summaryrefslogtreecommitdiffstats
path: root/src/com/android/providers/downloads/Helpers.java
blob: 7451a07cd51fad478ffd741d488133ca6ecc9767 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
/*
 * Copyright (C) 2008 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.android.providers.downloads;

import android.content.ContentUris;
import android.content.Context;
import android.content.Intent;
import android.content.pm.PackageManager;
import android.content.pm.ResolveInfo;
import android.database.Cursor;
import android.drm.mobile1.DrmRawContent;
import android.net.ConnectivityManager;
import android.net.NetworkInfo;
import android.net.Uri;
import android.os.Environment;
import android.os.StatFs;
import android.os.SystemClock;
import android.provider.Downloads;
import android.telephony.TelephonyManager;
import android.util.Config;
import android.util.Log;
import android.webkit.MimeTypeMap;

import java.io.File; 
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.util.Random;
import java.util.Set;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

/**
 * Some helper functions for the download manager
 */
public class Helpers {

    public static Random sRandom = new Random(SystemClock.uptimeMillis());

    /** Regex used to parse content-disposition headers */
    private static final Pattern CONTENT_DISPOSITION_PATTERN =
            Pattern.compile("attachment;\\s*filename\\s*=\\s*\"([^\"]*)\"");

    private Helpers() {
    }

    /*
     * Parse the Content-Disposition HTTP Header. The format of the header
     * is defined here: http://www.w3.org/Protocols/rfc2616/rfc2616-sec19.html
     * This header provides a filename for content that is going to be
     * downloaded to the file system. We only support the attachment type.
     */
    private static String parseContentDisposition(String contentDisposition) {
        try {
            Matcher m = CONTENT_DISPOSITION_PATTERN.matcher(contentDisposition);
            if (m.find()) {
                return m.group(1);
            }
        } catch (IllegalStateException ex) {
             // This function is defined as returning null when it can't parse the header
        }
        return null;
    }

    /**
     * Creates a filename (where the file should be saved) from a uri.
     */
    public static DownloadFileInfo generateSaveFile(
            Context context,
            String url,
            String hint,
            String contentDisposition,
            String contentLocation,
            String mimeType,
            int destination,
            int contentLength) throws FileNotFoundException {

        /*
         * Don't download files that we won't be able to handle
         */
        if (destination == Downloads.DESTINATION_EXTERNAL
                || destination == Downloads.DESTINATION_CACHE_PARTITION_PURGEABLE) {
            if (mimeType == null) {
                if (Config.LOGD) {
                    Log.d(Constants.TAG, "external download with no mime type not allowed");
                }
                return new DownloadFileInfo(null, null, Downloads.STATUS_NOT_ACCEPTABLE);
            }
        }
        String filename = chooseFilename(
                url, hint, contentDisposition, contentLocation, destination);

        // Split filename between base and extension
        // Add an extension if filename does not have one
        String extension = null;
        int dotIndex = filename.indexOf('.');
        if (dotIndex < 0) {
            extension = chooseExtensionFromMimeType(mimeType, true);
        } else {
            extension = chooseExtensionFromFilename(
                    mimeType, destination, filename, dotIndex);
            filename = filename.substring(0, dotIndex);
        }

        /*
         *  Locate the directory where the file will be saved
         */

        File base = null;
        StatFs stat = null;
        // DRM messages should be temporarily stored internally and then passed to 
        // the DRM content provider
        if (destination == Downloads.DESTINATION_CACHE_PARTITION
                || destination == Downloads.DESTINATION_CACHE_PARTITION_PURGEABLE
                || destination == Downloads.DESTINATION_CACHE_PARTITION_NOROAMING
                || DrmRawContent.DRM_MIMETYPE_MESSAGE_STRING.equalsIgnoreCase(mimeType)) {
            base = Environment.getDownloadCacheDirectory();
            stat = new StatFs(base.getPath());

            /*
             * Check whether there's enough space on the target filesystem to save the file.
             * Put a bit of margin (in case creating the file grows the system by a few blocks).
             */
            int blockSize = stat.getBlockSize();
            for (;;) {
                int availableBlocks = stat.getAvailableBlocks();
                if (blockSize * ((long) availableBlocks - 4) >= contentLength) {
                    break;
                }
                if (!discardPurgeableFiles(context,
                        contentLength - blockSize * ((long) availableBlocks - 4))) {
                    if (Config.LOGD) {
                        Log.d(Constants.TAG,
                                "download aborted - not enough free space in internal storage");
                    }
                    return new DownloadFileInfo(null, null, Downloads.STATUS_FILE_ERROR);
                }
                stat.restat(base.getPath());
            }

        } else {
            if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
                String root = Environment.getExternalStorageDirectory().getPath();
                base = new File(root + Constants.DEFAULT_DL_SUBDIR);
                if (!base.isDirectory() && !base.mkdir()) {
                    if (Config.LOGD) {
                        Log.d(Constants.TAG, "download aborted - can't create base directory "
                                + base.getPath());
                    }
                    return new DownloadFileInfo(null, null, Downloads.STATUS_FILE_ERROR);
                }
                stat = new StatFs(base.getPath());
            } else {
                if (Config.LOGD) {
                    Log.d(Constants.TAG, "download aborted - no external storage");
                }
                return new DownloadFileInfo(null, null, Downloads.STATUS_FILE_ERROR);
            }

            /*
             * Check whether there's enough space on the target filesystem to save the file.
             * Put a bit of margin (in case creating the file grows the system by a few blocks).
             */
            if (stat.getBlockSize() * ((long) stat.getAvailableBlocks() - 4) < contentLength) {
                if (Config.LOGD) {
                    Log.d(Constants.TAG, "download aborted - not enough free space");
                }
                return new DownloadFileInfo(null, null, Downloads.STATUS_FILE_ERROR);
            }

        }

        boolean recoveryDir = Constants.RECOVERY_DIRECTORY.equalsIgnoreCase(filename + extension);

        filename = base.getPath() + File.separator + filename;

        /*
         * Generate a unique filename, create the file, return it.
         */
        if (Constants.LOGVV) {
            Log.v(Constants.TAG, "target file: " + filename + extension);
        }

        String fullFilename = chooseUniqueFilename(
                destination, filename, extension, recoveryDir);
        if (fullFilename != null) {
            return new DownloadFileInfo(fullFilename, new FileOutputStream(fullFilename), 0);
        } else {
            return new DownloadFileInfo(null, null, Downloads.STATUS_FILE_ERROR);
        }
    }

    private static String chooseFilename(String url, String hint, String contentDisposition,
            String contentLocation, int destination) {
        String filename = null;

        // First, try to use the hint from the application, if there's one
        if (filename == null && hint != null && !hint.endsWith("/")) {
            if (Constants.LOGVV) {
                Log.v(Constants.TAG, "getting filename from hint");
            }
            int index = hint.lastIndexOf('/') + 1;
            if (index > 0) {
                filename = hint.substring(index);
            } else {
                filename = hint;
            }
        }

        // If we couldn't do anything with the hint, move toward the content disposition
        if (filename == null && contentDisposition != null) {
            filename = parseContentDisposition(contentDisposition);
            if (filename != null) {
                if (Constants.LOGVV) {
                    Log.v(Constants.TAG, "getting filename from content-disposition");
                }
                int index = filename.lastIndexOf('/') + 1;
                if (index > 0) {
                    filename = filename.substring(index);
                }
            }
        }

        // If we still have nothing at this point, try the content location
        if (filename == null && contentLocation != null) {
            String decodedContentLocation = Uri.decode(contentLocation);
            if (decodedContentLocation != null
                    && !decodedContentLocation.endsWith("/")
                    && decodedContentLocation.indexOf('?') < 0) {
                if (Constants.LOGVV) {
                    Log.v(Constants.TAG, "getting filename from content-location");
                }
                int index = decodedContentLocation.lastIndexOf('/') + 1;
                if (index > 0) {
                    filename = decodedContentLocation.substring(index);
                } else {
                    filename = decodedContentLocation;
                }
            }
        }

        // If all the other http-related approaches failed, use the plain uri
        if (filename == null) {
            String decodedUrl = Uri.decode(url);
            if (decodedUrl != null
                    && !decodedUrl.endsWith("/") && decodedUrl.indexOf('?') < 0) {
                int index = decodedUrl.lastIndexOf('/') + 1;
                if (index > 0) {
                    if (Constants.LOGVV) {
                        Log.v(Constants.TAG, "getting filename from uri");
                    }
                    filename = decodedUrl.substring(index);
                }
            }
        }

        // Finally, if couldn't get filename from URI, get a generic filename
        if (filename == null) {
            if (Constants.LOGVV) {
                Log.v(Constants.TAG, "using default filename");
            }
            filename = Constants.DEFAULT_DL_FILENAME;
        }

        filename = filename.replaceAll("[^a-zA-Z0-9\\.\\-_]+", "_");


        return filename;
    }

    private static String chooseExtensionFromMimeType(String mimeType, boolean useDefaults) {
        String extension = null;
        if (mimeType != null) {
            extension = MimeTypeMap.getSingleton().getExtensionFromMimeType(mimeType);
            if (extension != null) {
                if (Constants.LOGVV) {
                    Log.v(Constants.TAG, "adding extension from type");
                }
                extension = "." + extension;
            } else {
                if (Constants.LOGVV) {
                    Log.v(Constants.TAG, "couldn't find extension for " + mimeType);
                }
            }
        }
        if (extension == null) {
            if (mimeType != null && mimeType.toLowerCase().startsWith("text/")) {
                if (mimeType.equalsIgnoreCase("text/html")) {
                    if (Constants.LOGVV) {
                        Log.v(Constants.TAG, "adding default html extension");
                    }
                    extension = Constants.DEFAULT_DL_HTML_EXTENSION;
                } else if (useDefaults) {
                    if (Constants.LOGVV) {
                        Log.v(Constants.TAG, "adding default text extension");
                    }
                    extension = Constants.DEFAULT_DL_TEXT_EXTENSION;
                }
            } else if (useDefaults) {
                if (Constants.LOGVV) {
                    Log.v(Constants.TAG, "adding default binary extension");
                }
                extension = Constants.DEFAULT_DL_BINARY_EXTENSION;
            }
        }
        return extension;
    }

    private static String chooseExtensionFromFilename(String mimeType, int destination,
            String filename, int dotIndex) {
        String extension = null;
        if (mimeType != null) {
            // Compare the last segment of the extension against the mime type.
            // If there's a mismatch, discard the entire extension.
            int lastDotIndex = filename.lastIndexOf('.');
            String typeFromExt = MimeTypeMap.getSingleton().getMimeTypeFromExtension(
                    filename.substring(lastDotIndex + 1));
            if (typeFromExt == null || !typeFromExt.equalsIgnoreCase(mimeType)) {
                extension = chooseExtensionFromMimeType(mimeType, false);
                if (extension != null) {
                    if (Constants.LOGVV) {
                        Log.v(Constants.TAG, "substituting extension from type");
                    }
                } else {
                    if (Constants.LOGVV) {
                        Log.v(Constants.TAG, "couldn't find extension for " + mimeType);
                    }
                }
            }
        }
        if (extension == null) {
            if (Constants.LOGVV) {
                Log.v(Constants.TAG, "keeping extension");
            }
            extension = filename.substring(dotIndex);
        }
        return extension;
    }

    private static String chooseUniqueFilename(int destination, String filename,
            String extension, boolean recoveryDir) {
        String fullFilename = filename + extension;
        if (!new File(fullFilename).exists()
                && (!recoveryDir ||
                (destination != Downloads.DESTINATION_CACHE_PARTITION &&
                        destination != Downloads.DESTINATION_CACHE_PARTITION_PURGEABLE &&
                        destination != Downloads.DESTINATION_CACHE_PARTITION_NOROAMING))) {
            return fullFilename;
        }
        filename = filename + Constants.FILENAME_SEQUENCE_SEPARATOR;
        /*
        * This number is used to generate partially randomized filenames to avoid
        * collisions.
        * It starts at 1.
        * The next 9 iterations increment it by 1 at a time (up to 10).
        * The next 9 iterations increment it by 1 to 10 (random) at a time.
        * The next 9 iterations increment it by 1 to 100 (random) at a time.
        * ... Up to the point where it increases by 100000000 at a time.
        * (the maximum value that can be reached is 1000000000)
        * As soon as a number is reached that generates a filename that doesn't exist,
        *     that filename is used.
        * If the filename coming in is [base].[ext], the generated filenames are
        *     [base]-[sequence].[ext].
        */
        int sequence = 1;
        for (int magnitude = 1; magnitude < 1000000000; magnitude *= 10) {
            for (int iteration = 0; iteration < 9; ++iteration) {
                fullFilename = filename + sequence + extension;
                if (!new File(fullFilename).exists()) {
                    return fullFilename;
                }
                if (Constants.LOGVV) {
                    Log.v(Constants.TAG, "file with sequence number " + sequence + " exists");
                }
                sequence += sRandom.nextInt(magnitude) + 1;
            }
        }
        return null;
    }

    /**
     * Deletes purgeable files from the cache partition. This also deletes
     * the matching database entries. Files are deleted in LRU order until
     * the total byte size is greater than targetBytes.
     */
    public static final boolean discardPurgeableFiles(Context context, long targetBytes) {
        Cursor cursor = context.getContentResolver().query(
                Downloads.CONTENT_URI,
                null,
                "( " +
                Downloads.COLUMN_STATUS + " = '" + Downloads.STATUS_SUCCESS + "' AND " +
                Downloads.COLUMN_DESTINATION +
                        " = '" + Downloads.DESTINATION_CACHE_PARTITION_PURGEABLE + "' )",
                null,
                Downloads.COLUMN_LAST_MODIFICATION);
        if (cursor == null) {
            return false;
        }
        long totalFreed = 0;
        try {
            cursor.moveToFirst();
            while (!cursor.isAfterLast() && totalFreed < targetBytes) {
                File file = new File(cursor.getString(cursor.getColumnIndex(Downloads._DATA)));
                if (Constants.LOGVV) {
                    Log.v(Constants.TAG, "purging " + file.getAbsolutePath() + " for " +
                            file.length() + " bytes");
                }
                totalFreed += file.length();
                file.delete();
                long id = cursor.getLong(cursor.getColumnIndex(Downloads._ID));
                context.getContentResolver().delete(
                        ContentUris.withAppendedId(Downloads.CONTENT_URI, id), null, null);
                cursor.moveToNext();
            }
        } finally {
            cursor.close();
        }
        if (Constants.LOGV) {
            if (totalFreed > 0) {
                Log.v(Constants.TAG, "Purged files, freed " + totalFreed + " for " +
                        targetBytes + " requested");
            }
        }
        return totalFreed > 0;
    }

    /**
     * Returns whether the network is available
     */
    public static boolean isNetworkAvailable(Context context) {
        ConnectivityManager connectivity =
                (ConnectivityManager) context.getSystemService(Context.CONNECTIVITY_SERVICE);
        if (connectivity == null) {
            Log.w(Constants.TAG, "couldn't get connectivity manager");
        } else {
            NetworkInfo[] info = connectivity.getAllNetworkInfo();
            if (info != null) {
                for (int i = 0; i < info.length; i++) {
                    if (info[i].getState() == NetworkInfo.State.CONNECTED) {
                        if (Constants.LOGVV) {
                            Log.v(Constants.TAG, "network is available");
                        }
                        return true;
                    }
                }
            }
        }
        if (Constants.LOGVV) {
            Log.v(Constants.TAG, "network is not available");
        }
        return false;
    }

    /**
     * Returns whether the network is roaming
     */
    public static boolean isNetworkRoaming(Context context) {
        ConnectivityManager connectivity =
                (ConnectivityManager) context.getSystemService(Context.CONNECTIVITY_SERVICE);
        if (connectivity == null) {
            Log.w(Constants.TAG, "couldn't get connectivity manager");
        } else {
            NetworkInfo info = connectivity.getActiveNetworkInfo();
            if (info != null && info.getType() == ConnectivityManager.TYPE_MOBILE) {
                if (TelephonyManager.getDefault().isNetworkRoaming()) {
                    if (Constants.LOGVV) {
                        Log.v(Constants.TAG, "network is roaming");
                    }
                    return true;
                } else {
                    if (Constants.LOGVV) {
                        Log.v(Constants.TAG, "network is not roaming");
                    }
                }
            } else {
                if (Constants.LOGVV) {
                    Log.v(Constants.TAG, "not using mobile network");
                }
            }
        }
        return false;
    }

    /**
     * Checks whether the filename looks legitimate
     */
    public static boolean isFilenameValid(String filename) {
        File dir = new File(filename).getParentFile();
        return dir.equals(Environment.getDownloadCacheDirectory())
                || dir.equals(new File(Environment.getExternalStorageDirectory()
                        + Constants.DEFAULT_DL_SUBDIR));
    }

    /**
     * Checks whether this looks like a legitimate selection parameter
     */
    public static void validateSelection(String selection, Set<String> allowedColumns) {
        try {
            if (selection == null) {
                return;
            }
            Lexer lexer = new Lexer(selection, allowedColumns);
            parseExpression(lexer);
            if (lexer.currentToken() != Lexer.TOKEN_END) {
                throw new IllegalArgumentException("syntax error");
            }
        } catch (RuntimeException ex) {
            if (Constants.LOGV) {
                Log.d(Constants.TAG, "invalid selection [" + selection + "] triggered " + ex);
            } else if (Config.LOGD) {
                Log.d(Constants.TAG, "invalid selection triggered " + ex);
            }
            throw ex;
        }

    }

    // expression <- ( expression ) | statement [AND_OR ( expression ) | statement] *
    //             | statement [AND_OR expression]*
    private static void parseExpression(Lexer lexer) {
        for (;;) {
            // ( expression )
            if (lexer.currentToken() == Lexer.TOKEN_OPEN_PAREN) {
                lexer.advance();
                parseExpression(lexer);
                if (lexer.currentToken() != Lexer.TOKEN_CLOSE_PAREN) {
                    throw new IllegalArgumentException("syntax error, unmatched parenthese");
                }
                lexer.advance();
            } else {
                // statement
                parseStatement(lexer);
            }
            if (lexer.currentToken() != Lexer.TOKEN_AND_OR) {
                break;
            }
            lexer.advance();
        }
    }

    // statement <- COLUMN COMPARE VALUE
    //            | COLUMN IS NULL
    private static void parseStatement(Lexer lexer) {
        // both possibilities start with COLUMN
        if (lexer.currentToken() != Lexer.TOKEN_COLUMN) {
            throw new IllegalArgumentException("syntax error, expected column name");
        }
        lexer.advance();

        // statement <- COLUMN COMPARE VALUE
        if (lexer.currentToken() == Lexer.TOKEN_COMPARE) {
            lexer.advance();
            if (lexer.currentToken() != Lexer.TOKEN_VALUE) {
                throw new IllegalArgumentException("syntax error, expected quoted string");
            }
            lexer.advance();
            return;
        }

        // statement <- COLUMN IS NULL
        if (lexer.currentToken() == Lexer.TOKEN_IS) {
            lexer.advance();
            if (lexer.currentToken() != Lexer.TOKEN_NULL) {
                throw new IllegalArgumentException("syntax error, expected NULL");
            }
            lexer.advance();
            return;
        }

        // didn't get anything good after COLUMN
        throw new IllegalArgumentException("syntax error after column name");
    }

    /**
     * A simple lexer that recognizes the words of our restricted subset of SQL where clauses
     */
    private static class Lexer {
        public static final int TOKEN_START = 0;
        public static final int TOKEN_OPEN_PAREN = 1;
        public static final int TOKEN_CLOSE_PAREN = 2;
        public static final int TOKEN_AND_OR = 3;
        public static final int TOKEN_COLUMN = 4;
        public static final int TOKEN_COMPARE = 5;
        public static final int TOKEN_VALUE = 6;
        public static final int TOKEN_IS = 7;
        public static final int TOKEN_NULL = 8;
        public static final int TOKEN_END = 9;

        private final String mSelection;
        private final Set<String> mAllowedColumns;
        private int mOffset = 0;
        private int mCurrentToken = TOKEN_START;
        private final char[] mChars;

        public Lexer(String selection, Set<String> allowedColumns) {
            mSelection = selection;
            mAllowedColumns = allowedColumns;
            mChars = new char[mSelection.length()];
            mSelection.getChars(0, mChars.length, mChars, 0);
            advance();
        }

        public int currentToken() {
            return mCurrentToken;
        }

        public void advance() {
            char[] chars = mChars;

            // consume whitespace
            while (mOffset < chars.length && chars[mOffset] == ' ') {
                ++mOffset;
            }

            // end of input
            if (mOffset == chars.length) {
                mCurrentToken = TOKEN_END;
                return;
            }

            // "("
            if (chars[mOffset] == '(') {
                ++mOffset;
                mCurrentToken = TOKEN_OPEN_PAREN;
                return;
            }

            // ")"
            if (chars[mOffset] == ')') {
                ++mOffset;
                mCurrentToken = TOKEN_CLOSE_PAREN;
                return;
            }

            // "?"
            if (chars[mOffset] == '?') {
                ++mOffset;
                mCurrentToken = TOKEN_VALUE;
                return;
            }

            // "=" and "=="
            if (chars[mOffset] == '=') {
                ++mOffset;
                mCurrentToken = TOKEN_COMPARE;
                if (mOffset < chars.length && chars[mOffset] == '=') {
                    ++mOffset;
                }
                return;
            }

            // ">" and ">="
            if (chars[mOffset] == '>') {
                ++mOffset;
                mCurrentToken = TOKEN_COMPARE;
                if (mOffset < chars.length && chars[mOffset] == '=') {
                    ++mOffset;
                }
                return;
            }

            // "<", "<=" and "<>"
            if (chars[mOffset] == '<') {
                ++mOffset;
                mCurrentToken = TOKEN_COMPARE;
                if (mOffset < chars.length && (chars[mOffset] == '=' || chars[mOffset] == '>')) {
                    ++mOffset;
                }
                return;
            }

            // "!="
            if (chars[mOffset] == '!') {
                ++mOffset;
                mCurrentToken = TOKEN_COMPARE;
                if (mOffset < chars.length && chars[mOffset] == '=') {
                    ++mOffset;
                    return;
                }
                throw new IllegalArgumentException("Unexpected character after !");
            }

            // columns and keywords
            // first look for anything that looks like an identifier or a keyword
            //     and then recognize the individual words.
            // no attempt is made at discarding sequences of underscores with no alphanumeric
            //     characters, even though it's not clear that they'd be legal column names.
            if (isIdentifierStart(chars[mOffset])) {
                int startOffset = mOffset;
                ++mOffset;
                while (mOffset < chars.length && isIdentifierChar(chars[mOffset])) {
                    ++mOffset;
                }
                String word = mSelection.substring(startOffset, mOffset);
                if (mOffset - startOffset <= 4) {
                    if (word.equals("IS")) {
                        mCurrentToken = TOKEN_IS;
                        return;
                    }
                    if (word.equals("OR") || word.equals("AND")) {
                        mCurrentToken = TOKEN_AND_OR;
                        return;
                    }
                    if (word.equals("NULL")) {
                        mCurrentToken = TOKEN_NULL;
                        return;
                    }
                }
                if (mAllowedColumns.contains(word)) {
                    mCurrentToken = TOKEN_COLUMN;
                    return;
                }
                throw new IllegalArgumentException("unrecognized column or keyword");
            }

            // quoted strings
            if (chars[mOffset] == '\'') {
                ++mOffset;
                while (mOffset < chars.length) {
                    if (chars[mOffset] == '\'') {
                        if (mOffset + 1 < chars.length && chars[mOffset + 1] == '\'') {
                            ++mOffset;
                        } else {
                            break;
                        }
                    }
                    ++mOffset;
                }
                if (mOffset == chars.length) {
                    throw new IllegalArgumentException("unterminated string");
                }
                ++mOffset;
                mCurrentToken = TOKEN_VALUE;
                return;
            }

            // anything we don't recognize
            throw new IllegalArgumentException("illegal character");
        }

        private static final boolean isIdentifierStart(char c) {
            return c == '_' ||
                    (c >= 'A' && c <= 'Z') ||
                    (c >= 'a' && c <= 'z');
        }

        private static final boolean isIdentifierChar(char c) {
            return c == '_' ||
                    (c >= 'A' && c <= 'Z') ||
                    (c >= 'a' && c <= 'z') ||
                    (c >= '0' && c <= '9');
        }
    }
}