require 'mathn'
require 'rational'
require 'date'
require 'parsedate'
# = Ruby Units 0.2.1
#
# Copyright 2006 by Kevin C. Olbrich, Ph.D.
# 
# See http://rubyforge.org/ruby-units/
#
# http://www.sciwerks.org
#
# mailto://kevin.olbrich+ruby-units@gmail.com
#
# See README for detailed usage instructions and examples
#
# ==Unit Definition Format
#
#  '<name>'  => [%w{prefered_name synonyms}, conversion_to_base, :classification, %w{<base> <units> <in> <numerator>} , %w{<base> <units> <in> <denominator>} ],
#
# Prefixes (e.g., a :prefix classification) get special handling
# Note: The accuracy of unit conversions depends on the precision of the conversion factor.
# If you have more accurate estimates for particular conversion factors, please send them 
# to me and I will incorporate them into the next release.  It is also incumbent on the end-user
# to ensure that the accuracy of any conversions is sufficient for their intended application.
#
# While there are a large number of unit specified in the base package, 
# there are also a large number of units that are not included.
# This package covers nearly all SI, Imperial, and units commonly used
# in the United States. If your favorite units are not listed here, send me an email
#
# To add / override a unit definition, add a code block like this..
#
#  class Unit < Numeric
#   UNIT_DEFINITIONS = {
#    <name>'  => [%w{prefered_name synonyms}, conversion_to_base, :classification, %w{<base> <units> <in> <numerator>} , %w{<base> <units> <in> <denominator>} ]
#    }
#  end
#  Unit.setup
class Unit < Numeric
  require 'units'
  # pre-generate hashes from unit definitions for performance.  
  @@USER_DEFINITIONS = {}
  @@PREFIX_VALUES = {}
  @@PREFIX_MAP = {}
  @@UNIT_MAP = {}
  @@UNIT_VALUES = {}
  @@OUTPUT_MAP = {}
  @@UNIT_VECTORS = {}

  def self.setup
    (UNIT_DEFINITIONS.merge!(@@USER_DEFINITIONS)).each do |key, value|
      if value[2] == :prefix then
        @@PREFIX_VALUES[Regexp.escape(key)]=value[1]
        value[0].each {|x| @@PREFIX_MAP[Regexp.escape(x)]=key}    
      else
        @@UNIT_VALUES[Regexp.escape(key)]={}
        @@UNIT_VALUES[Regexp.escape(key)][:scalar]=value[1]
        @@UNIT_VALUES[Regexp.escape(key)][:numerator]=value[3] if value[3]
        @@UNIT_VALUES[Regexp.escape(key)][:denominator]=value[4] if value[4]
        value[0].each {|x| @@UNIT_MAP[Regexp.escape(x)]=key}
        @@UNIT_VECTORS[value[2]] = [] unless @@UNIT_VECTORS[value[2]]
        @@UNIT_VECTORS[value[2]] = @@UNIT_VECTORS[value[2]]+[Regexp.escape(key)]
      end
      @@OUTPUT_MAP[Regexp.escape(key)]=value[0][0]
    end
    @@PREFIX_REGEX = @@PREFIX_MAP.keys.sort_by {|prefix| prefix.length}.reverse.join('|')
    @@UNIT_REGEX = @@UNIT_MAP.keys.sort_by {|unit| unit.length}.reverse.join('|')
  end
  
  self.setup
  
  include Comparable
  attr_accessor :scalar, :numerator, :denominator, :signature, :base_scalar


  def to_yaml_properties
    %w{@scalar @numerator @denominator @signature @base_scalar}
  end
  
  # basically a copy of the basic to_yaml.  Needed because otherwise it ends up coercing the object to a string
  # before YAML'izing it.
  def to_yaml( opts = {} )
    YAML::quick_emit( object_id, opts ) do |out|
      out.map( taguri, to_yaml_style ) do |map|
        to_yaml_properties.each do |m|
          map.add( m[1..-1], instance_variable_get( m ) )
        end
      end
    end
  end
  
  # Create a new Unit object.  Can be initialized using a string, or a hash
  # Valid formats include:
  #  "5.6 kg*m/s^2"
  #  "5.6 kg*m*s^-2"
  #  "5.6 kilogram*meter*second^-2"
  #  "2.2 kPa"
  #  "37 degC"
  #  "1"  -- creates a unitless constant with value 1
  #  "GPa"  -- creates a unit with scalar 1 with units 'GPa'
  #  6'4"  -- recognized as 6 feet + 4 inches 
  #  8 lbs 8 oz -- recognized as 8 lbs + 8 ounces
  #
  def initialize(options)    
    case options
    when String: parse(options)
    when Hash:
      @scalar = options[:scalar] || 1
      @numerator = options[:numerator] || ["<1>"]
      @denominator = options[:denominator] || []
    when Array: 
      parse("#{options[0]} #{options[1]}/#{options[2]}")
    when Numeric:
      @scalar = options
      @numerator = @denominator = ['<1>']
    when Time:
      @scalar = options.to_f
      @numerator = ['<second>']
      @denominator = ['<1>']
    else
      raise ArgumentError, "Invalid Unit Format"
    end
    self.update_base_scalar
    self.freeze
  end
  
  def initialize_copy(other)
    @numerator = other.numerator.clone
    @denominator = other.denominator.clone
  end
    
  # Returns 'true' if the Unit is represented in base units
  def is_base?
    return true if @signature == 400 && @numerator.size == 1 && @numerator[0] =~ /(celcius|kelvin|farenheit|rankine)/
    n = @numerator + @denominator
    n.compact.each do |x|
      return false unless x == '<1>' || (@@UNIT_VALUES[Regexp.escape(x)] &&  @@UNIT_VALUES[Regexp.escape(x)][:denominator].nil? && @@UNIT_VALUES[Regexp.escape(x)][:numerator].include?(Regexp.escape(x)))
    end
    return true
  end  
  
  #convert to base SI units
  def to_base
    return self if self.is_base?
    num = []
    den = []
    q = @scalar
    @numerator.compact.each do |unit|
        if @@PREFIX_VALUES[Regexp.escape(unit)]
          q *= @@PREFIX_VALUES[Regexp.escape(unit)]
        else
          q *= @@UNIT_VALUES[Regexp.escape(unit)][:scalar] if @@UNIT_VALUES[Regexp.escape(unit)]
          num << @@UNIT_VALUES[Regexp.escape(unit)][:numerator] if @@UNIT_VALUES[Regexp.escape(unit)] && @@UNIT_VALUES[Regexp.escape(unit)][:numerator]
          den << @@UNIT_VALUES[Regexp.escape(unit)][:denominator] if @@UNIT_VALUES[Regexp.escape(unit)] && @@UNIT_VALUES[Regexp.escape(unit)][:denominator]
        end
    end
    @denominator.compact.each do |unit|
        if @@PREFIX_VALUES[Regexp.escape(unit)]
          q /= @@PREFIX_VALUES[Regexp.escape(unit)]
        else
          q /= @@UNIT_VALUES[Regexp.escape(unit)][:scalar] if @@UNIT_VALUES[Regexp.escape(unit)]
          den << @@UNIT_VALUES[Regexp.escape(unit)][:numerator] if @@UNIT_VALUES[Regexp.escape(unit)] && @@UNIT_VALUES[Regexp.escape(unit)][:numerator]
          num << @@UNIT_VALUES[Regexp.escape(unit)][:denominator] if @@UNIT_VALUES[Regexp.escape(unit)] && @@UNIT_VALUES[Regexp.escape(unit)][:denominator]
        end
    end
    
    num = num.flatten.compact
    den = den.flatten.compact
    num = ['<1>'] if num.empty?
   
    Unit.new(Unit.eliminate_terms(q,num,den))
  end
  
  # Generate human readable output.
  # If the name of a unit is passed, the scalar will first be converted to the target unit before output.
  # some named conversions are available
  #
  #  :ft - outputs in feet and inches (e.g., 6'4")
  #  :lbs - outputs in pounds and ounces (e.g, 8 lbs, 8 oz)
  def to_s(target_units=nil)
    case target_units
    when :ft:
      inches = self.to("in").scalar
      "#{(inches / 12).truncate}\'#{(inches % 12).round}\""
    when :lbs:
      ounces = self.to("oz").scalar
      "#{(ounces / 16).truncate} lbs, #{(ounces % 16).round} oz"
    when String
      begin #first try a standard format string
        target_units =~ /(%[\w\d#+-.]*)*\s*(.+)*/
        return self.to($2).to_s($1) if $2
        "#{($1 || '%g') % @scalar || 0} #{self.units}".strip
      rescue #if that is malformed, try a time string
        return (Time.gm(0) + self).strftime(target_units)
      end
    else
      "#{'%g' % @scalar} #{self.units}".strip
    end    
  end
  
  def inspect(option=nil)
    return super() if option == :dump
    self.to_s
  end
  
  # returns true if no associated units
  def unitless?
    (@numerator == ['<1>'] && @denominator == ['<1>'])
  end
  
  # Compare two Unit objects. Throws an exception if they are not of compatible types.
  # Comparisons are done based on the value of the unit in base SI units.
  def <=>(other)
    case other
    when Unit:
      raise ArgumentError, "Incompatible Units" unless self =~ other
      self.base_scalar <=> other.base_scalar
    else
      x,y = coerce(other)
      x <=> y
    end
  end
  
  # check to see if units are compatible, but not the scalar part
  # this check is done by comparing signatures for performance reasons
  # if passed a string, it will create a unit object with the string and then do the comparison
  # this permits a syntax like:  
  #  unit =~ "mm"
  # if you want to do a regexp on the unit string do this ...
  #  unit.units =~ /regexp/
  def =~(other)
    case other
    when Unit : self.signature == other.signature
    else
      x,y = coerce(other)
      x =~ y
    end 
  end
  
  alias :compatible? :=~
  alias :compatible_with? :=~
  
  # Compare two units.  Returns true if quantities and units match
  #
  # Unit("100 cm") === Unit("100 cm")   # => true
  # Unit("100 cm") === Unit("1 m")      # => false
  def ===(other)
    case other
    when Unit: (self.scalar == other.scalar) && (self.units == other.units)
    else
      x,y = coerce(other)
      x === y
    end
  end
  
  alias :same? :===
  alias :same_as? :===
  
  # Add two units together.  Result is same units as receiver and scalar and base_scalar are updated appropriately
  # throws an exception if the units are not compatible.
  def +(other)   
    if Unit === other 
      if self =~ other then
        q = @scalar + other.to(self).scalar
        Unit.new(:scalar=>q, :numerator=>@numerator, :denominator=>@denominator)
      else
         raise ArgumentError,  "Incompatible Units"
      end
    elsif Time === other
      other + self
    else
        x,y = coerce(other)
        y + x
    end
  end
  
  # Subtract two units. Result is same units as receiver and scalar and base_scalar are updated appropriately
  # throws an exception if the units are not compatible.
  def -(other)
    if Unit === other 
      if self =~ other then
        q = @scalar - other.to(self).scalar
        Unit.new(:scalar=>q, :numerator=>@numerator, :denominator=>@denominator)
      else
         raise ArgumentError,  "Incompatible Units"
      end
    elsif Time === other
      other + self
    else
        x,y = coerce(other)
        x - y
    end
  end
  
  # Multiply two units.
  def *(other)
    if Unit === other
      Unit.new(Unit.eliminate_terms(@scalar*other.scalar, @numerator + other.numerator ,@denominator + other.denominator))      
    else
      x,y = coerce(other)
      x * y
    end
  end
  
  # Divide two units.
  # Throws an exception if divisor is 0
  def /(other)
    if Unit === other
      raise ZeroDivisionError if other.zero?
      Unit.new(Unit.eliminate_terms(@scalar/other.scalar, @numerator + other.denominator ,@denominator + other.numerator))      
    else
      x,y = coerce(other)
      y / x
    end
  end
    
  # Exponentiate.  Only takes integer powers. 
  # Note that anything raised to the power of 0 results in a Unit object with a scalar of 1, and no units.
  # Throws an exception if exponent is not an integer.
  # Ideally this routine should accept a float for the exponent
  # It should then convert the float to a rational and raise the unit by the numerator and root it by the denominator
  # but, sadly, floats can't be converted to rationals.
  #
  # For now, if a rational is passed in, it will be used, otherwise we are stuck with integers and certain floats < 1
  def **(other)
    if Numeric === other
      return Unit("1") if other.zero?
      return self if other == 1
      return self.inverse if other == -1
    end
    case other
    when Rational:
      self.power(other.numerator).root(other.denominator)
    when Integer:
      self.power(other)
    when Float:
      return self**(other.to_i) if other == other.to_i
      valid = (1..9).map {|x| 1/x}
      raise ArgumentError, "Not a n-th root (1..9), use 1/n" unless valid.include? other.abs
      self.root((1/other).to_int)
    else
      raise ArgumentError, "Invalid Exponent"
    end
  end
  

  # returns the unit raised to the n-th power.  Integers only
  def power(n)
    raise ArgumentError, "Can only use Integer exponenents" unless Integer === n
    return self if n == 1
    return Unit("1") if n == 0
    return self.inverse if n == -1
    if n > 0 then 
      (1..n.to_i).inject(Unit.new("1")) {|product, x| product * self}
    else
      (1..-n.to_i).inject(Unit.new("1")) {|product, x| product / self}
    end
  end
  
  # Calculates the n-th root of a unit, where n = (1..9)
  # if n < 0, returns 1/unit^(1/n)
  def root(n)
    raise ArgumentError, "Exponent must an Integer" unless Integer === n
    raise ArgumentError, "0th root undefined" if n == 0
    return self if n == 1
    return self.root(n.abs).inverse if n < 0
    
    vec = self.unit_signature_vector
    vec=vec.map {|x| x % n}      
    raise ArgumentError, "Illegal root" unless vec.max == 0
    num = @numerator.clone
    den = @denominator.clone
      
    @numerator.uniq.each do |item|
      x = num.find_all {|i| i==item}.size
      r = ((x/n)*(n-1)).to_int
      r.times {|x| num.delete_at(num.index(item))}
    end
    
    @denominator.uniq.each do |item|
      x = den.find_all {|i| i==item}.size
      r = ((x/n)*(n-1)).to_int
      r.times {|x| den.delete_at(den.index(item))}
    end
    q = @scalar**(1/n)    
    Unit.new(:scalar=>q,:numerator=>num,:denominator=>den)    
  end
  
  # returns inverse of Unit (1/unit)
  def inverse
    Unit("1") / self
  end
  
  # convert to a specified unit string or to the same units as another Unit
  # 
  #  unit >> "kg"  will covert to kilograms
  #  unit1 >> unit2 converts to same units as unit2 object
  # 
  # To convert a Unit object to match another Unit object, use:
  #  unit1 >>= unit2
  # Throws an exception if the requested target units are incompatible with current Unit.
  #
  # Special handling for temperature conversions is supported.  If the Unit object is converted
  # from one temperature unit to another, the proper temperature offsets will be used.
  # Supports Kelvin, Celcius, Farenheit, and Rankine scales.
  #
  # Note that if temperature is part of a compound unit, the temperature will be treated as a differential
  # and the units will be scaled appropriately.
  def to(other)
    return self if other.nil? 
    return self if TrueClass === other
    return self if FalseClass === other
    if String === other && other =~ /temp(K|C|R|F)/
      raise ArgumentError, "Receiver is not a temperature unit" unless self.signature==400 
      return self.to_base.to(other) unless self.is_base?
      #return self.to_base.to("tempF") if @numerator.size > 1 || @denominator != ['<1>']
      q=case 
         when @numerator.include?('<celcius>'):
          case other
            when 'tempC'  : @scalar
            when 'tempK'  : @scalar + 273.15
            when 'tempF'  : @scalar * (9.0/5.0) + 32.0
            when 'tempR'  : @scalar * (9.0/5.0) + 491.67
          end
        when @numerator.include?( '<kelvin>'):
          case other
            when 'tempC'  : @scalar - 273.15
            when 'tempK'  : @scalar 
            when 'tempF'  : @scalar * (9.0/5.0) - 459.67
            when 'tempR'  : @scalar * (9.0/5.0) 
          end
        when @numerator.include?( '<farenheit>'):
          case other
            when 'tempC'  : (@scalar-32)*(5.0/9.0)
            when 'tempK'  : (@scalar+459.67)*(5.0/9.0)
            when 'tempF'  : @scalar 
            when 'tempR'  : @scalar  + 459.67
          end
        when @numerator.include?( '<rankine>'):
          case other
            when 'tempC'  : @scalar*(5.0/9.0) -273.15
            when 'tempK'  : @scalar*(5.0/9.0)
            when 'tempF'  : @scalar - 459.67
            when 'tempR'  : @scalar
          end
        else
          raise ArgumentError, "Unknown temperature conversion requested #{self.numerator}"
      end
      Unit.new("#{q} deg#{$1}")
    else
       case other
          when Unit: 
            return self if other.units == self.units
            target = other
          when String: target = Unit.new(other)
          else
            raise ArgumentError, "Unknown target units"
        end
      raise ArgumentError,  "Incompatible Units" unless self =~ target
      one = @numerator.map {|x| @@PREFIX_VALUES[Regexp.escape(x)] ? @@PREFIX_VALUES[Regexp.escape(x)] : x}.map {|i| i.kind_of?(Numeric) ? i : @@UNIT_VALUES[Regexp.escape(i)][:scalar] }.compact
      two = @denominator.map {|x| @@PREFIX_VALUES[Regexp.escape(x)] ? @@PREFIX_VALUES[Regexp.escape(x)] : x}.map {|i| i.kind_of?(Numeric) ? i : @@UNIT_VALUES[Regexp.escape(i)][:scalar] }.compact
      v = one.inject(1) {|product,n| product*n} / two.inject(1) {|product,n| product*n}
      one = target.numerator.map {|x| @@PREFIX_VALUES[Regexp.escape(x)] ? @@PREFIX_VALUES[Regexp.escape(x)] : x}.map {|x| x.kind_of?(Numeric) ? x : @@UNIT_VALUES[Regexp.escape(x)][:scalar] }.compact
      two = target.denominator.map {|x| @@PREFIX_VALUES[Regexp.escape(x)] ? @@PREFIX_VALUES[Regexp.escape(x)] : x}.map {|x| x.kind_of?(Numeric) ? x : @@UNIT_VALUES[Regexp.escape(x)][:scalar] }.compact
      y = one.inject(1) {|product,n| product*n} / two.inject(1) {|product,n| product*n}
      q = @scalar * v/y
      Unit.new(:scalar=>q, :numerator=>target.numerator, :denominator=>target.denominator)
    end
  end  
  
  alias :>> :to  
  # calculates the unit signature vector used by unit_signature
  def unit_signature_vector
      return self.to_base.unit_signature_vector unless self.is_base?
      result = self
      y = [:length, :time, :temperature, :mass, :current, :substance, :luminosity, :currency, :memory, :angle]
      vector = Array.new(y.size,0)
      y.each_with_index do |units,index|
        vector[index] = result.numerator.compact.find_all {|x| @@UNIT_VECTORS[units].include? Regexp.escape(x)}.size
        vector[index] -= result.denominator.compact.find_all {|x| @@UNIT_VECTORS[units].include? Regexp.escape(x)}.size  
      end
      vector
  end
  
  # calculates the unit signature id for use in comparing compatible units and simplification
  # the signature is based on a simple classification of units and is based on the following publication
  #  
  #  Novak, G.S., Jr. "Conversion of units of measurement", IEEE Transactions on Software Engineering,
  #  21(8), Aug 1995, pp.651-661
  #  doi://10.1109/32.403789
  #  http://ieeexplore.ieee.org/Xplore/login.jsp?url=/iel1/32/9079/00403789.pdf?isnumber=9079&prod=JNL&arnumber=403789&arSt=651&ared=661&arAuthor=Novak%2C+G.S.%2C+Jr.
  #
  def unit_signature
    vector = unit_signature_vector
    vector.each_with_index {|item,index| vector[index] = item * 20**index}
    @signature=vector.inject(0) {|sum,n| sum+n}
  end
  
  # Eliminates terms in the passed numerator and denominator.  Expands out prefixes and applies them to the 
  # scalar.  Returns a hash that can be used to initialize a new Unit object. 
  def self.eliminate_terms(q, n, d)
    num = n.clone
    den = d.clone
    
    num.delete_if {|v| v == '<1>'}
    den.delete_if {|v| v == '<1>'}
    combined = Hash.new(0)
 
    i = 0
    loop do 
      break if i > num.size
      if @@PREFIX_VALUES.has_key? num[i]
        k = [num[i],num[i+1]]
        i += 2
      else
        k = num[i]
        i += 1
      end
      combined[k] += 1 unless k.nil? || k == '<1>'
    end
    
    j = 0
    loop do
      break if j > den.size
        if @@PREFIX_VALUES.has_key? den[j]
          k = [den[j],den[j+1]]
          j += 2
        else
          k = den[j]
          j += 1
        end
      combined[k] -= 1 unless k.nil? || k == '<1>'
    end
 
    num = []
    den = []
    combined.each do |key,value| 
      case 
      when value > 0 : value.times {num << key}
      when value < 0 : value.abs.times {den << key}
      end
    end
    num = ["<1>"] if num.empty?
    den = ["<1>"] if den.empty?
    {:scalar=>q, :numerator=>num.flatten.compact, :denominator=>den.flatten.compact}
  end
      
  # converts the unit back to a float if it is unitless
  def to_f
    return @scalar.to_f if self.unitless?
    raise RuntimeError, "Can't convert to float unless unitless.  Use Unit#scalar"
  end
  
  # returns the 'unit' part of the Unit object without the scalar
  def units
    return "" if @numerator == ["<1>"] && @denominator == ["<1>"]
    output_n = []
    output_d =[] 
    num = @numerator.clone.compact
    den = @denominator.clone.compact
    if @numerator == ["<1>"]
      output_n << "1"
    else
      num.each_with_index do |token,index|
        if token && @@PREFIX_VALUES[Regexp.escape(token)] then
          output_n << "#{@@OUTPUT_MAP[Regexp.escape(token)]}#{@@OUTPUT_MAP[Regexp.escape(num[index+1])]}"
          num[index+1]=nil
        else
          output_n << "#{@@OUTPUT_MAP[Regexp.escape(token)]}" if token
        end
      end
    end
    if @denominator == ['<1>']
      output_d = ['1']
    else
      den.each_with_index do |token,index|
          if token && @@PREFIX_VALUES[Regexp.escape(token)] then
            output_d << "#{@@OUTPUT_MAP[Regexp.escape(token)]}#{@@OUTPUT_MAP[Regexp.escape(den[index+1])]}"
            den[index+1]=nil
          else
            output_d << "#{@@OUTPUT_MAP[Regexp.escape(token)]}" if token
          end
        end
    end
    on = output_n.reject {|x| x.empty?}.map {|x| [x, output_n.find_all {|z| z==x}.size]}.uniq.map {|x| ("#{x[0]}".strip+ (x[1] > 1 ? "^#{x[1]}" : ''))}
    od = output_d.reject {|x| x.empty?}.map {|x| [x, output_d.find_all {|z| z==x}.size]}.uniq.map {|x| ("#{x[0]}".strip+ (x[1] > 1 ? "^#{x[1]}" : ''))}
    "#{on.join('*')}#{od == ['1'] ? '': '/'+od.join('*')}".strip    
  end
  
  # negates the scalar of the Unit
  def -@
    Unit.new([-@scalar,@numerator,@denominator])    
  end
  
  # returns abs of scalar, without the units
  def abs
    return @scalar.abs
  end
  
  def ceil
    Unit.new([@scalar.ceil, @numerator, @denominator])    
  end
  
  def floor
    Unit.new([@scalar.floor, @numerator, @denominator])    
  end

  # changes internal scalar to an integer, but retains the units
  # if unitless, returns an int
  def to_int
    return @scalar.to_int if unitless?
    Unit.new([@scalar.to_int, @numerator, @denominator])    
  end
  
  # Tries to make a Time object from current unit
  def to_time
    Time.at(self)
  end
  alias :time :to_time
  alias :to_i :to_int
  alias :truncate :to_int
  
  def round
    Unit.new([@scalar.round, @numerator, @denominator])    
  end
   
  # true if scalar is zero
  def zero?
    return @scalar.zero?
  end
  
  # '5 min'.unit.ago 
  def ago
    Time.now - self
  end
  
  # '5 min'.before(time)
  def before(time_point = ::Time.now)
    raise ArgumentError, "Must specify a Time" unless time_point
    if String === time_point
      Time.local(*ParseDate.parsedate(time_point))-self 
    else
      time_point - self
    end
  end
  
  # 'min'.since(time)
  def since(time_point = ::Time.now)
    case time_point
    when Time:      (Time.now - time_point).unit('s').to(self)
    when DateTime:  (DateTime.now - time_point).unit('d').to(self)
    when String:    (Time.now - Time.local(*ParseDate.parsedate(time_point))).unit('s').to(self) 
    else
      raise ArgumentError, "Must specify a Time" unless time_point
    end
  end
  
  # 'min'.until(time)
  def until(time_point = ::Time.now)
    case time_point
    when Time:      (time_point - Time.now).unit('s').to(self)
    when DateTime:  (time_point - DateTime.now).unit('d').to(self)
    when String:    (Time.local(*ParseDate.parsedate(time_point)) - Time.now).unit('s').to(self)
    else
      raise ArgumentError, "Must specify a Time" unless time_point
    end
  end
  
  # '5 min'.from_now
  def from_now
    self.from
  end

  # '5 min'.from(time)
  def from(time_point = ::Time.now)
    raise ArgumentError, "Must specify a Time" unless time_point
    if String === time_point
      Time.local(*ParseDate.parsedate(time_point))+self
    else
      time_point + self
    end
  end
  alias :after :from
      
  def succ
    raise ArgumentError, "Non Integer Scalar" unless @scalar == @scalar.to_i
    q = @scalar.to_i.succ
    Unit.new([q, @numerator, @denominator])
  end
      
  def update_base_scalar
    if self.is_base?
      @base_scalar = @scalar
      @signature = unit_signature
    else
      base = self.to_base
      @base_scalar = base.scalar
      @signature = base.signature
    end
  end
  
  def coerce(other)
    case other
    when Unit : [other, self]
    else 
      [Unit.new(other), self]
    end
  end
  
  private
  
  # parse a string into a unit object.
  # Typical formats like : 
  #  "5.6 kg*m/s^2"
  #  "5.6 kg*m*s^-2"
  #  "5.6 kilogram*meter*second^-2"
  #  "2.2 kPa"
  #  "37 degC"
  #  "1"  -- creates a unitless constant with value 1
  #  "GPa"  -- creates a unit with scalar 1 with units 'GPa'
  #  6'4"  -- recognized as 6 feet + 4 inches
  #  8 lbs 8 oz -- recognized as 8 lbs + 8 ounces 
  def parse(unit_string="0")
    @numerator = ['<1>']
    @denominator = ['<1>']
    unit_string.gsub!(/[<>]/,"")
     
    # Special processing for unusual unit strings
    # feet -- 6'5"
    feet, inches = unit_string.scan(/(\d+)\s*(?:'|ft|feet)\s*(\d+)\s*(?:"|in|inches)/)[0]
    if (feet && inches)
      result = Unit.new("#{feet} ft") + Unit.new("#{inches} inches")
      @scalar = result.scalar
      @numerator = result.numerator
      @denominator = result.denominator
      @base_scalar = result.base_scalar
      return self
    end

    # weight -- 8 lbs 12 oz    
    pounds, oz = unit_string.scan(/(\d+)\s*(?:#|lbs|pounds)+[\s,]*(\d+)\s*(?:oz|ounces)/)[0]
    if (pounds && oz)
      result = Unit.new("#{pounds} lbs") + Unit.new("#{oz} oz")
      @scalar = result.scalar
      @numerator = result.numerator
      @denominator = result.denominator
      @base_scalar = result.base_scalar
      return self
    end
    @scalar, top, bottom = unit_string.scan(/([\dEe+.-]*)\s*([^\/]*)\/*(.+)*/)[0]  #parse the string into parts
    
    top.scan(/([^ \*]+)(?:\^|\*\*)([\d-]+)/).each do |item|
      n = item[1].to_i
      x = "#{item[0]} "
      case 
        when n>=0 : top.gsub!(/#{item[0]}(\^|\*\*)#{n}/) {|s| x * n}
        when n<0 : bottom = "#{bottom} #{x * -n}"; top.gsub!(/#{item[0]}(\^|\*\*)#{n}/,"")
      end
    end 

    bottom.gsub!(/([^* ]+)(?:\^|\*\*)(\d+)/) {|s| "#{$1} " * $2.to_i} if bottom
    if @scalar.empty?
      if top =~ /[\dEe+.-]+/
        @scalar = top.to_f  # need this for 'number only' initialization
      else
        @scalar = 1         # need this for 'unit only' intialization
      end
    else
      @scalar = @scalar.to_f
    end
            
    @numerator = top.scan(/(#{@@PREFIX_REGEX})*?(#{@@UNIT_REGEX})\b/).delete_if {|x| x.empty?}.compact if top
    @denominator = bottom.scan(/(#{@@PREFIX_REGEX})*?(#{@@UNIT_REGEX})\b/).delete_if {|x| x.empty?}.compact if bottom

    @numerator = @numerator.map do |item|
      item.map {|x| Regexp.escape(x) if x}
       @@PREFIX_MAP[item[0]] ? [@@PREFIX_MAP[item[0]], @@UNIT_MAP[item[1]]] : [@@UNIT_MAP[item[1]]]
    end.flatten.compact.delete_if {|x| x.empty?}

    @denominator = @denominator.map do |item|
       item.map {|x| Regexp.escape(x) if x}
       @@PREFIX_MAP[item[0]] ? [@@PREFIX_MAP[item[0]], @@UNIT_MAP[item[1]]] : [@@UNIT_MAP[item[1]]]
    end.flatten.compact.delete_if {|x| x.empty?}

    @numerator = ['<1>'] if @numerator.empty?    
    @denominator = ['<1>'] if @denominator.empty?
    self
  end 
end

# Need the 'Uncertain' gem for this to do anything helpful
if defined? Uncertain
  class Uncertain
    def to_unit(other=nil)
      other ? Unit.new(self).to(other) : Unit.new(self)
    end
    alias :unit :to_unit
    alias :u :to_unit
  end
end


# Allow date objects to do offsets by a time unit
# Date.today + U"1 week" => gives today+1 week
class Date
  alias :unit_date_add :+
  def +(unit)
    if Unit === unit
      unit_date_add(unit.to('day').scalar)
    else
      unit_date_add(unit)
    end
  end

  alias :unit_date_sub :-    
  def -(unit)
    if Unit === unit
      unit_date_sub(unit.to('day').scalar)
    else
      unit_date_sub(unit)
    end
  end
end

class Object
  def Unit(other)
    other.to_unit
  end
  alias :U :Unit
  alias :u :Unit
end

# make a unitless unit with a given scalar
class Numeric
  def to_unit(other = nil)
    other ? Unit.new(self) * Unit.new(other) : Unit.new(self)
  end
  alias :unit :to_unit
  alias :u :to_unit
end

# make a unit from an array
# [1, 'mm'].unit => 1 mm
class Array
  def to_unit(other = nil)
    other ? Unit.new(self).to(other) : Unit.new(self)
  end
  alias :unit :to_unit
  alias :u :to_unit
end

# make a string into a unit
class String
  def to_unit(other = nil)
    other ? Unit.new(self) >> other : Unit.new(self)
  end
  alias :unit :to_unit
  alias :u :to_unit
  alias :unit_format :%
  
  # format unit output using formating codes '%0.2f' % '1 mm'.unit => '1.00 mm'
  def %(*args)
    if Unit === args[0]
      args[0].to_s(self)
    else
      unit_format(*args)
    end
  end
    
  def from(time_point = ::Time.now)
    self.unit.from(time_point)
  end
  alias :after :from
  alias :from_now :from
  
  def ago
    self.unit.ago
  end
  
  def before(time_point = ::Time.now)
    self.unit.before(time_point)
  end
  alias :before_now :before
  
  def since(time_point = ::Time.now)
    self.unit.since(time_point)
  end
  
  def until(time_point = ::Time.now)
    self.unit.until(time_point)
  end
  
  def to(other)
    self.unit.to(other)
  end
end


# Allow time objects to use 
class Time
  
  class << self
    alias unit_time_at at
  end
  
  def self.at(*args)
    if Unit === args[0]
      unit_time_at(args[0].to("s").scalar)
    else
      unit_time_at(*args)
    end
  end
  
  def to_unit(other = "s")
    other ? Unit.new("#{self.to_f} s").to(other) : Unit.new("#{self.to_f} s")
  end
  alias :unit :to_unit
  alias :u :to_unit
  alias :unit_add :+
  def +(other)
    if Unit === other
      unit_add(other.to('s').scalar)
    else
      unit_add(other)
    end
  end
  
  alias :unit_sub :-
  def -(other)
    if Unit === other
      unit_sub(other.to('s').scalar)
    else
      unit_sub(other)
    end
  end
end

module Math
  alias unit_sin sin
   def sin(n)
     if Unit === n
       unit_sin(n.to('radian').scalar)
     else
       unit_sin(n)
     end
   end

   alias unit_cos cos
    def cos(n)
      if Unit === n
        unit_cos(n.to('radian').scalar)
      else
        unit_cos(n)
      end
    end
    
    alias unit_sinh sinh
     def sinh(n)
       if Unit === n
         unit_sinh(n.to('radian').scalar)
       else
         unit_sinh(n)
       end
     end

     alias unit_cosh cosh
      def cosh(n)
        if Unit === n
          unit_cosh(n.to('radian').scalar)
        else
          unit_cosh(n)
        end
      end

      alias unit_tan tan
       def tan(n)
         if Unit === n
           unit_tan(n.to('radian').scalar)
         else
           unit_tan(n)
         end
       end

       alias unit_tanh tanh
        def tanh(n)
          if Unit === n
            unit_tanh(n.to('radian').scalar)
          else
            unit_tanh(n)
          end
        end

   module_function :unit_sin
   module_function :sin
   module_function :unit_cos
   module_function :cos
   module_function :unit_sinh
   module_function :sinh
   module_function :unit_cosh
   module_function :cosh
   module_function :unit_tan
   module_function :tan
   module_function :unit_tanh
   module_function :tanh
end