Ruby script to truncate string, including multi-byte characters.

str_trunc.rb

#! /usr/local/bin/ruby
#**************************************************************
# 全角文字を2byte換算し、指定バイトで切り捨て（省略文字設定可）
# ※正確には、「全角文字」ではなく1byte超の文字
#**************************************************************
class String
  def trunc(trunc_at, om = "")
    om_size = (om.bytesize - om.size) / 2 + om.size
    if size == bytesize
      return size <= trunc_at ? self : "#{self[0, trunc_at - om_size]}#{om}"
    end
    return self if (self.bytesize - self.size) / 2 + self.size <= trunc_at
    size.times do |i|
      str_size = (self[0..i].bytesize - self[0..i].size) / 2 + self[0..i].size
      case
      when str_size <  trunc_at - om_size; next
      when str_size == trunc_at - om_size; return "#{self[0..i]}#{om}"
      else;                                return "#{self[0..(i - 1)]}#{om}"
      end
    end
    return self
  end
end

str = "abcdefg"
puts "str                 = #{str}"
puts "str.trunc(5)        = #{str.trunc(5)}"
puts "str.trunc(6)        = #{str.trunc(6)}"
puts "str.trunc(7)        = #{str.trunc(7)}"
puts "str.trunc(8)        = #{str.trunc(8)}"
puts "str.trunc(5, \"...\") = #{str.trunc(5, "...")}"
puts "str.trunc(6, \"...\") = #{str.trunc(6, "...")}"
puts "str.trunc(7, \"...\") = #{str.trunc(7, "...")}"
puts "str.trunc(8, \"...\") = #{str.trunc(8, "...")}"
puts
str = "AあBいCうDえEお"
puts "str                     = #{str}"
puts "str.trunc(12)           = #{str.trunc(12)}"
puts "str.trunc(13)           = #{str.trunc(13)}"
puts "str.trunc(14)           = #{str.trunc(14)}"
puts "str.trunc(15)           = #{str.trunc(15)}"
puts "str.trunc(16)           = #{str.trunc(16)}"
puts "str.trunc(12, \"...\")    = #{str.trunc(12, "...")}"
puts "str.trunc(13, \"...\")    = #{str.trunc(13, "...")}"
puts "str.trunc(14, \"...\")    = #{str.trunc(14, "...")}"
puts "str.trunc(15, \"...\")    = #{str.trunc(15, "...")}"
puts "str.trunc(16, \"...\")    = #{str.trunc(16, "...")}"
puts "str.trunc(12, \"(続く)\") = #{str.trunc(12, "(続く)")}"
puts "str.trunc(13, \"(続く)\") = #{str.trunc(13, "(続く)")}"
puts "str.trunc(14, \"(続く)\") = #{str.trunc(14, "(続く)")}"
puts "str.trunc(15, \"(続く)\") = #{str.trunc(15, "(続く)")}"
puts "str.trunc(16, \"(続く)\") = #{str.trunc(16, "(続く)")}"