;
 	/**
 	 * True if this adapter should have a special MediaView with custom text in
 	 * the first row.
@@ -454,14 +458,23 @@ public class MediaAdapter extends CursorAdapter implements SectionIndexer, Libra
 	@Override
 	public Object[] getSections()
 	{
-		if (mSortMode != 0)
-			return null;
-		return mIndexer.getSections();
+		if (mSections == null) {
+			if (mSortMode == 0)
+				mSections = mIndexer.getSections();
+			else
+				mSections = new String[] { " " };
+		}
+		return mSections;
 	}
 
 	@Override
 	public int getPositionForSection(int section)
 	{
+		if (section == 0)
+			return 0;
+		if (section == mSections.length)
+			return getCount();
+
 		int offset = 0;
 		if (mHasHeader) {
 			if (section == 0)
@@ -474,8 +487,16 @@ public class MediaAdapter extends CursorAdapter implements SectionIndexer, Libra
 	@Override
 	public int getSectionForPosition(int position)
 	{
-		// never called by FastScroller
-		return 0;
+		if (mSortMode != 0)
+			return 0;
+
+		if (mHasHeader) {
+			if (position == 0)
+				return 0;
+			else
+				position -= 1;
+		}
+		return mIndexer.getSectionForPosition(position);
 	}
 
 	/**

src/org/kreed/vanilla/MusicAlphabetIndexer.java

@@ -16,24 +16,162 @@
 
 package org.kreed.vanilla;
 
+import android.database.Cursor;
 import android.provider.MediaStore;
-import android.widget.AlphabetIndexer;
+import java.util.Arrays;
 
 /**
- * Handles comparisons in a different way because the Album, Song and Artist name
- * are stripped of some prefixes such as "a", "an", "the" and some symbols.
+ * Like android.widget.AlphabetIndexer, but handles MediaStore sorting order
+ * (strips "a", "the", etc).
  */
-class MusicAlphabetIndexer extends AlphabetIndexer {
+public class MusicAlphabetIndexer {
+	/**
+	 * The indexing sections.
+	 */
+	private static final String[] ALPHABET =
+		{ " ", "A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M",
+		       "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z" };
+	/**
+	 * The result of {@link MediaStore.Audio.keyFor(String)} called on each
+	 * letter of the alphabet.
+	 */
+	private static String[] ALPHABET_KEYS = null;
+	/**
+	 * Cached section postions.
+	 */
+	private int mAlphaMap[];
+	/**
+	 * Cursor that is used by the adapter of the list view.
+	 */
+	private Cursor mDataCursor;
+	/**
+	 * The index of the cursor column that this list is sorted on.
+	 */
+	private int mColumnIndex;
+
+	/**
+	 * Constructs the indexer.
+	 *
+	 * @param sortedColumnIndex the column number in the cursor that is sorted
+	 * alphabetically
+	 */
 	public MusicAlphabetIndexer(int sortedColumnIndex)
 	{
-		super(null, sortedColumnIndex, " ABCDEFGHIJKLMNOPQRSTUVWXYZ");
+		if (ALPHABET_KEYS == null) {
+			String[] keys = new String[ALPHABET.length];
+			for (int i = ALPHABET.length; --i != -1; ) {
+				keys[i] = MediaStore.Audio.keyFor(ALPHABET[i]);
+			}
+			ALPHABET_KEYS = keys;
+		}
+
+		mColumnIndex = sortedColumnIndex;
+		mAlphaMap = new int[ALPHABET.length];
 	}
 
-	@Override
-	protected int compare(String word, String letter)
+	/**
+	 * Returns the latin alphabet.
+	 */
+	public Object[] getSections()
 	{
-		String wordKey = MediaStore.Audio.keyFor(word);
-		String letterKey = MediaStore.Audio.keyFor(letter);
-		return wordKey.compareTo(letterKey);
+		return ALPHABET;
+	}
+
+	/**
+	 * Sets a new cursor as the data set and resets the cache of indices.
+	 *
+	 * @param cursor the new cursor to use as the data set
+	 */
+	public void setCursor(Cursor cursor)
+	{
+		mDataCursor = cursor;
+		Arrays.fill(mAlphaMap, -1);
+	}
+
+	/**
+	 * Performs a binary search or cache lookup to find the first row that
+	 * matches a given section's starting letter.
+	 *
+	 * @param sectionIndex the section to search for
+	 * @return the row index of the first occurrence, or the nearest next letter.
+	 * For instance, if searching for "T" and no "T" is found, then the first
+	 * row starting with "U" or any higher letter is returned. If there is no
+	 * data following "T" at all, then the list size is returned.
+	 */
+	public int getPositionForSection(int sectionIndex) {
+		Cursor cursor = mDataCursor;
+		if (cursor == null)
+			return 0;
+
+		if (sectionIndex <= 0)
+			return 0;
+		int count = cursor.getCount();
+		if (sectionIndex >= ALPHABET.length)
+			return count;
+
+		int pos = mAlphaMap[sectionIndex];
+		if (pos != -1)
+			return pos;
+
+		int savedCursorPos = cursor.getPosition();
+
+		int start = mAlphaMap[sectionIndex - 1];
+		int end = count;
+		if (start == -1)
+			start = 0;
+
+		String targetLetter = ALPHABET_KEYS[sectionIndex];
+		pos = (end + start) / 2;
+		while (pos < end) {
+			cursor.moveToPosition(pos);
+			String curName = cursor.getString(mColumnIndex);
+			String curLetter = MediaStore.Audio.keyFor(curName).substring(0, 3);
+			int diff = curLetter.compareTo(targetLetter);
+			if (diff != 0) {
+				if (diff < 0) {
+					start = pos + 1;
+					if (start >= count) {
+						pos = count;
+						break;
+					}
+				} else {
+					end = pos;
+				}
+			} else {
+				// They're the same, but that doesn't mean it's the start
+				if (start == pos) {
+					// This is it
+					break;
+				} else {
+					// Need to go further lower to find the starting row
+					end = pos;
+				}
+			}
+			pos = (start + end) / 2;
+		}
+
+		mAlphaMap[sectionIndex] = pos;
+		cursor.moveToPosition(savedCursorPos);
+		return pos;
+	}
+
+	/**
+	 * Returns the section index for a given position in the list by querying the item
+	 * and comparing it with all items in the section array.
+	 */
+	public int getSectionForPosition(int position)
+	{
+		int savedCursorPos = mDataCursor.getPosition();
+		mDataCursor.moveToPosition(position);
+		String key = MediaStore.Audio.keyFor(mDataCursor.getString(mColumnIndex));
+		mDataCursor.moveToPosition(savedCursorPos);
+
+		String[] alphabet = ALPHABET_KEYS;
+		for (int i = 1, len = alphabet.length; i != len; ++i) {
+			if (key.startsWith(alphabet[i]))
+				return i;
+		}
+
+		return 0;
 	}
 }