I was checking source code of StdRandom's uniform
And confused by part of its implementation.
The source code is below:
public static long uniform(long n) {
if (n <= 0L) throw new IllegalArgumentException("argument must be positive: " + n);
// https://docs.oracle.com/javase/8/docs/api/java/util/Random.html#longs-long-long-long-
long r = random.nextLong();
long m = n - 1;
// power of two
if ((n & m) == 0L) {
return r & m;
}
// reject over-represented candidates
long u = r >>> 1;
while (u + m - (r = u % n) < 0L) {
u = random.nextLong() >>> 1;
}
return r;
}
So my problem is about "// reject over-represented candidates" section
What is the meaning of (u+m - (r=u%n) < 0L) here? I can only think of one situation when this expression become true, that is when (u + m) become too big and overflow to be a negative number. But why then? Or am I wrong at very start?
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