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# encoding:utf-8
#--
# Copyright (C) 2006-2011 Bob Aman
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
#++


require "addressable/version"
require "addressable/uri"

module Addressable
  ##
  # This is an implementation of a URI template based on
  # RFC 6570 (http://tools.ietf.org/html/rfc6570).
  class Template
    # Constants used throughout the template code.
    anything =
      Addressable::URI::CharacterClasses::RESERVED +
      Addressable::URI::CharacterClasses::UNRESERVED


    variable_char_class =
      Addressable::URI::CharacterClasses::ALPHA +
      Addressable::URI::CharacterClasses::DIGIT + '_'

    var_char =
      "(?:(?:[#{variable_char_class}]|%[a-fA-F0-9][a-fA-F0-9])+)"
    RESERVED =
      "(?:[#{anything}]|%[a-fA-F0-9][a-fA-F0-9])"
    UNRESERVED =
      "(?:[#{
        Addressable::URI::CharacterClasses::UNRESERVED
      }]|%[a-fA-F0-9][a-fA-F0-9])"
    variable =
      "(?:#{var_char}(?:\\.?#{var_char})*)"
    varspec =
      "(?:(#{variable})(\\*|:\\d+)?)"
    VARNAME =
      /^#{variable}$/
    VARSPEC =
      /^#{varspec}$/
    VARIABLE_LIST =
      /^#{varspec}(?:,#{varspec})*$/
    operator =
      "+#./;?&=,!@|"
    EXPRESSION =
      /\{([#{operator}])?(#{varspec}(?:,#{varspec})*)\}/


    LEADERS = {
      '?' => '?',
      '/' => '/',
      '#' => '#',
      '.' => '.',
      ';' => ';',
      '&' => '&'
    }
    JOINERS = {
      '?' => '&',
      '.' => '.',
      ';' => ';',
      '&' => '&',
      '/' => '/'
    }

    ##
    # Raised if an invalid template value is supplied.
    class InvalidTemplateValueError < StandardError
    end

    ##
    # Raised if an invalid template operator is used in a pattern.
    class InvalidTemplateOperatorError < StandardError
    end

    ##
    # Raised if an invalid template operator is used in a pattern.
    class TemplateOperatorAbortedError < StandardError
    end

    ##
    # This class represents the data that is extracted when a Template
    # is matched against a URI.
    class MatchData
      ##
      # Creates a new MatchData object.
      # MatchData objects should never be instantiated directly.
      #
      # @param [Addressable::URI] uri
      # The URI that the template was matched against.
      def initialize(uri, template, mapping)
        @uri = uri.dup.freeze
        @template = template
        @mapping = mapping.dup.freeze
      end

      ##
      # @return [Addressable::URI]
      # The URI that the Template was matched against.
      attr_reader :uri

      ##
      # @return [Addressable::Template]
      # The Template used for the match.
      attr_reader :template

      ##
      # @return [Hash]
      # The mapping that resulted from the match.
      # Note that this mapping does not include keys or values for
      # variables that appear in the Template, but are not present
      # in the URI.
      attr_reader :mapping

      ##
      # @return [Array]
      # The list of variables that were present in the Template.
      # Note that this list will include variables which do not appear
      # in the mapping because they were not present in URI.
      def variables
        self.template.variables
      end
      alias_method :keys, :variables

      ##
      # @return [Array]
      # The list of values that were captured by the Template.
      # Note that this list will include nils for any variables which
      # were in the Template, but did not appear in the URI.
      def values
        @values ||= self.variables.inject([]) do |accu, key|
          accu << self.mapping[key]
          accu
        end
      end
      alias_method :captures, :values

      ##
      # Returns a <tt>String</tt> representation of the MatchData's state.
      #
      # @return [String] The MatchData's state, as a <tt>String</tt>.
      def inspect
        sprintf("#<%s:%#0x RESULT:%s>",
          self.class.to_s, self.object_id, self.mapping.inspect)
      end
    end

    ##
    # Creates a new <tt>Addressable::Template</tt> object.
    #
    # @param [#to_str] pattern The URI Template pattern.
    #
    # @return [Addressable::Template] The initialized Template object.
    def initialize(pattern)
      if !pattern.respond_to?(:to_str)
        raise TypeError, "Can't convert #{pattern.class} into String."
      end
      @pattern = pattern.to_str.freeze
    end

    ##
    # @return [String] The Template object's pattern.
    attr_reader :pattern

    ##
    # Returns a <tt>String</tt> representation of the Template object's state.
    #
    # @return [String] The Template object's state, as a <tt>String</tt>.
    def inspect
      sprintf("#<%s:%#0x PATTERN:%s>",
        self.class.to_s, self.object_id, self.pattern)
    end

    ##
    # Extracts a mapping from the URI using a URI Template pattern.
    #
    # @param [Addressable::URI, #to_str] uri
    # The URI to extract from.
    #
    # @param [#restore, #match] processor
    # A template processor object may optionally be supplied.
    #
    # The object should respond to either the <tt>restore</tt> or
    # <tt>match</tt> messages or both. The <tt>restore</tt> method should
    # take two parameters: `[String] name` and `[String] value`.
    # The <tt>restore</tt> method should reverse any transformations that
    # have been performed on the value to ensure a valid URI.
    # The <tt>match</tt> method should take a single
    # parameter: `[String] name`. The <tt>match</tt> method should return
    # a <tt>String</tt> containing a regular expression capture group for
    # matching on that particular variable. The default value is `".*?"`.
    # The <tt>match</tt> method has no effect on multivariate operator
    # expansions.
    #
    # @return [Hash, NilClass]
    # The <tt>Hash</tt> mapping that was extracted from the URI, or
    # <tt>nil</tt> if the URI didn't match the template.
    #
    # @example
    # class ExampleProcessor
    # def self.restore(name, value)
    # return value.gsub(/\+/, " ") if name == "query"
    # return value
    # end
    #
    # def self.match(name)
    # return ".*?" if name == "first"
    # return ".*"
    # end
    # end
    #
    # uri = Addressable::URI.parse(
    # "http://example.com/search/an+example+search+query/"
    # )
    # Addressable::Template.new(
    # "http://example.com/search/{query}/"
    # ).extract(uri, ExampleProcessor)
    # #=> {"query" => "an example search query"}
    #
    # uri = Addressable::URI.parse("http://example.com/a/b/c/")
    # Addressable::Template.new(
    # "http://example.com/{first}/{second}/"
    # ).extract(uri, ExampleProcessor)
    # #=> {"first" => "a", "second" => "b/c"}
    #
    # uri = Addressable::URI.parse("http://example.com/a/b/c/")
    # Addressable::Template.new(
    # "http://example.com/{first}/{-list|/|second}/"
    # ).extract(uri)
    # #=> {"first" => "a", "second" => ["b", "c"]}
    def extract(uri, processor=nil)
      match_data = self.match(uri, processor)
      return (match_data ? match_data.mapping : nil)
    end

    ##
    # Extracts match data from the URI using a URI Template pattern.
    #
    # @param [Addressable::URI, #to_str] uri
    # The URI to extract from.
    #
    # @param [#restore, #match] processor
    # A template processor object may optionally be supplied.
    #
    # The object should respond to either the <tt>restore</tt> or
    # <tt>match</tt> messages or both. The <tt>restore</tt> method should
    # take two parameters: `[String] name` and `[String] value`.
    # The <tt>restore</tt> method should reverse any transformations that
    # have been performed on the value to ensure a valid URI.
    # The <tt>match</tt> method should take a single
    # parameter: `[String] name`. The <tt>match</tt> method should return
    # a <tt>String</tt> containing a regular expression capture group for
    # matching on that particular variable. The default value is `".*?"`.
    # The <tt>match</tt> method has no effect on multivariate operator
    # expansions.
    #
    # @return [Hash, NilClass]
    # The <tt>Hash</tt> mapping that was extracted from the URI, or
    # <tt>nil</tt> if the URI didn't match the template.
    #
    # @example
    # class ExampleProcessor
    # def self.restore(name, value)
    # return value.gsub(/\+/, " ") if name == "query"
    # return value
    # end
    #
    # def self.match(name)
    # return ".*?" if name == "first"
    # return ".*"
    # end
    # end
    #
    # uri = Addressable::URI.parse(
    # "http://example.com/search/an+example+search+query/"
    # )
    # match = Addressable::Template.new(
    # "http://example.com/search/{query}/"
    # ).match(uri, ExampleProcessor)
    # match.variables
    # #=> ["query"]
    # match.captures
    # #=> ["an example search query"]
    #
    # uri = Addressable::URI.parse("http://example.com/a/b/c/")
    # match = Addressable::Template.new(
    # "http://example.com/{first}/{+second}/"
    # ).match(uri, ExampleProcessor)
    # match.variables
    # #=> ["first", "second"]
    # match.captures
    # #=> ["a", "b/c"]
    #
    # uri = Addressable::URI.parse("http://example.com/a/b/c/")
    # match = Addressable::Template.new(
    # "http://example.com/{first}{/second*}/"
    # ).match(uri)
    # match.variables
    # #=> ["first", "second"]
    # match.captures
    # #=> ["a", ["b", "c"]]
    def match(uri, processor=nil)
      uri = Addressable::URI.parse(uri)
      mapping = {}

      # First, we need to process the pattern, and extract the values.
      expansions, expansion_regexp =
        parse_template_pattern(pattern, processor)

      return nil unless uri.to_str.match(expansion_regexp)
      unparsed_values = uri.to_str.scan(expansion_regexp).flatten

      if uri.to_str == pattern
        return Addressable::Template::MatchData.new(uri, self, mapping)
      elsif expansions.size > 0
        index = 0
        expansions.each do |expansion|
          _, operator, varlist = *expansion.match(EXPRESSION)
          varlist.split(',').each do |varspec|
            _, name, modifier = *varspec.match(VARSPEC)
            case operator
            when nil, '+', '#', '/', '.'
              unparsed_value = unparsed_values[index]
              name = varspec[VARSPEC, 1]
              value = unparsed_value
              value = value.split(JOINERS[operator]) if value && modifier == '*'
            when ';', '?', '&'
              if modifier == '*'
                value = unparsed_values[index].split(JOINERS[operator])
                value = value.inject({}) do |acc, v|
                  key, val = v.split('=')
                  val = "" if val.nil?
                  acc[key] = val
                  acc
                end
              else
                if (unparsed_values[index])
                  name, value = unparsed_values[index].split('=')
                  value = "" if value.nil?
                end
              end
            end
            if processor != nil && processor.respond_to?(:restore)
              value = processor.restore(name, value)
            end
            if processor == nil
              if value.is_a?(Hash)
                value = value.inject({}){|acc, (k, v)|
                  acc[Addressable::URI.unencode_component(k)] =
                    Addressable::URI.unencode_component(v)
                  acc
                }
              elsif value.is_a?(Array)
                value = value.map{|v| Addressable::URI.unencode_component(v) }
              else
                value = Addressable::URI.unencode_component(value)
              end
            end
            if mapping[name] == nil || mapping[name] == value
              mapping[name] = value
            else
              return nil
            end
            index = index + 1
          end
        end
        return Addressable::Template::MatchData.new(uri, self, mapping)
      else
        return nil
      end
    end

    ##
    # Expands a URI template into another URI template.
    #
    # @param [Hash] mapping The mapping that corresponds to the pattern.
    # @param [#validate, #transform] processor
    # An optional processor object may be supplied.
    #
    # The object should respond to either the <tt>validate</tt> or
    # <tt>transform</tt> messages or both. Both the <tt>validate</tt> and
    # <tt>transform</tt> methods should take two parameters: <tt>name</tt> and
    # <tt>value</tt>. The <tt>validate</tt> method should return <tt>true</tt>
    # or <tt>false</tt>; <tt>true</tt> if the value of the variable is valid,
    # <tt>false</tt> otherwise. An <tt>InvalidTemplateValueError</tt>
    # exception will be raised if the value is invalid. The <tt>transform</tt>
    # method should return the transformed variable value as a <tt>String</tt>.
    # If a <tt>transform</tt> method is used, the value will not be percent
    # encoded automatically. Unicode normalization will be performed both
    # before and after sending the value to the transform method.
    #
    # @return [Addressable::Template] The partially expanded URI template.
    #
    # @example
    # Addressable::Template.new(
    # "http://example.com/{one}/{two}/"
    # ).partial_expand({"one" => "1"}).pattern
    # #=> "http://example.com/1/{two}/"
    #
    # Addressable::Template.new(
    # "http://example.com/{?one,two}/"
    # ).partial_expand({"one" => "1"}).pattern
    # #=> "http://example.com/?one=1{&two}/"
    #
    # Addressable::Template.new(
    # "http://example.com/{?one,two,three}/"
    # ).partial_expand({"one" => "1", "three" => 3}).pattern
    # #=> "http://example.com/?one=1{&two}&three=3"
    def partial_expand(mapping, processor=nil)
      result = self.pattern.dup
      result.gsub!( EXPRESSION ) do |capture|
        transform_partial_capture(mapping, capture, processor)
      end
      return Addressable::Template.new(result)
    end

    ##
    # Expands a URI template into a full URI.
    #
    # @param [Hash] mapping The mapping that corresponds to the pattern.
    # @param [#validate, #transform] processor
    # An optional processor object may be supplied.
    #
    # The object should respond to either the <tt>validate</tt> or
    # <tt>transform</tt> messages or both. Both the <tt>validate</tt> and
    # <tt>transform</tt> methods should take two parameters: <tt>name</tt> and
    # <tt>value</tt>. The <tt>validate</tt> method should return <tt>true</tt>
    # or <tt>false</tt>; <tt>true</tt> if the value of the variable is valid,
    # <tt>false</tt> otherwise. An <tt>InvalidTemplateValueError</tt>
    # exception will be raised if the value is invalid. The <tt>transform</tt>
    # method should return the transformed variable value as a <tt>String</tt>.
    # If a <tt>transform</tt> method is used, the value will not be percent
    # encoded automatically. Unicode normalization will be performed both
    # before and after sending the value to the transform method.
    #
    # @return [Addressable::URI] The expanded URI template.
    #
    # @example
    # class ExampleProcessor
    # def self.validate(name, value)
    # return !!(value =~ /^[\w ]+$/) if name == "query"
    # return true
    # end
    #
    # def self.transform(name, value)
    # return value.gsub(/ /, "+") if name == "query"
    # return value
    # end
    # end
    #
    # Addressable::Template.new(
    # "http://example.com/search/{query}/"
    # ).expand(
    # {"query" => "an example search query"},
    # ExampleProcessor
    # ).to_str
    # #=> "http://example.com/search/an+example+search+query/"
    #
    # Addressable::Template.new(
    # "http://example.com/search/{query}/"
    # ).expand(
    # {"query" => "an example search query"}
    # ).to_str
    # #=> "http://example.com/search/an%20example%20search%20query/"
    #
    # Addressable::Template.new(
    # "http://example.com/search/{query}/"
    # ).expand(
    # {"query" => "bogus!"},
    # ExampleProcessor
    # ).to_str
    # #=> Addressable::Template::InvalidTemplateValueError
    def expand(mapping, processor=nil)
      result = self.pattern.dup
      mapping = normalize_keys(mapping)
      result.gsub!( EXPRESSION ) do |capture|
        transform_capture(mapping, capture, processor)
      end
      return Addressable::URI.parse(result)
    end

    ##
    # Returns an Array of variables used within the template pattern.
    # The variables are listed in the Array in the order they appear within
    # the pattern. Multiple occurrences of a variable within a pattern are
    # not represented in this Array.
    #
    # @return [Array] The variables present in the template's pattern.
    def variables
      @variables ||= ordered_variable_defaults.map { |var, val| var }.uniq
    end
    alias_method :keys, :variables

    ##
    # Returns a mapping of variables to their default values specified
    # in the template. Variables without defaults are not returned.
    #
    # @return [Hash] Mapping of template variables to their defaults
    def variable_defaults
      @variable_defaults ||=
        Hash[*ordered_variable_defaults.reject { |k, v| v.nil? }.flatten]
    end

  private
    def ordered_variable_defaults
      @ordered_variable_defaults ||= (
        expansions, expansion_regexp = parse_template_pattern(pattern)
        expansions.map do |capture|
          _, operator, varlist = *capture.match(EXPRESSION)
          varlist.split(',').map do |varspec|
            name = varspec[VARSPEC, 1]
          end
        end.flatten
      )
    end


    ##
    # Loops through each capture and expands any values available in mapping
    #
    # @param [Hash] mapping
    # Set of keys to expand
    # @param [String] capture
    # The expression to expand
    # @param [#validate, #transform] processor
    # An optional processor object may be supplied.
    #
    # The object should respond to either the <tt>validate</tt> or
    # <tt>transform</tt> messages or both. Both the <tt>validate</tt> and
    # <tt>transform</tt> methods should take two parameters: <tt>name</tt> and
    # <tt>value</tt>. The <tt>validate</tt> method should return <tt>true</tt>
    # or <tt>false</tt>; <tt>true</tt> if the value of the variable is valid,
    # <tt>false</tt> otherwise. An <tt>InvalidTemplateValueError</tt> exception
    # will be raised if the value is invalid. The <tt>transform</tt> method
    # should return the transformed variable value as a <tt>String</tt>. If a
    # <tt>transform</tt> method is used, the value will not be percent encoded
    # automatically. Unicode normalization will be performed both before and
    # after sending the value to the transform method.
    #
    # @return [String] The expanded expression
    def transform_partial_capture(mapping, capture, processor = nil)
      _, operator, varlist = *capture.match(EXPRESSION)
      is_first = true
      varlist.split(',').inject('') do |acc, varspec|
        _, name, modifier = *varspec.match(VARSPEC)
        value = mapping[name]
        if value
          operator = '&' if !is_first && operator == '?'
          acc << transform_capture(mapping, "{#{operator}#{varspec}}", processor)
        else
          operator = '&' if !is_first && operator == '?'
          acc << "{#{operator}#{varspec}}"
        end
        is_first = false
        acc
      end
    end

    ##
    # Transforms a mapped value so that values can be substituted into the
    # template.
    #
    # @param [Hash] mapping The mapping to replace captures
    # @param [String] capture
    # The expression to replace
    # @param [#validate, #transform] processor
    # An optional processor object may be supplied.
    #
    # The object should respond to either the <tt>validate</tt> or
    # <tt>transform</tt> messages or both. Both the <tt>validate</tt> and
    # <tt>transform</tt> methods should take two parameters: <tt>name</tt> and
    # <tt>value</tt>. The <tt>validate</tt> method should return <tt>true</tt>
    # or <tt>false</tt>; <tt>true</tt> if the value of the variable is valid,
    # <tt>false</tt> otherwise. An <tt>InvalidTemplateValueError</tt> exception
    # will be raised if the value is invalid. The <tt>transform</tt> method
    # should return the transformed variable value as a <tt>String</tt>. If a
    # <tt>transform</tt> method is used, the value will not be percent encoded
    # automatically. Unicode normalization will be performed both before and
    # after sending the value to the transform method.
    #
    # @return [String] The expanded expression
    def transform_capture(mapping, capture, processor=nil)
      _, operator, varlist = *capture.match(EXPRESSION)
      return_value = varlist.split(',').inject([]) do |acc, varspec|
        _, name, modifier = *varspec.match(VARSPEC)
        value = mapping[name]
        unless value == nil || value == {}
          allow_reserved = %w(+ #).include?(operator)
          value = value.to_s if Numeric === value || Symbol === value
          length = modifier.gsub(':', '').to_i if modifier =~ /^:\d+/

          unless (Hash === value) ||
            value.respond_to?(:to_ary) || value.respond_to?(:to_str)
            raise TypeError,
              "Can't convert #{value.class} into String or Array."
          end

          value = normalize_value(value)

          if processor == nil || !processor.respond_to?(:transform)
            # Handle percent escaping
            if allow_reserved
              encode_map =
                Addressable::URI::CharacterClasses::RESERVED +
                Addressable::URI::CharacterClasses::UNRESERVED
            else
              encode_map = Addressable::URI::CharacterClasses::UNRESERVED
            end
            if value.kind_of?(Array)
              transformed_value = value.map do |val|
                if length
                  Addressable::URI.encode_component(val[0...length], encode_map)
                else
                  Addressable::URI.encode_component(val, encode_map)
                end
              end
              unless modifier == "*"
                transformed_value = transformed_value.join(',')
              end
            elsif value.kind_of?(Hash)
              transformed_value = value.map do |key, val|
                if modifier == "*"
                  "#{
                    Addressable::URI.encode_component( key, encode_map)
                  }=#{
                    Addressable::URI.encode_component( val, encode_map)
                  }"
                else
                  "#{
                    Addressable::URI.encode_component( key, encode_map)
                  },#{
                    Addressable::URI.encode_component( val, encode_map)
                  }"
                end
              end
              unless modifier == "*"
                transformed_value = transformed_value.join(',')
              end
            else
              if length
                transformed_value = Addressable::URI.encode_component(
                  value[0...length], encode_map)
              else
                transformed_value = Addressable::URI.encode_component(
                  value, encode_map)
              end
            end
          end

          # Process, if we've got a processor
          if processor != nil
            if processor.respond_to?(:validate)
              if !processor.validate(name, value)
                display_value = value.kind_of?(Array) ? value.inspect : value
                raise InvalidTemplateValueError,
                  "#{name}=#{display_value} is an invalid template value."
              end
            end
            if processor.respond_to?(:transform)
              transformed_value = processor.transform(name, value)
              transformed_value = normalize_value(transformed_value)
            end
          end
          acc << [name, transformed_value]
        end
        acc
      end
      return "" if return_value.empty?
      join_values(operator, return_value)
    end

    ##
    # Takes a set of values, and joins them together based on the
    # operator.
    #
    # @param [String, Nil] operator One of the operators from the set
    # (?,&,+,#,;,/,.), or nil if there wasn't one.
    # @param [Array] return_value
    # The set of return values (as [variable_name, value] tuples) that will
    # be joined together.
    #
    # @return [String] The transformed mapped value
    def join_values(operator, return_value)
      leader = LEADERS.fetch(operator, '')
      joiner = JOINERS.fetch(operator, ',')
      case operator
      when '&', '?'
        leader + return_value.map{|k,v|
          if v.is_a?(Array) && v.first =~ /=/
            v.join(joiner)
          elsif v.is_a?(Array)
            v.map{|inner_value| "#{k}=#{inner_value}"}.join(joiner)
          else
            "#{k}=#{v}"
          end
        }.join(joiner)
      when ';'
        return_value.map{|k,v|
          if v.is_a?(Array) && v.first =~ /=/
            ';' + v.join(";")
          elsif v.is_a?(Array)
            ';' + v.map{|inner_value| "#{k}=#{inner_value}"}.join(";")
          else
            v && v != '' ? ";#{k}=#{v}" : ";#{k}"
          end
        }.join
      else
        leader + return_value.map{|k,v| v}.join(joiner)
      end
    end

    ##
    # Takes a set of values, and joins them together based on the
    # operator.
    #
    # @param [Hash, Array, String] value
    # Normalizes keys and values with IDNA#unicode_normalize_kc
    #
    # @return [Hash, Array, String] The normalized values
    def normalize_value(value)
      unless value.is_a?(Hash)
        value = value.respond_to?(:to_ary) ? value.to_ary : value.to_str
      end

      # Handle unicode normalization
      if value.kind_of?(Array)
        value.map! { |val| Addressable::IDNA.unicode_normalize_kc(val) }
      elsif value.kind_of?(Hash)
        value = value.inject({}) { |acc, (k, v)|
          acc[Addressable::IDNA.unicode_normalize_kc(k)] =
            Addressable::IDNA.unicode_normalize_kc(v)
          acc
        }
      else
        value = Addressable::IDNA.unicode_normalize_kc(value)
      end
      value
    end

    ##
    # Generates a hash with string keys
    #
    # @param [Hash] mapping A mapping hash to normalize
    #
    # @return [Hash]
    # A hash with stringified keys
    def normalize_keys(mapping)
      return mapping.inject({}) do |accu, pair|
        name, value = pair
        if Symbol === name
          name = name.to_s
        elsif name.respond_to?(:to_str)
          name = name.to_str
        else
          raise TypeError,
            "Can't convert #{name.class} into String."
        end
        accu[name] = value
        accu
      end
    end

    ##
    # Generates the <tt>Regexp</tt> that parses a template pattern.
    #
    # @param [String] pattern The URI template pattern.
    # @param [#match] processor The template processor to use.
    #
    # @return [Regexp]
    # A regular expression which may be used to parse a template pattern.
    def parse_template_pattern(pattern, processor=nil)
      # Escape the pattern. The two gsubs restore the escaped curly braces
      # back to their original form. Basically, escape everything that isn't
      # within an expansion.
      escaped_pattern = Regexp.escape(
        pattern
      ).gsub(/\\\{(.*?)\\\}/) do |escaped|
        escaped.gsub(/\\(.)/, "\\1")
      end

      expansions = []

      # Create a regular expression that captures the values of the
      # variables in the URI.
      regexp_string = escaped_pattern.gsub( EXPRESSION ) do |expansion|

        expansions << expansion
        _, operator, varlist = *expansion.match(EXPRESSION)
        leader = Regexp.escape(LEADERS.fetch(operator, ''))
        joiner = Regexp.escape(JOINERS.fetch(operator, ','))
        leader + varlist.split(',').map do |varspec|
          _, name, modifier = *varspec.match(VARSPEC)
          if processor != nil && processor.respond_to?(:match)
            "(#{ processor.match(name) })"
          else
            group = case operator
            when '+'
              "#{ RESERVED }*?"
            when '#'
              "#{ RESERVED }*?"
            when '/'
              "#{ UNRESERVED }*?"
            when '.'
              "#{ UNRESERVED.gsub('\.', '') }*?"
            when ';'
              "#{ UNRESERVED }*=?#{ UNRESERVED }*?"
            when '?'
              "#{ UNRESERVED }*=#{ UNRESERVED }*?"
            when '&'
              "#{ UNRESERVED }*=#{ UNRESERVED }*?"
            else
              "#{ UNRESERVED }*?"
            end
            if modifier == '*'
              "(#{group}(?:#{joiner}?#{group})*)?"
            else
              "(#{group})?"
            end
          end
        end.join("#{joiner}?")
      end

      # Ensure that the regular expression matches the whole URI.
      regexp_string = "^#{regexp_string}$"
      return expansions, Regexp.new(regexp_string)
    end

  end
end
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