Iterating through a binary sequence with some bits stuck to 1

Say I have a binary number 01100 = 12, what's an efficient way to iterate starting with this number such that the bits already set to one remain set?

In this example the sequence would go

01100 = 12

01101 = 13

01110 = 14

01111 = 15

开发者_运维知识库11100 = 28

11101 = 29

11110 = 30

11111 = 31

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Save the original value. Then every time you increment the dynamic value, or it with the original. In Java:

int orig = val;
while (true) {
    System.out.println(val);
    val = (val+1) | orig;
}

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If you just increment and look at the binary, whenever a block of 1s gets cleared, it will also increment the 0 past the end of that block. So you can just count, and set the bits again on each iteration:

const unsigned n = 12;
unsigned i = n;

while (1) {
    // print i (or whatever)
    i = (i + 1) | n;
}

---

int origin = 12;
for (int i = 0; i < n; i++) {
    if ((origin & i) != origin)
        continue;
    do_something(i);
}