require "diff-lcs"
require "hyp_diff"

module Scrivito

class Diff; class << self

  def for(mode, field_type, old, new)
    case field_type
    when "widget"
      ListComparison.for(mode, old || [], new || [])
    when "html"
      for_html(mode, old, new)
    when "text"
      for_string(mode, old, new)
    when "string"
      for_string(mode, old, new)
    end
  end

  private

  def for_string(mode, old, new)
    for_html(mode, ERB::Util.h(old), ERB::Util.h(new))
  end

  def for_html(mode, old, new)
    return nil if !old
    return nil if !new

    hyp_diff_options =
      case mode
      when "added"
        {render_deletion: proc {}, markup_from: "after"}
      when "deleted"
        {render_insertion: proc {}, markup_from: "before"}
      when "diff"
        {}
      else
        return nil
      end

    diff = HypDiff.compare(old, new, hyp_diff_options)
    [StringTagging.tag_as_html(diff)]
  end

  class ListComparison
    def self.for(mode, old, new)
      new(mode).compare(old, new).result
    end

    def initialize(mode)
      @mode = mode
      @list = []
      @modifications = []
    end

    def compare(old, new)
      ::Diff::LCS.sdiff(old, new).map do |change|
        case change.action
        when "-" then
          note_deletion(change)
        when "+" then
          note_insertion(change)
        when "=" then
          note_equal(change)
        when "!" then
          note_deletion(change)
          note_insertion(change)
        else
          raise "unexpected change.action #{change.action}"
        end
      end

      self
    end

    def result
      [@list, @modifications]
    end

    private

    def note_equal(change)
      @modifications << nil
      if @mode == "deleted"
        @list << change.old_element
      else
        @list << change.new_element
      end
    end

    def note_deletion(change)
      unless @mode == "added"
        @modifications << "deleted"
        @list << change.old_element
      end
    end

    def note_insertion(change)
      unless @mode == "deleted"
        @modifications << "new"
        @list << change.new_element
      end
    end
  end

end; end

end