 			# for invalid build id, return top level
 			return file
 
+	generateBuildId: (callback = (error, buildId) ->) ->
+		# generate a secure build id from Date.now() and 8 random bytes in hex
+		crypto.randomBytes 8, (err, buf) ->
+			return callback(err) if err?
+			random = buf.toString('hex')
+			date = Date.now().toString(16)
+			callback err, "#{date}-#{random}"
+
 	saveOutputFiles: (outputFiles, compileDir, callback = (error) ->) ->
+		OutputCacheManager.generateBuildId (err, buildId) ->
+			return callback(err) if err?
+			OutputCacheManager.saveOutputFilesInBuildDir outputFiles, compileDir, buildId, callback
+
+	saveOutputFilesInBuildDir: (outputFiles, compileDir, buildId, callback = (error) ->) ->
 		# make a compileDir/CACHE_SUBDIR/build_id directory and
 		# copy all the output files into it
 		cacheRoot = Path.join(compileDir, OutputCacheManager.CACHE_SUBDIR)
 		# Put the files into a new cache subdirectory
-		buildId = Date.now().toString(16)
 		cacheDir = Path.join(compileDir, OutputCacheManager.CACHE_SUBDIR, buildId)
 
 		# let file expiry run in the background

				return false if options?.keep == dir
				# remove any directories over the hard limit
				return true if index > OutputCacheManager.CACHE_LIMIT
				# we can get the build time from the directory name
				dirTime = parseInt(dir, 16)
				age = currentTime - dirTime
				return age > OutputCacheManager.CACHE_AGE


logger = require "logger-sharelatex"
_ = require "underscore"
Settings = require "settings-sharelatex"
crypto = require "crypto"

OutputFileOptimiser = require "./OutputFileOptimiser"

module.exports = OutputCacheManager =
	CACHE_SUBDIR: '.cache/clsi'
	ARCHIVE_SUBDIR: '.archive/clsi'
	# build id is HEXDATE-HEXRANDOM from Date.now()and RandomBytes
	# for backwards compatibility, make the randombytes part optional
	BUILD_REGEX: /^[0-9a-f]+(-[0-9a-f]+)?$/
	CACHE_LIMIT: 2  # maximum number of cache directories
	CACHE_AGE: 60*60*1000 # up to one hour old

