summaryrefslogtreecommitdiff
path: root/macros/luatex/generic/lua-widow-control/lua-widow-control.lua
blob: 1ef86174c4ff014ccc0385ff61114688f888d457 (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
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
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
--[[
    lua-widow-control
    https://github.com/gucci-on-fleek/lua-widow-control
    SPDX-License-Identifier: MPL-2.0+
    SPDX-FileCopyrightText: 2022 Max Chernoff
  ]]

--- Tell the linter about node attributes
--- @class node
--- @field depth integer
--- @field height integer
--- @field id integer
--- @field list node
--- @field next node
--- @field penalty integer
--- @field prev node
--- @field subtype integer

-- Initial setup
lwc = lwc or {}
lwc.name = "lua-widow-control"

-- Locals for `debug_print`
local debug_lib = debug
local string_rep = string.rep
local write_nl = texio.write_nl

local write_log
if status.luatex_engine == "luametatex" then
    write_log = "logfile"
else
    write_log = "log"
end

--- Prints debugging messages to the log, only if `debug` is set to `true`.
---
--- @param title string The "title" to use
--- @param text string? The "content" to print
--- @return nil
local function debug(title, text)
    if not lwc.debug then return end

    -- The number of spaces we need
    local filler = 15 - #title

    if text then
        write_nl(write_log, "LWC (" .. title .. string_rep(" ", filler) .. "): " .. text)
    else
        write_nl(write_log, "LWC: " .. string_rep(" ", 18) .. title)
    end
end


--[[
    \lwc/ is intended to be format-agonistic. It only runs on Lua\TeX{},
    but there are still some slight differences between formats. Here, we
    detect the format name then set some flags for later processing.
  ]]
local format = tex.formatname
local context, latex, plain, optex, lmtx

if format:find("cont") then -- cont-en, cont-fr, cont-nl, ...
    context = true
    if status.luatex_engine == "luametatex" then
        lmtx = true
    end
elseif format:find("latex") then -- lualatex, lualatex-dev, ...
    latex = true
elseif format == "luatex" or format == "luahbtex" then -- Plain
    plain = true
elseif format:find("optex") then -- OpTeX
    optex = true
end

--[[
    Save some local copies of the node library to reduce table lookups.
    This is probably a useless micro-optimization, but it is done in all of the
    ConTeXt and expl3 Lua code, so I should probably do it here too.
  ]]
-- Node ID's
-- (We need to hardcode the subid's sadly)
local id_from_name = node.id
local baselineskip_subid = 2
local glue_id = id_from_name("glue")
local glyph_id = id_from_name("glyph")
local hlist_id = id_from_name("hlist")
local insert_id = id_from_name("insert") or id_from_name("ins")
local line_subid = 1
local linebreakpenalty_subid = 1
local par_id = id_from_name("par") or id_from_name("local_par")
local penalty_id = id_from_name("penalty")

-- Local versions of globals
local abs = math.abs
local copy = node.copy
local copy_list = node.copy_list or node.copylist
local find_attribute = node.find_attribute or node.findattribute
local free = node.free
local free_list = node.flush_list or node.flushlist
local get_attribute = node.get_attribute or node.getattribute
local insert_token = token.put_next or token.putnext
local last = node.slide
local linebreak = tex.linebreak
local new_node = node.new
local remove = node.remove
local set_attribute = node.set_attribute or node.setattribute
local string_char = string.char
local tex_box = tex.box
local tex_count = tex.count
local tex_dimen = tex.dimen
local tex_lists = tex.lists
local traverse = node.traverse
local traverse_id = node.traverse_id or node.traverseid
local vpack = node.vpack

-- Misc. Constants
local iffalse = token.create("iffalse")
local iftrue = token.create("iftrue")
local INFINITY = 10000
local INSERT_CLASS_MULTIPLE = 1000 * 1000
local INSERT_FIRST_MULTIPLE = 1000
local llap_offset = math.max(tex.dimen.parindent, tex.sp("12pt"))
local min_col_width = tex.sp("250pt")
local PAGE_MULTIPLE = 100
local SINGLE_LINE = 50

lwc.colours = {
    expanded = {0.00, 0.70, 0.25},
    failure  = {0.90, 0.00, 0.25},
    moved    = {0.25, 0.25, 1.00},
}

--[[ Package/module initialization.

     Here, we replace any format/engine-specific variables/functions with some
     generic equivalents. This way, we can write the rest of the module without
     worrying about any format/engine differences.
  ]]
local contrib_head,
      emergencystretch,
      hold_head,
      info,
      insert_attribute,
      max_cost,
      pagenum,
      paragraph_attribute,
      shrink_order,
      stretch_order,
      warning

if lmtx then
    -- LMTX has removed underscores from most of the Lua parts
    debug("LMTX")
    contrib_head = "contributehead"
    shrink_order = "shrinkorder"
    stretch_order = "stretchorder"
    hold_head = "holdhead"
else
    contrib_head = "contrib_head"
    shrink_order = "shrink_order"
    stretch_order = "stretch_order"
    hold_head = "hold_head"
end

if context then
    debug("ConTeXt")

    warning = logs.reporter(lwc.name, "warning")
    local _info = logs.reporter(lwc.name, "info")
    --[[ We don't want the info messages on the terminal, but ConTeXt doesn't
         provide any logfile-only reporters, so we need this hack.
      ]]
    info = function (text)
        logs.pushtarget("logfile")
        _info(text)
        logs.poptarget()
    end
    paragraph_attribute = attributes.public(lwc.name .. "_paragraph")
    insert_attribute = attributes.public(lwc.name .. "_insert")
    pagenum = function() return tex_count["realpageno"] end

    -- Dimen names
    emergencystretch = "lwc_emergency_stretch"
    max_cost = "lwc_max_cost"
elseif plain or latex or optex then
    pagenum = function() return tex_count[0] end

    -- Dimen names
    if tex.isdimen("g__lwc_emergencystretch_dim") then
        emergencystretch = "g__lwc_emergencystretch_dim"
        max_cost = "g__lwc_maxcost_int"
    else
        emergencystretch = "lwcemergencystretch"
        max_cost = "lwcmaxcost"
    end

    if plain or latex then
        debug("Plain/LaTeX")
        luatexbase.provides_module {
            name = lwc.name,
            date = "2022/08/23", --%%slashdate
            version = "2.2.2", --%%version
            description = [[

This module provides a LuaTeX-based solution to prevent
widows and orphans from appearing in a document. It does
so by increasing or decreasing the lengths of previous
paragraphs.]],
        }
        warning = function(str) luatexbase.module_warning(lwc.name, str) end
        info = function(str) luatexbase.module_info(lwc.name, str) end
        paragraph_attribute = luatexbase.new_attribute(lwc.name .. "_paragraph")
        insert_attribute = luatexbase.new_attribute(lwc.name .. "_insert")
    elseif optex then
        debug("OpTeX")

        warning = function(str) write_nl(lwc.name .. " Warning: " .. str) end
        info = function(str) write_nl("log", lwc.name .. " Info: " .. str) end
        paragraph_attribute = alloc.new_attribute(lwc.name .. "_paragraph")
        insert_attribute = alloc.new_attribute(lwc.name .. "_insert")
    end
else -- This shouldn't ever happen
    error [[Unsupported format.

Please use LaTeX, Plain TeX, ConTeXt or OpTeX.]]
end

--[[ Select the fonts

     We want to use cmr7 for the draft mode cost displays, and the easiest
     way to do so is to just hardcode the font id's. This relies on some
     implementation details; however, it is very unlikely to ever be an issue
  ]]
local SMALL_FONT
if plain then
    SMALL_FONT = 4
elseif latex then
    SMALL_FONT = 7
elseif optex then
    SMALL_FONT = 7
elseif context then
    SMALL_FONT = 3
end

--[[ Table to hold the alternate paragraph versions.

     This is global(ish) mutable state, which isn't ideal, but any other way of
     passing this data around would be even worse.
  ]]
local paragraphs = {}
local inserts = {}

--[[ Function definitions
  ]]

--- Gets the current paragraph and page locations
--- @return string
local function get_location()
    return "At " .. pagenum() .. "/" .. #paragraphs
end


--- Prints the starting glyphs and glue of an `hlist`
---
--- @param head node
--- @return nil
local function get_chars(head)
    if not lwc.debug then return end

    local chars = ""
    for n in traverse(head) do
        if n.id == glyph_id then
            if n.char < 127 then -- Only ASCII
                chars = chars .. string_char(n.char)
            else
                chars = chars .. "#"  -- Replacement for an unknown glyph
            end
        elseif n.id == glue_id then
            chars = chars .. " " -- Any glue goes to a space
        end
        if #chars > 25 then
            break
        end
    end

    debug(get_location(), chars)
end


--- The "cost function" to use. Users can redefine this if they wish.
---
--- @param demerits number The demerits of the broken paragraph
--- @param lines number The number of lines in the broken paragraph
--- @return number The cost of the broken paragraph
function lwc.paragraph_cost(demerits, lines)
    return demerits / math.sqrt(lines)
end


--- Checks if the ConTeXt "grid snapping" is active
---
--- @return boolean
local function grid_mode_enabled()
    -- Compare the token "mode" to see if `\\ifgridsnapping` is `\\iftrue`
    return token.create("ifgridsnapping").mode == iftrue.mode
end


--- Gets the next node of a specified type/subtype in a node list
---
--- @param head node The head of the node list
--- @param id number The node type
--- @param args table?
---     subtype: number = The node subtype
---     reverse: bool = Whether we should iterate backwards
--- @return node?
local function next_of_type(head, id, args)
    args = args or {}

    if lmtx or not args.reverse then
        for n, subtype in traverse_id(id, head, args.reverse) do
            if (subtype == args.subtype) or (args.subtype == nil) then
                return n
            end
        end
    else
        --[[ Only LMTX has the built-in backwards traverser, so we need to do it
             ourselves here.
          ]]
        while head do
            if head.id == id and
               (head.subtype == args.subtype or args.subtype == nil)
            then
                return head
            end
            head = head.prev
        end
    end

    -- Needed for the special `tex.lists` nodes
    if head and head.id == id and
       (head.subtype == args.subtype or args.subtype == nil)
    then
        return head
    end
end


--- Breaks a paragraph one line longer than natural
---
--- @param head node The unbroken paragraph
--- @return node long_node The broken paragraph
--- @return table long_info An info table about the broken paragraph
local function long_paragraph(head)
    -- We can't modify the original paragraph
    head = copy_list(head)

    if lmtx then
        tex.preparelinebreak(head)
    end

    -- Prevent ultra-short last lines (\TeX{}Book p. 104), except with narrow columns
    -- Equivalent to \\parfillskip=0pt plus 0.8\\hsize
    local parfillskip = last(head)

    if tex.hsize > min_col_width then
        parfillskip[stretch_order] = 0
        parfillskip.stretch = 0.8 * tex.hsize -- Last line must be at least 20% long
    end

    -- Break the paragraph 1 line longer than natural
    return linebreak(head, {
        looseness = 1,
        emergencystretch = tex_dimen[emergencystretch],
    })
end


--- Breaks a paragraph at its natural length
---
--- @param head node The unbroken paragraph
--- @return table natural_info An info table about the broken paragraph
local function natural_paragraph(head)
    -- We can't modify the original paragraph
    head = copy_list(head)

    if lmtx then
        tex.preparelinebreak(head)
    end

    -- Break the paragraph naturally to get \\prevgraf
    local natural_node, natural_info = linebreak(head)
    free_list(natural_node)

    return natural_info
end


lwc.draft_mode = false
--- Changes the text colour in a node list if draft mode is active
---
--- @param head node The first node to colour
--- @param colour table<number> A 3-tuple of RGB values
--- @return node head The coloured node
local function colour_list(head, colour)
    if not lwc.draft_mode then
        return head
    end

    -- Adapted from https://tex.stackexchange.com/a/372437
    -- \\pdfextension colorstack is ignored in LMTX
    local start_colour = new_node("whatsit", "pdf_colorstack")
    start_colour.stack = 0
    start_colour.command = 1
    start_colour.data = string.format("%.2f %.2f %.2f rg", table.unpack(colour))

    local end_colour = new_node("whatsit", "pdf_colorstack")
    end_colour.stack = 0
    end_colour.command = 2

    start_colour.next = head
    last(head).next = end_colour

    return start_colour
end


--- Generate an \\llap'ed box containing the provided string
---
--- @param str string The string to typeset
--- @return node head The box node
local function llap_string(str)
    local first = new_node("glue")
    first.width = llap_offset

    local m = first
    for letter in str:gmatch(".")  do
        local n = new_node("glyph")
        n.font = SMALL_FONT
        n.char = string.byte(letter)

        m.next = n
        m = n
    end

    local hss = new_node("glue")
    hss.stretch = 1
    hss[stretch_order] = 1
    hss.shrink = 1
    hss[shrink_order] = 1
    m.next = hss

    return node.hpack(first, 0, "exactly")
end


--- Typesets the cost of a paragraph beside it in draft mode
---
--- @param paragraph node
--- @param cost number
--- @return nil
local function show_cost(paragraph, cost)
    if not lwc.draft_mode then
        return
    end

    local last_hlist_end = last(next_of_type(
        last(paragraph),
        hlist_id,
        { subtype = line_subid, reverse = true }
    ).list)

    local cost_str
    if cost < math.maxinteger then
        cost_str = string.format("%.0f", cost)
    else
        cost_str = "infinite"
    end

    last_hlist_end.next = llap_string(cost_str)
end


--- Saves each paragraph, but lengthened by 1 line
---
--- Called by the `pre_linebreak_filter` callback
---
--- @param head node
--- @return node
function lwc.save_paragraphs(head)
    if (head.id ~= par_id and context) or -- Ensure that we were actually given a par
        status.output_active or -- Don't run during the output routine
        tex.nest.ptr > 1 -- Don't run inside boxes
    then
        return head
    end

    -- Prevent the "underfull hbox" warnings when we store a potential paragraph
    local renable_box_warnings
    if (context or optex) or
       #luatexbase.callback_descriptions("hpack_quality") == 0
    then -- See #18 and michal-h21/linebreaker#3
        renable_box_warnings = true
        lwc.callbacks.disable_box_warnings.enable()
    end

    long_node, long_info = long_paragraph(head)

    natural_info = natural_paragraph(head)

    if renable_box_warnings then
        lwc.callbacks.disable_box_warnings.disable()
    end

    if not grid_mode_enabled() then
        -- Offset the \\prevdepth differences between natural and long
        local prevdepth = new_node("glue")
        prevdepth.width = natural_info.prevdepth - long_info.prevdepth
        last(long_node).next = prevdepth
    end

    local long_cost = lwc.paragraph_cost(long_info.demerits, long_info.prevgraf)

    if long_info.prevgraf ~= natural_info.prevgraf + 1 or
       long_cost < 10 -- Any paragraph that is "free" to expand is suspicious
    then
        -- This paragraph is infinitely bad
        long_cost = math.maxinteger
    end

    local saved_node = next_of_type(long_node, hlist_id, { subtype = line_subid })

    show_cost(saved_node, long_cost)
    for n in traverse_id(hlist_id, saved_node) do
        n.list = colour_list(n.list, lwc.colours.expanded)
    end

    table.insert(paragraphs, {
        cost = long_cost,
        node = copy_list(saved_node)
    })

    free_list(long_node)

    -- Print some debugging information
    get_chars(head)
    debug(get_location(), "nat  lines    " .. natural_info.prevgraf)
    debug(
        get_location(),
        "nat  cost " ..
        lwc.paragraph_cost(natural_info.demerits, natural_info.prevgraf)
    )
    debug(get_location(), "long lines    " .. long_info.prevgraf)
    debug(
        get_location(),
        "long cost " ..
        lwc.paragraph_cost(long_info.demerits, long_info.prevgraf)
    )

    -- \ConTeXt{} crashes if we return `true`
    return head
end


--- Tags the beginning and the end of each paragraph as it is added to the page.
---
--- We add an attribute to the first and last node of each paragraph. The ID is
--- some arbitrary number for \lwc/, and the value corresponds to the
--- paragraphs index, which is negated for the end of the paragraph.
---
--- @param head node
--- @return nil
local function mark_paragraphs(head)
    -- Tag the paragraphs
    if not status.output_active then -- Don't run during the output routine
        -- Get the start and end of the paragraph
        local top = next_of_type(head, hlist_id, { subtype = line_subid })
        local bottom = last(head)

        while bottom.id == insert_id do
            bottom = bottom.prev
        end

        if top ~= bottom then
            set_attribute(
                top,
                paragraph_attribute,
                #paragraphs + (PAGE_MULTIPLE * pagenum())
            )
            set_attribute(
                bottom,
                paragraph_attribute,
                -1 * (#paragraphs + (PAGE_MULTIPLE * pagenum()))
            )
        else
            -- We need a special tag for a 1-line paragraph since the node can only
            -- have a single attribute value
            set_attribute(
                top,
                paragraph_attribute,
                #paragraphs + (PAGE_MULTIPLE * pagenum()) + SINGLE_LINE
            )
        end

        if #paragraphs > 0 then
            show_cost(head, paragraphs[#paragraphs].cost)
        end
    end
end


--- Tags the each line with the indices of any corresponding inserts.
---
--- We need to tag the first element of the hlist before the any insert nodes
--- since the insert nodes are removed before `pre_output_filter` gets called.
---
--- @param head node
--- @return nil
local function mark_inserts(head)
    local insert_indices = {}
    for insert in traverse_id(insert_id, head) do
        -- Save the found insert nodes for later
        inserts[#inserts+1] = copy(insert)

        -- Tag the insert's content so that we can find it later
        set_attribute(insert.list, insert_attribute, #inserts)

        for n in traverse(insert.list.next) do
            set_attribute(n, insert_attribute, -1 * #inserts)
        end

        --[[ Each hlist/line can have multiple inserts, but so we can't just tag
             the hlist as we go. Instead, we need save up all of their indices,
             then tag the hlist with the first and last indices.
          ]]
        insert_indices[#insert_indices+1] = #inserts

        if not insert.next or
           insert.next.id ~= insert_id
        then
            local hlist_before = next_of_type(insert, hlist_id, { reverse = true} )

            local insert_class
            if lmtx then
                -- FIXME: temporarily hardcode the main "footnote" class
                insert_class = 4 -- insert.index
            else
                insert_class = insert.subtype
            end

            --[[ We tag the first element of the hlist/line with an integer
                 that holds the insert class and the first and last indices
                 of the inserts contained in the line. This won't work if
                 the line has multiple classes of inserts, but I don't think
                 that happens in real-world documents.
              ]]
            set_attribute(
                hlist_before.list,
                insert_attribute,
                insert_class      * INSERT_CLASS_MULTIPLE +
                insert_indices[1] * INSERT_FIRST_MULTIPLE +
                insert_indices[#insert_indices]
            )

            -- Clear the indices to prepare for the next line
            insert_indices = {}
        end
    end
end


--- Saves the inserts and tags a typeset paragraph. Called by the
--- `post_linebreak_filter` callback.
---
--- @param head node
--- @return node
function lwc.mark_paragraphs(head)
    mark_paragraphs(head)
    mark_inserts(head)

    return head
end


--- Checks to see if a penalty matches the widow/orphan/broken penalties
---
--- @param penalty number
--- @return boolean
function is_matching_penalty(penalty)
    local widowpenalty = tex.widowpenalty
    local clubpenalty = tex.clubpenalty
    local displaywidowpenalty = tex.displaywidowpenalty
    local brokenpenalty = tex.brokenpenalty

    penalty = penalty - tex.interlinepenalty

    -- https://tug.org/TUGboat/tb39-3/tb123mitt-widows-code.pdf#subsection.0.2.1
    return penalty ~= 0 and
           penalty <  INFINITY and (
               penalty == widowpenalty or
               penalty == displaywidowpenalty or
               penalty == clubpenalty or
               penalty == clubpenalty + widowpenalty or
               penalty == clubpenalty + displaywidowpenalty or
               penalty == brokenpenalty or
               penalty == brokenpenalty + widowpenalty or
               penalty == brokenpenalty + displaywidowpenalty or
               penalty == brokenpenalty + clubpenalty or
               penalty == brokenpenalty + clubpenalty + widowpenalty or
               penalty == brokenpenalty + clubpenalty + displaywidowpenalty
           )
end


--- Reset any state saved between pages
---
--- @return nil
local function reset_state()
    for _, paragraph in ipairs(paragraphs) do
        free_list(paragraph.node)
    end
    paragraphs = {}

    for _, insert in ipairs(inserts) do
        free(insert)
    end
    inserts = {}
end


--- When we are unable to remove a widow/orphan, print a warning
---
--- @return nil
local function remove_widows_fail()
    warning("Widow/Orphan/broken hyphen NOT removed on page " .. pagenum())

    local last_line = next_of_type(
        last(tex_lists.page_head),
        hlist_id,
        { subtype = line_subid, reverse = true }
    )
    if last_line then
        last_line.list = colour_list(last_line.list, lwc.colours.failure)
    end

    local next_first_line = next_of_type(
        tex_lists[contrib_head],
        hlist_id,
        { subtype = line_subid }
    )
    if next_first_line then
        next_first_line.list = colour_list(next_first_line.list, lwc.colours.failure)
    end

    reset_state()
end


--- Finds the first and last paragraphs present on a page
---
--- @param head node The node representing the start of the page
--- @return number first_index The index of the first paragraph on the page in
---                            the `paragraphs` table
--- @return number last_index The index of the last paragraph on the page in the
---                           `paragraphs` table
local function first_last_paragraphs(head)
    local first_index, last_index

    -- Find the last paragraph on the page, starting at the end, heading in reverse
    local n = last(head)
    while n do
        local value = get_attribute(n, paragraph_attribute)
        if value then
            last_index = value % PAGE_MULTIPLE
            break
        end

        n = n.prev
    end

    -- Find the first paragraph on the page, from the top
    local first_val, first_head = find_attribute(head, paragraph_attribute)
    while abs(first_val) // PAGE_MULTIPLE == pagenum() - 1 do
        --[[ If the first complete paragraph on the page was initially broken on the
             previous page, then we can't expand it here so we need to skip it.
          ]]
        first_val, first_head = find_attribute(
            first_head.next,
            paragraph_attribute
        )
    end

    first_index = first_val % PAGE_MULTIPLE

    if first_index >= SINGLE_LINE then
        first_index = first_index - SINGLE_LINE
    end

    debug("first/last", first_index .. "/" .. last_index)

    return first_index, last_index
end


--- Selects the "best" paragraph on the page to expand
---
--- @param head node The node representing the start of the page
--- @return number? best_index The index of the paragraph to expand in the
---                           `paragraphs` table
local function best_paragraph(head)
    local first_paragraph_index, last_paragraph_index = first_last_paragraphs(head)

    -- Find the paragraph on the page with the least cost.
    local best_index = 1
    local best_cost = paragraphs[best_index].cost

    -- We find the current "best" replacement
    for index, paragraph in pairs(paragraphs) do
        if paragraph.cost < best_cost and
           index <  last_paragraph_index and
           index >= first_paragraph_index
        then
            best_index, best_cost = index, paragraph.cost
        end
    end

    debug(
        "selected para",
        pagenum() .. "/" .. best_index .. " (" .. best_cost .. ")"
    )

    if best_cost  >  tex_count[max_cost] or
       best_index == last_paragraph_index or
       best_cost  == math.maxinteger
    then
        return nil
    else
        return best_index
    end
end


--- Gets any inserts present in the moved line
---
--- @param last_line node The moved last line
--- @return table<node> inserts A list of the present inserts
local function get_inserts(last_line)
    local selected_inserts = {}

    local n = last_line.list
    while n do -- Iterate through the last line
        local line_value
        line_value, n = find_attribute(n, insert_attribute)

        if not n then
            break
        end

        -- Demux the insert values
        local class = line_value // INSERT_CLASS_MULTIPLE
        local first_index = (line_value % INSERT_CLASS_MULTIPLE) // INSERT_FIRST_MULTIPLE
        local last_index = line_value % INSERT_FIRST_MULTIPLE

        -- Get the output box containing the insert boxes
        local insert_box

        if lmtx then
            insert_box = tex.getinsertcontent(class)
        else
            insert_box = tex_box[class]
        end

        -- Get any portions of the insert held over until the next page
        local split_insert = next_of_type(
            tex_lists[hold_head],
            insert_id,
            { subtype = class }
        )

        for i, insert in ipairs { insert_box, split_insert } do
            local m = insert and insert.list

            while m do -- Iterate through the insert box
                local box_value
                box_value, m = find_attribute(m, insert_attribute)

                if not m then
                    break
                end

                if abs(box_value) >= first_index and
                   abs(box_value) <= last_index
                then
                    -- Remove the respective contents from the insert box
                    insert.list = remove(insert.list, m)

                    if box_value > 0 and i == 1 then
                        table.insert(selected_inserts, copy(inserts[box_value]))
                    end

                    m = free(m)
                else
                    m = m.next
                end
            end
        end

        if not insert_box.list then
            tex_box[class] = nil
        end

        if split_insert and not split_insert.list then
            remove(tex_lists[hold_head], split_insert)
        end

        n = n.next
    end

    if #selected_inserts ~= 0 then
        info("Moving footnotes on page " .. pagenum())
    end

    return selected_inserts
end


lwc.nobreak_behaviour = "keep"
--- Moves the last line of the page onto the following page.
---
--- This is the most complicated function of the module since it needs to
--- look back to see if there is a heading preceding the last line, then it does
--- some low-level node shuffling.
---
--- @param head node The node representing the start of the page
--- @return boolean success
local function move_last_line(head)
    -- Start of final paragraph
    debug("remove_widows", "moving last line")

    -- Here we check to see if the widow/orphan was preceded by a large penalty
    local big_penalty_found, last_line, hlist_head
    local n = last(head).prev
    while n do
        if n.id == glue_id then
            -- Ignore any glue nodes
        elseif n.id == penalty_id and n.penalty >= INFINITY then
            -- Infinite break penalty
            big_penalty_found = true
        elseif big_penalty_found and n.id == hlist_id then
            -- Line before the penalty
            if lwc.nobreak_behaviour == "keep" then
                hlist_head = n
                big_penalty_found = false
            elseif lwc.nobreak_behaviour == "split" then
                n = last(head)
                break
            elseif lwc.nobreak_behaviour == "warn" then
                debug("last line", "heading found")
                return false
            end
        else
            -- Not found
            if hlist_head then
                n = hlist_head
            else
                n = last(head)
            end
            break
        end
        n = n.prev
    end

    local potential_penalty = n.prev.prev

    if potential_penalty and
       potential_penalty.id      == penalty_id and
       potential_penalty.subtype == linebreakpenalty_subid and
       is_matching_penalty(potential_penalty.penalty)
    then
        warning("Making a new widow/orphan/broken hyphen on page " .. pagenum())

        local second_last_line = next_of_type(
            potential_penalty,
            hlist_id,
            { subtype = line_subid, reverse = true }
        )
        second_last_line.list = colour_list(second_last_line.list, lwc.colours.failure)
    end

    last_line = copy_list(n)

    last_line.list = colour_list(last_line.list, lwc.colours.moved)

    -- Reinsert any inserts originally present in this moved line
    local selected_inserts = get_inserts(last_line)
    for _, insert in ipairs(selected_inserts) do
        last(last_line).next = insert
    end

    -- Add back in the content from the next page
    last(last_line).next = copy_list(tex_lists[contrib_head])

    free_list(n.prev.prev.next)
    n.prev.prev.next = nil

    -- Set the content of the next page
    free_list(tex_lists[contrib_head])
    tex_lists[contrib_head] = last_line

    return true
end


--- Replace the chosen paragraph with its expanded version.
---
--- This is the "core function" of the module since it is what ultimately causes
--- the expansion to occur.
---
--- @param head node
--- @param paragraph_index number
local function replace_paragraph(head, paragraph_index)
    local target_node = copy_list(paragraphs[paragraph_index].node)

    local start_found = false
    local end_found = false
    local free_nodes_begin

    -- Loop through all of the nodes on the page with the lwc attribute
    local n = head
    while n do
        local value
        value, n = find_attribute(n, paragraph_attribute)

        if not n then
            break
        end

        debug("remove_widows", "found " .. value)

        -- Insert the start of the replacement paragraph
        if value == paragraph_index + (PAGE_MULTIPLE * pagenum()) or
           value == paragraph_index + (PAGE_MULTIPLE * pagenum()) + SINGLE_LINE
        then
            debug("remove_widows", "replacement start")
            start_found = true

            -- Fix the `\\baselineskip` glue between paragraphs
            height_difference = (
                next_of_type(n, hlist_id, { subtype = line_subid }).height -
                next_of_type(target_node, hlist_id, { subtype = line_subid }).height
            )

            local prev_bls = next_of_type(
                n,
                glue_id,
                { subtype = baselineskip_subid, reverse = true }
            )

            if prev_bls then
                prev_bls.width = prev_bls.width + height_difference
            end

            n.prev.next = target_node
            free_nodes_begin = n
        end

        -- Insert the end of the replacement paragraph
        if value == -1 * (paragraph_index + (PAGE_MULTIPLE * pagenum())) or
           value ==       paragraph_index + (PAGE_MULTIPLE * pagenum()) + SINGLE_LINE
        then
            debug("remove_widows", "replacement end")
            end_found = true

            local target_node_last = last(target_node)

            if grid_mode_enabled() then
                -- Account for the difference in depth
                local after_glue = new_node("glue")
                after_glue.width = n.depth - target_node_last.depth
                target_node_last.next = after_glue

                after_glue.next = n.next
            else
                target_node_last.next = n.next
            end

            n.next = nil

            break
        end

        n = n.next
    end

    if start_found and end_found then
        free_list(free_nodes_begin)
    else
        warning("Paragraph NOT expanded on page " .. pagenum())
    end
end


--- Remove the widows and orphans from the page, just after the output routine.
---
--- This is called just after the end of the output routine, before the page is
--- shipped out. If the output penalty indicates that the page was broken at a
--- widow or an orphan, we replace one paragraph with the same paragraph, but
--- lengthened by one line. Then, we can push the bottom line of the page to the
--- next page.
---
--- @param head node
--- @return node
function lwc.remove_widows(head)
    debug("outputpenalty", tex.outputpenalty .. " " .. #paragraphs)

    -- See if there is a widow/orphan for us to remove
    if not is_matching_penalty(tex.outputpenalty) then
        reset_state()
        return head
    end

    info("Widow/orphan/broken hyphen detected. Attempting to remove")

    -- Nothing that we can do if there aren't any paragraphs available to expand
    if #paragraphs == 0 then
        debug("failure", "no paragraphs to expand")
        remove_widows_fail()
        return head
    end

    -- Check the original height of \\box255
    local vsize = tex_dimen.vsize
    local orig_vpack = vpack(head)
    local orig_height_diff = orig_vpack.height - vsize
    orig_vpack.list = nil
    free(orig_vpack)

    -- Find the paragraph to expand
    local paragraph_index = best_paragraph(head)

    if not paragraph_index then
        debug("failure", "no good paragraph")
        remove_widows_fail()
        return head
    end

    -- Move the last line of the page to the next page
    if not move_last_line(head) then
        debug("failure", "can't move last line")
        remove_widows_fail()
        return head
    end

    -- Replace the chosen paragraph with its expanded version
    replace_paragraph(head, paragraph_index)

    --[[ The final \\box255 needs to be exactly \\vsize tall to avoid
         over/underfull box warnings, so we correct any discrepancies
         here.
      ]]
    local new_vpack = vpack(head)
    local new_height_diff = new_vpack.height - vsize
    new_vpack.list = nil
    free(new_vpack)
    -- We need the original height discrepancy in case there are \\vfill's
    local net_height_diff = orig_height_diff - new_height_diff
    local bls = tex.skip.baselineskip
    local bls_width = bls.width
    free(bls)

    if abs(net_height_diff) > 0 and
       -- A difference larger than 0.25\\baselineskip is probably not from \lwc/
       abs(net_height_diff) < bls_width / 4
    then
        local bottom_glue = new_node("glue")
        bottom_glue.width = net_height_diff
        last(head).next = bottom_glue
    end

    info(
        "Widow/orphan/broken hyphen successfully removed at paragraph "
        .. paragraph_index
        .. " on page "
        .. pagenum()
    )

    reset_state()

    return head
end


--- Create a table of functions to enable or disable a given callback
---
--- @param t table Parameters of the callback to create
---     callback: string = The \LuaTeX{} callback name
---     func: function = The function to call
---     name: string = The name/ID of the callback
---     category: string = The category for a \ConTeXt{} "Action"
---     position: string = The "position" for a \ConTeXt{} "Action"
---     lowlevel: boolean = If we should use a lowlevel \LuaTeX{} callback instead of a
---                      \ConTeXt{} "Action"
--- @return table t Enablers/Disablers for the callback
---     enable: function = Enable the callback
---     disable: function = Disable the callback
local function register_callback(t)
    if plain or latex then -- Both use \LuaTeX{}Base for callbacks
        return {
            enable = function()
                luatexbase.add_to_callback(t.callback, t.func, t.name)
            end,
            disable = function()
                luatexbase.remove_from_callback(t.callback, t.name)
            end,
        }
    elseif context and not t.lowlevel then
        return {
            --[[ Register the callback when the table is created,
                 but activate it when `enable()` is called.
              ]]
            enable = nodes.tasks.appendaction(t.category, t.position, "lwc." .. t.name)
                or function()
                    nodes.tasks.enableaction(t.category, "lwc." .. t.name)
                end,
            disable = function()
                nodes.tasks.disableaction(t.category, "lwc." .. t.name)
            end,
        }
    elseif context and t.lowlevel then
        --[[ Some of the callbacks in \ConTeXt{} have no associated "actions". Unlike
             with \LuaTeX{}base, \ConTeXt{} leaves some \LuaTeX{} callbacks unregistered
             and unfrozen. Because of this, we need to register some callbacks at the
             engine level. This is fragile though, because a future \ConTeXt{} update
             may decide to register one of these functions, in which case
             \lwc/ will crash with a cryptic error message.
          ]]
        return {
            enable = function() callback.register(t.callback, t.func) end,
            disable = function() callback.register(t.callback, nil) end,
        }
    elseif optex then -- Op\TeX{} is very similar to luatexbase
        return {
            enable = function()
                callback.add_to_callback(t.callback, t.func, t.name)
            end,
            disable = function()
                callback.remove_from_callback(t.callback, t.name)
            end,
        }
    end
end


-- Add all of the callbacks
lwc.callbacks = {
    disable_box_warnings = register_callback({
        callback = "hpack_quality",
        func     = function() end,
        name     = "disable_box_warnings",
        lowlevel = true,
    }),
    remove_widows = register_callback({
        callback = "pre_output_filter",
        func     = lwc.remove_widows,
        name     = "remove_widows",
        lowlevel = true,
    }),
    save_paragraphs = register_callback({
        callback = "pre_linebreak_filter",
        func     = lwc.save_paragraphs,
        name     = "save_paragraphs",
        category = "processors",
        position = "after",
    }),
    mark_paragraphs = register_callback({
        callback = "post_linebreak_filter",
        func     = lwc.mark_paragraphs,
        name     = "mark_paragraphs",
        category = "finalizers",
        position = "after",
    }),
}


local lwc_enabled = false
--- Enables the paragraph callbacks
function lwc.enable_callbacks()
    debug("callbacks", "enabling")
    if not lwc_enabled then
        lwc.callbacks.save_paragraphs.enable()
        lwc.callbacks.mark_paragraphs.enable()

        lwc_enabled = true
    else
        info("Already enabled")
    end
end


--- Disables the paragraph callbacks
function lwc.disable_callbacks()
    debug("callbacks", "disabling")
    if lwc_enabled then
        lwc.callbacks.save_paragraphs.disable()
        lwc.callbacks.mark_paragraphs.disable()
        --[[ We do \emph{not} disable `remove_widows` callback, since we still want
             to expand any of the previously-saved paragraphs if we hit an orphan
             or a widow.
          ]]

        lwc_enabled = false
    else
        info("Already disabled")
    end
end

function lwc.if_lwc_enabled()
    debug("iflwc")
    if lwc_enabled then
        insert_token(iftrue)
    else
        insert_token(iffalse)
    end
end


--- Mangles a macro name so that it's suitable for a specific format
---
--- @param name string The plain name
--- @param args table<string> The TeX types of the function arguments
--- @return string name The mangled name
local function mangle_name(name, args)
    if plain then
        return "lwc@" .. name:gsub("_", "@")
    elseif optex then
        return "_lwc_" .. name
    elseif context then
        return "lwc_" .. name
    elseif latex then
        return "__lwc_" .. name .. ":" .. string_rep("n", #args)
    end
end


--- Creates a TeX command that evaluates a Lua function
---
--- @param name string The name of the csname to define
--- @param func function
--- @param args table<string> The TeX types of the function arguments
--- @return nil
local function register_tex_cmd(name, func, args)
    local scanning_func
    name = mangle_name(name, args)

    if not context then
        local scanners = {}
        for _, arg in ipairs(args) do
            scanners[#scanners+1] = token['scan_' .. arg]
        end

        scanning_func = function()
            local values = {}
            for _, scanner in ipairs(scanners) do
                values[#values+1] = scanner()
            end

            func(table.unpack(values))
        end
    end

    if optex then
        define_lua_command(name, scanning_func)
        return
    elseif plain or latex then
        local index = luatexbase.new_luafunction(name)
        lua.get_functions_table()[index] = scanning_func
        token.set_lua(name, index)
    elseif context then
        interfaces.implement {
            name = name,
            public = true,
            arguments = args,
            actions = func
        }
    end
end


register_tex_cmd("if_enabled", lwc.if_lwc_enabled, {})
register_tex_cmd("enable", lwc.enable_callbacks, {})
register_tex_cmd("disable", lwc.disable_callbacks, {})
register_tex_cmd(
    "nobreak",
    function(str)
        lwc.nobreak_behaviour = str
    end,
    { "string" }
)
register_tex_cmd(
    "debug",
    function(str)
        lwc.debug = str ~= "0" and str ~= "false" and str ~= "stop"
    end,
    { "string" }
)
register_tex_cmd(
    "draft",
    function(str)
        lwc.draft_mode = str ~= "0" and str ~= "false" and str ~= "stop"
    end,
    { "string" }
)

--- Silence the luatexbase "Enabling/Removing <callback>" info messages
---
--- Every time that a paragraph is typeset, \lwc/ hooks in
--- and typesets the paragraph 1 line longer. Some of these longer paragraphs
--- will have pretty bad badness values, so TeX will issue an over/underfull
--- hbox warning. To block these warnings, we hook into the `hpack_quality`
--- callback and disable it so that no warning is generated.
---
--- However, each time that we enable/disable the null `hpack_quality` callback,
--- luatexbase puts an info message in the log. This completely fills the log file
--- with useless error messages, so we disable it here.
---
--- This uses the Lua `debug` library to internally modify the log upvalue in the
--- `add_to_callback` function. This is almost certainly a terrible idea, but I don't
--- know of a better way.
---
--- @return nil
local function silence_luatexbase()
    local nups = debug_lib.getinfo(luatexbase.add_to_callback).nups

    for i = 1, nups do
        local name, func = debug_lib.getupvalue(luatexbase.add_to_callback, i)
        if name == "luatexbase_log" then
            debug_lib.setupvalue(
                luatexbase.add_to_callback,
                i,
                function(text)
                    if text:match("^Inserting") or text:match("^Removing") then
                        return
                    else
                        func(text)
                    end
                end
            )
            return
        end
    end
end


--[[ Call `silence_luatexbase` in Plain and LaTeX, unless the undocmented global
     `LWC_NO_DEBUG` is set. We provide this opt-out in case something goes awry
     with the `debug` library calls.
  ]]
if (plain or latex) and
   not LWC_NO_DEBUG --- @diagnostic disable-line
then
    silence_luatexbase()
end

-- Activate \lwc/
lwc.callbacks.remove_widows.enable()

return lwc