module Kernel
  def format(format_string, *args)
    if args.length == 1 && args[0].respond_to?(:to_ary)
      ary = Opal.coerce_to?(args[0], Array, :to_ary)
      args = ary.to_a unless ary.nil?
    end

    %x{
      var result = '',
          //used for slicing:
          begin_slice = 0,
          end_slice,
          //used for iterating over the format string:
          i,
          len = format_string.length,
          //used for processing field values:
          arg,
          str,
          //used for processing %g and %G fields:
          exponent,
          //used for keeping track of width and precision:
          width,
          precision,
          //used for holding temporary values:
          tmp_num,
          //used for processing %{} and %<> fileds:
          hash_parameter_key,
          closing_brace_char,
          //used for processing %b, %B, %o, %x, and %X fields:
          base_number,
          base_prefix,
          base_neg_zero_regex,
          base_neg_zero_digit,
          //used for processing arguments:
          next_arg,
          seq_arg_num = 1,
          pos_arg_num = 0,
          //used for keeping track of flags:
          flags,
          FNONE  = 0,
          FSHARP = 1,
          FMINUS = 2,
          FPLUS  = 4,
          FZERO  = 8,
          FSPACE = 16,
          FWIDTH = 32,
          FPREC  = 64,
          FPREC0 = 128;

      function CHECK_FOR_FLAGS() {
        if (flags&FWIDTH) { #{raise ArgumentError, 'flag after width'} }
        if (flags&FPREC0) { #{raise ArgumentError, 'flag after precision'} }
      }

      function CHECK_FOR_WIDTH() {
        if (flags&FWIDTH) { #{raise ArgumentError, 'width given twice'} }
        if (flags&FPREC0) { #{raise ArgumentError, 'width after precision'} }
      }

      function GET_NTH_ARG(num) {
        if (num >= args.length) { #{raise ArgumentError, 'too few arguments'} }
        return args[num];
      }

      function GET_NEXT_ARG() {
        switch (pos_arg_num) {
        case -1: #{raise ArgumentError, "unnumbered(#{`seq_arg_num`}) mixed with numbered"}
        case -2: #{raise ArgumentError, "unnumbered(#{`seq_arg_num`}) mixed with named"}
        }
        pos_arg_num = seq_arg_num++;
        return GET_NTH_ARG(pos_arg_num - 1);
      }

      function GET_POS_ARG(num) {
        if (pos_arg_num > 0) {
          #{raise ArgumentError, "numbered(#{`num`}) after unnumbered(#{`pos_arg_num`})"}
        }
        if (pos_arg_num === -2) {
          #{raise ArgumentError, "numbered(#{`num`}) after named"}
        }
        if (num < 1) {
          #{raise ArgumentError, "invalid index - #{`num`}$"}
        }
        pos_arg_num = -1;
        return GET_NTH_ARG(num - 1);
      }

      function GET_ARG() {
        return (next_arg === undefined ? GET_NEXT_ARG() : next_arg);
      }

      function READ_NUM(label) {
        var num, str = '';
        for (;; i++) {
          if (i === len) {
            #{raise ArgumentError, 'malformed format string - %*[0-9]'}
          }
          if (format_string.charCodeAt(i) < 48 || format_string.charCodeAt(i) > 57) {
            i--;
            num = parseInt(str, 10) || 0;
            if (num > 2147483647) {
              #{raise ArgumentError, "#{`label`} too big"}
            }
            return num;
          }
          str += format_string.charAt(i);
        }
      }

      function READ_NUM_AFTER_ASTER(label) {
        var arg, num = READ_NUM(label);
        if (format_string.charAt(i + 1) === '$') {
          i++;
          arg = GET_POS_ARG(num);
        } else {
          arg = GET_NEXT_ARG();
        }
        return #{`arg`.to_int};
      }

      for (i = format_string.indexOf('%'); i !== -1; i = format_string.indexOf('%', i)) {
        str = undefined;

        flags = FNONE;
        width = -1;
        precision = -1;
        next_arg = undefined;

        end_slice = i;

        i++;

        switch (format_string.charAt(i)) {
        case '%':
          begin_slice = i;
        case '':
        case '\n':
        case '\0':
          i++;
          continue;
        }

        format_sequence: for (; i < len; i++) {
          switch (format_string.charAt(i)) {

          case ' ':
            CHECK_FOR_FLAGS();
            flags |= FSPACE;
            continue format_sequence;

          case '#':
            CHECK_FOR_FLAGS();
            flags |= FSHARP;
            continue format_sequence;

          case '+':
            CHECK_FOR_FLAGS();
            flags |= FPLUS;
            continue format_sequence;

          case '-':
            CHECK_FOR_FLAGS();
            flags |= FMINUS;
            continue format_sequence;

          case '0':
            CHECK_FOR_FLAGS();
            flags |= FZERO;
            continue format_sequence;

          case '1':
          case '2':
          case '3':
          case '4':
          case '5':
          case '6':
          case '7':
          case '8':
          case '9':
            tmp_num = READ_NUM('width');
            if (format_string.charAt(i + 1) === '$') {
              if (i + 2 === len) {
                str = '%';
                i++;
                break format_sequence;
              }
              if (next_arg !== undefined) {
                #{raise ArgumentError, "value given twice - %#{`tmp_num`}$"}
              }
              next_arg = GET_POS_ARG(tmp_num);
              i++;
            } else {
              CHECK_FOR_WIDTH();
              flags |= FWIDTH;
              width = tmp_num;
            }
            continue format_sequence;

          case '<':
          case '\{':
            closing_brace_char = (format_string.charAt(i) === '<' ? '>' : '\}');
            hash_parameter_key = '';

            i++;

            for (;; i++) {
              if (i === len) {
                #{raise ArgumentError, 'malformed name - unmatched parenthesis'}
              }
              if (format_string.charAt(i) === closing_brace_char) {

                if (pos_arg_num > 0) {
                  #{raise ArgumentError, "named #{`hash_parameter_key`} after unnumbered(#{`pos_arg_num`})"}
                }
                if (pos_arg_num === -1) {
                  #{raise ArgumentError, "named #{`hash_parameter_key`} after numbered"}
                }
                pos_arg_num = -2;

                if (args[0] === undefined || !args[0].$$is_hash) {
                  #{raise ArgumentError, 'one hash required'}
                }

                next_arg = #{`args[0]`.fetch(`hash_parameter_key`)};

                if (closing_brace_char === '>') {
                  continue format_sequence;
                } else {
                  str = next_arg.toString();
                  if (precision !== -1) { str = str.slice(0, precision); }
                  if (flags&FMINUS) {
                    while (str.length < width) { str = str + ' '; }
                  } else {
                    while (str.length < width) { str = ' ' + str; }
                  }
                  break format_sequence;
                }
              }
              hash_parameter_key += format_string.charAt(i);
            }

          case '*':
            i++;
            CHECK_FOR_WIDTH();
            flags |= FWIDTH;
            width = READ_NUM_AFTER_ASTER('width');
            if (width < 0) {
              flags |= FMINUS;
              width = -width;
            }
            continue format_sequence;

          case '.':
            if (flags&FPREC0) {
              #{raise ArgumentError, 'precision given twice'}
            }
            flags |= FPREC|FPREC0;
            precision = 0;
            i++;
            if (format_string.charAt(i) === '*') {
              i++;
              precision = READ_NUM_AFTER_ASTER('precision');
              if (precision < 0) {
                flags &= ~FPREC;
              }
              continue format_sequence;
            }
            precision = READ_NUM('precision');
            continue format_sequence;

          case 'd':
          case 'i':
          case 'u':
            arg = #{Integer(`GET_ARG()`)};
            if (arg >= 0) {
              str = arg.toString();
              while (str.length < precision) { str = '0' + str; }
              if (flags&FMINUS) {
                if (flags&FPLUS || flags&FSPACE) { str = (flags&FPLUS ? '+' : ' ') + str; }
                while (str.length < width) { str = str + ' '; }
              } else {
                if (flags&FZERO && precision === -1) {
                  while (str.length < width - ((flags&FPLUS || flags&FSPACE) ? 1 : 0)) { str = '0' + str; }
                  if (flags&FPLUS || flags&FSPACE) { str = (flags&FPLUS ? '+' : ' ') + str; }
                } else {
                  if (flags&FPLUS || flags&FSPACE) { str = (flags&FPLUS ? '+' : ' ') + str; }
                  while (str.length < width) { str = ' ' + str; }
                }
              }
            } else {
              str = (-arg).toString();
              while (str.length < precision) { str = '0' + str; }
              if (flags&FMINUS) {
                str = '-' + str;
                while (str.length < width) { str = str + ' '; }
              } else {
                if (flags&FZERO && precision === -1) {
                  while (str.length < width - 1) { str = '0' + str; }
                  str = '-' + str;
                } else {
                  str = '-' + str;
                  while (str.length < width) { str = ' ' + str; }
                }
              }
            }
            break format_sequence;

          case 'b':
          case 'B':
          case 'o':
          case 'x':
          case 'X':
            switch (format_string.charAt(i)) {
            case 'b':
            case 'B':
              base_number = 2;
              base_prefix = '0b';
              base_neg_zero_regex = /^1+/;
              base_neg_zero_digit = '1';
              break;
            case 'o':
              base_number = 8;
              base_prefix = '0';
              base_neg_zero_regex = /^3?7+/;
              base_neg_zero_digit = '7';
              break;
            case 'x':
            case 'X':
              base_number = 16;
              base_prefix = '0x';
              base_neg_zero_regex = /^f+/;
              base_neg_zero_digit = 'f';
              break;
            }
            arg = #{Integer(`GET_ARG()`)};
            if (arg >= 0) {
              str = arg.toString(base_number);
              while (str.length < precision) { str = '0' + str; }
              if (flags&FMINUS) {
                if (flags&FPLUS || flags&FSPACE) { str = (flags&FPLUS ? '+' : ' ') + str; }
                if (flags&FSHARP && arg !== 0) { str = base_prefix + str; }
                while (str.length < width) { str = str + ' '; }
              } else {
                if (flags&FZERO && precision === -1) {
                  while (str.length < width - ((flags&FPLUS || flags&FSPACE) ? 1 : 0) - ((flags&FSHARP && arg !== 0) ? base_prefix.length : 0)) { str = '0' + str; }
                  if (flags&FSHARP && arg !== 0) { str = base_prefix + str; }
                  if (flags&FPLUS || flags&FSPACE) { str = (flags&FPLUS ? '+' : ' ') + str; }
                } else {
                  if (flags&FSHARP && arg !== 0) { str = base_prefix + str; }
                  if (flags&FPLUS || flags&FSPACE) { str = (flags&FPLUS ? '+' : ' ') + str; }
                  while (str.length < width) { str = ' ' + str; }
                }
              }
            } else {
              if (flags&FPLUS || flags&FSPACE) {
                str = (-arg).toString(base_number);
                while (str.length < precision) { str = '0' + str; }
                if (flags&FMINUS) {
                  if (flags&FSHARP) { str = base_prefix + str; }
                  str = '-' + str;
                  while (str.length < width) { str = str + ' '; }
                } else {
                  if (flags&FZERO && precision === -1) {
                    while (str.length < width - 1 - (flags&FSHARP ? 2 : 0)) { str = '0' + str; }
                    if (flags&FSHARP) { str = base_prefix + str; }
                    str = '-' + str;
                  } else {
                    if (flags&FSHARP) { str = base_prefix + str; }
                    str = '-' + str;
                    while (str.length < width) { str = ' ' + str; }
                  }
                }
              } else {
                str = (arg >>> 0).toString(base_number).replace(base_neg_zero_regex, base_neg_zero_digit);
                while (str.length < precision - 2) { str = base_neg_zero_digit + str; }
                if (flags&FMINUS) {
                  str = '..' + str;
                  if (flags&FSHARP) { str = base_prefix + str; }
                  while (str.length < width) { str = str + ' '; }
                } else {
                  if (flags&FZERO && precision === -1) {
                    while (str.length < width - 2 - (flags&FSHARP ? base_prefix.length : 0)) { str = base_neg_zero_digit + str; }
                    str = '..' + str;
                    if (flags&FSHARP) { str = base_prefix + str; }
                  } else {
                    str = '..' + str;
                    if (flags&FSHARP) { str = base_prefix + str; }
                    while (str.length < width) { str = ' ' + str; }
                  }
                }
              }
            }
            if (format_string.charAt(i) === format_string.charAt(i).toUpperCase()) {
              str = str.toUpperCase();
            }
            break format_sequence;

          case 'f':
          case 'e':
          case 'E':
          case 'g':
          case 'G':
            arg = #{Float(`GET_ARG()`)};
            if (arg >= 0 || isNaN(arg)) {
              if (arg === Infinity) {
                str = 'Inf';
              } else {
                switch (format_string.charAt(i)) {
                case 'f':
                  str = arg.toFixed(precision === -1 ? 6 : precision);
                  break;
                case 'e':
                case 'E':
                  str = arg.toExponential(precision === -1 ? 6 : precision);
                  break;
                case 'g':
                case 'G':
                  str = arg.toExponential();
                  exponent = parseInt(str.split('e')[1], 10);
                  if (!(exponent < -4 || exponent >= (precision === -1 ? 6 : precision))) {
                    str = arg.toPrecision(precision === -1 ? (flags&FSHARP ? 6 : undefined) : precision);
                  }
                  break;
                }
              }
              if (flags&FMINUS) {
                if (flags&FPLUS || flags&FSPACE) { str = (flags&FPLUS ? '+' : ' ') + str; }
                while (str.length < width) { str = str + ' '; }
              } else {
                if (flags&FZERO && arg !== Infinity && !isNaN(arg)) {
                  while (str.length < width - ((flags&FPLUS || flags&FSPACE) ? 1 : 0)) { str = '0' + str; }
                  if (flags&FPLUS || flags&FSPACE) { str = (flags&FPLUS ? '+' : ' ') + str; }
                } else {
                  if (flags&FPLUS || flags&FSPACE) { str = (flags&FPLUS ? '+' : ' ') + str; }
                  while (str.length < width) { str = ' ' + str; }
                }
              }
            } else {
              if (arg === -Infinity) {
                str = 'Inf';
              } else {
                switch (format_string.charAt(i)) {
                case 'f':
                  str = (-arg).toFixed(precision === -1 ? 6 : precision);
                  break;
                case 'e':
                case 'E':
                  str = (-arg).toExponential(precision === -1 ? 6 : precision);
                  break;
                case 'g':
                case 'G':
                  str = (-arg).toExponential();
                  exponent = parseInt(str.split('e')[1], 10);
                  if (!(exponent < -4 || exponent >= (precision === -1 ? 6 : precision))) {
                    str = (-arg).toPrecision(precision === -1 ? (flags&FSHARP ? 6 : undefined) : precision);
                  }
                  break;
                }
              }
              if (flags&FMINUS) {
                str = '-' + str;
                while (str.length < width) { str = str + ' '; }
              } else {
                if (flags&FZERO && arg !== -Infinity) {
                  while (str.length < width - 1) { str = '0' + str; }
                  str = '-' + str;
                } else {
                  str = '-' + str;
                  while (str.length < width) { str = ' ' + str; }
                }
              }
            }
            if (format_string.charAt(i) === format_string.charAt(i).toUpperCase() && arg !== Infinity && arg !== -Infinity && !isNaN(arg)) {
              str = str.toUpperCase();
            }
            str = str.replace(/([eE][-+]?)([0-9])$/, '$10$2');
            break format_sequence;

          case 'a':
          case 'A':
            // Not implemented because there are no specs for this field type.
            #{raise NotImplementedError, '`A` and `a` format field types are not implemented in Opal yet'}

          case 'c':
            arg = GET_ARG();
            if (#{`arg`.respond_to?(:to_ary)}) { arg = #{`arg`.to_ary}[0]; }
            if (#{`arg`.respond_to?(:to_str)}) {
              str = #{`arg`.to_str};
            } else {
              str = String.fromCharCode(#{Opal.coerce_to(`arg`, Integer, :to_int)});
            }
            if (str.length !== 1) {
              #{raise ArgumentError, '%c requires a character'}
            }
            if (flags&FMINUS) {
              while (str.length < width) { str = str + ' '; }
            } else {
              while (str.length < width) { str = ' ' + str; }
            }
            break format_sequence;

          case 'p':
            str = #{`GET_ARG()`.inspect};
            if (precision !== -1) { str = str.slice(0, precision); }
            if (flags&FMINUS) {
              while (str.length < width) { str = str + ' '; }
            } else {
              while (str.length < width) { str = ' ' + str; }
            }
            break format_sequence;

          case 's':
            str = #{`GET_ARG()`.to_s};
            if (precision !== -1) { str = str.slice(0, precision); }
            if (flags&FMINUS) {
              while (str.length < width) { str = str + ' '; }
            } else {
              while (str.length < width) { str = ' ' + str; }
            }
            break format_sequence;

          default:
            #{raise ArgumentError, "malformed format string - %#{`format_string.charAt(i)`}"}
          }
        }

        if (str === undefined) {
          #{raise ArgumentError, 'malformed format string - %'}
        }

        result += format_string.slice(begin_slice, end_slice) + str;
        begin_slice = i + 1;
      }

      if (#{$DEBUG} && pos_arg_num >= 0 && seq_arg_num < args.length) {
        #{raise ArgumentError, 'too many arguments for format string'}
      }

      return result + format_string.slice(begin_slice);
    }
  end

  alias sprintf format
end