--------------------------------------------------------------------------------
-- Format link
--
-- Makes a wikilink from the given link and display values. Links are escaped
-- with colons if necessary, and links to sections are detected and displayed
-- with " § " as a separator rather than the standard MediaWiki "#". Used in
-- the ((format link)) template.
--------------------------------------------------------------------------------
local libraryUtil = require('libraryUtil')
local checkType = libraryUtil.checkType
local checkTypeForNamedArg = libraryUtil.checkTypeForNamedArg
local mArguments -- lazily initialise [[Module:Arguments]]
local mError -- lazily initialise [[Module:Error]]
local yesno -- lazily initialise [[Module:Yesno]]

local p = {}

--------------------------------------------------------------------------------
-- Helper functions
--------------------------------------------------------------------------------

local function getArgs(frame)
	-- Fetches the arguments from the parent frame. Whitespace is trimmed and
	-- blanks are removed.
	mArguments = require('Module:Arguments')
	return mArguments.getArgs(frame, {parentOnly = true})
end

local function removeInitialColon(s)
	-- Removes the initial colon from a string, if present.
	return s:match('^:?(.*)')
end

local function maybeItalicize(s, shouldItalicize)
	-- Italicize s if s is a string and the shouldItalicize parameter is true.
	if s and shouldItalicize then
		return '<i>' .. s .. '</i>'
	else
		return s
	end
end

local function parseLink(link)
	-- Parse a link and return a table with the link's components.
	-- These components are:
	-- - link: the link, stripped of any initial colon (always present)
	-- - page: the page name (always present)
	-- - section: the page name (may be nil)
	-- - display: the display text, if manually entered after a pipe (may be nil)
	link = removeInitialColon(link)

	-- Find whether a faux display value has been added with the ((!)) magic
	-- word.
	local prePipe, display = link:match('^(.-)|(.*)$')
	link = prePipe or link

	-- Find the page, if it exists.
	-- For links like [[#Bar]], the page will be nil.
	local preHash, postHash = link:match('^(.-)#(.*)$')
	local page
	if not preHash then
		-- We have a link like [[Foo]].
		page = link
	elseif preHash ~= '' then
		-- We have a link like [[Foo#Bar]].
		page = preHash
	end

	-- Find the section, if it exists.
	local section
	if postHash and postHash ~= '' then
		section = postHash
	end
	
	return {
		link = link,
		page = page,
		section = section,
		display = display,
	}
end

local function formatDisplay(parsed, options)
	-- Formats a display string based on a parsed link table (matching the
	-- output of parseLink) and an options table (matching the input options for
	-- _formatLink).
	local page = maybeItalicize(parsed.page, options.italicizePage)
	local section = maybeItalicize(parsed.section, options.italicizeSection)
	if (not section) then
		return page
	elseif (not page) then
		return mw.ustring.format('§&nbsp;%s', section)
	else
		return mw.ustring.format('%s §&nbsp;%s', page, section)
	end
end

local function missingArgError(target)
	mError = require('Module:Error')
	return mError.error{message =
		'Error: no link or target specified! ([[' .. target .. '#Errors|help]])'
	}
end

--------------------------------------------------------------------------------
-- Main functions
--------------------------------------------------------------------------------

function p.formatLink(frame)
	-- The formatLink export function, for use in templates.
	yesno = require('Module:Yesno')
	local args = getArgs(frame)
	local link = args[1] or args.link
	local target = args[3] or args.target
	if not (link or target) then
		return missingArgError('Template:Format link')
	end

	return p._formatLink{
		link = link,
		display = args[2] or args.display,
		target = target,
		italicizePage = yesno(args.italicizepage),
		italicizeSection = yesno(args.italicizesection),
		categorizeMissing = args.categorizemissing
	}
end

function p._formatLink(options)
	-- The formatLink export function, for use in modules.
	checkType('_formatLink', 1, options, 'table')
	local function check(key, expectedType) --for brevity
		checkTypeForNamedArg(
			'_formatLink', key, options[key], expectedType or 'string', true
		)
	end
	check('link')
	check('display')
	check('target')
	check('italicizePage', 'boolean')
	check('italicizeSection', 'boolean')
	check('categorizeMissing')

	-- Normalize link and target and check that at least one is present
	if options.link == '' then options.link = nil end
	if options.target == '' then options.target = nil end
	if not (options.link or options.target) then
		return missingArgError('Module:Format link')
	end

	local parsed = parseLink(options.link)
	local display = options.display or parsed.display
	local catMissing = options.categorizeMissing
	local category = ''

	-- Find the display text
	if not display then display = formatDisplay(parsed, options) end

	-- Handle the target option if present
	if options.target then
		local parsedTarget = parseLink(options.target)
		parsed.link = parsedTarget.link
		parsed.page = parsedTarget.page
	end

	-- Test if page exists if a diagnostic category is specified
	if catMissing and (mw.ustring.len(catMissing) > 0) then
		local title = nil
		if parsed.page then title = mw.title.new(parsed.page) end
		if title and (not title.isExternal) then
			local success, exists = pcall(function() return title.exists end)
			if success and not exists then
				category = mw.ustring.format('[[Category:%s]]', catMissing)
			end
		end
	end
	
	-- Format the result as a link
	if parsed.link == display then
		return mw.ustring.format('[[:%s]]%s', parsed.link, category)
	else
		return mw.ustring.format('[[:%s|%s]]%s', parsed.link, display, category)
	end
end

--------------------------------------------------------------------------------
-- Derived convenience functions
--------------------------------------------------------------------------------

function p.formatPages(options, pages)
	-- Formats an array of pages using formatLink and the given options table,
	-- and returns it as an array. Nil values are not allowed.
	local ret = {}
	for i, page in ipairs(pages) do
		ret[i] = p._formatLink{
			link = page,
			categorizeMissing = options.categorizeMissing,
			italicizePage = options.italicizePage,
			italicizeSection = options.italicizeSection
		}
	end
	return ret
end

return p