This file is indexed.

/usr/lib/python3/dist-packages/urwid/canvas.py is in python3-urwid 2.0.1-2.

This file is owned by root:root, with mode 0o644.

The actual contents of the file can be viewed below.

   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
 768
 769
 770
 771
 772
 773
 774
 775
 776
 777
 778
 779
 780
 781
 782
 783
 784
 785
 786
 787
 788
 789
 790
 791
 792
 793
 794
 795
 796
 797
 798
 799
 800
 801
 802
 803
 804
 805
 806
 807
 808
 809
 810
 811
 812
 813
 814
 815
 816
 817
 818
 819
 820
 821
 822
 823
 824
 825
 826
 827
 828
 829
 830
 831
 832
 833
 834
 835
 836
 837
 838
 839
 840
 841
 842
 843
 844
 845
 846
 847
 848
 849
 850
 851
 852
 853
 854
 855
 856
 857
 858
 859
 860
 861
 862
 863
 864
 865
 866
 867
 868
 869
 870
 871
 872
 873
 874
 875
 876
 877
 878
 879
 880
 881
 882
 883
 884
 885
 886
 887
 888
 889
 890
 891
 892
 893
 894
 895
 896
 897
 898
 899
 900
 901
 902
 903
 904
 905
 906
 907
 908
 909
 910
 911
 912
 913
 914
 915
 916
 917
 918
 919
 920
 921
 922
 923
 924
 925
 926
 927
 928
 929
 930
 931
 932
 933
 934
 935
 936
 937
 938
 939
 940
 941
 942
 943
 944
 945
 946
 947
 948
 949
 950
 951
 952
 953
 954
 955
 956
 957
 958
 959
 960
 961
 962
 963
 964
 965
 966
 967
 968
 969
 970
 971
 972
 973
 974
 975
 976
 977
 978
 979
 980
 981
 982
 983
 984
 985
 986
 987
 988
 989
 990
 991
 992
 993
 994
 995
 996
 997
 998
 999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
#!/usr/bin/python
#
# Urwid canvas class and functions
#    Copyright (C) 2004-2011  Ian Ward
#
#    This library is free software; you can redistribute it and/or
#    modify it under the terms of the GNU Lesser General Public
#    License as published by the Free Software Foundation; either
#    version 2.1 of the License, or (at your option) any later version.
#
#    This library 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
#    Lesser General Public License for more details.
#
#    You should have received a copy of the GNU Lesser General Public
#    License along with this library; if not, write to the Free Software
#    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
#
# Urwid web site: http://excess.org/urwid/

from __future__ import division, print_function

import weakref

from urwid.util import rle_len, rle_append_modify, rle_join_modify, rle_product, \
    calc_width, calc_text_pos, apply_target_encoding, trim_text_attr_cs
from urwid.text_layout import trim_line, LayoutSegment
from urwid.compat import bytes


class CanvasCache(object):
    """
    Cache for rendered canvases.  Automatically populated and
    accessed by Widget render() MetaClass magic, cleared by
    Widget._invalidate().

    Stores weakrefs to the canvas objects, so an external class
    must maintain a reference for this cache to be effective.
    At present the Screen classes store the last topmost canvas
    after redrawing the screen, keeping the canvases from being
    garbage collected.

    _widgets[widget] = {(wcls, size, focus): weakref.ref(canvas), ...}
    _refs[weakref.ref(canvas)] = (widget, wcls, size, focus)
    _deps[widget} = [dependent_widget, ...]
    """
    _widgets = {}
    _refs = {}
    _deps = {}
    hits = 0
    fetches = 0
    cleanups = 0

    def store(cls, wcls, canvas):
        """
        Store a weakref to canvas in the cache.

        wcls -- widget class that contains render() function
        canvas -- rendered canvas with widget_info (widget, size, focus)
        """
        if not canvas.cacheable:
            return

        assert canvas.widget_info, "Can't store canvas without widget_info"
        widget, size, focus = canvas.widget_info
        def walk_depends(canv):
            """
            Collect all child widgets for determining who we
            depend on.
            """
            # FIXME: is this recursion necessary?  The cache
            # invalidating might work with only one level.
            depends = []
            for x, y, c, pos in canv.children:
                if c.widget_info:
                    depends.append(c.widget_info[0])
                elif hasattr(c, 'children'):
                    depends.extend(walk_depends(c))
            return depends

        # use explicit depends_on if available from the canvas
        depends_on = getattr(canvas, 'depends_on', None)
        if depends_on is None and hasattr(canvas, 'children'):
            depends_on = walk_depends(canvas)
        if depends_on:
            for w in depends_on:
                if w not in cls._widgets:
                    return
            for w in depends_on:
                cls._deps.setdefault(w,[]).append(widget)

        ref = weakref.ref(canvas, cls.cleanup)
        cls._refs[ref] = (widget, wcls, size, focus)
        cls._widgets.setdefault(widget, {})[(wcls, size, focus)] = ref
    store = classmethod(store)

    def fetch(cls, widget, wcls, size, focus):
        """
        Return the cached canvas or None.

        widget -- widget object requested
        wcls -- widget class that contains render() function
        size, focus -- render() parameters
        """
        cls.fetches += 1 # collect stats

        sizes = cls._widgets.get(widget, None)
        if not sizes:
            return None
        ref = sizes.get((wcls, size, focus), None)
        if not ref:
            return None
        canv = ref()
        if canv:
            cls.hits += 1 # more stats
        return canv
    fetch = classmethod(fetch)

    def invalidate(cls, widget):
        """
        Remove all canvases cached for widget.
        """
        try:
            for ref in cls._widgets[widget].values():
                try:
                    del cls._refs[ref]
                except KeyError:
                    pass
            del cls._widgets[widget]
        except KeyError:
            pass
        if widget not in cls._deps:
            return
        dependants = cls._deps.get(widget, [])
        try:
            del cls._deps[widget]
        except KeyError:
            pass
        for w in dependants:
            cls.invalidate(w)
    invalidate = classmethod(invalidate)

    def cleanup(cls, ref):
        cls.cleanups += 1 # collect stats

        w = cls._refs.get(ref, None)
        del cls._refs[ref]
        if not w:
            return
        widget, wcls, size, focus = w
        sizes = cls._widgets.get(widget, None)
        if not sizes:
            return
        try:
            del sizes[(wcls, size, focus)]
        except KeyError:
            pass
        if not sizes:
            try:
                del cls._widgets[widget]
                del cls._deps[widget]
            except KeyError:
                pass
    cleanup = classmethod(cleanup)

    def clear(cls):
        """
        Empty the cache.
        """
        cls._widgets = {}
        cls._refs = {}
        cls._deps = {}
    clear = classmethod(clear)



class CanvasError(Exception):
    pass

class Canvas(object):
    """
    base class for canvases
    """
    cacheable = True

    _finalized_error = CanvasError("This canvas has been finalized. "
        "Use CompositeCanvas to wrap this canvas if "
        "you need to make changes.")
    _renamed_error = CanvasError("The old Canvas class is now called "
        "TextCanvas. Canvas is now the base class for all canvas "
        "classes.")
    _old_repr_error = CanvasError("The internal representation of "
        "canvases is no longer stored as .text, .attr, and .cs "
        "lists, please see the TextCanvas class for the new "
        "representation of canvas content.")

    def __init__(self, value1=None, value2=None, value3=None):
        """
        value1, value2, value3 -- if not None, raise a helpful error:
            the old Canvas class is now called TextCanvas.
        """
        if value1 is not None:
            raise self._renamed_error
        self._widget_info = None
        self.coords = {}
        self.shortcuts = {}

    def finalize(self, widget, size, focus):
        """
        Mark this canvas as finalized (should not be any future
        changes to its content). This is required before caching
        the canvas.  This happens automatically after a widget's
        'render call returns the canvas thanks to some metaclass
        magic.

        widget -- widget that rendered this canvas
        size -- size parameter passed to widget's render method
        focus -- focus parameter passed to widget's render method
        """
        if self.widget_info:
            raise self._finalized_error
        self._widget_info = widget, size, focus

    def _get_widget_info(self):
        return self._widget_info
    widget_info = property(_get_widget_info)

    def _raise_old_repr_error(self, val=None):
        raise self._old_repr_error

    def _text_content(self):
        """
        Return the text content of the canvas as a list of strings,
        one for each row.
        """
        return [bytes().join([text for (attr, cs, text) in row])
            for row in self.content()]

    text = property(_text_content, _raise_old_repr_error)
    attr = property(_raise_old_repr_error, _raise_old_repr_error)
    cs = property(_raise_old_repr_error, _raise_old_repr_error)

    def content(self, trim_left=0, trim_top=0, cols=None, rows=None,
            attr=None):
        raise NotImplementedError()

    def cols(self):
        raise NotImplementedError()

    def rows(self):
        raise NotImplementedError()

    def content_delta(self):
        raise NotImplementedError()

    def get_cursor(self):
        c = self.coords.get("cursor", None)
        if not c:
            return
        return c[:2] # trim off data part
    def set_cursor(self, c):
        if self.widget_info and self.cacheable:
            raise self._finalized_error
        if c is None:
            try:
                del self.coords["cursor"]
            except KeyError:
                pass
            return
        self.coords["cursor"] = c + (None,) # data part
    cursor = property(get_cursor, set_cursor)

    def get_pop_up(self):
        c = self.coords.get("pop up", None)
        if not c:
            return
        return c
    def set_pop_up(self, w, left, top, overlay_width, overlay_height):
        """
        This method adds pop-up information to the canvas.  This information
        is intercepted by a PopUpTarget widget higher in the chain to
        display a pop-up at the given (left, top) position relative to the
        current canvas.

        :param w: widget to use for the pop-up
        :type w: widget
        :param left: x position for left edge of pop-up >= 0
        :type left: int
        :param top: y position for top edge of pop-up >= 0
        :type top: int
        :param overlay_width: width of overlay in screen columns > 0
        :type overlay_width: int
        :param overlay_height: height of overlay in screen rows > 0
        :type overlay_height: int
        """
        if self.widget_info and self.cacheable:
            raise self._finalized_error

        self.coords["pop up"] = (left, top, (
            w, overlay_width, overlay_height))

    def translate_coords(self, dx, dy):
        """
        Return coords shifted by (dx, dy).
        """
        d = {}
        for name, (x, y, data) in self.coords.items():
            d[name] = (x+dx, y+dy, data)
        return d



class TextCanvas(Canvas):
    """
    class for storing rendered text and attributes
    """
    def __init__(self, text=None, attr=None, cs=None,
        cursor=None, maxcol=None, check_width=True):
        """
        text -- list of strings, one for each line
        attr -- list of run length encoded attributes for text
        cs -- list of run length encoded character set for text
        cursor -- (x,y) of cursor or None
        maxcol -- screen columns taken by this canvas
        check_width -- check and fix width of all lines in text
        """
        Canvas.__init__(self)
        if text == None:
            text = []

        if check_width:
            widths = []
            for t in text:
                if type(t) != bytes:
                    raise CanvasError("Canvas text must be plain strings encoded in the screen's encoding", repr(text))
                widths.append( calc_width( t, 0, len(t)) )
        else:
            assert type(maxcol) == int
            widths = [maxcol] * len(text)

        if maxcol is None:
            if widths:
                # find maxcol ourselves
                maxcol = max(widths)
            else:
                maxcol = 0

        if attr == None:
            attr = [[] for x in range(len(text))]
        if cs == None:
            cs = [[] for x in range(len(text))]

        # pad text and attr to maxcol
        for i in range(len(text)):
            w = widths[i]
            if w > maxcol:
                raise CanvasError("Canvas text is wider than the maxcol specified \n%r\n%r\n%r"%(maxcol,widths,text))
            if w < maxcol:
                text[i] = text[i] + bytes().rjust(maxcol-w)
            a_gap = len(text[i]) - rle_len( attr[i] )
            if a_gap < 0:
                raise CanvasError("Attribute extends beyond text \n%r\n%r" % (text[i],attr[i]) )
            if a_gap:
                rle_append_modify( attr[i], (None, a_gap))

            cs_gap = len(text[i]) - rle_len( cs[i] )
            if cs_gap < 0:
                raise CanvasError("Character Set extends beyond text \n%r\n%r" % (text[i],cs[i]) )
            if cs_gap:
                rle_append_modify( cs[i], (None, cs_gap))

        self._attr = attr
        self._cs = cs
        self.cursor = cursor
        self._text = text
        self._maxcol = maxcol

    def rows(self):
        """Return the number of rows in this canvas."""
        rows = len(self._text)
        assert isinstance(rows, int)
        return rows

    def cols(self):
        """Return the screen column width of this canvas."""
        return self._maxcol

    def translated_coords(self,dx,dy):
        """
        Return cursor coords shifted by (dx, dy), or None if there
        is no cursor.
        """
        if self.cursor:
            x, y = self.cursor
            return x+dx, y+dy
        return None

    def content(self, trim_left=0, trim_top=0, cols=None, rows=None,
            attr_map=None):
        """
        Return the canvas content as a list of rows where each row
        is a list of (attr, cs, text) tuples.

        trim_left, trim_top, cols, rows may be set by
        CompositeCanvas when rendering a partially obscured
        canvas.
        """
        maxcol, maxrow = self.cols(), self.rows()
        if not cols:
            cols = maxcol - trim_left
        if not rows:
            rows = maxrow - trim_top

        assert trim_left >= 0 and trim_left < maxcol
        assert cols > 0 and trim_left + cols <= maxcol
        assert trim_top >=0 and trim_top < maxrow
        assert rows > 0 and trim_top + rows <= maxrow

        if trim_top or rows < maxrow:
            text_attr_cs = zip(
                self._text[trim_top:trim_top+rows],
                self._attr[trim_top:trim_top+rows],
                self._cs[trim_top:trim_top+rows])
        else:
            text_attr_cs = zip(self._text, self._attr, self._cs)

        for text, a_row, cs_row in text_attr_cs:
            if trim_left or cols < self._maxcol:
                text, a_row, cs_row = trim_text_attr_cs(
                    text, a_row, cs_row, trim_left,
                    trim_left + cols)
            attr_cs = rle_product(a_row, cs_row)
            i = 0
            row = []
            for (a, cs), run in attr_cs:
                if attr_map and a in attr_map:
                    a = attr_map[a]
                row.append((a, cs, text[i:i+run]))
                i += run
            yield row


    def content_delta(self, other):
        """
        Return the differences between other and this canvas.

        If other is the same object as self this will return no
        differences, otherwise this is the same as calling
        content().
        """
        if other is self:
            return [self.cols()]*self.rows()
        return self.content()



class BlankCanvas(Canvas):
    """
    a canvas with nothing on it, only works as part of a composite canvas
    since it doesn't know its own size
    """
    def __init__(self):
        Canvas.__init__(self, None)

    def content(self, trim_left, trim_top, cols, rows, attr):
        """
        return (cols, rows) of spaces with default attributes.
        """
        def_attr = None
        if attr and None in attr:
            def_attr = attr[None]
        line = [(def_attr, None, bytes().rjust(cols))]
        for i in range(rows):
            yield line

    def cols(self):
        raise NotImplementedError("BlankCanvas doesn't know its own size!")

    def rows(self):
        raise NotImplementedError("BlankCanvas doesn't know its own size!")

    def content_delta(self):
        raise NotImplementedError("BlankCanvas doesn't know its own size!")

blank_canvas = BlankCanvas()


class SolidCanvas(Canvas):
    """
    A canvas filled completely with a single character.
    """
    def __init__(self, fill_char, cols, rows):
        Canvas.__init__(self)
        end, col = calc_text_pos(fill_char, 0, len(fill_char), 1)
        assert col == 1, "Invalid fill_char: %r" % fill_char
        self._text, cs = apply_target_encoding(fill_char[:end])
        self._cs = cs[0][0]
        self.size = cols, rows
        self.cursor = None

    def cols(self):
        return self.size[0]

    def rows(self):
        return self.size[1]

    def content(self, trim_left=0, trim_top=0, cols=None, rows=None,
            attr=None):
        if cols is None:
            cols = self.size[0]
        if rows is None:
            rows = self.size[1]
        def_attr = None
        if attr and None in attr:
            def_attr = attr[None]

        line = [(def_attr, self._cs, self._text*cols)]
        for i in range(rows):
            yield line

    def content_delta(self, other):
        """
        Return the differences between other and this canvas.
        """
        if other is self:
            return [self.cols()]*self.rows()
        return self.content()




class CompositeCanvas(Canvas):
    """
    class for storing a combination of canvases
    """
    def __init__(self, canv=None):
        """
        canv -- a Canvas object to wrap this CompositeCanvas around.

        if canv is a CompositeCanvas, make a copy of its contents
        """
        # a "shard" is a (num_rows, list of cviews) tuple, one for
        # each cview starting in this shard

        # a "cview" is a tuple that defines a view of a canvas:
        # (trim_left, trim_top, cols, rows, attr_map, canv)

        # a "shard tail" is a list of tuples:
        # (col_gap, done_rows, content_iter, cview)

        # tuples that define the unfinished cviews that are part of
        # shards following the first shard.
        Canvas.__init__(self)

        if canv is None:
            self.shards = []
            self.children = []
        else:
            if hasattr(canv, "shards"):
                self.shards = canv.shards
            else:
                self.shards = [(canv.rows(), [
                    (0, 0, canv.cols(), canv.rows(),
                    None, canv)])]
            self.children = [(0, 0, canv, None)]
            self.coords.update(canv.coords)
            for shortcut in canv.shortcuts:
                self.shortcuts[shortcut] = "wrap"

    def rows(self):
        for r,cv in self.shards:
            try:
                assert isinstance(r, int)
            except AssertionError:
                raise AssertionError(r, cv)
        rows = sum([r for r,cv in self.shards])
        assert isinstance(rows, int)
        return rows

    def cols(self):
        if not self.shards:
            return 0
        cols = sum([cv[2] for cv in self.shards[0][1]])
        assert isinstance(cols, int)
        return cols


    def content(self):
        """
        Return the canvas content as a list of rows where each row
        is a list of (attr, cs, text) tuples.
        """
        shard_tail = []
        for num_rows, cviews in self.shards:
            # combine shard and shard tail
            sbody = shard_body(cviews, shard_tail)

            # output rows
            for i in range(num_rows):
                yield shard_body_row(sbody)

            # prepare next shard tail
            shard_tail = shard_body_tail(num_rows, sbody)



    def content_delta(self, other):
        """
        Return the differences between other and this canvas.
        """
        if not hasattr(other, 'shards'):
            for row in self.content():
                yield row
            return

        shard_tail = []
        for num_rows, cviews in shards_delta(
                self.shards, other.shards):
            # combine shard and shard tail
            sbody = shard_body(cviews, shard_tail)

            # output rows
            row = []
            for i in range(num_rows):
                # if whole shard is unchanged, don't keep
                # calling shard_body_row
                if len(row) != 1 or type(row[0]) != int:
                    row = shard_body_row(sbody)
                yield row

            # prepare next shard tail
            shard_tail = shard_body_tail(num_rows, sbody)


    def trim(self, top, count=None):
        """Trim lines from the top and/or bottom of canvas.

        top -- number of lines to remove from top
        count -- number of lines to keep, or None for all the rest
        """
        assert top >= 0, "invalid trim amount %d!"%top
        assert top < self.rows(), "cannot trim %d lines from %d!"%(
            top, self.rows())
        if self.widget_info:
            raise self._finalized_error

        if top:
            self.shards = shards_trim_top(self.shards, top)

        if count == 0:
            self.shards = []
        elif count is not None:
            self.shards = shards_trim_rows(self.shards, count)

        self.coords = self.translate_coords(0, -top)


    def trim_end(self, end):
        """Trim lines from the bottom of the canvas.

        end -- number of lines to remove from the end
        """
        assert end > 0, "invalid trim amount %d!"%end
        assert end <= self.rows(), "cannot trim %d lines from %d!"%(
            end, self.rows())
        if self.widget_info:
            raise self._finalized_error

        self.shards = shards_trim_rows(self.shards, self.rows() - end)


    def pad_trim_left_right(self, left, right):
        """
        Pad or trim this canvas on the left and right

        values > 0 indicate screen columns to pad
        values < 0 indicate screen columns to trim
        """
        if self.widget_info:
            raise self._finalized_error
        shards = self.shards
        if left < 0 or right < 0:
            trim_left = max(0, -left)
            cols = self.cols() - trim_left - max(0, -right)
            shards = shards_trim_sides(shards, trim_left, cols)

        rows = self.rows()
        if left > 0 or right > 0:
            top_rows, top_cviews = shards[0]
            if left > 0:
                new_top_cviews = (
                    [(0,0,left,rows,None,blank_canvas)] +
                    top_cviews)
            else:
                new_top_cviews = top_cviews[:] #copy

            if right > 0:
                new_top_cviews.append(
                    (0,0,right,rows,None,blank_canvas))
            shards = [(top_rows, new_top_cviews)] + shards[1:]

        self.coords = self.translate_coords(left, 0)
        self.shards = shards


    def pad_trim_top_bottom(self, top, bottom):
        """
        Pad or trim this canvas on the top and bottom.
        """
        if self.widget_info:
            raise self._finalized_error
        orig_shards = self.shards

        if top < 0 or bottom < 0:
            trim_top = max(0, -top)
            rows = self.rows() - trim_top - max(0, -bottom)
            self.trim(trim_top, rows)

        cols = self.cols()
        if top > 0:
            self.shards = [(top,
                [(0,0,cols,top,None,blank_canvas)])] + \
                self.shards
            self.coords = self.translate_coords(0, top)

        if bottom > 0:
            if orig_shards is self.shards:
                self.shards = self.shards[:]
            self.shards.append((bottom,
                [(0,0,cols,bottom,None,blank_canvas)]))


    def overlay(self, other, left, top ):
        """Overlay other onto this canvas."""
        if self.widget_info:
            raise self._finalized_error

        width = other.cols()
        height = other.rows()
        right = self.cols() - left - width
        bottom = self.rows() - top - height

        assert right >= 0, "top canvas of overlay not the size expected!" + repr((other.cols(),left,right,width))
        assert bottom >= 0, "top canvas of overlay not the size expected!" + repr((other.rows(),top,bottom,height))

        shards = self.shards
        top_shards = []
        side_shards = self.shards
        bottom_shards = []
        if top:
            side_shards = shards_trim_top(shards, top)
            top_shards = shards_trim_rows(shards, top)
        if bottom:
            bottom_shards = shards_trim_top(side_shards, height)
            side_shards = shards_trim_rows(side_shards, height)

        left_shards = []
        right_shards = []
        if left > 0:
            left_shards = [shards_trim_sides(side_shards, 0, left)]
        if right > 0:
            right_shards = [shards_trim_sides(side_shards,
                max(0, left + width), right)]

        if not self.rows():
            middle_shards = []
        elif left or right:
            middle_shards = shards_join(left_shards +
                [other.shards] + right_shards)
        else:
            middle_shards = other.shards

        self.shards = top_shards + middle_shards + bottom_shards

        self.coords.update(other.translate_coords(left, top))


    def fill_attr(self, a):
        """
        Apply attribute a to all areas of this canvas with default
        attribute currently set to None, leaving other attributes
        intact."""
        self.fill_attr_apply({None:a})

    def fill_attr_apply(self, mapping):
        """
        Apply an attribute-mapping dictionary to the canvas.

        mapping -- dictionary of original-attribute:new-attribute items
        """
        if self.widget_info:
            raise self._finalized_error

        shards = []
        for num_rows, original_cviews in self.shards:
            new_cviews = []
            for cv in original_cviews:
                # cv[4] == attr_map
                if cv[4] is None:
                    new_cviews.append(cv[:4] +
                        (mapping,) + cv[5:])
                else:
                    combined = dict(mapping)
                    combined.update([
                        (k, mapping.get(v, v)) for k,v in cv[4].items()])
                    new_cviews.append(cv[:4] +
                        (combined,) + cv[5:])
            shards.append((num_rows, new_cviews))
        self.shards = shards

    def set_depends(self, widget_list):
        """
        Explicitly specify the list of widgets that this canvas
        depends on.  If any of these widgets change this canvas
        will have to be updated.
        """
        if self.widget_info:
            raise self._finalized_error

        self.depends_on = widget_list


def shard_body_row(sbody):
    """
    Return one row, advancing the iterators in sbody.

    ** MODIFIES sbody by calling next() on its iterators **
    """
    row = []
    for done_rows, content_iter, cview in sbody:
        if content_iter:
            row.extend(next(content_iter))
        else:
            # need to skip this unchanged canvas
            if row and type(row[-1]) == int:
                row[-1] = row[-1] + cview[2]
            else:
                row.append(cview[2])

    return row


def shard_body_tail(num_rows, sbody):
    """
    Return a new shard tail that follows this shard body.
    """
    shard_tail = []
    col_gap = 0
    done_rows = 0
    for done_rows, content_iter, cview in sbody:
        cols, rows = cview[2:4]
        done_rows += num_rows
        if done_rows == rows:
            col_gap += cols
            continue
        shard_tail.append((col_gap, done_rows, content_iter, cview))
        col_gap = 0
    return shard_tail


def shards_delta(shards, other_shards):
    """
    Yield shards1 with cviews that are the same as shards2
    having canv = None.
    """
    other_shards_iter = iter(other_shards)
    other_num_rows = other_cviews = None
    done = other_done = 0
    for num_rows, cviews in shards:
        if other_num_rows is None:
            other_num_rows, other_cviews = next(other_shards_iter)
        while other_done < done:
            other_done += other_num_rows
            other_num_rows, other_cviews = next(other_shards_iter)
        if other_done > done:
            yield (num_rows, cviews)
            done += num_rows
            continue
        # top-aligned shards, compare each cview
        yield (num_rows, shard_cviews_delta(cviews, other_cviews))
        other_done += other_num_rows
        other_num_rows = None
        done += num_rows

def shard_cviews_delta(cviews, other_cviews):
    """
    """
    other_cviews_iter = iter(other_cviews)
    other_cv = None
    cols = other_cols = 0
    for cv in cviews:
        if other_cv is None:
            other_cv = next(other_cviews_iter)
        while other_cols < cols:
            other_cols += other_cv[2]
            other_cv = next(other_cviews_iter)
        if other_cols > cols:
            yield cv
            cols += cv[2]
            continue
        # top-left-aligned cviews, compare them
        if cv[5] is other_cv[5] and cv[:5] == other_cv[:5]:
            yield cv[:5]+(None,)+cv[6:]
        else:
            yield cv
        other_cols += other_cv[2]
        other_cv = None
        cols += cv[2]



def shard_body(cviews, shard_tail, create_iter=True, iter_default=None):
    """
    Return a list of (done_rows, content_iter, cview) tuples for
    this shard and shard tail.

    If a canvas in cviews is None (eg. when unchanged from
    shard_cviews_delta()) or if create_iter is False then no
    iterator is created for content_iter.

    iter_default is the value used for content_iter when no iterator
    is created.
    """
    col = 0
    body = [] # build the next shard tail
    cviews_iter = iter(cviews)
    for col_gap, done_rows, content_iter, tail_cview in shard_tail:
        while col_gap:
            try:
                cview = next(cviews_iter)
            except StopIteration:
                raise CanvasError("cviews do not fill gaps in"
                    " shard_tail!")
            (trim_left, trim_top, cols, rows, attr_map, canv) = \
                cview[:6]
            col += cols
            col_gap -= cols
            if col_gap < 0:
                raise CanvasError("cviews overflow gaps in"
                    " shard_tail!")
            if create_iter and canv:
                new_iter = canv.content(trim_left, trim_top,
                    cols, rows, attr_map)
            else:
                new_iter = iter_default
            body.append((0, new_iter, cview))
        body.append((done_rows, content_iter, tail_cview))
    for cview in cviews_iter:
        (trim_left, trim_top, cols, rows, attr_map, canv) = \
            cview[:6]
        if create_iter and canv:
            new_iter = canv.content(trim_left, trim_top, cols, rows,
                attr_map)
        else:
            new_iter = iter_default
        body.append((0, new_iter, cview))
    return body


def shards_trim_top(shards, top):
    """
    Return shards with top rows removed.
    """
    assert top > 0

    shard_iter = iter(shards)
    shard_tail = []
    # skip over shards that are completely removed
    for num_rows, cviews in shard_iter:
        if top < num_rows:
            break
        sbody = shard_body(cviews, shard_tail, False)
        shard_tail = shard_body_tail(num_rows, sbody)
        top -= num_rows
    else:
        raise CanvasError("tried to trim shards out of existence")

    sbody = shard_body(cviews, shard_tail, False)
    shard_tail = shard_body_tail(num_rows, sbody)
    # trim the top of this shard
    new_sbody = []
    for done_rows, content_iter, cv in sbody:
        new_sbody.append((0, content_iter,
            cview_trim_top(cv, done_rows+top)))
    sbody = new_sbody

    new_shards = [(num_rows-top,
        [cv for done_rows, content_iter, cv in sbody])]

    # write out the rest of the shards
    new_shards.extend(shard_iter)

    return new_shards

def shards_trim_rows(shards, keep_rows):
    """
    Return the topmost keep_rows rows from shards.
    """
    assert keep_rows >= 0, keep_rows

    new_shards = []
    done_rows = 0
    for num_rows, cviews in shards:
        if done_rows >= keep_rows:
            break
        new_cviews = []
        for cv in cviews:
            if cv[3] + done_rows > keep_rows:
                new_cviews.append(cview_trim_rows(cv,
                    keep_rows - done_rows))
            else:
                new_cviews.append(cv)

        if num_rows + done_rows > keep_rows:
            new_shards.append((keep_rows - done_rows, new_cviews))
        else:
            new_shards.append((num_rows, new_cviews))
        done_rows += num_rows

    return new_shards

def shards_trim_sides(shards, left, cols):
    """
    Return shards with starting from column left and cols total width.
    """
    assert left >= 0 and cols > 0, (left, cols)
    shard_tail = []
    new_shards = []
    right = left + cols
    for num_rows, cviews in shards:
        sbody = shard_body(cviews, shard_tail, False)
        shard_tail = shard_body_tail(num_rows, sbody)
        new_cviews = []
        col = 0
        for done_rows, content_iter, cv in sbody:
            cv_cols = cv[2]
            next_col = col + cv_cols
            if done_rows or next_col <= left or col >= right:
                col = next_col
                continue
            if col < left:
                cv = cview_trim_left(cv, left - col)
                col = left
            if next_col > right:
                cv = cview_trim_cols(cv, right - col)
            new_cviews.append(cv)
            col = next_col
        if not new_cviews:
            prev_num_rows, prev_cviews = new_shards[-1]
            new_shards[-1] = (prev_num_rows+num_rows, prev_cviews)
        else:
            new_shards.append((num_rows, new_cviews))
    return new_shards

def shards_join(shard_lists):
    """
    Return the result of joining shard lists horizontally.
    All shards lists must have the same number of rows.
    """
    shards_iters = [iter(sl) for sl in shard_lists]
    shards_current = [next(i) for i in shards_iters]

    new_shards = []
    while True:
        new_cviews = []
        num_rows = min([r for r,cv in shards_current])

        shards_next = []
        for rows, cviews in shards_current:
            if cviews:
                new_cviews.extend(cviews)
            shards_next.append((rows - num_rows, None))

        shards_current = shards_next
        new_shards.append((num_rows, new_cviews))

        # advance to next shards
        try:
            for i in range(len(shards_current)):
                if shards_current[i][0] > 0:
                    continue
                shards_current[i] = next(shards_iters[i])
        except StopIteration:
            break
    return new_shards


def cview_trim_rows(cv, rows):
    return cv[:3] + (rows,) + cv[4:]

def cview_trim_top(cv, trim):
    return (cv[0], trim + cv[1], cv[2], cv[3] - trim) + cv[4:]

def cview_trim_left(cv, trim):
    return (cv[0] + trim, cv[1], cv[2] - trim,) + cv[3:]

def cview_trim_cols(cv, cols):
    return cv[:2] + (cols,) + cv[3:]




def CanvasCombine(l):
    """Stack canvases in l vertically and return resulting canvas.

    :param l: list of (canvas, position, focus) tuples:

              position
                a value that widget.set_focus will accept or None if not
                allowed
              focus
                True if this canvas is the one that would be in focus
                if the whole widget is in focus
    """
    clist = [(CompositeCanvas(c),p,f) for c,p,f in l]

    combined_canvas = CompositeCanvas()
    shards = []
    children = []
    row = 0
    focus_index = 0
    n = 0
    for canv, pos, focus in clist:
        if focus:
            focus_index = n
        children.append((0, row, canv, pos))
        shards.extend(canv.shards)
        combined_canvas.coords.update(canv.translate_coords(0, row))
        for shortcut in canv.shortcuts.keys():
            combined_canvas.shortcuts[shortcut] = pos
        row += canv.rows()
        n += 1

    if focus_index:
        children = [children[focus_index]] + children[:focus_index] + \
            children[focus_index+1:]

    combined_canvas.shards = shards
    combined_canvas.children = children
    return combined_canvas


def CanvasOverlay(top_c, bottom_c, left, top):
    """
    Overlay canvas top_c onto bottom_c at position (left, top).
    """
    overlayed_canvas = CompositeCanvas(bottom_c)
    overlayed_canvas.overlay(top_c, left, top)
    overlayed_canvas.children = [(left, top, top_c, None),
        (0, 0, bottom_c, None)]
    overlayed_canvas.shortcuts = {} # disable background shortcuts
    for shortcut in top_c.shortcuts.keys():
        overlayed_canvas.shortcuts[shortcut]="fg"
    return overlayed_canvas


def CanvasJoin(l):
    """
    Join canvases in l horizontally. Return result.

    :param l: list of (canvas, position, focus, cols) tuples:

              position
                value that widget.set_focus will accept or None if not allowed
              focus
                True if this canvas is the one that would be in focus if
                the whole widget is in focus
              cols
                is the number of screen columns that this widget will require,
                if larger than the actual canvas.cols() value then this widget
                will be padded on the right.
    """

    l2 = []
    focus_item = 0
    maxrow = 0
    n = 0
    for canv, pos, focus, cols in l:
        rows = canv.rows()
        pad_right = cols - canv.cols()
        if focus:
            focus_item = n
        if rows > maxrow:
            maxrow = rows
        l2.append((canv, pos, pad_right, rows))
        n += 1

    shard_lists = []
    children = []
    joined_canvas = CompositeCanvas()
    col = 0
    for canv, pos, pad_right, rows in l2:
        canv = CompositeCanvas(canv)
        if pad_right:
            canv.pad_trim_left_right(0, pad_right)
        if rows < maxrow:
            canv.pad_trim_top_bottom(0, maxrow - rows)
        joined_canvas.coords.update(canv.translate_coords(col, 0))
        for shortcut in canv.shortcuts.keys():
            joined_canvas.shortcuts[shortcut] = pos
        shard_lists.append(canv.shards)
        children.append((col, 0, canv, pos))
        col += canv.cols()

    if focus_item:
        children = [children[focus_item]] + children[:focus_item] + \
            children[focus_item+1:]

    joined_canvas.shards = shards_join(shard_lists)
    joined_canvas.children = children
    return joined_canvas


def apply_text_layout(text, attr, ls, maxcol):
    t = []
    a = []
    c = []

    class AttrWalk:
        pass
    aw = AttrWalk
    aw.k = 0 # counter for moving through elements of a
    aw.off = 0 # current offset into text of attr[ak]

    def arange( start_offs, end_offs ):
        """Return an attribute list for the range of text specified."""
        if start_offs < aw.off:
            aw.k = 0
            aw.off = 0
        o = []
        while aw.off < end_offs:
            if len(attr)<=aw.k:
                # run out of attributes
                o.append((None,end_offs-max(start_offs,aw.off)))
                break
            at,run = attr[aw.k]
            if aw.off+run <= start_offs:
                # move forward through attr to find start_offs
                aw.k += 1
                aw.off += run
                continue
            if end_offs <= aw.off+run:
                o.append((at, end_offs-max(start_offs,aw.off)))
                break
            o.append((at, aw.off+run-max(start_offs, aw.off)))
            aw.k += 1
            aw.off += run
        return o


    for line_layout in ls:
        # trim the line to fit within maxcol
        line_layout = trim_line( line_layout, text, 0, maxcol )

        line = []
        linea = []
        linec = []

        def attrrange( start_offs, end_offs, destw ):
            """
            Add attributes based on attributes between
            start_offs and end_offs.
            """
            if start_offs == end_offs:
                [(at,run)] = arange(start_offs,end_offs)
                rle_append_modify( linea, ( at, destw ))
                return
            if destw == end_offs-start_offs:
                for at, run in arange(start_offs,end_offs):
                    rle_append_modify( linea, ( at, run ))
                return
            # encoded version has different width
            o = start_offs
            for at, run in arange(start_offs, end_offs):
                if o+run == end_offs:
                    rle_append_modify( linea, ( at, destw ))
                    return
                tseg = text[o:o+run]
                tseg, cs = apply_target_encoding( tseg )
                segw = rle_len(cs)

                rle_append_modify( linea, ( at, segw ))
                o += run
                destw -= segw


        for seg in line_layout:
            #if seg is None: assert 0, ls
            s = LayoutSegment(seg)
            if s.end:
                tseg, cs = apply_target_encoding(
                    text[s.offs:s.end])
                line.append(tseg)
                attrrange(s.offs, s.end, rle_len(cs))
                rle_join_modify( linec, cs )
            elif s.text:
                tseg, cs = apply_target_encoding( s.text )
                line.append(tseg)
                attrrange( s.offs, s.offs, len(tseg) )
                rle_join_modify( linec, cs )
            elif s.offs:
                if s.sc:
                    line.append(bytes().rjust(s.sc))
                    attrrange( s.offs, s.offs, s.sc )
            else:
                line.append(bytes().rjust(s.sc))
                linea.append((None, s.sc))
                linec.append((None, s.sc))

        t.append(bytes().join(line))
        a.append(linea)
        c.append(linec)

    return TextCanvas(t, a, c, maxcol=maxcol)