truncate_from_center.rb

require "test/unit"

class TruncateFromCenter

  def self.truncate(string, length = 30, separator = "...")
    return string if length > string.length

    if string.length <= separator.length || length <= separator.length
      return string[0...length]
    end

    middle          = (length - separator.length) / 2.0
    separator_start = middle.ceil
    separator_end   = string.length - middle.floor

    string[0...separator_start] + separator + string[separator_end..-1]
  end

end

class TruncateTest < Test::Unit::TestCase

  def test_truncate_from_center
    assert_equal TruncateFromCenter.truncate("foo",       1),      "f"
    assert_equal TruncateFromCenter.truncate("foo",       3),      "foo"
    assert_equal TruncateFromCenter.truncate("foo",       5),      "foo"
    assert_equal TruncateFromCenter.truncate("foobar",    3),      "foo"
    assert_equal TruncateFromCenter.truncate("foobar",    4),      "f..."
    assert_equal TruncateFromCenter.truncate("foobar",    5),      "f...r"
    assert_equal TruncateFromCenter.truncate("foobarbaz", 6),      "fo...z"
    assert_equal TruncateFromCenter.truncate("foobarbaz", 7),      "fo...az"
    assert_equal TruncateFromCenter.truncate("foobar",    4, "-"), "fo-r"
  end

end