Make modInv throw Exception on incompatible operands.

sdk/lib/core/int.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart.core;
 
 /**
  * An arbitrarily large integer.
  *
  * **Note:** When compiling to JavaScript, integers are
@@ skipping 98 matching lines @@
    * The [exponent] must be non-negative and [modulus] must be
    * positive.
    */
   int modPow(int exponent, int modulus);
 
   /**
    * Returns the modular multiplicative inverse of this integer
    * modulo [modulus].
    *
    * The [modulus] must be positive.
-   * Throws a RangeError if no modular inverse exists.
+   * Throws an [UnsupportedError] if no modular inverse exists.

[regis, 2015/06/24 15:40:43]

It is an error if ...

[Lasse Reichstein Nielsen, 2015/06/25 07:22:21]

As stated elsewhere, I think we might want to tell the user how they can detect
the case where no modular inverse exists.

On the other hand, I'm leaning more and more towards returning 0 in that case
and avoid throwing. So, let's iterate this some more. The current documentation
in 1.11 leaves us free to think some more.

[regis, 2015/06/25 16:51:45]

It was my thinking in the first version, where I was returning 0 in that case,
so the user does not have to catch an exception. However, Srdjan did not like it
and I then realized that 0 is a valid answer for a modulus of 1, so I gave in
and threw in that case.

I imagine that this function will be used for crypto to choose RSA keys. The
question is whether the user will use this function to iterate and find coprime
numbers (returning -1 would be better), or will verify with other means (gcd?)
that the operands are coprime (throwing an exception is then OK).

[Lasse Reichstein Nielsen, 2015/06/30 05:50:48]

I can see that 0 can be seen as a valid answer. After all, mod 0, most equality
requiements hold. Heck, 0 is even a multiplicative unit on all the elements :)

So -1, null or throw Exception or ArithmeticException seems to be the options.

    */
   int modInverse(int modulus);
 
   /**
    * Returns the greatest common divisor of the absolute value of
    * this integer and the absolute value of [other].
    *
    * Both this and [other] must be non-zero.
    */
   int gcd(int other);
@@ skipping 171 matching lines @@
    *     var value = int.parse(text, onError: (source) => null);
    *     if (value == null) ... handle the problem
    *
    * The [onError] function is only invoked if [source] is a [String]. It is
    * not invoked if the [source] is, for example, `null`.
    */
   external static int parse(String source,
                             { int radix,
                               int onError(String source) });
 }