DouKaku?

#include <string.h>

// なにもフォーマットしてない
int mysprintf(char *str, const char *format, ... ){
    strcpy(str, format);
    return strlen(str);
}

#%[フラグ][最小フィールド幅][.精度][長さ修飾子]変換指定子
MATCH_STR = '%([\+\-\s#0\*]*)(\d*)((?:\.\d+)?)([hliztl]*)([\w%]*)'
def mysprintf(*argv)
  def check_flag(str, flag, width, precision)
    str = str.to_s
    width = width.to_i
    pos = flag.include?('-')
    sign = (/\A([\-\+])\w+\z/ =~ str or flag.include?('+'))
    zero = flag.include?('0')
    w = (width.to_i > str.size ? width-str.size : 0)
    if sign
      s = (str.to_i.abs >= 0 ? '+' : '-')
      w = (w >= 1 ? w - 1 : 0)
    else
      s = ''
    end
    num = (/\.(\d+)/ =~ precision and $1.to_i <= w and $1.to_i > 0) ? $i.to_i+1 : 0
    filler = (zero ? '0' : ' ')*(w-num) + '0'*num
    "#{pos ? '' : filler}#{s}#{str}#{pos ? filler : ''}"
  end
  def check_spec(str, spec, opt)
    def exp_expr(n) #有理数を指数表示に変換
      i = 0
      break if eval("10**#{n.abs > 1 ? '1' : '0'}*10**(i#{n.abs > 1 ? '+= 1) >' : '-= 1) <='}#{'-1*' if n < 0}n") while 1
      "#{n.quo(10**(i+1))}e#{i+1}"
    end
    def e_flag(str)
      if str.include?('e')
        if str.include('.')
          str
        else
          "#{str.split('e')[0]}.e#{str.split('e')[1]}"
        end
      else
        exp_expr(str.to_f)
      end
    end
    def g_flag(str)
      str.to_f < 10e-3 ? e_flag(str) : str.to_f.to_s
    end
    if opt
      case spec
      when 'b': "0b#{str.to_i.to_s(2)}"
      when 'o': "0#{str.to_i.oct}"
      when 'x': "0x#{str.to_i.hex}"
      when 'X': "0x#{str.to_i.hex}".upcase
      when 'f': str.include('.') ? str : "#{str}."
      when 'F': (str.include('.') ? str : "#{str}.").upcase
      when 'e': e_flag(str)
      when 'E': e_flag(str).upcase
      when 'g': g_flag(str)
      when 'G': g_flag(str).upcase
      else
      end
    else
      case spec
      when 'd': str.to_i
      when 'i': str.to_i
      when 'u': (str.to_i < 0 ? 2**32 + str.to_i : str.to_i)
      when 'o': str.to_i.oct
      when 'x': str.to_i.hex
      when 'X': str.to_i.hex.to_s.upcase
      when 'e': e_flag(str)
      when 'E': e_flag(str).upcase
      when 'f': str.to_f
      when 'F': str.to_f.to_s.upcase
      when 'g': g_flag(str)
      when 'G': g_flag(str).upcase
      when 'c': str[0].chr
      when 's': str.to_s
      when '%': '%'
      else
        raise ArgumentError.new("malformed format string - #{spec}")
      end
    end
  end
  f = argv[0].match(/\A#{MATCH_STR}\z/)[1..-1]
  if f[0].include?('*')
    str = argv[2].to_s
    width = argv[1]
  else
    str = argv[1].to_s
    width = f[1]
  end
  if /\A[di]\z/ =~ f[4] and str.to_f != str.to_i
    raise ArgumentError.new("invalid value for Integer: #{str}")
  end
  flag = f[0]
  prec = f[2]
  spec = f[4]
  opt = f[0].include?('#')
  check_flag(check_spec(str, spec, opt), flag, width, prec)
end
p mysprintf("%%", "testtest") # => "%"
p sprintf("%%", "testtest")   # => "%"
p mysprintf("%5.0u", '-1')    # => "4294967295"
p sprintf("%5.0u", '-1')      # => "4294967295"
p mysprintf("% 2d", "1", '2') # => " 1"
p sprintf("% 2d", "1", '2')   # => " 1"