implement Cache to store info about target dependencies and checksums across invocations

lib/rscons/cache.rb

@@ -1,8 +1,36 @@
 require 'yaml'
 require 'fileutils'
 require 'digest/md5'
+require 'set'
 
 module Rscons
+  # Example cache:
+  # {
+  #   'program' => {
+  #     'checksum' => 'A1B2C3D4',
+  #     'command' => ['gcc', '-o', 'program', 'program.o'],
+  #     'deps' => [
+  #       {
+  #         'fname' => 'program.o',
+  #         'checksum' => '87654321',
+  #       }
+  #     ],
+  #   }
+  #   'program.o' => {
+  #     'checksum' => '87654321',
+  #     'command' => ['gcc', '-c', '-o', 'program.o', 'program.c'],
+  #     'deps' => [
+  #       {
+  #         'fname' => 'program.c',
+  #         'checksum' => '456789ABC',
+  #       },
+  #       {
+  #         'fname' => 'program.h',
+  #         'checksum' => '7979764643',
+  #       }
+  #     ]
+  #   }
+  # }
   class Cache
     # Constants
     CACHE_FILE = '.rsconscache'
@@ -15,6 +43,7 @@ module Rscons
     # Instance Methods
     def initialize
       @cache = YAML.load(File.read(CACHE_FILE)) rescue {}
+      @lookup_checksums = {}
     end
 
     def write
@@ -23,20 +52,43 @@ module Rscons
       end
     end
 
-    def up_to_date?(file, deps = nil)
-      # TODO
-      false
+    def up_to_date?(target, command, deps)
+      # target file must exist on disk
+      return false unless File.exists?(target)
+      # target must be registered in the cache
+      return false unless @cache.has_key?(target)
+      # command line used to build target must be identical
+      return false unless @cache[target][:command] == command
+      # all dependencies passed in must exist in cache (but cache may have more)
+      return false unless (Set.new(deps) - Set.new(@cache[target][:deps])).empty?
+      # all cached dependencies must have their checksums match
+      @cache[target][:deps].map do |dep_cache|
+        dep_cache[:checksum] == lookup_checksum(dep_cache[:fname])
+      end.all?
     end
 
-    def register_build(target, deps)
-      # TODO
+    def register_build(target, command, deps)
+      @cache[target] = {
+        command: command,
+        checksum: calculate_checksum(target),
+        deps: deps.map do |dep|
+          {
+            fname: dep,
+            checksum: lookup_checksum(dep),
+          }
+        end
+      }
     end
 
     # Private Instance Methods
     private
 
+    def lookup_checksum(file)
+      @lookup_checksums[file] || calculate_checksum(file)
+    end
+
     def calculate_checksum(file)
-      Digest::MD5.hexdigest(File.read(file)).encode(__ENCODING__)
+      @lookup_checksums[file] = Digest::MD5.hexdigest(File.read(file)).encode(__ENCODING__) rescue ''
     end
   end
 end