# Copyright (c) 2020 Contrast Security, Inc. See https://www.contrastsecurity.com/enduser-terms-0317a for more details.
# frozen_string_literal: true

module Contrast
  module Agent
    module Assess
      # A Tag represents a range in a given piece of data. It is used by the
      # Agent to determine if a vulnerable dataflow has occurred.
      class Tag
        attr_reader :label,     # the label of this tag
                    :length,    # length of tagged text within string
                    :start_idx, # start of range
                    :end_idx    # end of range (calculated from start + length), exclusive

        # Initialize a new tag
        #
        # @param label [String] the label of the tag
        # @param length [Integer] the length of the string described with this
        #   tag
        # @param start_idx [Integer] (0) the starting position in the string for
        #   this tag
        def initialize label, length, start_idx = 0
          @label = label
          update_range(start_idx, start_idx + length)
        end

        # Return true if the tag covers the given position in the string
        #
        # @param idx [Integer] the index to check
        # @return [Boolean]
        def covers? idx
          idx >= start_idx && idx < end_idx
        end

        # Return true if the tag is above the given position in the string
        # @param idx [Integer] the index to check
        # @return [Boolean]
        def above? idx
          idx < start_idx
        end

        # Return the range that this tag covers, from start (inclusive) to
        # end (exclusive).
        #
        # @return [Range]
        def range
          start_idx...end_idx
        end

        def extends_beyond_string_size? string_length
          @end_idx > string_length
        end

        # Return if a given tag overlaps this one
        def overlaps? start_idx, end_idx
          return true if @start_idx <  start_idx && @end_idx >= start_idx  # we start below range & end in it
          return true if @start_idx >= start_idx && @end_idx <= end_idx    # we start and end in range

          @start_idx <= end_idx && @end_idx > end_idx                      # we start in range & end above it
        end

        def shift idx
          update_range(@start_idx + idx, @end_idx + idx)
        end

        def shift_end idx
          update_range(@start_idx, @end_idx + idx)
        end

        def update_start start_idx
          update_range(start_idx, @end_idx)
        end

        def update_end end_idx
          update_range(@start_idx, end_idx)
        end

        def repurpose start_idx, end_idx
          update_range(start_idx, end_idx)
        end

        # Given a tag, merge its ranges with this one
        # such that the lowest start and highest end
        # become the values of this tag
        #
        # Returns true if the other tag was merged into
        # this tag
        def merge other
          return unless overlaps?(other.start_idx, other.end_idx)

          start = other.start_idx < @start_idx ? other.start_idx : @start_idx
          finish = other.end_idx > @end_idx ? other.end_idx : @end_idx
          update_range(start, finish)
        end

        # Modification to tracked String can change the position and length of the tracked tag
        # shift : negative value moves left
        def copy_modified shift
          start = start_idx + shift
          # Tags cannot start below 0
          new_start_idx = start >= 0 ? start : 0
          # If a tag were to go negative, cut off the negative portion from length
          new_length = start >= 0 ? length : (length + start)
          Contrast::Agent::Assess::Tag.new(label, new_length, new_start_idx)
        end

        def str_val
          @_str_val ||= "[#{ start_idx },#{ end_idx }]"
        end
        alias_method :to_s, :str_val

        BELOW = 'BELOW'
        LOW_SPAN = 'LOW_SPAN'
        WITHIN = 'WITHIN'
        WITHOUT = 'WITHOUT'
        HIGH_SPAN = 'HIGH_SPAN'
        ABOVE = 'ABOVE'

        # The tag is ______ the range
        # rrrrrrr == self.range, the range of the tag
        def compare_range start, stop
          # the range starts below the given values
          if @start_idx < start
            # r starts and stops below
            # rrrrrrrrrrrrr
            #               start       stop
            return BELOW if @end_idx <= start
            # r starts below and finishes within
            # rrrrrrrrrrrrr
            #    start       stop
            return LOW_SPAN if @end_idx > start && @end_idx <= stop
            # r starts below and finishes above stop
            #  rrrrrrrrrrrrrrrrrrrrrrrr
            #     start       stop
            return WITHOUT if @end_idx > stop
          end

          # the range starts at or above the given values
          # r is between start and stop
          #        rrrrrrrrrrrrrrr
          # start                   stop
          return WITHIN if @start_idx < stop && @end_idx <= stop
          # r starts within and finishes above stop
          #           rrrrrrrrrrrrr
          #   start       stop
          return HIGH_SPAN if @start_idx < stop && @end_idx > stop

          # the range is above the given values
          # starts and stops above
          #                   rrrrrrrrrrrrr
          #  start       stop
          ABOVE
        end

        private

        # Update range should be how start and end index of tags are changed,
        # as it includes validation
        #
        # Note that we allow start_idx == end_idx b/c this is how we determine
        # if a tag range is 'covered' in trigger detection
        ERROR_NEGATIVE_START = 'Unable to set start idx negative'
        ERROR_END_BEFORE_START = 'Unable to set start idx after end idx'
        def update_range start_idx, end_idx
          raise(ArgumentError, ERROR_NEGATIVE_START) if start_idx.negative?
          raise(ArgumentError, ERROR_END_BEFORE_START) if end_idx < start_idx

          @start_idx = start_idx
          @end_idx = end_idx
          @length = @end_idx - @start_idx
          @_str_val = nil
        end
      end
    end
  end
end