Fix encoding_for_model to return nil for unknown encoding (#29)

lib/tiktoken_ruby.rb

@@ -28,7 +28,7 @@ def get_encoding(name)
 
     # Gets the encoding for an OpenAI model
     # @param model_name [Symbol|String] The name of the model to get the encoding for
-    # @return [Tiktoken::Encoding] The encoding instance
+    # @return [Tiktoken::Encoding, nil] The encoding instance, or nil if no encoding is found
     # @example Count tokens for text
     #   enc = Tiktoken.encoding_for_model("gpt-4")
     #   enc.encode("hello world").length #=> 2
@@ -37,10 +37,12 @@ def encoding_for_model(model_name)
         return get_encoding(MODEL_TO_ENCODING_NAME[model_name.to_sym])
       end
 
-      MODEL_PREFIX_TO_ENCODING.each do |prefix, encoding|
-        if model_name.start_with?(prefix.to_s)
-          return get_encoding(encoding)
-        end
+      _prefix, encoding = MODEL_PREFIX_TO_ENCODING.find do |prefix, _encoding|
+        model_name.start_with?(prefix.to_s)
+      end
+
+      if encoding
+        get_encoding(encoding)
       end
     end
 

spec/tiktoken_ruby_spec.rb

@@ -17,6 +17,10 @@
     expect(Tiktoken.encoding_for_model("ft:gpt-3.5-turbo:org:suffix:abc123")).to be_a(Tiktoken::Encoding)
   end
 
+  it "fails gracefully when getting an encoding for an unknown model" do
+    expect(Tiktoken.encoding_for_model("bad-model-name")).to be_nil
+  end
+
   it "lists available encodings" do
     expect(Tiktoken.list_encoding_names).to be_a(Array)
   end