1 files changed, 113 insertions, 0 deletions
diff --git a/lib/syntax_suggest/scan_history.rb b/lib/syntax_suggest/scan_history.rb
new file mode 100644
index 0000000000..c5664d0f65
--- /dev/null
+++ b/lib/syntax_suggest/scan_history.rb
@@ -0,0 +1,113 @@
+# frozen_string_literal: true
+
+module SyntaxSuggest
+  # Scans up/down from the given block
+  #
+  # You can snapshot a change by committing it and rolling back.
+  #
+  class ScanHistory
+    attr_reader :before_index, :after_index
+
+    def initialize(code_lines:, block:)
+      @code_lines = code_lines
+      @history = [block]
+      refresh_index
+    end
+
+    def commit_if_changed
+      if changed?
+        @history << CodeBlock.new(lines: @code_lines[before_index..after_index])
+        refresh_index
+      end
+
+      self
+    end
+
+    def try_rollback
+      if @history.length > 1
+        @history.pop
+        refresh_index
+      end
+
+      self
+    end
+
+    def changed?
+      @before_index != current.lines.first.index ||
+        @after_index != current.lines.last.index
+    end
+
+    # Iterates up and down
+    #
+    # Returns line, kw_count, end_count for each iteration
+    def scan(up:, down:)
+      kw_count = 0
+      end_count = 0
+
+      up_index = before_lines.reverse_each.take_while do |line|
+        kw_count += 1 if line.is_kw?
+        end_count += 1 if line.is_end?
+        up.call(line, kw_count, end_count)
+      end.last&.index
+
+      kw_count = 0
+      end_count = 0
+
+      down_index = after_lines.each.take_while do |line|
+        kw_count += 1 if line.is_kw?
+        end_count += 1 if line.is_end?
+        down.call(line, kw_count, end_count)
+      end.last&.index
+
+      @before_index = if up_index && up_index < @before_index
+        up_index
+      else
+        @before_index
+      end
+
+      @after_index = if down_index && down_index > @after_index
+        down_index
+      else
+        @after_index
+      end
+
+      self
+    end
+
+    def next_up
+      return nil if @before_index <= 0
+
+      @code_lines[@before_index - 1]
+    end
+
+    def next_down
+      return nil if @after_index >= @code_lines.length
+
+      @code_lines[@after_index + 1]
+    end
+
+    def lines
+      @code_lines[@before_index..@after_index]
+    end
+
+    private def before_lines
+      @code_lines[0...@before_index] || []
+    end
+
+    # Returns an array of all the CodeLines that exist after
+    # the currently scanned block
+    private def after_lines
+      @code_lines[@after_index.next..-1] || []
+    end
+
+    private def current
+      @history.last
+    end
+
+    private def refresh_index
+      @before_index = current.lines.first.index
+      @after_index = current.lines.last.index
+      self
+    end
+  end
+end