Detecting single one-bit streams within an integer

Question

I have to check a number if it satisfies the following criteria:

• in binary, all one-bits must be successive.
• the number must have at least one bit set.
• the successive one-bits may start at the MSB or end at the LSB, so it's perfectly valid if the number is made up of a single one-bit stream followed by a zero-bit stream or vice versa.

I wrote a code that checks for these conditions for a real-world problem (checking data-file integrity).

It works without problems and it's anything but time-critical, but I'm an old bit-twiddeling freak and love such puzzles, so I've tried to came up with a more clever way to check for single-one-bit streams.

Cases where the string is surrounded by zeros is easy, but that one can't deal with the special cases.

Any ideas, binary hacks and partial solutions are welcome!

---

To make my requirements more clear some examples: The following numbers satisfy my criteria:

  0x80000000
  0x00000001
  0xff000000
  0x000000ff
  0xffffffff
  0x000ff000

The following numbers don't (as they have more than one successive string of ones):

  0xf00000f <- one-bit streams should not wrap-around at 2^n
  0x0001700 <- a trivial example.
  0x0000000 <- no one bit at all.

Answer 1

This should do what you want.

if(i == 0)
    return false;
while(i % 2 == 0) {
    i = i / 2;
}
return (i & (i + 1)) == 0;

Answer 2

bool isOK(uint val) {
   while (val != 0 && (val & 1u) == 0) val >>= 1;
   if (val == 0) return false;
   while (val != 0 && (val & 1u) == 1) val >>= 1;
   return val == 0;
}

; x86 assembly
mov eax, THE_NUMBER ; store the number in eax
bsf ecx, eax
jz .notok
mov edi, 1
shl edi, cl
mov esi, eax
add esi, edi
test esi, eax
jnz .notok
mov eax, 1
jmp .end
.notok:
mov eax, 0
.end: ; eax = 1 if satisfies the criteria, otherwise it's 0