String#unpack format string width

format-width.rb

# http://www.ruby-doc.org/core/classes/String.html#M001112
#
# Calculate how many bytes String#unpack will read for a format string.
#
# Only guaranteed to work with the characters defined in classes, which are
# the sensible ones you should be using anyway. So there.
#
# Notably will not work with x, X, @, and so on.

def width(format)
  classes = { "AazCc" => 1, "Ssn" => 2, "LlNVFfeg" => 4, "QqDdEG" => 8 }
  bytes = Hash[classes.map {|k,v| k.each_char.map {|x| [x, v] } }.reduce(:+)]
  directives = format.scan(/([A-Za-z])(\d*)/)
  directives.map { |c,n| b = bytes[c] * [1, n.to_i].max }.reduce(:+)
end

p width("NnccNcc")
p width("N2s48")