Preprocess an exim4 config file, expanding .ifdefs etc.

preprocess-exim4.conf

#!/usr/bin/ruby -w
# Won't expand .include!
# Usage on Debian 7: update-exim4.conf && ./preprocess-exim4.conf </var/lib/exim4/config.autogenerated

lines = STDIN.each_line.to_a

# simplify ".if(n)def foo bar" to ".if(n) true/false"
definition = /^([A-Z]\S*)\s*=\s*(.*?)\s*$/
defined = {}
lines.map! do |line|
	l = line.strip

	if (m = definition.match l)
		defined[m[1]] = m[2]
		line
	elsif (m = /^(\.ifn?)def\s+(.+?)\s*$/.match l)
		vars = m[2].split(/\s+/)
		m[1] + " " + (vars.any? {|v| defined[v] } ? "true" : "false")
	else
		line
	end
end.compact!

# canonicalize
lines.map! {|l| l.gsub(/^\.ifn false$/, '.if true') }
lines.map! {|l| l.gsub(/^\.ifn true$/,  '.if false') }

# remove unused branches
print = []
lines.map! do |line|
	case line.strip
		when ".if true"  then print << true;          nil
		when ".if false" then print << false;         nil
		when ".else"     then print[-1] = !print[-1]; nil
		when ".endif"    then print.pop;              nil
		else line if print.all?
	end
end.compact!

# merge split lines
lines = lines.join.gsub("\\\n", "{LINE_CONTINUATION}").each_line.to_a

# rebuild definition cache
defined = {}
lines.each do |line|
	l = line.strip

	if (m = definition.match l)
		defined[m[1]] = m[2]
	end
end

# count macro uses
usages = Hash.new 0
defined.keys.each do |k|
	usages[k] = lines.join.scan(/\b#{k}\b/).length - 1
end

# expand macros if used only once
lines.map! do |line|
	l = line.strip

	if (m = definition.match l)
		line if usages[m[1]] > 1
	else
		defined.each_pair do |k, v|
			line.gsub!(/\b#{k}\b/, v) if usages[k] == 1
		end
		line
	end
end.compact!

# remove comments
lines.reject!{|line| line.match(/^\s*#/) }

# split merged lines again
lines = lines.join.gsub("{LINE_CONTINUATION}", "\\\n")

puts lines