class Query

  attr_reader :options

  # Options: page, per_page
  def initialize(options={})
    @options = HashWithIndifferentAccess.new(options)
  end

  def search
    begin
      $elasticsearch.search(
        index: index_name,
        body: generated_query,
        track_total_hits: true
      )
    rescue Elastic::Transport::Transport::Errors::BadRequest
      { error: "Your search failed because of a syntax error. Please try again." }
    end
  end

  def search_results
    response = search
    QueryResult.new(klass, response, { page: page, per_page: per_page })
  end

  # Perform a count query based on the given options
  def count
    $elasticsearch.count(
      index: index_name,
      body: { query: generated_query[:query] }
    )['count']
  end

  # Retrieve a randomly sampled selection of results:
  def sample(count: 5)
    response = $elasticsearch.search(
      index: index_name,
      body: {
        query: {
          function_score: {
            query: filtered_query,
            random_score: {},
            boost_mode: "replace"
          }
        },
        size: count
      }
    )

    QueryResult.new(klass, response, { page: 1, per_page: count })
  end

  # Perform a specific aggregation:
  def aggregation_search(aggregation)
    $elasticsearch.search(
      index: index_name,
      body: {
        query: filtered_query,
        size: 0, # aggregations only
        aggs: { aggregation: aggregation }
      }
    ).dig("aggregations", "aggregation")
  end

  # Use a composite aggregation to get all values that a particular field can
  # take on. Returns a hash mapping from values to counts.
  def field_values(field_name, batch_size: 100)
    aggregation = {
      composite: {
        size: batch_size,
        sources: [{ field_value: { terms: { field: field_name } } }]
      }
    }

    counts = {}

    loop do
      results = aggregation_search(aggregation)

      results["buckets"].each do |info|
        counts[info.dig("key", "field_value")] = info.dig("doc_count")
      end

      after_key = results["after_key"]
      return counts if after_key.nil?

      aggregation[:composite][:after] = after_key
    end
  end

  # Return (an approximation of) the number of distinct values that a
  # particular field can take on:
  def field_count(field_name, precision_threshold: 1000)
    aggregation_search(
      cardinality: {
        field: field_name,
        precision_threshold: precision_threshold
      }
    ).dig("value")
  end

  # Sort by relevance by default, override in subclasses as necessary
  def sort
    { _score: { order: "desc" } }
  end

  # Search query with filters
  def generated_query
    q = {
      query: filtered_query,
      size: per_page,
      from: pagination_offset,
      sort: sort
    }
    if (aggs = aggregations).present?
      q.merge!(aggs)
    end
    q
  end

  # Combine the filters and queries, with a fallback in case there are no
  # filters or queries:
  def filtered_query
    make_bool(
      must: queries, # required, score calculated
      filter: filters, # required, score ignored
      must_not: exclusion_filters # disallowed, score ignored
    ) || { match_all: {} }
  end

  # Define specifics in subclasses

  def filters
    @filters
  end

  def term_filter(field, value, options={})
    { term: options.merge(field => value) }
  end

  def terms_filter(field, value, options={})
    { terms: options.merge(field => value) }
  end

  def exists_filter(field)
    { exists: { field: field } }
  end

  # A filter used to match all words in a particular field, most frequently
  # used for matching non-existent tags. The match query doesn't allow
  # negation/or/and/wildcards, so it should only be used on fields where the
  # users are expected to enter, e.g. canonical tags.
  def match_filter(field, value, options = {})
    { match: { field => { query: value, operator: "and" }.merge(options) } }
  end

  # Replaces the existing scores for a query with the value of a field. The
  # optional value "missing" determines what score value should be used if the
  # specified field is missing from a document.
  def field_value_score(field, query, missing: 0)
    {
      function_score: {
        query: query,
        field_value_factor: {
          field: field,
          missing: missing
        },
        boost_mode: :replace
      }
    }
  end

  def bool_value(str)
    %w(true 1 T).include?(str.to_s)
  end

  def exclusion_filters
    @exclusion_filters
  end

  def queries
  end

  def aggregations
  end

  def index_name
  end

  def document_type
  end

  def per_page
    options[:per_page] || ArchiveConfig.ITEMS_PER_PAGE
  end

  # Example: if the limit is 3 results, and we're displaying 2 per page,
  # disallow pages beyond page 2.
  def page
    [
      options[:page] || 1,
      (ArchiveConfig.MAX_SEARCH_RESULTS / per_page.to_f).ceil
    ].min
  end

  def pagination_offset
    (page * per_page) - per_page
  end

  # Only escape if it isn't already escaped
  def escape_slashes(word)
    word.gsub(/([^\\])\//) { |s| $1 + '\\/' }
  end

  def escape_reserved_characters(word)
    word = escape_slashes(word)
    word.gsub!('!', '\\!')
    word.gsub!('+', '\\\\+')
    word.gsub!('-', '\\-')
    word.gsub!('?', '\\?')
    word.gsub!("~", '\\~')
    word.gsub!("(", '\\(')
    word.gsub!(")", '\\)')
    word.gsub!("[", '\\[')
    word.gsub!("]", '\\]')
    word.gsub!(':', '\\:')
    word
  end

  def split_query_text_phrases(fieldname, text)
    str = +""
    return str if text.blank?
    text.split(",").map(&:squish).each do |phrase|
      str << " #{fieldname}:\"#{phrase}\""
    end
    str
  end

  def split_query_text_words(fieldname, text)
    str = +""
    return str if text.blank?
    text.split(" ").each do |word|
      if word.length >= 2 && word[0] == "-"
        str << " NOT"
        word.slice!(0)
      end
      word = escape_reserved_characters(word)
      str << " #{fieldname}:#{word}"
    end
    str
  end

  def make_bool(query)
    query.reject! { |_, value| value.blank? }
    query[:minimum_should_match] = 1 if query[:should].present?

    if query.empty?
      nil
    elsif query.values.flatten.size == 1 && (query[:must] || query[:should])
      # There's only one clause in our boolean, so we might as well skip the
      # bool and just require it.
      query.values.flatten.first
    else
      { bool: query }
    end
  end

  def make_list(*args)
    args.flatten.compact
  end
end
