fn replace(text string, start int, len int, repl string) string {
	return text[..start] + repl + text[start + len..]
}

fn parse_header(line string) (int, string) {
	if line.len == 0 {
		return 0, line
	}

	mut lvl := 0
	for lvl < line.len && line[lvl] == '!'.bytes()[0] {
		lvl++
	}
	if lvl == 0 || lvl >= line.len || line[lvl] != ' '.bytes()[0] {
		return 0, line
	}
	return lvl, line[lvl + 1..]
}

fn parse(lines []string) string {
	mut output := '<p>'
	for line in lines {
		mut trimmed := line.trim_space()

		if trimmed.len == 0 {
			output += '<br>\n'
			continue
		}

		lvl, mut text := parse_header(trimmed)

		is_header := lvl > 0

		if is_header {
			output += '<h' + lvl.str() + '>'
			output += text + '</h' + lvl.str() + '>'
		} else {
			output += text
			output += '<br>'
		}

		output += '\n'
	}

	output += '</p>'
	return output
}