From 4f69da14cce037eaf09faf3c5f59632d06f353ef Mon Sep 17 00:00:00 2001
From: OdenTakashi <odentakashidesu2@gmail.com>
Date: Sat, 6 Jun 2026 17:10:00 +0900
Subject: [PATCH] Fix duplicate annotations when a file ends with a trailing
 blank line

## Problem
Given a model whose file ends with two newlines (`end\n\n`):

```rb
# == Schema Information
#
# Table name: users
#
class User < ApplicationRecord
end
     <= 1st newline (ends "end")
     <= 2nd newline (blank line; file ends with \n\n)
```

running `annotaterb models --force` duplicates the schema block on every run:

```rb
# == Schema Information
#
# Table name: users
#

# == Schema Information   # <- duplicated, grows by one every run
#
# Table name: users
#
class User < ApplicationRecord
end
```

## Cause
With an extra trailing newline (`end\n\n`), `@file_lines[-1]` is a real blank
line (`"\n"`). For a top-of-file annotation (annotation_start == 0), the
leading-whitespace check reads `@file_lines[annotation_start - 1]`, i.e.
`@file_lines[-1]` via Ruby's negative index, and treats that blank line as
leading whitespace. `annotation_start` becomes -1, which makes
`@file_lines[-1..annotation_end]` an empty range, so
`annotations_with_whitespace` is `""`. Removal then runs `sub("", "")`, a
no-op, so the old annotation is never removed and a new one is appended.

## Solution
Add an `annotation_start > 0` guard so a top-of-file annotation never reads
`@file_lines[-1]`.
---
 .../file_parser/parsed_file.rb                |  2 +-
 .../file_parser/parsed_file_spec.rb           | 48 +++++++++++++++++++
 2 files changed, 49 insertions(+), 1 deletion(-)
 create mode 100644 spec/lib/annotate_rb/model_annotator/file_parser/parsed_file_spec.rb

diff --git a/lib/annotate_rb/model_annotator/file_parser/parsed_file.rb b/lib/annotate_rb/model_annotator/file_parser/parsed_file.rb
index 5ad8320b..0d62718e 100644
--- a/lib/annotate_rb/model_annotator/file_parser/parsed_file.rb
+++ b/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
diff --git a/spec/lib/annotate_rb/model_annotator/file_parser/parsed_file_spec.rb b/spec/lib/annotate_rb/model_annotator/file_parser/parsed_file_spec.rb
new file mode 100644
index 00000000..3c3caccb
--- /dev/null
+++ b/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