Selectable checksums and matching tests

Note: the initializer is removed here, because it *does nothing*.
If all a method does is call `super` with the same args, it is better
for it not to exist.

lib/moab/file_signature.rb

@@ -44,11 +44,6 @@ class FileSignature < Serializer::Serializable
     # The name of the XML element used to serialize this objects data
     tag 'fileSignature'
 
-    # (see Serializable#initialize)
-    def initialize(opts = {})
-      super(opts)
-    end
-
     # @attribute
     # @return [Integer] The size of the file in bytes
     attribute :size, Integer, :on_save => Proc.new { |n| n.to_s }
@@ -65,6 +60,30 @@ def initialize(opts = {})
     # @return [String] The SHA256 checksum value of the file
     attribute :sha256, String, :on_save => Proc.new { |n| n.nil? ? "" : n.to_s }
 
+    KNOWN_ALGOS = {
+      md5: proc { Digest::MD5.new },
+      sha1: proc { Digest::SHA1.new },
+      sha256: proc { Digest::SHA2.new(256) }
+    }.freeze
+
+    # Reads the file once for ALL (requested) algorithms, not once per.
+    # @param [Pathname] pathname
+    # @param [Array<Symbol>] one or more keys of KNOWN_ALGOS to be computed
+    # @return [Moab::FileSignature] object populated with (requested) checksums
+    def self.from_file(pathname, algos_to_use = KNOWN_ALGOS.keys)
+      raise 'Unrecognized algorithm requested' unless algos_to_use.all? { |a| KNOWN_ALGOS.include?(a) }
+
+      signatures = algos_to_use.map { |k| [k, KNOWN_ALGOS[k].call] }.to_h
+
+      pathname.open("r") do |stream|
+        while (buffer = stream.read(8192))
+          signatures.each_value { |digest| digest.update(buffer) }
+        end
+      end
+
+      new(signatures.map { |k, digest| [k, digest.hexdigest] }.to_h.merge(size: pathname.size) )
+    end
+
     # @param type [Symbol,String] The type of checksum
     # @param value [String] The checksum value
     # @return [void] Set the value of the specified checksum type
@@ -139,24 +158,17 @@ def hash
       @size.to_i
     end
 
-    # @api internal
+    # @deprecated
+    # this method is a holdover from an earlier version.  use the class method .from_file going forward.
+    # @api external
     # @param pathname [Pathname] The location of the file to be digested
     # @return [FileSignature] Generate a FileSignature instance containing size and checksums for a physical file
     def signature_from_file(pathname)
-      @size = pathname.size
-      md5_digest = Digest::MD5.new
-      sha1_digest = Digest::SHA1.new
-      sha256_digest = Digest::SHA2.new(256)
-      pathname.open("r") do |stream|
-        while (buffer = stream.read(8192))
-          md5_digest.update(buffer)
-          sha1_digest.update(buffer)
-          sha256_digest.update(buffer)
-        end
-      end
-      @md5 = md5_digest.hexdigest
-      @sha1 = sha1_digest.hexdigest
-      @sha256 = sha256_digest.hexdigest
+      file_signature = self.class.from_file(pathname)
+      @size = file_signature.size
+      @md5 = file_signature.md5
+      @sha1 = file_signature.sha1
+      @sha256 = file_signature.sha256
       self
     end
 

spec/unit_tests/moab/file_signature_spec.rb

@@ -14,6 +14,24 @@
     described_class.new.signature_from_file(page1_v2_pathname)
   end
 
+  describe '.from_file' do
+    it 'raises if unrecognized checksum requested' do
+      expect { described_class.from_file('whatever', [:rot13]) }.to raise_exception /Unrecognized algorithm/
+    end
+    it 'computes all checksums by default' do
+      sig = described_class.from_file(title_v1_pathname)
+      expect(sig.md5).to eq '1a726cd7963bd6d3ceb10a8c353ec166'
+      expect(sig.sha1).to eq '583220e0572640abcd3ddd97393d224e8053a6ad'
+      expect(sig.sha256).to eq '8b0cee693a3cf93cf85220dd67c5dc017a7edcdb59cde8fa7b7f697be162b0c5'
+    end
+    it 'only computes requested checksum(s)' do
+      sig = described_class.from_file(title_v1_pathname, [:md5, :sha1])
+      expect(sig.md5).to eq '1a726cd7963bd6d3ceb10a8c353ec166'
+      expect(sig.sha1).to eq '583220e0572640abcd3ddd97393d224e8053a6ad'
+      expect(sig.sha256).to be_nil
+    end
+  end
+
   specify '#initialize' do
     opts = {
       size: 75,