$ include "seed7_05.s7i";

const string: DIGITS is "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";

const func string: encodeNegativeBase (in var integer: number, in integer: base) is func
  result
    var string: encoded is "";
  local
    var integer: remainder is 0;
  begin
    if base < -62 or base > -1 then
      raise RANGE_ERROR;
    elsif number = 0 then
      encoded := "0";
    else
      while number <> 0 do
        remainder := number rem base;
        number := number div base;
        if remainder < 0 then
          incr(number);
          remainder -:= base;
        end if;
        encoded &:= DIGITS[succ(remainder)];
      end while;
      encoded := reverse(encoded);
    end if;
  end func;

const func integer: decodeNegativeBase (in string: encoded, in integer: base) is func
  result
    var integer: decoded is 0;
  local
    var integer: factor is 1;
    var integer: index is 0;
    var integer: digit is 0;
  begin
    if base < -62 or base > -1 then
      raise RANGE_ERROR;
    elsif encoded = "0" then
      decoded := 0;
    else
      for index range length(encoded) downto 1 do
        digit := pred(pos(DIGITS, encoded[index]));
        if digit = -1 then
          raise RANGE_ERROR;
        else
          decoded +:= digit * factor;
          factor *:= base;
        end if;
      end for;
    end if;
  end func;

const proc: doCheck (in integer: number, in integer: base) is func
  local
    var string: encoded is "";
    var integer: decoded is 0;
  begin
    encoded := encodeNegativeBase(number, base);
    writeln(number lpad 10 <& " encoded in base " <& base lpad 3 <& " = " <& encoded);
    decoded := decodeNegativeBase(encoded, base);
    writeln(encoded lpad 10 <& " decoded in base " <& base lpad 3 <& " = " <& decoded);
  end func;

const proc: main is func
  begin
    doCheck(10, -2);
    doCheck(146, -3);
    doCheck(15, -10);
    doCheck(404355637, -62);
  end func;