refactor: simplify batch_slice_columns column-narrowing (#186)

Replace the per-call memoized index array and cursor with a plain reset:
batch_import restores the full header set at the start of each batch, so
batch_slice_columns just computes indices from the current headers and slices.
Successive calls still compose and multi-batch still works, with far less state.

Author: Fivell

lib/active_admin_import/importer.rb

@@ -82,29 +82,15 @@ def batch_replace(header_key, options)
     # end
     #
     def batch_slice_columns(slice_columns)
-      @batch_slices ||= []
-      @batch_slice_cursor ||= 0
-
-      # Indices are computed once (on the first batch) against the columns present
-      # *at this point in the call chain*, then replayed for later batches. Keeping
-      # one entry per call lets successive calls narrow the result of the previous
-      # one instead of re-applying the first call's indices (issue #186).
-      indexes = @batch_slices[@batch_slice_cursor]
-      unless indexes
-        columns = headers.values
-        indexes = columns.each_index.select { |i| columns[i].in?(slice_columns) }
-        @batch_slices[@batch_slice_cursor] = indexes
-        # slice CSV headers
-        @headers = headers.to_a.values_at(*indexes).to_h unless indexes.empty?
-      end
-      @batch_slice_cursor += 1
-
+      columns = headers.values
+      indexes = columns.each_index.select { |i| columns[i].in?(slice_columns) }
       return csv_lines if indexes.empty?
 
-      # slice CSV values
-      csv_lines.map! do |line|
-        line.values_at(*indexes)
-      end
+      # @headers is reset to the full set at the start of every batch (see
+      # #batch_import), so each call narrows the previous call's result and every
+      # batch slices the same way — calling this more than once now composes (#186).
+      @headers = headers.to_a.values_at(*indexes).to_h
+      csv_lines.map! { |line| line.values_at(*indexes) }
     end
 
     def values_at(header_key)
@@ -138,7 +124,7 @@ def prepare_headers
       headers = self.headers.present? ? self.headers.map(&:to_s) : yield
       @headers = Hash[headers.zip(headers.map { |el| el.underscore.gsub(/\s+/, '_') })].with_indifferent_access
       @headers.merge!(options[:headers_rewrites].symbolize_keys.slice(*@headers.symbolize_keys.keys))
-      @headers
+      @full_headers = @headers
     end
 
     def run_callback(name)
@@ -147,9 +133,9 @@ def run_callback(name)
 
     def batch_import
       batch_result = nil
-      # Restart the batch_slice_columns pipeline so its memoized indices are
-      # replayed from the first call for every batch (see #batch_slice_columns).
-      @batch_slice_cursor = 0
+      # Every batch re-parses full-width rows, so restore the full header set
+      # before slicing; batch_slice_columns then narrows it the same way each time.
+      @headers = @full_headers.dup if @full_headers
       @resource.transaction do
         run_callback(:before_batch_import)
         batch_result = resource.import(headers.values, csv_lines, import_options)