Fix duplicate annotations when a file ends with a trailing blank line

lib/annotate_rb/model_annotator/file_parser/parsed_file.rb

@@ -45,7 +45,7 @@ def parse
               annotation_start = @finder.annotation_start
               annotation_end = @finder.annotation_end
 
-              if @file_lines[annotation_start - 1]&.strip&.empty?
+              if annotation_start > 0 && @file_lines[annotation_start - 1]&.strip&.empty?
                 annotation_start -= 1
                 has_leading_whitespace = true
               end

spec/lib/annotate_rb/model_annotator/file_parser/parsed_file_spec.rb

@@ -0,0 +1,48 @@
+# frozen_string_literal: true
+
+RSpec.describe AnnotateRb::ModelAnnotator::FileParser::ParsedFile do
+  describe "#parse" do
+    subject { described_class.new(file_content, new_annotations, parser_klass, options).parse }
+
+    let(:parser_klass) { AnnotateRb::ModelAnnotator::FileParser::CustomParser }
+    let(:options) { AnnotateRb::Options.new({}) }
+    let(:new_annotations) do
+      <<~ANNOTATIONS
+        # == Schema Information
+        #
+        # Table name: users
+        #
+        #  id                     :bigint           not null, primary key
+        #
+      ANNOTATIONS
+    end
+
+    # Regression: when the annotation is at the very top of the file (annotation_start == 0),
+    # `@file_lines[annotation_start - 1]` used to wrap around to `@file_lines[-1]` (the last
+    # line). If the file ended with a blank line, this was mistaken for leading whitespace,
+    # decremented annotation_start to -1, and produced an empty `annotations_with_whitespace`.
+    # An empty removal string is a no-op, so old annotations were never removed and duplicated.
+    context "when the annotation is at the top of the file and the file ends with a blank line" do
+      let(:file_content) do
+        "# == Schema Information\n" \
+          "#\n" \
+          "# Table name: users\n" \
+          "#\n" \
+          "#  id                     :bigint           not null, primary key\n" \
+          "#\n" \
+          "class User < ApplicationRecord\n" \
+          "end\n" \
+          "\n"
+      end
+
+      it "does not treat the trailing blank line as leading whitespace" do
+        expect(subject.has_leading_whitespace?).to eq(false)
+      end
+
+      it "captures the annotation in annotations_with_whitespace" do
+        expect(subject.annotations_with_whitespace).not_to be_empty
+        expect(subject.annotations_with_whitespace).to include("# == Schema Information")
+      end
+    end
+  end
+end