module GdsApi::PartMethods

def find_part(slug)
  return nil unless index = part_index(slug)
  parts[index]
end

def has_next_part?(part)
  index = part_index(part.slug)
  !! (index && (index + 1) < parts.length)
end

def has_parts?(part)
  !! (has_previous_part?(part) || has_next_part?(part))
end

def has_previous_part?(part)
  index = part_index(part.slug)
  !! (index && index > 0)
end

def part_after(part)
  part_at(part, 1)
end

def part_at(part, relative_offset)
  return nil unless current_index = part_index(part.slug)
  other_index = current_index + relative_offset
  return nil unless (0 ... parts.length).include?(other_index)
  parts[other_index]
end

def part_before(part)
  part_at(part, -1)
end

def part_index(slug)
  parts.index { |p| p.slug == slug }
end