ANY MUTABLE_BIG_INTEGER NONE NUMBER
class interface MUTABLE_BIG_INTEGER
   --
   -- A class used to represent multiprecision integers that makes efficient use
   -- of allocated space by allowing a number to occupy only part of an array so
   -- that the arrays do not have to be reallocated as often. When performing an
   -- operation with many iterations the array used to hold a number is only
   -- reallocated when necessary and does not have to be the same size as the
   -- number it represents. A mutable number allows calculations to occur on the
   -- same number without having to create a new number for every step of the
   -- calculation as it occurs with NUMBERs.
   --

creation
   from_integer (value: INTEGER)
      -- Create or initialize Current using value as an initializer.
      ensure
         to_integer = value

   from_integer_64 (value: INTEGER_64)
      -- Create or set Current using value as an initializer.
      ensure
         to_integer_64 = value

   copy (other: like Current)
      -- Update current object using fields of object attached
      -- to other, so as to yield equal objects.
      -- Note: you can't copy object from a different dynamic type.
      require
         same_dynamic_type(other)
      ensure
         is_equal(other)

feature(s) from HASHABLE
   hash_code: INTEGER
      -- The hash-code value of Current.
      ensure
         good_hash_value: Result >= 0

feature(s) from COMPARABLE
   is_equal (other: like Current): BOOLEAN
      -- Is other attached to an object considered equal to
      -- current object ?
      require
         other /= Void
      ensure
         trichotomy: Result = (not (Current < other) and not (other < Current));
         generating_type = other.generating_type implies Result = other.is_equal(Current)

   infix "<" (other: like Current): BOOLEAN
      -- Is Current strictly less than other?
      require
         other_exists: other /= Void
      ensure
         asymmetric: Result implies not (other < Current)

   infix "<=" (other: like Current): BOOLEAN
      -- Is Current less than or equal other?
      require
         other_exists: other /= Void
      ensure
         definition: Result = (Current < other or is_equal(other))

   infix ">" (other: like Current): BOOLEAN
      -- Is Current strictly greater than other?
      require
         other_exists: other /= Void
      ensure
         definition: Result = (other < Current)

   infix ">=" (other: like Current): BOOLEAN
      -- Is Current greater than or equal than other?
      require
         other_exists: other /= Void
      ensure
         definition: Result = (other <= Current)

   in_range (lower, upper: like Current): BOOLEAN
      -- Return True if Current is in range [lower..upper]
      ensure
         Result = (Current >= lower and Current <= upper)

   compare (other: like Current): INTEGER
      -- If current object equal to other, 0
      -- if smaller,  -1; if greater, 1.
      require
         other_exists: other /= Void
      ensure
         equal_zero: Result = 0 = is_equal(other);
         smaller_negative: Result = -1 = (Current < other);
         greater_positive: Result = 1 = (Current > other)

   three_way_comparison (other: like Current): INTEGER
      -- If current object equal to other, 0
      -- if smaller,  -1; if greater, 1.
      require
         other_exists: other /= Void
      ensure
         equal_zero: Result = 0 = is_equal(other);
         smaller_negative: Result = -1 = (Current < other);
         greater_positive: Result = 1 = (Current > other)

   min (other: like Current): like Current
      -- Minimum of Current and other.
      require
         other /= Void
      ensure
         Result <= Current and then Result <= other;
         compare(Result) = 0 or else other.compare(Result) = 0

   max (other: like Current): like Current
      -- Maximum of Current and other.
      require
         other /= Void
      ensure
         Result >= Current and then Result >= other;
         compare(Result) = 0 or else other.compare(Result) = 0

feature(s) from MUTABLE_BIG_INTEGER
   -- Creation / initialization from INTEGER_32 or INTEGER_64:

   from_integer (value: INTEGER)
      -- Create or initialize Current using value as an initializer.
      ensure
         to_integer = value

   is_integer: BOOLEAN
      -- Do Current fit on an INTEGER_32?
      ensure
         Result implies is_integer_64;
         Result implies integer_length <= 1

   to_integer: INTEGER
      -- Convert Current as a 32 bit INTEGER.
      require
         is_integer

   from_integer_64 (value: INTEGER_64)
      -- Create or set Current using value as an initializer.
      ensure
         to_integer_64 = value

   is_integer_64: BOOLEAN
      -- Do Current fit on an INTEGER_64?
      ensure
         not Result implies not is_integer;
         Result implies integer_length <= 2

   to_integer_64: INTEGER_64
      -- Convert Current as a INTEGER_64.
      require
         is_integer_64

feature(s) from MUTABLE_BIG_INTEGER
   -- Conversion tool

   to_double: DOUBLE
      -- only a tool
      -- unsigned conversion

feature(s) from MUTABLE_BIG_INTEGER
   -- Addition:

   add (other: like Current)
      -- Add other into Current.
      -- (See also add_integer add_integer_64 and add_natural.)
      require
         other /= Void

   add_to (other, res: like Current)
      -- Add other and Current, and put the result in res.
      require
         other /= Void;
         res /= Void;
         res /= Current;
         res /= other

   add_integer (other: INTEGER)
      -- Add other into Current.

   add_integer_64 (other: INTEGER_64)
      -- Add other into Current.

   add_natural (other: like Current)
      -- Same behavior as add, but this one works only when Current
      -- and other are both positive numbers and are both greater than
      -- zero. The only one advantage of using add_natural instead of the
      -- general add is the gain of efficiency.
      require
         not is_zero and not is_negative;
         not other.is_zero and not other.is_negative

feature(s) from MUTABLE_BIG_INTEGER
   -- Subtract:

   subtract (other: like Current)
      -- Subtract other from Current.
      require
         other /= Void

   subtract_to (other, res: like Current)
      -- Subtract other from Current and put it in res.
      require
         other /= Void;
         res /= Void;
         res /= Current;
         res /= other

   subtract_integer (other: INTEGER)

feature(s) from MUTABLE_BIG_INTEGER
   -- To divide:

   divide (other: like Current)
      -- Put the the quotient of the Euclidian division of
      -- Current by other in Current.
      -- (The contents of other is not changed.)
      require
         not other.is_zero;
         other /= Current

   mod (other: like Current)
      -- Put the the remainder of the Euclidian division of
      -- Current by other in Current.
      -- (The contents of other is not changed.)
      require
         not other.is_zero;
         other /= Current
      ensure
         not negative and abs_compare(other) = -1

   divide_with_remainder_to (other, remainder: like Current)
      -- Euclidian division.
      -- Calculates the quotient and remainder of Current
      -- divided by other.
      -- Quotient is put in Current.
      -- (The contents of other is not changed.)
      require
         not other.is_zero;
         remainder /= Void;
         remainder /= other;
         remainder /= Current
      ensure
         not remainder.negative and remainder.abs_compare(other) = -1

   remainder_with_quotient_to (other, quotient: like Current)
      -- Euclidian division.
      -- Calculates the quotient and remainder of Current
      -- divided by other.
      -- Remainder is put in Current.
      -- (The contents of other is not changed.)
      --
      -- Note: Uses Algorithm D in Knuth section 4.3.1.
      require
         not other.is_zero;
         quotient /= Void;
         quotient /= other;
         quotient /= Current
      ensure
         not negative and abs_compare(other) = -1

   divide_to (other, quotient, remainder: like Current)
      -- Euclidian division.
      -- Calculates the quotient and remainder of Current
      -- divided by other. (The contents of Current and other are
      -- not changed.)
      --
      -- Note: Uses Algorithm D in Knuth section 4.3.1.
      require
         not other.is_zero;
         quotient /= Void;
         remainder /= Void;
         quotient /= other;
         quotient /= Current;
         remainder /= other;
         remainder /= Current
      ensure
         is_a_good_divide(other,quotient,remainder);
         not remainder.negative and remainder.abs_compare(other) = -1

   shift_left (n: INTEGER)
      -- Shift bits of magnitude by n position left. (Note that no bit can
      -- be lost because the storage area is automatically extended when
      -- it is necessary.)
      require
         n > 0

   shift_right (n: INTEGER)
      -- Right shift Current n bits. (Current is left in normal form.)
      require
         n > 0

feature(s) from MUTABLE_BIG_INTEGER
   -- GCD

   gcd (other: like Current)
      -- Compute GCD of Current and other.
      -- GCD is put in Current (other is not modified).
      require
         other /= Void
      ensure
         is_positive;
         is_zero implies other.is_zero

feature(s) from MUTABLE_BIG_INTEGER
   -- To multiply:

   multiply (other: like Current)
      -- Multiply Current by other.
      require
         other /= Void

   multiply_to (other, res: like Current)
      -- Multiply the contents of Current and other and place the
      -- result  in res. (Current and other are not modified.)
      require
         other /= Void;
         res /= Void;
         res /= Current

   multiply_integer (other: INTEGER; res: like Current)
      -- Multiply the contents of Current and other and place the
      -- result  in res. (Current is not modified.)
      require
         res /= Current;
         res /= Void

feature(s) from MUTABLE_BIG_INTEGER
   -- Comparison:

   is_zero: BOOLEAN
      -- Is it 0?
      ensure
         Result implies not is_negative

   is_one: BOOLEAN
      -- Is it 1?
      ensure
         Result implies not is_negative

   is_one_negative: BOOLEAN
      -- Is it -1 ?
      ensure
         Result implies is_negative

   is_negative: BOOLEAN
      -- Is Current negative integer?

   is_positive: BOOLEAN
      -- Is Current positive (or equal to zero) integer?

   is_even: BOOLEAN
      -- Is Current even?
      ensure
         Result = not Current.is_odd

   is_odd: BOOLEAN
      -- Is Current odd?
      ensure
         Result = not Current.is_even

   abs_compare (other: like Current): INTEGER
      -- Compare the magnitude of Current and other. Returns -1, 0 or 1
      -- as this MutableBigInteger is numerically less than, equal to, or
      -- greater than other.

feature(s) from MUTABLE_BIG_INTEGER
   -- Printing:

   to_string: STRING
      -- Create a decimal view of the Current big integer.
      -- Note: see also append_in to save memory.

   append_in (buffer: STRING)
      -- Append in the buffer the equivalent of to_string. No new
      -- STRING creation during the process.
      require
         is_printable

   is_printable: BOOLEAN
      -- True if decimal view of Current is short enougth
      -- to be put in a STRING.

   out_in_tagged_out_memory
      -- Append terse printable represention of current object
      -- in tagged_out_memory.
      ensure
         not_cleared: tagged_out_memory.count >= old tagged_out_memory.count;
         append_only: (old tagged_out_memory.twin).is_equal(tagged_out_memory.substring(1,old tagged_out_memory.count))

   fill_tagged_out_memory
      -- Append a viewable information in tagged_out_memory in
      -- order to affect the behavior of out, tagged_out, etc.

feature(s) from MUTABLE_BIG_INTEGER
   -- Miscellaneous:

   negate
      -- Negate the sign of Current.

   abs
      -- Absolute value of Current.

   set_with_zero
      ensure
         is_zero

   copy (other: like Current)
      -- Update current object using fields of object attached
      -- to other, so as to yield equal objects.
      -- Note: you can't copy object from a different dynamic type.
      require
         same_dynamic_type(other)
      ensure
         is_equal(other)

   swap_with (other: like Current)
      -- Swap the value of Current with the value of other.
      ensure
         -- *** other.to_string.is_equal(old to_string)
         -- *** Current.to_string.is_equal(old other.to_string)


invariant

    capacity > 0;

    storage.is_not_null;

    integer_length.in_range(0,capacity);

    integer_length /= 0 implies offset.in_range(0,capacity - integer_length) and item(offset + integer_length - 1) /= 0;

    integer_length = 0 implies not negative;

    capacity.is_a_power_of_2;

end of MUTABLE_BIG_INTEGER