Do not rebuild targets when they are up to date

Store dependencies and file checksums in a cache file

lib/rscons.rb

@@ -1,6 +1,7 @@
 require "rscons/version"
 require "rscons/builder"
 require "rscons/environment"
+require "rscons/cache"
 
 # default builders
 require "rscons/builders/cc"

lib/rscons/builders/cc.rb

@@ -17,14 +17,17 @@ module Rscons
     def run(env, target, source)
       raise "String expected, not #{source.inspect}" unless source.is_a?(String)
       o_file = "#{env.stem(source)}#{env['OBJSUFFIX']}"
-      command = [
-        env['CC'],
-        *env['CPPFLAGS'],
-        *env['CFLAGS'],
-        '-o', o_file,
-        source
-      ]
-      env.execute("CC #{o_file}", command)
+      unless Cache.open.up_to_date?(target, [source])
+        command = [
+          env['CC'],
+          *env['CPPFLAGS'],
+          *env['CFLAGS'],
+          '-o', o_file,
+          source
+        ]
+        env.execute("CC #{o_file}", command)
+        Cache.open.register_build(target, [source])
+      end
       o_file
     end
   end

lib/rscons/builders/program.rb

@@ -11,6 +11,7 @@ module Rscons
 
     def run(env, target, sources)
       sources = [sources] if sources.is_a?(String)
+      # convert sources to object file names
       sources = sources.map do |source|
         if source =~ /#{env['OBJSUFFIX']}$/ or source =~ /#{env['LIBSUFFIX']}$/
           source
@@ -24,15 +25,18 @@ module Rscons
           end
         end
       end
-      command = [
-        env['LD'] || env['CC'],
-        '-o', target,
-        *env['LDFLAGS'],
-        *sources,
-        *env['LIBPATHS'].map {|lp| "-L#{lp}"},
-        *env['LIBS'].map {|lib| "-l#{lib}"}
-      ]
-      env.execute("LINK #{target}", command)
+      unless Cache.open.up_to_date?(target, sources)
+        command = [
+          env['LD'] || env['CC'],
+          '-o', target,
+          *env['LDFLAGS'],
+          *sources,
+          *env['LIBPATHS'].map {|lp| "-L#{lp}"},
+          *env['LIBS'].map {|lib| "-l#{lib}"}
+        ]
+        env.execute("LINK #{target}", command)
+        Cache.open.register_build(target, sources)
+      end
       target
     end
   end

lib/rscons/cache.rb

@@ -0,0 +1,74 @@
+require 'yaml'
+require 'fileutils'
+require 'digest/md5'
+require 'set'
+
+module Rscons
+  class Cache
+    CACHE_FILE = '.rscons'
+
+    private_class_method :new
+    def initialize
+      @cache = YAML.load(File.read(CACHE_FILE)) rescue {}
+      @lookup_checksums = {}
+      @stored_checksums = {}
+    end
+
+    def self.open
+      @@cache ||= new
+    end
+
+    def clear
+      FileUtils.rm_f(CACHE_FILE)
+      initialize
+    end
+
+    def write
+      File.open(CACHE_FILE, 'w') do |fh|
+        fh.puts(YAML.dump(@cache))
+      end
+    end
+
+    def up_to_date?(file, deps = nil)
+      return false unless File.exists?(file)
+      stored_md5 = @cache[:checksums][file] rescue nil
+      return false unless stored_md5
+      return false unless lookup_checksum(file) == stored_md5
+      cached_deps = @cache[:deps][file] || [] rescue []
+      return false if deps and Set.new(cached_deps) != Set.new(deps)
+      cached_deps.map {|d| up_to_date?(d)}.all?
+    end
+
+    def register_build(target, deps)
+      @cache[:deps] ||= {}
+      @cache[:deps][target] = deps
+      store_checksum(target)
+      deps.each do |dep|
+        store_checksum(dep)
+      end
+      # it is unfortunate to write the file here since many file writes may
+      # occur before the final version, but I don't know where else to do so
+      write
+    end
+
+    private
+
+    def lookup_checksum(file)
+      @lookup_checksums[file] ||= calculate_checksum(file)
+    end
+
+    def store_checksum(file)
+      cs = calculate_checksum(file)
+      if @stored_checksums[file] and @stored_checksums[file] != cs
+        $stderr.puts "Warning: file #{file.inspect} changed after previously used by builder"
+      end
+      @cache[:checksums] ||= {}
+      @cache[:checksums][file] = cs
+      @lookup_checksums[file] = cs
+    end
+
+    def calculate_checksum(file)
+      Digest::MD5.hexdigest(File.read(file))
+    end
+  end
+end

spec/build_tests_spec.rb

@@ -17,6 +17,7 @@ describe Rscons do
 
   before do
     $stdout.stub(:puts) { nil }
+    Rscons::Cache.open.clear
   end
 
   ###########################################################################
@@ -31,4 +32,14 @@ describe Rscons do
       `./simple`.should =~ /This is a simple C program/
     end
   end
+
+  it 'does not rebuild the program if no sources changed' do
+    $stdout.should_receive(:puts).once.with('gcc -c -o simple.o simple.c')
+    $stdout.should_receive(:puts).once.with('gcc -o simple simple.o')
+    setup_testdir(['simple.c']) do
+      env = Rscons::Environment.new
+      env.Program('simple', 'simple.c')
+      env.Program('simple', 'simple.c')
+    end
+  end
 end