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--[[
lua-widow-control
https://github.com/gucci-on-fleek/lua-widow-control
SPDX-License-Identifier: MPL-2.0+
SPDX-FileCopyrightText: 2021 Max Chernoff
]]
lwc = {}
lwc.name = "lua-widow-control"
--[[
\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
if format:find('cont') then -- cont-en, cont-fr, cont-nl, ...
lwc.context = true
elseif format:find('latex') then -- lualatex, lualatex-dev, ...
lwc.latex = true
elseif format == 'luatex' then -- Plain
lwc.plain = true
end
--[[
Save some local copies of the node library to reduce table lookups.
This is probably a useless micro-optimization, but it can't hurt.
]]
local last = node.slide
local copy = node.copy_list
local par_id = node.id("par")
local glue_id = node.id("glue")
local set_attribute = node.set_attribute
local has_attribute = node.has_attribute
local flush_list = node.flush_list or node.flushlist
local free = node.free
local min_col_width = tex.sp("25em")
local maxdimen = 1073741823 -- \\maxdimen in sp
--[[
This error is raised in the following circumstances:
- When the user manually loads the Lua module without loading Lua\TeX{}Base
- When the package is used with an unsupported format
Both of these are pretty unlikely, but it can't hurt to check.
]]
assert(lwc.context or luatexbase, [[
This module requires a supported callback library. Please
follow the following format-dependant instructions:
- LaTeX: Use a version built after 2015-01-01, or include
`\usepackage{luatexbase}' before loading this module.
- Plain: Include `\input ltluatex' before loading this module.
- ConTeXt: Use the LMTX version.
]])
--[[
Package/module initialization
]]
if lwc.context then
lwc.warning = logs.reporter("module", lwc.name)
lwc.attribute = attributes.public(lwc.name)
lwc.contrib_head = 'contributehead' -- For \LuaMetaTeX{}
lwc.stretch_order = "stretchorder"
elseif lwc.plain or lwc.latex then
luatexbase.provides_module {
name = lwc.name,
date = "2021/12/14", --%%date
version = "1.1.2", --%%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.
]],
}
lwc.warning = function(str) luatexbase.module_warning(lwc.name, str) end
lwc.attribute = luatexbase.new_attribute(lwc.name)
lwc.contrib_head = 'contrib_head' -- For \LuaTeX{}
lwc.stretch_order = "stretch_order"
else -- uh oh
error [[
Unsupported format.
Please use (Lua)LaTeX, Plain (Lua)TeX, or ConTeXt (MKXL/LMTX)
]]
end
lwc.paragraphs = {} -- List to hold the alternate paragraph versions
if tex.interlinepenalty ~= 0 then
lwc.warning [[
\interlinepenalty is set to a non-zero value.
This may prevent lua-widow-control from
properly functioning.
]]
end
--[[
Function definitions
]]
--- Create a table of functions to enable or disable a given callback
--- @param t table Parameters of the callback to create
--- callback: str = The \LuaTeX{} callback name
--- func: function = The function to call
--- name: str = The name/ID of the callback
--- category: str = The category for a \ConTeXt{} "Action"
--- position: str = The "position" for a \ConTeXt{} "Action"
--- lowlevel: bool = 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
function lwc.register_callback(t)
if lwc.plain or lwc.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 lwc.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 lwc.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,
}
end
end
--- Saves each paragraph, but lengthened by 1 line
function lwc.save_paragraphs(head)
-- Prevent the "underfull hbox" warnings when we store a potential paragraph
lwc.callbacks.disable_box_warnings.enable()
-- Ensure that we were actually given a par (only under \ConTeXt{} for some reason)
if head.id ~= par_id and lwc.context then
return head
end
-- We need to return the unmodified head at the end, so we make a copy here
local new_head = copy(head)
-- Prevent ultra-short last lines (\TeX{}Book p. 104), except with narrow columns
local parfillskip = last(new_head)
if parfillskip.id == glue_id and tex.hsize > min_col_width then
parfillskip[lwc.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
local long_node, long_info = tex.linebreak(new_head, {
looseness = 1,
emergencystretch = tex.dimen.lwcemergencystretch,
})
-- Break the natural paragraph so we know how long it was
local natural_node, natural_info = tex.linebreak(copy(head))
flush_list(natural_node)
lwc.callbacks.disable_box_warnings.disable()
-- If we can't lengthen the paragraph, assign a \emph{very} large demerit value
local long_demerits
if long_info.prevgraf == natural_info.prevgraf then
long_demerits = maxdimen
else
long_demerits = long_info.demerits
end
-- Offset the accumulated \\prevdepth
local prevdepth = node.new("glue")
prevdepth.width = -1 * long_info.prevdepth
last(long_node).next = prevdepth
table.insert(lwc.paragraphs, {demerits = long_demerits, node = long_node})
-- \LuaMetaTeX{} 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.
function lwc.mark_paragraphs(head)
set_attribute(head, lwc.attribute, #lwc.paragraphs)
set_attribute(last(head), lwc.attribute, -1 * #lwc.paragraphs)
return head
end
--- A "safe" version of the last/slide function.
---
--- Sometimes the node list can form a loop. Since there is no last element
--- of a looped linked-list, the `last()` function will never terminate. This
--- function provides a "safe" version of the `last()` function that returns
--- `nil` if the list is circular.
local function safe_last(head)
local ids = {}
while head.next do
local id = node.is_node(head) -- Returns the internal node id
if ids[id] then
lwc.warning("Circular node list detected!")
return nil
end
ids[id] = true
head = head.next
end
return head
end
--- Remove the widows and orphans from the page, just after the output routine.
---
--- This function holds the "meat" of the module. It 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.
function lwc.remove_widows(head)
local penalty = tex.outputpenalty
local paragraphs = lwc.paragraphs
--[[
We only need to process pages that have orphans or widows. If `paragraphs`
is empty, then there is nothing that we can do.
]]
if penalty >= 10000 or
penalty <= -10000 or
penalty == 0 or
#paragraphs == 0 then
return head
end
local head_save = head -- Save the start of the `head` linked-list
--[[
Find the paragraph on the page with the minimum penalty.
This would be a 1-liner in Python or JavaScript, but Lua is pretty low-level,
so there's quite a bit of code here.
]]
local paragraph_index = 1
local minimum_demerits = paragraphs[paragraph_index].demerits
-- We find the current "best" replacement, then free the unused ones
for i, paragraph in pairs(paragraphs) do
if paragraph.demerits < minimum_demerits and i <= #paragraphs - 1 then
flush_list(paragraphs[paragraph_index].node)
paragraphs[paragraph_index].node = nil
paragraph_index, minimum_demerits = i, paragraph.demerits
elseif i > 1 then
-- Not sure why `i > 1` is required?
flush_list(paragraph.node)
paragraph.node = nil
end
end
local target_node = paragraphs[paragraph_index].node
local clear_flag = false
-- Loop through all of the nodes on the page
while head do
-- Insert the start of the replacement paragraph
if has_attribute(head, lwc.attribute, paragraph_index) then
local current = head
head.prev.next = target_node
-- Abort if we've created a loop
if not safe_last(target_node) then
head.prev.next = current
lwc.warning("Widow/Orphan NOT removed!")
break
end
clear_flag = true
end
-- Insert the end of the replacement paragraph
if has_attribute(head, lwc.attribute, -1 * paragraph_index) then
last(target_node).next = head.next
clear_flag = false
end
-- Start of final paragraph
if has_attribute(head, lwc.attribute, #paragraphs) then
local last_line = copy(last(head))
last(last_line).next = copy(tex.lists[lwc.contrib_head])
last(head).prev.prev.next = nil
-- Move the last line to the next page
tex.lists[lwc.contrib_head] = last_line
end
if clear_flag then
head = free(head)
else
head = head.next
end
end
lwc.paragraphs = {} -- Clear paragraphs array at the end of the page
return head_save
end
-- Add all of the callbacks
lwc.callbacks = {
disable_box_warnings = lwc.register_callback({
callback = "hpack_quality",
func = function() end,
name = "disable_box_warnings",
lowlevel = true,
}),
remove_widows = lwc.register_callback({
callback = "pre_output_filter",
func = lwc.remove_widows,
name = "remove_widows",
lowlevel = true,
}),
save_paragraphs = lwc.register_callback({
callback = "pre_linebreak_filter",
func = lwc.save_paragraphs,
name = "save_paragraphs",
category = "processors",
position = "after",
}),
mark_paragraphs = lwc.register_callback({
callback = "post_linebreak_filter",
func = lwc.mark_paragraphs,
name = "mark_paragraphs",
category = "finalizers",
position = "after",
}),
}
local enabled = false
function lwc.enable_callbacks()
if not enabled then
lwc.callbacks.remove_widows.enable()
lwc.callbacks.save_paragraphs.enable()
lwc.callbacks.mark_paragraphs.enable()
enabled = true
else
lwc.warning("Already enabled")
end
end
function lwc.disable_callbacks()
if 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.
]]
enabled = false
else
lwc.warning("Already disabled")
end
end
function lwc.if_lwc_enabled()
if enabled then
tex.sprint("\\iftrue")
else
tex.sprint("\\iffalse")
end
end
return lwc
|