Add support for ABK opening books

Move probabilities are not exactly the same as in the Arena Chess GUI
because it is unknown how the computation in Arena works.

DroidFishApp/src/androidTest/java/org/petero/droidfish/book/BookTest.java

@@ -23,7 +23,7 @@ import java.util.ArrayList;
 
 import junit.framework.TestCase;
 
-import org.petero.droidfish.book.DroidBook;
+import org.petero.droidfish.book.IOpeningBook.BookPosInput;
 import org.petero.droidfish.gamelogic.ChessParseError;
 import org.petero.droidfish.gamelogic.Move;
 import org.petero.droidfish.gamelogic.MoveGen;
@@ -38,19 +38,21 @@ public class BookTest extends TestCase {
     public void testGetBookMove() throws ChessParseError {
         Position pos = TextIO.readFEN(TextIO.startPosFEN);
         DroidBook book = DroidBook.getInstance();
-        Move move = book.getBookMove(pos);
+        BookPosInput posInput = new BookPosInput(pos, null, null);
+        Move move = book.getBookMove(posInput);
         checkValid(pos, move);
 
         // Test "out of book" condition
         pos.setCastleMask(0);
-        move = book.getBookMove(pos);
+        move = book.getBookMove(posInput);
         assertEquals(null, move);
     }
 
     public void testGetAllBookMoves() throws ChessParseError {
         Position pos = TextIO.readFEN(TextIO.startPosFEN);
         DroidBook book = DroidBook.getInstance();
-        ArrayList<Move> moves = book.getAllBookMoves(pos, false).second;
+        BookPosInput posInput = new BookPosInput(pos, null, null);
+        ArrayList<Move> moves = book.getAllBookMoves(posInput, false).second;
         assertTrue(moves.size() > 1);
         for (Move m : moves) {
             checkValid(pos, m);

DroidFishApp/src/main/java/org/petero/droidfish/DroidFish.java

@@ -2500,7 +2500,7 @@ public class DroidFish extends Activity
             if (dotIdx < 0)
                 return false;
             String ext = filename.substring(dotIdx+1);
-            return ("ctg".equals(ext) || "bin".equals(ext));
+            return ("ctg".equals(ext) || "bin".equals(ext) || "abk".equals(ext));
         });
         final int numFiles = fileNames.length;
         final String[] items = new String[numFiles + 3];

DroidFishApp/src/main/java/org/petero/droidfish/book/AbkBook.java

@@ -0,0 +1,294 @@
+/*
+    DroidFish - An Android chess program.
+    Copyright (C) 2020  Peter Österlund, peterosterlund2@gmail.com
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+package org.petero.droidfish.book;
+
+import java.io.File;
+import java.io.IOException;
+import java.io.RandomAccessFile;
+import java.util.ArrayList;
+
+import org.petero.droidfish.book.DroidBook.BookEntry;
+import org.petero.droidfish.gamelogic.ChessParseError;
+import org.petero.droidfish.gamelogic.Move;
+import org.petero.droidfish.gamelogic.Piece;
+import org.petero.droidfish.gamelogic.Position;
+import org.petero.droidfish.gamelogic.TextIO;
+
+/** Handle Arena Chess GUI opening books. */
+class AbkBook implements IOpeningBook {
+    private File abkFile; // The ".abk" file
+    private Position startPos;
+
+    /** Constructor. */
+    public AbkBook() {
+        try {
+            startPos = TextIO.readFEN(TextIO.startPosFEN);
+        } catch (ChessParseError ex) {
+            throw new RuntimeException(ex);
+        }
+    }
+
+    static boolean canHandle(BookOptions options) {
+        String filename = options.filename;
+        return filename.endsWith(".abk");
+    }
+
+    @Override
+    public boolean enabled() {
+        return abkFile.canRead();
+    }
+
+    @Override
+    public void setOptions(BookOptions options) {
+        abkFile = new File(options.filename);
+    }
+
+    private static class MoveData {
+        Move move;
+        double weightPrio;
+        double weightNGames;
+        double weightScore;
+    }
+
+    @Override
+    public ArrayList<BookEntry> getBookEntries(BookPosInput posInput) {
+        if (!startPos.equals(posInput.getPrevPos()))
+            return null;
+
+        try (RandomAccessFile abkF = new RandomAccessFile(abkFile, "r")) {
+            ArrayList<Move> gameMoves = posInput.getMoves();
+
+            BookSettings bs = new BookSettings(abkF);
+            if (gameMoves.size() >= bs.maxPly)
+                return null;
+
+            AbkBookEntry ent = new AbkBookEntry();
+            int entNo = 900;
+            for (Move m : gameMoves) {
+                int iter = 0;
+                while (true) {
+                    if (entNo < 0)
+                        return null;
+                    ent.read(abkF, entNo);
+                    if (ent.getMove().equals(m) && ent.isValid()) {
+                        entNo = ent.nextMove;
+                        break;
+                    }
+                    entNo = ent.nextSibling;
+                    iter++;
+                    if (iter > 255)
+                        return null; // Corrupt book
+                }
+            }
+            if (entNo < 0)
+                return null;
+
+            boolean wtm = (gameMoves.size() % 2) == 0;
+            ArrayList<MoveData> moves = new ArrayList<>();
+            while (entNo >= 0) {
+                ent.read(abkF, entNo);
+                MoveData md = new MoveData();
+                md.move = ent.getMove();
+
+                int nWon  = wtm ? ent.nWon  : ent.nLost;
+                int nLost = wtm ? ent.nLost : ent.nWon;
+                int nDraw = ent.nGames - nWon - nLost;
+                md.weightPrio = scaleWeight(ent.priority, bs.prioImportance);
+                md.weightNGames = scaleWeight(ent.nGames, bs.nGamesImportance);
+                double score = (nWon + nDraw * 0.5) / ent.nGames;
+                md.weightScore = scaleWeight(score, bs.scoreImportance);
+
+                if (ent.isValid() &&
+                    (!bs.skipPrio0Moves || ent.priority > 0) &&
+                    (ent.nGames >= bs.minGames) &&
+                    (nWon >= bs.minWonGames) &&
+                    (score * 100 >= (wtm ? bs.minWinPercentWhite : bs.minWinPercentBlack))) {
+                    moves.add(md);
+                }
+
+                if (moves.size() > 255)
+                    return null; // Corrupt book
+                entNo = ent.nextSibling;
+            }
+
+            double sumWeightPrio = 0;
+            double sumWeightNGames = 0;
+            double sumWeightScore = 0;
+            for (MoveData md : moves) {
+                sumWeightPrio += md.weightPrio;
+                sumWeightNGames += md.weightNGames;
+                sumWeightScore += md.weightScore;
+            }
+
+            ArrayList<BookEntry> ret = new ArrayList<>();
+            boolean hasNonZeroWeight = false;
+            for (MoveData md : moves) {
+                BookEntry be = new BookEntry(md.move);
+                double wP = sumWeightPrio   > 0 ? md.weightPrio   / sumWeightPrio   : 0.0;
+                double wN = sumWeightNGames > 0 ? md.weightNGames / sumWeightNGames : 0.0;
+                double wS = sumWeightScore  > 0 ? md.weightScore  / sumWeightScore  : 0.0;
+                double a = 0.624;
+                double w = wP * Math.exp(a * bs.prioImportance) +
+                           wN * Math.exp(a * bs.nGamesImportance) +
+                           wS * Math.exp(a * bs.scoreImportance) * 1.4;
+                hasNonZeroWeight |= w > 0;
+                be.weight = (float)w;
+                ret.add(be);
+            }
+            if (!hasNonZeroWeight)
+                for (BookEntry be : ret)
+                    be.weight = 1;
+            return ret;
+        } catch (IOException e) {
+            return null;
+        }
+    }
+
+    private static class AbkBookEntry {
+        private byte[] data = new byte[28];
+
+        private byte from;       // From square, 0 = a1, 7 = h1, 8 = a2, 63 = h8
+        private byte to;         // To square
+        private byte promotion;  // 0 = none, +-1 = rook, +-2 = knight, +-3 = bishop, +-4 = queen
+        byte priority;           // 0 = bad, >0 better, 9 best
+        int nGames;              // Number of times games in which move was played
+        int nWon;                // Number of won games for white
+        int nLost;               // Number of lost games for white
+        int flags;               // Value is 0x01000000 if move has been deleted
+        int nextMove;            // First following move (by opposite color)
+        int nextSibling;         // Next alternative move (by same color)
+
+        AbkBookEntry() {
+        }
+
+        void read(RandomAccessFile f, long entNo) throws IOException {
+            f.seek(entNo * 28);
+            f.readFully(data);
+
+            from = data[0];
+            to = data[1];
+            promotion = data[2];
+            priority = data[3];
+            nGames = extractInt(4);
+            nWon = extractInt(8);
+            nLost = extractInt(12);
+            flags = extractInt(16);
+            nextMove = extractInt(20);
+            nextSibling = extractInt(24);
+        }
+
+        Move getMove() {
+            int prom;
+            switch (promotion) {
+            case 0:  prom = Piece.EMPTY;   break;
+            case -1: prom = Piece.WROOK;   break;
+            case -2: prom = Piece.WKNIGHT; break;
+            case -3: prom = Piece.WBISHOP; break;
+            case -4: prom = Piece.WQUEEN;  break;
+            case 1:  prom = Piece.BROOK;   break;
+            case 2:  prom = Piece.BKNIGHT; break;
+            case 3:  prom = Piece.BBISHOP; break;
+            case 4:  prom = Piece.BQUEEN;  break;
+            default: prom = -1; break;
+            }
+            return new Move(from, to, prom);
+        }
+
+        boolean isValid() {
+            return flags != 0x01000000;
+        }
+
+        private int extractInt(int offs) {
+            return AbkBook.extractInt(data, offs);
+        }
+    }
+
+    /** Convert 4 bytes starting at "offs" in buf[] to an integer. */
+    private static int extractInt(byte[] buf, int offs) {
+        int ret = 0;
+        for (int i = 3; i >= 0; i--) {
+            int b = buf[offs + i];
+            if (b < 0) b += 256;
+            ret = (ret << 8) + b;
+        }
+        return ret;
+    }
+
+    private static class BookSettings {
+        private byte[] buf = new byte[256];
+
+        int minGames;
+        int minWonGames;
+        int minWinPercentWhite;     // 0 - 100
+        int minWinPercentBlack;     // 0 - 100
+
+        int prioImportance;         // 0 - 15
+        int nGamesImportance;       // 0 - 15
+        int scoreImportance;        // 0 - 15
+
+        int maxPly;
+
+        boolean skipPrio0Moves = false; // Not stored in abk file
+
+        public BookSettings(RandomAccessFile abkF) throws IOException {
+            abkF.seek(0);
+            abkF.readFully(buf);
+
+            minGames = getInt(0xde, Integer.MAX_VALUE);
+            minWonGames = getInt(0xe2, Integer.MAX_VALUE);
+            minWinPercentWhite = getInt(0xe6, 100);
+            minWinPercentBlack = getInt(0xea, 100);
+
+            prioImportance   = getInt(0xee, 15);
+            nGamesImportance = getInt(0xf2, 15);
+            scoreImportance  = getInt(0xf6, 15);
+
+            maxPly = getInt(0xfa, 9999);
+
+            if (prioImportance == 0 && nGamesImportance == 0 && scoreImportance == 0) {
+                minGames = 0;
+                minWonGames = 0;
+                minWinPercentWhite = 0;
+                minWinPercentBlack = 0;
+            }
+        }
+
+        private int getInt(int offs, int maxVal) {
+            int val = extractInt(buf, offs);
+            return Math.min(Math.max(val, 0), maxVal);
+        }
+    }
+
+    private static double scaleWeight(double w, int importance) {
+        double e;
+        switch (importance) {
+        case 0:
+            return 0;
+        case 1:
+            e = 0.66;
+            break;
+        case 2:
+            e = 0.86;
+            break;
+        default:
+            e = 1 + ((double)importance - 3) / 6;
+            break;
+        }
+        return Math.pow(w, e);
+    }
+}

DroidFishApp/src/main/java/org/petero/droidfish/book/CtgBook.java

@@ -62,7 +62,8 @@ class CtgBook implements IOpeningBook {
     }
 
     @Override
-    public ArrayList<BookEntry> getBookEntries(Position pos) {
+    public ArrayList<BookEntry> getBookEntries(BookPosInput posInput) {
+        Position pos = posInput.getCurrPos();
         try (RandomAccessFile ctgF = new RandomAccessFile(ctgFile, "r");
              RandomAccessFile ctbF = new RandomAccessFile(ctbFile, "r");
              RandomAccessFile ctoF = new RandomAccessFile(ctoFile, "r")) {

DroidFishApp/src/main/java/org/petero/droidfish/book/DroidBook.java

@@ -28,6 +28,7 @@ import java.util.List;
 import java.util.Random;
 
 import org.petero.droidfish.Util;
+import org.petero.droidfish.book.IOpeningBook.BookPosInput;
 import org.petero.droidfish.gamelogic.Move;
 import org.petero.droidfish.gamelogic.MoveGen;
 import org.petero.droidfish.gamelogic.Position;
@@ -64,7 +65,6 @@ public final class DroidBook {
     }
 
     private DroidBook() {
-        rndGen.setSeed(System.currentTimeMillis());
     }
 
     /** Set opening book options. */
@@ -74,6 +74,8 @@ public final class DroidBook {
             externalBook = new CtgBook();
         else if (PolyglotBook.canHandle(options))
             externalBook = new PolyglotBook();
+        else if (AbkBook.canHandle(options))
+            externalBook = new AbkBook();
         else
             externalBook = new NullBook();
         externalBook.setOptions(options);
@@ -83,10 +85,11 @@ public final class DroidBook {
     }
 
     /** Return a random book move for a position, or null if out of book. */
-    public final synchronized Move getBookMove(Position pos) {
+    public final synchronized Move getBookMove(BookPosInput posInput) {
+        Position pos = posInput.getCurrPos();
         if ((options != null) && (pos.fullMoveCounter > options.maxLength))
             return null;
-        List<BookEntry> bookMoves = getBook().getBookEntries(pos);
+        List<BookEntry> bookMoves = getBook().getBookEntries(posInput);
         if (bookMoves == null || bookMoves.isEmpty())
             return null;
 
@@ -116,11 +119,12 @@ public final class DroidBook {
     }
 
     /** Return all book moves, both as a formatted string and as a list of moves. */
-    public final synchronized Pair<String,ArrayList<Move>> getAllBookMoves(Position pos,
+    public final synchronized Pair<String,ArrayList<Move>> getAllBookMoves(BookPosInput posInput,
                                                                            boolean localized) {
+        Position pos = posInput.getCurrPos();
         StringBuilder ret = new StringBuilder();
         ArrayList<Move> bookMoveList = new ArrayList<>();
-        ArrayList<BookEntry> bookMoves = getBook().getBookEntries(pos);
+        ArrayList<BookEntry> bookMoves = getBook().getBookEntries(posInput);
 
         // Check legality
         if (bookMoves != null) {

DroidFishApp/src/main/java/org/petero/droidfish/book/EcoBook.java

@@ -43,7 +43,8 @@ public class EcoBook implements IOpeningBook {
     }
 
     @Override
-    public ArrayList<BookEntry> getBookEntries(Position pos) {
+    public ArrayList<BookEntry> getBookEntries(BookPosInput posInput) {
+        Position pos = posInput.getCurrPos();
         ArrayList<Move> moves = EcoDb.getInstance().getMoves(pos);
         ArrayList<BookEntry> entries = new ArrayList<>();
         for (int i = 0; i < moves.size(); i++) {

DroidFishApp/src/main/java/org/petero/droidfish/book/IOpeningBook.java

@@ -18,18 +18,61 @@
 
 package org.petero.droidfish.book;
 
+import android.util.Pair;
+
 import java.util.ArrayList;
 
 import org.petero.droidfish.book.DroidBook.BookEntry;
+import org.petero.droidfish.gamelogic.Game;
+import org.petero.droidfish.gamelogic.Move;
 import org.petero.droidfish.gamelogic.Position;
 
-interface IOpeningBook {
+public interface IOpeningBook {
     /** Return true if book is currently enabled. */
     boolean enabled();
 
     /** Set book options, including filename. */
     void setOptions(BookOptions options);
 
+    /** Information required to query an opening book. */
+    class BookPosInput {
+        private final Position currPos;
+
+        private Game game;
+        private Position prevPos;
+        private ArrayList<Move> moves;
+
+        public BookPosInput(Position currPos, Position prevPos, ArrayList<Move> moves) {
+            this.currPos = currPos;
+            this.prevPos = prevPos;
+            this.moves = moves;
+        }
+
+        public BookPosInput(Game game) {
+            currPos = game.currPos();
+            this.game = game;
+        }
+
+        public Position getCurrPos() {
+            return currPos;
+        }
+        public Position getPrevPos() {
+            lazyInit();
+            return prevPos;
+        }
+        public ArrayList<Move> getMoves() {
+            lazyInit();
+            return moves;
+        }
+
+        private void lazyInit() {
+            if (prevPos == null) {
+                Pair<Position, ArrayList<Move>> ph = game.getUCIHistory();
+                prevPos = ph.first;
+                moves = ph.second;
+            }
+        }
+    }
     /** Get all book entries for a position. */
-    ArrayList<BookEntry> getBookEntries(Position pos);
+    ArrayList<BookEntry> getBookEntries(BookPosInput posInput);
 }

DroidFishApp/src/main/java/org/petero/droidfish/book/InternalBook.java

@@ -52,7 +52,8 @@ final class InternalBook implements IOpeningBook {
     }
 
     @Override
-    public ArrayList<BookEntry> getBookEntries(Position pos) {
+    public ArrayList<BookEntry> getBookEntries(BookPosInput posInput) {
+        Position pos = posInput.getCurrPos();
         initInternalBook();
         ArrayList<BookEntry> ents = bookMap.get(pos.zobristHash());
         if (ents == null)

DroidFishApp/src/main/java/org/petero/droidfish/book/NoBook.java

@@ -37,7 +37,7 @@ class NoBook implements IOpeningBook {
     }
 
     @Override
-    public ArrayList<BookEntry> getBookEntries(Position pos) {
+    public ArrayList<BookEntry> getBookEntries(BookPosInput posInput) {
         return null;
     }
 }

DroidFishApp/src/main/java/org/petero/droidfish/book/NullBook.java

@@ -35,7 +35,7 @@ class NullBook implements IOpeningBook {
     }
 
     @Override
-    public ArrayList<BookEntry> getBookEntries(Position pos) {
+    public ArrayList<BookEntry> getBookEntries(BookPosInput posInput) {
         return null;
     }
 }

DroidFishApp/src/main/java/org/petero/droidfish/book/PolyglotBook.java

@@ -370,7 +370,8 @@ class PolyglotBook implements IOpeningBook {
     }
 
     @Override
-    public final ArrayList<BookEntry> getBookEntries(Position pos) {
+    public final ArrayList<BookEntry> getBookEntries(BookPosInput posInput) {
+        Position pos = posInput.getCurrPos();
         try (RandomAccessFile f = new RandomAccessFile(bookFile, "r")) {
             long numEntries = f.length() / 16;
             long key = getHashKey(pos);

DroidFishApp/src/main/java/org/petero/droidfish/engine/DroidComputerPlayer.java

@@ -28,6 +28,7 @@ import java.util.TreeMap;
 import org.petero.droidfish.EngineOptions;
 import org.petero.droidfish.book.BookOptions;
 import org.petero.droidfish.book.DroidBook;
+import org.petero.droidfish.book.IOpeningBook.BookPosInput;
 import org.petero.droidfish.gamelogic.Move;
 import org.petero.droidfish.gamelogic.MoveGen;
 import org.petero.droidfish.gamelogic.Position;
@@ -95,7 +96,7 @@ public class DroidComputerPlayer {
         int searchId;           // Unique identifier for this search request
         long startTime;         // System time (milliseconds) when search request was created
 
-        Position prevPos;       // Position at last irreversible move
+        Position prevPos;       // Position at last null move
         ArrayList<Move> mList;  // Moves after prevPos, including ponderMove
         Position currPos;       // currPos = prevPos + mList - ponderMove
         boolean drawOffer;      // True if other side made draw offer
@@ -361,8 +362,9 @@ public class DroidComputerPlayer {
     }
 
     /** Return all book moves, both as a formatted string and as a list of moves. */
-    public final Pair<String, ArrayList<Move>> getBookHints(Position pos, boolean localized) {
+    public final Pair<String, ArrayList<Move>> getBookHints(BookPosInput posInput,
-        return book.getAllBookMoves(pos, localized);
+                                                            boolean localized) {
+        return book.getAllBookMoves(posInput, localized);
     }
 
     /** Get engine reported name. */
@@ -454,7 +456,8 @@ public class DroidComputerPlayer {
 
         if (sr.ponderMove == null) {
             // If we have a book move, play it.
-            Move bookMove = book.getBookMove(sr.currPos);
+            BookPosInput posInput = new BookPosInput(sr.currPos, sr.prevPos, sr.mList);
+            Move bookMove = book.getBookMove(posInput);
             if (bookMove != null) {
                 if (canClaimDraw(sr.currPos, posHashList, posHashListSize, bookMove).isEmpty()) {
                     listener.notifySearchResult(sr.searchId,

DroidFishApp/src/main/java/org/petero/droidfish/gamelogic/DroidChessController.java

@@ -39,6 +39,7 @@ import org.petero.droidfish.PGNOptions;
 import org.petero.droidfish.Util;
 import org.petero.droidfish.book.BookOptions;
 import org.petero.droidfish.book.EcoDb;
+import org.petero.droidfish.book.IOpeningBook.BookPosInput;
 import org.petero.droidfish.engine.DroidComputerPlayer;
 import org.petero.droidfish.engine.UCIOptions;
 import org.petero.droidfish.engine.DroidComputerPlayer.EloData;
@@ -955,7 +956,8 @@ public class DroidChessController {
 
     private void updateBookHints() {
         if (game != null) {
-            Pair<String, ArrayList<Move>> bi = computerPlayer.getBookHints(game.currPos(), localPt());
+            BookPosInput posInput = new BookPosInput(game);
+            Pair<String, ArrayList<Move>> bi = computerPlayer.getBookHints(posInput, localPt());
             EcoDb.Result ecoData = EcoDb.getInstance().getEco(game.tree);
             String eco = ecoData.getName();
             listener.notifyBookInfo(searchId, bi.first, bi.second, eco, ecoData.distToEcoTree);

DroidFishApp/src/main/java/org/petero/droidfish/gamelogic/Game.java

@@ -112,7 +112,7 @@ public class Game {
         return ret;
     }
 
-    final Position currPos() {
+    public final Position currPos() {
         return tree.currentPos;
     }
 

DroidFishApp/src/main/res/values/strings.xml

@@ -389,7 +389,7 @@ If you are running on battery power, it is recommended that you change settings
     <string name="prefs_bookTournamentMode_summary">Ignore moves marked as not for tournament play</string>
     <string name="prefs_bookRandom_title">Book Randomization</string>
     <string name="prefs_bookFile_title">Book Filename</string>
-    <string name="prefs_bookFile_summary">Polyglot or CTG book file in DroidFish directory on SD Card</string>
+    <string name="prefs_bookFile_summary">Polyglot, ABK or CTG book file in DroidFish directory on SD Card</string>
     <string name="prefs_pgnSettings_title">PGN Settings</string>
     <string name="prefs_pgnSettings_summary">Settings for import and export of portable game notation (PGN) data</string>
     <string name="prefs_pgn_viewer">PGN viewer</string>

README.md

@@ -560,16 +560,19 @@ restored.
 
 ## Installing additional opening books
 
-To use *polyglot* or *CTG* book files:
+To use *polyglot*, *CTG* or *ABK* book files:
 
-1. Copy one or more polyglot book files to the `DroidFish/book` directory on the
+1. Copy one or more opening book files to the `DroidFish/book` directory on the
-   external storage. Polyglot books must have the file extension `.bin`.  
+   external storage.
-   **Note!** The Android file system is case sensitive, so the extension must be
-   `.bin`, not `.Bin` or `.BIN`.
 
-1. Copy one or more CTG book files to the `DroidFish/book` directory. A CTG
+   1. Polyglot books must have the file extension `.bin`.  
-   book consists of three files with file extensions `.ctg`, `.ctb` and
+     **Note!** The Android file system may be case sensitive, in which case the
-   `.cto`. You must copy all three files.
+     extension must be `.bin`, not `.Bin` or `.BIN`.
+
+   1. A *CTG* book consists of three files with file extensions `.ctg`, `.ctb`
+      and `.cto`. You must copy all three files.
+
+   1. an *ABK* book must have the file extension `.abk`.
 
 1. Go to *Left drawer menu* -> *Select opening book*.
 
@@ -862,11 +865,11 @@ You can change aspects of the opening book from *Left drawer menu* -> *Settings*
 * *Prefer main lines*: When enabled, moves that are marked as main line moves in
   the book are given a higher weight so they will be played more often by the
   chess engine.  
-  **Note!** This option only has an effect for CTG opening books.
+  **Note!** This option only has an effect for *CTG* opening books.
 
 * *Tournament mode*: When enabled, only book moves that are marked for
   tournament play are played by the chess engine.  
-  **Note!** This option only has an effect for CTG opening books.
+  **Note!** This option only has an effect for *CTG* opening books.
 
 * *Book randomization*: Controls how often different book moves are played by
   the engine. The default is 50% which means that the statistics from the
@@ -882,6 +885,10 @@ You can change aspects of the opening book from *Left drawer menu* -> *Settings*
   very big opening book stored somewhere on the device but it would be
   impractical to copy it to the `DroidFish/book` directory.
 
-**Note!** The move percentages calculated by *DroidFish* for CTG books are
+**Note!** The move percentages calculated by *DroidFish* for *CTG* books are
 unlikely to agree with percentages calculated by other chess programs that can
-use CTG books.
+use *CTG* books.
+
+**Note!** The move percentages calculated by *DroidFish* for *ABK* books are not
+always equal to percentages shown in the Arena Chess GUI, because the algorithm
+used by Arena to compute the percentages is unknown.