tmp.rb

# Matrix#det2 does what Matrix#det doesn't do: calculate correctly a matrix determinant
require 'matrix'

module FixnumExt
  def square?
    sqrt = self ** (1.0/2)
    (sqrt - sqrt.to_i).zero?
  end
end

module MatrixDeterminant

  def det2
    return nil unless self.square?
    return self[0,0] if self.row_size == 1
    return self[0,0] * self[1,1] - self[0,1] * self[1,0] if self.row_size == 2
    return self[0,0] * self[1,1] * self[2,2] + self[1,0] * self[2,1] * self[0,2] + self[2,0] * self[0,1] * self[1,2] -
      self[0,2] * self[1,1] * self[2,0] - self[0,0] * self[1,2] * self[2,1] - self[0,1] * self[1,0] *
      self[2,2] if self.row_size == 3

    # calculating determinant for 4x4 or bigger
    most_zeros_line = self.lowest_cost_line
    _row            = most_zeros_line > 0
    line_index      = _row ? most_zeros_line - 1 : most_zeros_line.abs - 1
    line            = (_row ? self.row(line_index) : self.column(line_index)).to_a
    sum             = 0
    line.each_index {|k| sum += line[k].zero? ? 0 : line[k] * (_row ? self.minor2(line_index, k).det2 : self.minor2(k, line_index).det2)}
    sum
  end

  def lowest_cost_line
    rows = self.to_a.map {|e| e - [0]}
    cols = self.transpose.to_a.map {|e| e - [0]}

    lowest_row = rows.index(rows.inject(rows.first) {|lowest, e| lowest.size > e.size ? e : lowest})
    lowest_col = cols.index(cols.inject(cols.first) {|lowest, e| lowest.size > e.size ? e : lowest})
    rows[lowest_row].size <= cols[lowest_col].size ? lowest_row + 1 : - (lowest_col + 1)
  end

  def minor2(row, col)
    r = []

    self.row_size.times do |i|
      next if i == row
      r << (crow = [])

      self.column_size.times do |j|
        next if j == col
        crow << self[i,j]
      end

    end
    Matrix[*r]
  end
end

module MatrixCoreExt

  def self.included(base)
    base.extend ClassMethods
  end

  module ClassMethods
    def square_from_rows *members
      return nil unless members.size.square?

      msize    = (members.size ** (1.0/2)).to_i
      matrix  = []
      members.each_slice(msize) {|s| matrix << s}
      Matrix[*matrix]
    end
  end

  def % num
    r = []

    self.row_size.times do |i|
      r << (crow = [])
      self.column_size.times do |j|
        crow << self[i,j] % num
      end
    end
    Matrix[*r]
  end

end

module Main

  class ::Matrix
    include MatrixDeterminant
    include MatrixCoreExt
  end

  class ::Fixnum
    include FixnumExt
  end

end

include Main
puts Matrix.square_from_rows(0,6,0,0,0,0,7,0,0,5,4,5,1,2,1,6,2,1,4,2,3,2,0,2,1).det2