= Constants

Libraries of physical and chemical constants for scientific calculations in Ruby.

== Description

Constants provides libraries of constant values such as the precise mass of carbon 13, or 
the spin of a strange quark.  When applicable, the constant values include uncertainty and
units (via {ruby-units}[http://rubyforge.org/projects/ruby-units]).  Also provides 
Constants::Library to index and generate common collections of constant values.

I have attempted to use reputable sources for the constants data (see below).  Please notify 
me of any errors and send me suggestions for other constants to include.

* Rubyforge[http://rubyforge.org/projects/bioactive]
* Lighthouse[http://bahuvrihi.lighthouseapp.com/projects/13504-constants/overview]
* Github[http://github.com/bahuvrihi/constants/tree/master]

== Usage

  require 'constants'
  include Constants::Libraries
  
  # Element predefines all chemical elements
  c = Element::C
  c.name                 # => "Carbon"
  c.symbol               # => "C"
  c.atomic_number        # => 6
  c.mass                 # => 12.0
  c.mass(13)             # => 13.0033548378
  
  # A smorgasbord of lookups methods
  Element['Carbon']      # => Element::C
  Element['C']           # => Element::C
  Element[6]             # => Element::C

=== Custom Libraries

Making a new constants library is straightforward using the Constants::Library module.
The following example is adapted from the molecule[http://github.com/bahuvrihi/molecules/tree/master]
library (a subproject of constants).

  # A library of amino acid residues.
  class Residue
    attr_reader :letter, :abbr, :name

    def initialize(letter, abbr, name)
      @letter = letter
      @abbr = abbr
      @name = name
    end

    A = Residue.new('A', "Ala", "Alanine")
    C = Residue.new('C', "Cys", "Cysteine")
    D = Residue.new('D', "Asp", "Aspartic Acid")
    # ... normally you'd add the rest here ...

    include Constants::Library

    # add an index by an attribute or method
    library.index_by_attribute :letter

    # add an index where keys are calculated by a block
    library.index_by 'upcase abbr' do |residue|
      residue.abbr.upcase
    end
    
    # add a collection (same basic idea, but using an array)
    library.collect_attribute 'name'
  end

  # index access through []
  Residue['D']                   # => Residue::D
  Residue['ALA']                 # => Residue::A

  # access an index hash or collection array
  Residue.index('upcase abbr')   # => {'ALA' => Residue::A, 'CYS' => Residue::C, 'ASP' => Residue::D}
  Residue.collection('name')     # => ["Alanine", "Cysteine", "Aspartic Acid"]

As you can see, Constants::Library allows the predefinition of common views generated
for a set of constants.  Nothing you couldn't do yourself, but very handy.

== Known Issues

* Particle data is from an unreliable source
* Constants::Constant could use some development; constants should support mathematical 
  operations and comparisons based on uncertainty as well as value.
* Ruby doesn't track of the order of constant declaration until Ruby 1.9.  Constants
  are indexed/collected as they appear in [module].constants

== Installation

Constants is available as a gem through RubyForge[http://rubyforge.org/projects/bioactive].  Use:

  % gem install constants

== Info 

Copyright (c) 2006-2008, Regents of the University of Colorado.
Developer:: {Simon Chiang}[http://bahuvrihi.wordpress.com], {Biomolecular Structure Program}[http://biomol.uchsc.edu/], {Hansen Lab}[http://hsc-proteomics.uchsc.edu/hansenlab/] 
Support:: CU Denver School of Medicine Deans Academic Enrichment Fund
Licence:: MIT-Style

=== Element Data

Element isotope, mass, and abundance information was obtained from the NIST {Atomic Weights and Isotopic Compositions}[http://www.physics.nist.gov/PhysRefData/Compositions/index.html] reference.  All isotopes 
with a non-nil isotopic composition (ie relative abundance) were compiled from this {view}[http://www.physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=&all=all&ascii=ascii2&isotype=all] 
on 2008-01-22. 

=== Physical Constant Data

The physical constant data is assembled from the NIST {Fundamental Physical Constants}[http://www.physics.nist.gov/cuu/Constants/Table/allascii.txt]
on 2008-04-28.

Constants adds several units to ruby-units to support the physical constants 
reported in the NIST data.  These are (as reported in the NIST data):

electron-volt:: 1.602176487e-19 joules
kelvin:: 1.3806504e-23 joules
hartree:: 4.35974394e-18 joules

=== Particle Data

Particle data was assembled from a non-ideal source, wikipedia[http://www.wikipedia.org/], and will remain
so until I have time to update it with something better.