How do you randomly zero a bit in an integer?
Updated with newer answer and better test
Let's say I have the number 382 which is 101111110.
How could I randomly turn a bit which is not 0 to 0?
The why;
Since people ask me why, I simply need to do this, removing a bit from an integer.
based on the answer here is the result(working one)
I ran thisusing System;
using System.Collections.Generic;
using System.Collections;
using System.Linq;
using System.Diagnostics;
namespace ConsoleApplication1
{
class Program
{
static Random random;
static void Main(string[] args)
{
Stopwatch sw;
int[] test = new int[10] { 382, 256, 1, 257, 999, 555, 412, 341, 682, 951 };
random = new Random(42);
for (int j = 0; j < 10; j++)
{
sw = Stopwatch.StartNew();
for (int i = 0; i < 1000000; i++)
Perturb(test[j]);
sw.Stop();
Console.WriteLine("Perturb " + sw.Elapsed.TotalSeconds.ToString("0.#######") + " seconds for " + test[j].ToString());
Debug.WriteLine("> Perturb " + sw.Elapsed.TotalSeconds.ToString("0.#######") + " seconds for " + test[j].ToString() + " ");
}
random = new Random(42);
for (int j = 0; j < 10; j++)
{
sw = Stopwatch.StartNew();
for (int i = 0; i < 1000000; i++)
FastPerturb(test[j]);
sw.Stop();
Console.WriteLine("FastPerturb " + sw.Elapsed.TotalSeconds.ToString("0.#######") + " seconds for " + test[j].ToString());
Debug.WriteLine("> FastPerturb " + sw.Elapsed.TotalSeconds.ToString("0.#######") + " seconds for " + test[j].ToString() + " ");
}
random = new Random(42);
for (int j = 0; j < 10; j++)
{
sw = Stopwatch.StartNew();
for (int i = 0; i < 1000000; i++)
SetRandomTrueBitToFalse(test[j]);
sw.Stop();
Console.WriteLine("SetRandomTrueBitToFalse " + sw.Elapsed.TotalSeconds.ToString("0.#######") + " seconds for " + test[j].ToString());
Debug.WriteLine("> SetRandomTrueBitToFalse " + sw.Elapsed.TotalSeconds.ToString("0.#######") + " seconds for " + test[j].ToString() + " ");
}
random = new Random(42);
for (int j = 0; j < 10; j++)
{
sw = Stopwatch.StartNew();
for (int i = 0; i < 1000000; i++)
flipRandomBit(test[j]);
sw.Stop();
Console.WriteLine("flipRandomBit " + sw.Elapsed.TotalSeconds.ToString("0.#######") + " seconds for " + test[j].ToString());
Debug.WriteLine("> flipRandomBit " + sw.Elapsed.TotalSeconds.ToString("0.#######") + " seconds for " + test[j].ToString() + " ");
}
random = new Random(42);
for (int j = 0; j < 10; j++)
{
sw = Stopwatch.StartNew();
for (int i = 0; i < 1000000; i++)
oneBitsIndexes(test[j]);
sw.Stop();
Console.WriteLine("oneBitsIndexes " + sw.Elapsed.TotalSeconds.ToString("0.#######") + " seconds for " + test[j].ToString());
Debug.WriteLine("> oneBitsIndexes " + sw.Elapsed.TotalSeconds.ToString("0.#######") + " seconds for " + test[j].ToString() + " ");
}
random = new Random(42);
for (int j = 0; j < 10; j++)
{
sw = Stopwatch.StartNew();
for (int i = 0; i < 1000000; i++)
ClearOneBit(test[j]);
sw.Stop();
Console.WriteLine("ClearOneBit " + sw.Elapsed.TotalSeconds.ToString("0.#######") + " seconds for " + test[j].ToString());
Debug.WriteLine("> ClearOneBit " + sw.Elapsed.TotalSeconds.ToString("0.#######") + " seconds for " + test[j].ToString() + " ");
}
random = new Random(42);
for (int j = 0; j < 10; j++)
{
sw = Stopwatch.StartNew();
for (int i = 0; i < 1000000; i++)
FlipRandomTrueBit(test[j]);
sw.Stop();
Console.WriteLine("FlipRandomTrueBit " + sw.Elapsed.TotalSeconds.ToString("0.#######") + " seconds for " + test[j].ToString());
Debug.WriteLine("> FlipRandomTrueBit " + sw.Elapsed.TotalSeconds.ToString("0.#######") + " seconds for " + test[j].ToString() + " ");
}
random = new Random(42);
for (int j = 0; j < 10; j++)
{
sw = Stopwatch.StartNew();
for (int i = 0; i < 1000000; i++)
ClearRandomBit(test[j]);
sw.Stop();
Console.WriteLine("ClearRandomBit " + sw.Elapsed.TotalSeconds.ToString("0.#######") + " seconds for " + test[j].ToString());
Debug.WriteLine("> ClearRandomBit " + sw.Elapsed.TotalSeconds.ToString("0.#######") + " seconds for " + test[j].ToString() + " ");
}
Console.Read();
}
public static int Perturb(int data)
{
if (data == 0) return 0;
int minBits = (data & 0xFFFF0000) == 0 ? 16 : 32;
int newData = data;
do
{
newData &= ~(1 << random.Next(minBits));
} while (newData == data);
return newData;
}
public static int FastPerturb(int data)
{
if (data == 0) return 0;
int bit = 0;
while (0 == (data & (bit = 1 << random.Next(32)))) ;
return data & ~bit;
}
private static Int32 SetRandomTrueBitToFalse(Int32 p)
{
List<int> trueBits = new List<int>();
for (int i = 0; i < 31; i++)
{
if ((p >> i & 1) == 1)
{
trueBits.Add(i);
}
}
if (trueBits.Count > 0)
{
int index = random.Next(0, trueBits.Count);
return p & ~(1 << trueBits[index]);
}
return p;
}
public static int getBitCount(int bits)
{
bits = bits - ((bits >> 1) & 0x55555555);
bits = (bits & 0x33333333) + ((bits >> 2) & 0x33333333);
return ((bits + (bits >> 4) & 0xF0F0F0F) * 0x1010101) >> 24;
}
public static int flipRandomBit(int data)
{
int index = random.Next(getBitCount(data));
int mask = data;
for (int i = 0; i < index; i++)
mask &= mask - 1;
mask ^= mask & (mask - 1);
return data ^ mask;
}
public static int oneBitsIndexes(int data)
{
if (data > 0)
{
var oneBitsIndexes = Enumerable.Range(0, 31)
.Where(i => ((data >> i) & 0x1) != 0).ToList();
// pick a random index and update the source value bit there from 1 to 0
data &= ~(1 << oneBitsIndexes[random.Next(oneBitsIndexes.Count)]);
}
return data;
}
static private int ClearOneBit(int originalValue)
{
if (originalValue == 0)
return 0; // All bits are already set to 0, nothing to do
int mask = 0;
do
{
int n = random.Next(32);
mask = 1 << n;
} while ((mask & originalValue) == 0); // check that this bit is not 0
int newValue = originalValue & ~mask; // clear this bit
return newValue;
}
public static BitArray FlipRandomTrueBit(BitArray bits)
{
List<int> trueBits = new List<int>();
for (int i = 0; i < bits.Count; i++)
if (bits[i])
trueBits.Add(i);
if (trueBits.Count > 0)
{
int index = random.Next(0, trueBits.Count);
bits[trueBits[index]] = false;
}
return bits;
}
public static int FlipRandomTrueBit(int input)
{
BitArray bits = new BitArray(new int[] { input });
BitArray flipedBits = FlipRandomTrueBit(bits);
byte[] bytes = new byte[4];
flipedBits.CopyTo(bytes, 0);
int result = BitConverter.ToInt32(bytes, 0);
return result;
}
static int ClearRandomBit(int value)
{
return unchecked((int)ClearRandomBit((ulong)(uint)value));
}
static ulong ClearRandomBit(ulong value)
{
// Algorithm from http://graphics.stanford.edu/~seander/bithacks.html
//
// "Select the bit position (from the most-significant bit) with the
// given count (rank)."
//
// The following 64-bit code selects the position of the rth 1 bit when
// counting from the left. In other words if we start at the most
// significant bit and proceed to the right, counting the number of bits
// set to 1 until we reach the desired rank, r, then the position where
// we stop will be the final value given to s.
// Do a normal parallel bit count for a 64-bit integer,
// but store all intermediate steps.
ulong v = value;
ulong a = v - ((v >> 1) & ~0UL / 3);
ulong b = (a & ~0UL / 5) + ((a >> 2) & ~0UL / 5);
ulong c = (b + (b >> 4)) & ~0UL / 0x11;
ulong d = (c + (c >> 8)) & ~0UL / 0x101;
ulong t = (uint)((d >> 32) + (d >> 48));
// Choose a random r in the range [1-bitCount]
int bitCount = (int)((d * (~0UL / 255)) >> 56);
int randomRank = 1 + random.Next(bitCount);
ulong r = (ulong)randomRank;
// Compute s
ulong s = 64;
s -= ((t - r) & 256UL) >> 3;
r -= (t & ((t - r) >> 8));
t = (d >> (int)(s - 16)) & 0xff;
s -= ((t - r) & 256UL) >> 4;
r -= (t & ((t - r) >> 8));
t = (c >> (int)(s - 8)) & 0xf;
s -= ((t - r) & 256UL) >> 5;
r -= (t & ((t - r) >> 8));
t = (b >> (int)(s - 4)) & 0xf;
s -= ((t - r) & 256UL) >> 6;
r -= (t & ((t - r) >> 8));
t = (a >> (int)(s - 2)) & 0x3;
s -= ((t - r) & 256UL) >> 7;
r -= (t & ((t - r) >> 8));
t = (v >> (int)(s - 1)) & 0x1;
s -= ((t - r) & 256UL) >> 8;
s = 65 - s;
// Clear the selected bit
return value & ~(1UL << (int)(64 - s));
}
}
}
result;
Perturb 0.1704681 seconds for 382
Perturb 0.9307034 seconds for 256 Perturb 0.932266 seconds for 1 Perturb 0.4896138 seconds for 257 Perturb 0.1541828 seconds for 999 Perturb 0.2222421 seconds for 555 Perturb 0.2370868 seconds for 412 Perturb 0.2229154 seconds for 341 Perturb 0.2233445 seconds for 682 Perturb 0.1554396 seconds for 951 FastPerturb 0.2988974 seconds for 382 FastPerturb 1.8008209 seconds for 256 FastPerturb 1.7966043 seconds for 1 FastPerturb 0.9255025 seconds for 257 FastPerturb 0.2708695 seconds for 999 FastPerturb 0.4036553 seconds for 555 FastPerturb 0.401872 seconds for 412 FastPerturb 0.4042984 seconds for 341 FastPerturb 0.4028209 seconds for 682 FastPerturb 0.2688467 seconds for 951 SetRandomTrueBitToFalse 0.6127648 seconds for 382 SetRandomTrueBitToFalse 0.4432519 seconds for 256 SetRandomTrueBitToFalse 0.4193295 seconds for 1 SetRandomTrueBitToFalse 0.4543657 seconds for 257 SetRandomTrueBitToFalse 0.6270696 seconds for 999 SetRandomTrueBitToFalse 0.5891294 seconds for 555 SetRandomTrueBitToFalse 0.5910375 seconds for 412 SetRandomTrueBitToFalse 0.6104247 seconds for 341 SetRandomTrueBitToFalse 0.6249519 seconds for 682 SetRandomTrueBitToFalse 0开发者_运维知识库.6142904 seconds for 951 flipRandomBit 0.1624584 seconds for 382 flipRandomBit 0.1284565 seconds for 256 flipRandomBit 0.13208 seconds for 1 flipRandomBit 0.1383649 seconds for 257 flipRandomBit 0.1658636 seconds for 999 flipRandomBit 0.1563506 seconds for 555 flipRandomBit 0.1588513 seconds for 412 flipRandomBit 0.1561841 seconds for 341 flipRandomBit 0.1562256 seconds for 682 flipRandomBit 0.167605 seconds for 951 oneBitsIndexes 2.1871352 seconds for 382 oneBitsIndexes 1.8677352 seconds for 256 oneBitsIndexes 1.8389871 seconds for 1 oneBitsIndexes 1.8729746 seconds for 257 oneBitsIndexes 2.1821771 seconds for 999 oneBitsIndexes 2.1300304 seconds for 555 oneBitsIndexes 2.1098191 seconds for 412 oneBitsIndexes 2.0836421 seconds for 341 oneBitsIndexes 2.0803612 seconds for 682 oneBitsIndexes 2.1684378 seconds for 951 ClearOneBit 0.3005068 seconds for 382 ClearOneBit 1.7872318 seconds for 256 ClearOneBit 1.7902597 seconds for 1 ClearOneBit 0.9243212 seconds for 257 ClearOneBit 0.2666008 seconds for 999 ClearOneBit 0.3929297 seconds for 555 ClearOneBit 0.3964557 seconds for 412 ClearOneBit 0.3945432 seconds for 341 ClearOneBit 0.3936286 seconds for 682 ClearOneBit 0.2686803 seconds for 951 FlipRandomTrueBit 1.5828644 seconds for 382 FlipRandomTrueBit 1.3162437 seconds for 256 FlipRandomTrueBit 1.2944724 seconds for 1 FlipRandomTrueBit 1.3305612 seconds for 257 FlipRandomTrueBit 1.5845461 seconds for 999 FlipRandomTrueBit 1.5252726 seconds for 555 FlipRandomTrueBit 1.5786568 seconds for 412 FlipRandomTrueBit 1.5314749 seconds for 341 FlipRandomTrueBit 1.5311035 seconds for 682 FlipRandomTrueBit 1.6164142 seconds for 951 ClearRandomBit 0.2681578 seconds for 382 ClearRandomBit 0.2728117 seconds for 256 ClearRandomBit 0.2685423 seconds for 1 ClearRandomBit 0.2626029 seconds for 257 ClearRandomBit 0.2623253 seconds for 999 ClearRandomBit 0.274382 seconds for 555 ClearRandomBit 0.2644288 seconds for 412 ClearRandomBit 0.2667171 seconds for 341 ClearRandomBit 0.264912 seconds for 682 ClearRandomBit 0.2666491 seconds for 951
so in the end, Kyteland is now the winner.
static Random random = new Random();
public static int Perturb(int data)
{
if (data == 0) return 0;
// attempt to pick a more narrow search space
int minBits = (data & 0xFFFF0000) == 0 ? 16 : 32;
// int used = 0; // Uncomment for more-bounded performance
int newData = data;
do
{
// Unbounded performance guarantees
newData &= ~(1 << random.Next(minBits));
// // More-bounded performance:
// int bit = 1 << random.Next(minBits);
// if ((used & bit) == bit) continue;
// used |= bit;
// newData &= ~bit;
} while (newData == data); // XXX: we know we've inverted at least one 1
// when the new value differs
return newData;
}
Update: added code above which can be used for more-bounded performance guarantees (or less unbounded if you want to think of it that way). Interestingly enough this performs better than the original uncommented version.
Below is an alternate approach that is fast but without bounded performance guarantees:
public static int FastPerturb(int data)
{
if (data == 0) return 0;
int bit = 0;
while (0 == (data & (bit = 1 << random.Next(32))));
return data & ~bit;
}
Here's a slightly more efficient version using bit twiddling.
public static int getBitCount(int bits)
{
bits = bits - ((bits >> 1) & 0x55555555);
bits = (bits & 0x33333333) + ((bits >> 2) & 0x33333333);
return ((bits + (bits >> 4) & 0xF0F0F0F) * 0x1010101) >> 24;
}
public static int flipRandomBit(int data)
{
int index = random.Next(getBitCount(data));
int mask = data;
for (int i = 0; i < index; i++)
mask &= mask - 1;
mask ^= mask & (mask - 1);
return data ^ mask;
}
EDIT : fixed to take into account the constraint "a bit which is not 0"
Pick a random number N between 0 and 31 (for a 32 bit integer), and use it to generate a bitmask by shifting 1 N times to the left. Repeat until bit N is not 0 in the original number. Negate the bitmask to have only 1 bit set to 0 and combine it with your original number with the & operator :
private int ClearOneBit(int originalValue)
{
if (originalValue == 0)
return 0; // All bits are already set to 0, nothing to do
Random rnd = new Random();
int mask = 0;
do
{
int n = rnd.Next(32);
mask = 1 << n;
} while ((mask & originalValue) == 0); // check that this bit is not 0
int newValue = originalValue & ~mask; // clear this bit
return newValue;
}
OK:
private static Random rnd = new Random((int)DateTime.Now.Ticks);
private static Int32 SetRandomTrueBitToFalse(Int32 p)
{
List<int> trueBits = new List<int>();
for (int i = 0; i < 31; i++)
{
if ((p>>i&1) == 1){
trueBits.Add(i);
}
}
if (trueBits.Count>0){
int index = rnd.Next(0, trueBits.Count);
return p & ~(1 << trueBits[index]);
}
return p;
}
But I would love to know: Why do you need/want this?
You can turn on any bit by OR'ing it with 1 and turn it off by AND'ing with the bitwise complement.
Here's an example that selects a random 1-bit and turns it off.
var rand = new Random();
int myValue = 0x017E; // 101111110b
// identify which indexes are one-bits (if any, thanks Doc)
if( myValue > 0 )
{
var oneBitsIndexes = Enumerable.Range( 0, 31 )
.Where(i => ((myValue >> i) & 0x1) !=0).ToList();
// pick a random index and update the source value bit there from 1 to 0
myValue &= ~(1 << oneBitsIndexes[rand.Next(oneBitsIndexes.Count)]);
}
// otherwise, there are no bits to turn off...
You can generalize this by using BitArray.
public static BitArray FlipRandomTrueBit(BitArray bits)
{
List<int> trueBits = new List<int>();
for (int i = 0; i < bits.Count; i++)
if (bits[i])
trueBits.Add(i);
if (trueBits.Count > 0)
{
int index = rnd.Next(0, trueBits.Count);
bits[trueBits[index]] = false;
}
return bits;
}
However then you will have to write helper functions for simple data types.
public static int FlipRandomTrueBit(int input)
{
BitArray bits = new BitArray(new int[] { input });
BitArray flipedBits = FlipRandomTrueBit(bits);
byte[] bytes = new byte[4];
flipedBits.CopyTo(bytes, 0);
int result = BitConverter.ToInt32(bytes, 0);
return result;
}
If your using a large bit array you could save memory by iterating twice.
public static void FlipRandomTrueBitLowMem(ref BitArray bits)
{
int trueBits = 0;
for (int i = 0; i < bits.Count; i++)
if (bits[i])
trueBits++;
if (trueBits > 0)
{
int flip = rnd.Next(0, trueBits);
for (int i = 0; i < bits.Count; i++)
{
if (bits[i])
{
if (flip == 0)
{
bits[i] = false;
break;
}
flip--;
}
}
}
}
Test Program.
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
namespace bitarray
{
class Program
{
private static Random rnd = new Random((int)DateTime.Now.Ticks);
public static BitArray FlipRandomTrueBit(BitArray bits)
{
List<int> trueBits = new List<int>();
for (int i = 0; i < bits.Count; i++)
if (bits[i])
trueBits.Add(i);
if (trueBits.Count > 0)
{
int index = rnd.Next(0, trueBits.Count);
bits[trueBits[index]] = false;
}
return bits;
}
public static int FlipRandomTrueBit(int input)
{
BitArray bits = new BitArray(new int[] { input });
BitArray flipedBits = FlipRandomTrueBit(bits);
byte[] bytes = new byte[4];
flipedBits.CopyTo(bytes, 0);
int result = BitConverter.ToInt32(bytes, 0);
return result;
}
static void Main(string[] args)
{
int test = 382;
for (int n = 0; n < 200; n++)
{
int result = FlipRandomTrueBit(test);
Console.WriteLine(result);
}
Console.ReadLine();
}
}
}
Count all the 1's in your integer. Choose a random number using your favorite random number generator between 1 and the first count. Create a mask for Random-th 1 in your integer. OR your integer with the mask.
EDIT: Fixed some logic.
BitArray bits = new BitArray(new int[] { number } );
randomIndex = new Random().Next(32);
// check if bit is true, if not, goes to next bit and wraps around as well.
for(int i = 0; i < 32; i++)
{
if(bits[randomIndex] == false)
{
randomIndex = (randomIndex + 1) % 32;
}
else
{
break;
}
}
bits[randomIndex] = false;
Try the following code
public static int ChangeOneBit(int data)
{
if (data == 0)
{
return data;
}
var random = new Random();
int bit = 0;
do
{
var shift = random.Next(31);
bit = data >> shift;
bit = bit & 0x00000001;
} while (bit == 0);
var ret = data & (~(1 << bit));
return ret;
}
int changeBit(int a)
{
a = ~a;
int temp = a;
while(temp == a)
{
r = Math.pow(2,(int)(32*random.next()));
a = a || r;
}
return ~a;
}
Ok, a lot of wrong answers. Here's one that works:
- determine which bit to flip. Do this randomly. I won't supply the code, it's pretty straightforward.
- setup a bitmask with all zeros, with a 1 for the bit in question. So for example, if it's the 3rd bit, your bitmask might be 00000100. Again, this doesn't require code.
- bitwise XOR your number with the bit mask. If you're unfamiliar with the operator it's the hat operator:
^
Here's some sample code:
int myInt; // set this up with your original value
int myBitmask; // set this up with the bit mask via steps 1 and 2.
int randomlyZeroedBitInt = myInt ^ myBitmask;
Edit: On a fifth read of the question, I have a question in return: you are wanting to do which of the following:
- Randomly zero a bit, but only if that bit is already 1. In other words, if the bit in question isn't already 1, the operation is a no-op.
- Randomly choose a bit that is 1 and zero it. This operation always chooses a bit that is already 1 and always zeros it. The operation is only a no-op if the original value is 0.
Edit 2:
2 is correct,(15chars) – Fredou
In that case, my general algorithm stands; merely choose the bit in step 1 with internal logic. Alternatively, choose a fully random bit in step 1 and repeat until the value of myInt and randomlyZeroedBitInt are not equal.
Unfortunately either case means a more complex algorithm, as you'll either need to iterate over every bit in your value to determine which to flip, or you'll need to loop the algorithm until a bit is flipped.
Here is a version based on an algorithm from Bit Twiddling Hacks to select the nth set bit of an integer. For this case, we simply select n at random.
The code has been ported to C#, made to work directly on 32-bit signed integers, and count from the right instead of the left. Furthermore, the optimization to remove all branches has not been preserved here as it yielded slower code on my machine (Intel Core 2 Quad Q9450).
The description on the Bit Twiddling Hacks page does not give much insight into how the algorithm works. I have taken the time to step through and reverse engineer it and what I found is described in detail in the comments below.
In my tests, this algorithm performs very similarly to Kyteland's excellent flipRandomBit over input that is distributed randomly across the full range of 32-bit integers. However, flipRandomBit is slightly faster for numbers with significantly fewer set bits than cleared bits. Conversely, this algorithm is slightly faster for numbers with significantly more set bits than cleared bits.
The OP's benchmark consists entirely of small positive integers, which do not stress flipRandomBit's worst case. If this is an indication of the expected input, then all the more reason to prefer flipRandomBit.
static int ClearRandomSetBit(int input) {
///////////////////////////////////////////////////////////////////////
// ** Step 1 **
// Count the set bits
////////////////////////////////////////////////////////////////////////
// magic numbers
const int m2 = 0x55555555; // 1 zero, 1 one, ...
const int m4 = 0x33333333; // 2 zeros, 2 ones, ...
const int m8 = 0x0f0f0f0f; // 4 zeros, 4 ones, ...
// sequence of 2-bit values representing the counts of each 2 bits.
int c2 = input - ((input >> 1) & m2);
// sequence of 4-bit values representing the counts of each 4 bits.
int c4 = (c2 & m4) + ((c2 >> 2) & m4);
// sequence of 8-bit values representing the counts of each 8 bits.
int c8 = (c4 + (c4 >> 4)) & m8;
// count set bits in input.
int bitCount = (c8 * 0x1010101) >> 24;
///////////////////////////////////////////////////////////////////////////////////
// ** Step 2 **
// Select a random set bit to clear and find it using binary search with our
// knowledge of the bit counts in the various regions.
///////////////////////////////////////////////////////////////////////////////////
// count 16 right-most bits where we'll begin our search
int count = (c8 + (c8 >> 8)) & 0xff;
// position of target bit among the set bits
int target = random.Next(bitCount);
// distance in set bits from the current position to the target
int distance = target + 1;
// current bit position
int pos = 0;
// if the target is not in the right-most 16 bits, move past them
if (distance > count) { pos += 16; distance -= count; }
// if the target is not in the next 8 bits, move past them
count = (c8 >> pos) & 0xff;
if (distance > count) { pos += 8; distance -= count; }
// if the target is not in the next 4 bits, move past them
count = (c4 >> pos) & 0xf;
if (distance > count) { pos += 4; distance -= count; }
// if the target is not in the next 2 bits, move past them
count = (c2 >> pos) & 0x3;
if (distance > count) { pos += 2; distance -= count; }
// if the bit is not the next bit, move past it.
//
// Note that distance and count must be single bits by now.
// As such, distance is greater than count if and only if
// distance equals 1 and count equals 0. This obversation
// allows us to optimize away the final branch.
Debug.Assert((distance & 0x1) == distance);
Debug.Assert((count & 0x1) == count);
count = (input >> pos) & 0x1;
pos += (distance & (count ^ 1));
Debug.Assert((input & (1 << pos)) != 0);
return input ^ (1 << pos);
}
int val=382
int mask = ~(1 << N)
// this would turn-off nth bit (0 to 31)
NewVal = (int) ((uint)val & (uint)mask}
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