reports_controller.rb

class ReportsController < ApplicationController
  def group_columns columns, items
    numitems = items.length
    rows = numitems / columns
    rows += 1 if numitems % columns > 0
    extras = numitems % rows if rows > 0

    1.upto(rows).map { |rownum|
      # distribute the extras evenly
      extra = 0
      if extras > 0
        extra = 1
        extras -= 1
      end

      row = items.shift(numitems/rows + extra)
      row + [nil] * (columns - row.length)
    }
  end
end

reports_controller_spec.rb

describe ReportsController do
  let(:it) { described_class.new }

  it "should group columns" do
    def self.expect_this n, a
      expect(it.group_columns(n, a.to_a))
    end
    n = nil

    expect_this(3, []         ).to eq [                               ]
    expect_this(3, [1]        ).to eq [[1, n, n]                      ]
    expect_this(3, [1,2]      ).to eq [[1, 2, n]                      ]
    expect_this(3, [1,2,3]    ).to eq [[1, 2, 3]                      ]
    expect_this(3, [1,2,3,4]  ).to eq [[1, 2, n], [3, 4, n]           ]
    expect_this(3, 1.upto(5)  ).to eq [[1, 2, 3], [4, 5, n]           ]
    expect_this(3, 1.upto(6)  ).to eq [[1, 2, 3], [4, 5, 6]           ]
    expect_this(3, 1.upto(7)  ).to eq [[1, 2, 3], [4, 5, n], [6, 7, n]]
    expect_this(3, 1.upto(8)  ).to eq [[1, 2, 3], [4, 5, 6], [7, 8, n]]
    expect_this(3, 1.upto(9)  ).to eq [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

    expect_this(4, []         ).to eq [                                           ]
    expect_this(4, [1]        ).to eq [[1, n, n, n]                               ]
    expect_this(4, [1,2]      ).to eq [[1, 2, n, n]                               ]
    expect_this(4, [1,2,3]    ).to eq [[1, 2, 3, n]                               ]
    expect_this(4, [1,2,3,4]  ).to eq [[1, 2, 3, 4]                               ]
    expect_this(4, 1.upto(5)  ).to eq [[1, 2, 3, n], [4, 5, n, n]                 ]
    expect_this(4, 1.upto(6)  ).to eq [[1, 2, 3, n], [4, 5, 6, n]                 ]
    expect_this(4, 1.upto(7)  ).to eq [[1, 2, 3, 4], [5, 6, 7, n]                 ]
    expect_this(4, 1.upto(8)  ).to eq [[1, 2, 3, 4], [5, 6, 7, 8]                 ]
    expect_this(4, 1.upto(9)  ).to eq [[1, 2, 3, n], [4, 5, 6, n], [7,  8,  9,  n]]
    expect_this(4, 1.upto(10) ).to eq [[1, 2, 3, 4], [5, 6, 7, n], [8,  9, 10,  n]]
    expect_this(4, 1.upto(11) ).to eq [[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11,  n]]
    expect_this(4, 1.upto(12) ).to eq [[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12]]
  end
end