remove_redundant_indices.rb

# frozen_string_literal: true

module ActiveRecord
  module Schema
    class Context
      class Table
        class Index
          attr_reader :columns, :options

          def initialize(columns, **options)
            @columns = columns
            @options = options
          end
        end

        attr_reader :name

        def initialize(name, **, &block)
          @name    = name
          @indices = []

          instance_eval(&block)
        end

        def index(columns, **options)
          @indices << Index.new(columns, **options)
        end

        def redundant_indices
          Enumerator.new do |y|
            longest_index = nil

            @indices.sort_by(&:columns).reverse_each do |index|
              next longest_index = index unless longest_index
              next longest_index = index unless index.columns == (index.columns & longest_index.columns)

              y.yield(index, longest_index)
            end
          end
        end

        %i[bigint boolean date datetime json integer string text].each do |dsl|
          define_method(dsl) do |*, **|
            # do nothing
          end
        end
      end

      def initialize(&block)
        @tables = []

        instance_eval(&block)
      end

      def create_table(name, **, &block)
        @tables << Table.new(name, &block)
      end

      def print_suggestions
        @tables.each do |t|
          memo = Hash.new { |h, k| h[k] = [] }

          t.redundant_indices.each { |index, covered_by| memo[covered_by] << index }
          next if memo.empty?

          memo.each do |longer_index, redundant_indices|
            puts "# covered by: #{longer_index.options[:name]}"

            redundant_indices.each do |index|
              puts "remove_index :#{t.name}, %i[#{index.columns.join ' '}]"
            end
          end
        end
      end
    end

    def self.define(**, &block)
      Context.new(&block).print_suggestions
    end
  end
end

require_relative "db/schema"