# Status:
# [.] Inline HTML
# [x] Autoescape
# [ ] Block elements
#     [x] Paragraph
#     [x] Line break
#     [x] Headers (atx and setex)
#     [x] Blockquotes
#     [x] Lists
#     [ ] Code blocks
#     [x] Horizontal rules
# [ ] Span elements
#     [.] Links
#         [x] Inline
#         [ ] Reference
#     [x] Emphasis
#     [x] Code
#     [.] Images
#         [x] Inline
#         [ ] Reference
# [ ] Other
#     [ ] Autolinks
#     [ ] Escapes
#
# [ ] Non-standard features
#     [x] Strikethrough ~~...~~
#     [x] Superscript ^...^
#     [x] Subscript ~...~
#     [ ] Footnote [^...] (with user customizations)
#     [x] Task list - [ ] (readonly)
#     [.] Definition lists
#         [x] Single line
#         [ ] Multi-line
#     [ ] Fenced code blocks ```lang ```
#     [ ] Tables
# No character classes, since they aren't supported on every system

BEGIN {
	prev_blankline = blankline = 1
	skipprint = 1
	newStack("bq-level")
	newStack("list-level")
}

function println(line) {
	if (0 == line) line = $0
	print prev
	prev = line
}

function autoinsert(arrname, isbefore) {
	if (isbefore == "") isbefore = 0
	for (confstr in arrname) {
		split(confstr, conf, SUBSEP)

		## FILENAME regex, ".*" (default)
		if (2 in conf && FILENAME !~ conf[2]) { continue }
		## Text regex, ".*" (default)
		if (3 in conf && text !~ conf[3]) { continue }

		if (isbefore) prev = arrname[confstr] prev
		else          prev = prev arrname[confstr]

		## When, FIRST | ALL
		if (conf[1] == FIRST) { delete arrname[confstr] }
	}
}

{
	prev_blankline = blankline
	blankline = 1
}

/[^ \t]/ {
	blankline = 0
}

# Inline HTML
/^[ \t]*<\/?[0-9A-Za-z]+/ {
	println()
	next
}

# Link definition
/^\[.+\]:/ {
	blankline = 1
	next
}

# Auto-escape &
/&/ {
	ampstart = 0
	## gsub(/&(?![[:alnum:]#]+;)/, "&amp;")
	while (match(substr($0, ampstart + 1), /&/)) {
		ampstart += RSTART
		if (match(substr($0, ampstart), /&[0-9A-Za-z#]+;/)) { continue }

		$0 = substr($0, 1, ampstart - 1) "&amp;" substr($0, ampstart + 1)
	}
}
# Auto-escape <
/</ {
	gsub(/</, "\\&lt;")
}

# Blockquote Part 1/2
/^>[ \t]*$/ { next }
/^> / {
	level = 0
	while (match($0, /^> [ \t]*/)) {
		$0 = substr($0, RLENGTH + 1)
		level += RSTART
		while (topStack("bq-level") > level) {
			println("</blockquote>")
			autoinsert(user_BLOCKQUOTE_AFTER)

			popStack("bq-level")
		}
		if (topStack("bq-level") < level) {
			autoinsert(user_BLOCKQUOTE_BEFORE, 1)
			println("<blockquote>")

			pushStack("bq-level", level)
		}
	}
}

# atx-style headers
/^[ \t]*#+/ {
	match($0, /#+/)
	if (RLENGTH <= 6) {
		## Remove prefix/suffix
		gsub(/^[ \t]*#+[ \t]*/, "")
		gsub(/[ \t]*#+[ \t]*$/, "")

		     if (RLENGTH == 1) autoinsert(user_H1_BEFORE, 1)
		else if (RLENGTH == 2) autoinsert(user_H2_BEFORE, 1)
		else if (RLENGTH == 3) autoinsert(user_H3_BEFORE, 1)
		else if (RLENGTH == 4) autoinsert(user_H4_BEFORE, 1)
		else if (RLENGTH == 5) autoinsert(user_H5_BEFORE, 1)
		else if (RLENGTH == 6) autoinsert(user_H6_BEFORE, 1)
		autoinsert(user_H_BEFORE, 1)

		println("<h" RLENGTH ">" $0 "</h" RLENGTH ">")

		autoinsert(user_H_AFTER)
		     if (RLENGTH == 1) autoinsert(user_H1_AFTER)
		else if (RLENGTH == 2) autoinsert(user_H2_AFTER)
		else if (RLENGTH == 3) autoinsert(user_H3_AFTER)
		else if (RLENGTH == 4) autoinsert(user_H4_AFTER)
		else if (RLENGTH == 5) autoinsert(user_H5_AFTER)
		else if (RLENGTH == 6) autoinsert(user_H6_AFTER)

		blankline = 1
		next
	}
}
# setex-style headers
!prev_blankline && /^[ \t]*(=|-)+[ \t]*$/ {
	sub(/^<p>/, "", prev) ## from paragraph logic
	hlevel = 1 + ($0 ~ /-/)

	prev =  "<h" hlevel ">" prev "</h" hlevel ">"

	if (hlevel == 1) autoinsert(user_H1_BEFORE, 1)
	else             autoinsert(user_H2_BEFORE, 1)
	autoinsert(user_H_BEFORE, 1)

	autoinsert(user_H_AFTER)
	if (hlevel == 1) autoinsert(user_H1_AFTER)
	else             autoinsert(user_H2_AFTER)

	blankline = 1
	next
}

# Horizontal rules
/^(-|\*|_) ?(-|\*|_) ?(-|\*|_|[ \t])+$/ {
	autoinsert(user_HR_BEFORE, 1)
	println("<hr>")
	autoinsert(user_HR_AFTER)
	blankline = 1
	next
}

# Line break
/  $/ {
	sub(/[ \t]*  $/, "</br>")
}


# Code blocks
/^(    |\t)[ \t]*/ {
}

# Check boxes
/^[ \t]*- \[[ x]\][ \t]/ {
	match($0, /- \[[ x]\][ \t]/)
	if (substr($0, RSTART + 3, 1) == "x") checked = " checked=\"checked\""
	else                                  checked = ""
	$0 = "<input type=\"checkbox\" disabled=\"disabled\"" checked ">" substr($0, RSTART + RLENGTH) "</br>"
}

# Unordered lists
function abs(x) { return ((x < 0) ? -x : x) }
!blankline && lengthStack("list-level") > 0 {
	match($0, /^ */)
	indent = RLENGTH
	match($0, /(\*|\+|-|[0-9]+\.) /)
	if (RLENGTH == -1) RLENGTH = 0
	## indent + RLENGTH = length("  * " or " 1. ")
	while (indent + RLENGTH < abs(topStack("list-level"))) {
		if (topStack("list-level") < 0) {
			println("</ol>")
			autoinsert(user_L_AFTER)
			autoinsert(user_OL_AFTER)
		}
		else {
			println("</ul>")
			autoinsert(user_L_AFTER)
			autoinsert(user_UL_AFTER)
		}
		popStack("list-level")
	}
}
/^[ \t]*(\*|\+|-|[0-9]+\.) / {
	match($0, /(\*|\+|-|[0-9]+\.) /)
	if (abs(topStack("list-level")) < RSTART) {
		if (RLENGTH > 2) {
			xl = "<ol>"; mult = -1
			autoinsert(user_OL_BEFORE, 1)
		}
		else {
			xl = "<ul>"; mult = 1
			autoinsert(user_UL_BEFORE, 1)
		}
		autoinsert(user_L_BEFORE, 1)
		println(xl "<li>")
		pushStack("list-level", mult * (RSTART + RLENGTH - 1))
	}
	else {
		println("</li><li>")
	}
	$0 = substr($0, RSTART + RLENGTH)
}

# Paragraphs
prev_blankline && !blankline && lengthStack("bq-level") == 0 && lengthStack("list-level") == 0 { $0 = "<p>" $0 }
!prev_blankline && blankline && lengthStack("bq-level") == 0 && lengthStack("list-level") == 0 { prev = prev "</p>" }

# Blockquote Part 2/2
blankline {
	while (lengthStack("bq-level") > 0) {
		println("</blockquote>")
		autoinsert(user_BLOCKQUOTE_AFTER)
		popStack("bq-level")
	}
}

# Links
/!?\[[^\]]*\] ?[(\[][^)\]]*[\])]/ {
	isimg = 0
	while (match($0, /!?\[[^\]]*\] ?[(\[]/)) {
		if (substr($0, RSTART, 1) == "!") {
			isimg = 1
			RSTART++
			RLENGTH--
		}
		astart = RSTART ## astart = indexof("[")
		has_space = substr($0, RSTART + RLENGTH - 2, 1) == " "
		inner_anchor = substr($0, RSTART + 1, RLENGTH - 3 - has_space) ## ...[xxxxx](...)...

		aend = RSTART + RLENGTH - 1
		match(substr($0, astart), /\] ?[(\[][^)\]]*[\])]/)
		aend += RLENGTH - 2 - has_space ## aend = indexof(")")
		href = substr($0, astart + RSTART + 1 + has_space, RLENGTH - 3 - has_space) ## ...[...](xxxxx)...

		title = "\"\""
		if (substr($0, astart + RSTART + has_space, 1) == "[") { ## is reference link
			if (href == "") href = inner_anchor
			title = reference[href, "title"]
			href = reference[href, "href"]
		}
		else if (match(href, /[ \t]"[^"]*"[ \t]*$/)) { ## has title
			title = substr(href, RSTART + 1, RLENGTH - 1)
			href  = substr(href, 1, RSTART - 1)
		}
		sub(/[ \t]+/, "", href)

		if (isimg) elem = "<img src=\"" href "\" alt=\"" inner_anchor "\" title=" title ">"
		else       elem = "<a href=\"" href "\" title=" title ">" inner_anchor "</a>"

		$0 = substr($0, 1, astart - 1 - isimg) elem substr($0, aend + 1)
		isimg = 0
	}
}

# Emphasis
function surround(brace, opening, closing) {
	while ($0 ~ brace ".*" brace) {
		sub(brace, opening)
		sub(brace, closing)
	}
}
{
	surround("__"    , "<strong>", "</strong>")
	surround("_"     , "<em>"    , "</em>")
	surround("\\*\\*", "<strong>", "</strong>")
	surround("\\*"   , "<em>"    , "</em>")
	surround("~~"    , "<s>"     , "</s>")
	surround("~"     , "<sub>"   , "</sub>")
	surround("\\^"   , "<sup>"   , "</sup>")
	surround("``"    , "<code>"  , "</code>")
	surround("`"     , "<code>"  , "</code>")
}

!prev_blankline && /^: / {
	sub(/^<p>/, "", prev)
	sub(/^: /, "")
	prev = "<dl><dt>" prev "</dt>" ORS "<dd>" $0 "</dd></dl>"
	blankline = 1
	next
}

{
	println()
}

END {
	print prev
	if (!blankline && lengthStack("bq-level") == 0) { print "</p>" }
	## Flush stacks
	while (lengthStack("bq-level") > 0) {
		print "</blockquote>"
		popStack("bq-level")
	}
	while (lengthStack("list-level") > 0) {
		if (topStack("list-level") < 0) print "</ol>"
		else                            print "</ul>"
		popStack("list-level")
	}
}