=begin
    Copyright 2010-2016 Tasos Laskos <tasos.laskos@arachni-scanner.com>

    This file is part of the Arachni Framework project and is subject to
    redistribution and commercial restrictions. Please see the Arachni Framework
    web site for more information on licensing and terms of use.
=end

module Arachni::Support

# Represents a signature, used to maintain a lightweight representation of a
# {String} and refine it using similar {String}s to remove noise.
#
# @author Tasos "Zapotek" Laskos <tasos.laskos@arachni-scanner.com>
class Signature

    CACHE = {
        tokens: Cache::LeastRecentlyPushed.new( 100 )
    }

    attr_reader :tokens

    # @note The string will be tokenized based on whitespace.
    #
    # @param    [String, Signature]    data
    #   Seed data to use to initialize the signature.
    # @param    [Hash]    options
    # @option   options :threshold  [Float]
    #   Sets the maximum allowed {#differences} when performing
    #   {#similar? similarity} comparisons.
    def initialize( data, options = {} )
        @tokens  = tokenize( data )
        @options = options

        if @options[:threshold] && !@options[:threshold].is_a?( Numeric )
            fail ArgumentError, 'Option :threshold must be a number.'
        end
    end

    # @note The string will be tokenized based on whitespace.
    #
    # @param    [String, Signature]    data
    #   Data to use to refine the signature.
    #
    # @return   [Signature]
    #   `self`
    def refine!( data )
        @tokens &= tokenize( data )
        self
    end

    # @note The string will be tokenized based on whitespace.
    #
    # @param    [String, Signature]    data
    #   Data to use to refine the signature.
    #
    # @return   [Signature]
    #   New, refined signature.
    def refine( data )
        dup.refine!( data )
    end

    # @param    [Signature] other
    #
    # @return   [Float]
    #   Ratio of difference between signatures.
    def differences( other )
        return 1 if other.nil?
        return 0 if self == other

        ((tokens - other.tokens) | (other.tokens - tokens)).size /
            Float((other.tokens | tokens).size)
    end

    # @param    [Signature] other
    # @param    [Integer] threshold
    #   Threshold of {#differences differences}.
    #
    # @return   [Bool]
    def similar?( other, threshold = @options[:threshold] )
        fail 'No threshold given.' if !threshold
        self == other || differences( other ) < threshold
    end

    # @return [Signature]
    #   Copy of `self`.
    def dup
        self.class.new( '' ).tap { |s| s.copy( tokens, @options ) }
    end

    def hash
        tokens.hash
    end

    # @param [Signature]    other
    def ==( other )
        hash == other.hash
    end

    protected

    def copy( tokens, options )
        @tokens  = tokens.dup
        @options = options.dup
    end

    private

    # @param    [Signature, String] data
    #
    # @return [Array<String,Integer>]
    #   Words as tokens.
    def tokenize( data )
        return data.tokens if data.is_a? self.class

        if CACHE[:tokens][data]
            CACHE[:tokens][data].dup
        else
            CACHE[:tokens][data] = compress( data.split( /\W/ ) )
        end
    end

    # Compresses the tokens by only storing unique #hash values.
    # Seems kinda silly but this can actually save us GB of RAM when comparing
    # large signatures, not to mention CPU cycles.
    def compress( tokens )
        s = Set.new
        tokens.each do |token|
            # Left-over non-word characters will be on their own, this is a
            # low-overhead way to dispose of them.
            next if token.empty?

            s << token.hash
        end
        s
    end

end
end