Binomial coefficient function C++ incorrect answer n>13 - c++

I'm trying to learn C++ and hence I'm trying to do a function to calculate the binomial coefficient. The code works up to a n of 12, for larger values the generated result is incorrect. I'm grateful for your input.
long double binomial(int n, int k) {
int d = n-k;
int i = 1, t = 1, n1 = 1, n2 = 1;
if (d == 0) {
return 1;
} else if (n==0) {
return 1;
} else {
while (i <=n) {
t *= i;
if (i == d) {
n1 = t;
cout << t;
}
if (i == k) {
n2 = t;
cout << t;
}
i++;
}
}
return t/n1/n2;
}
int main() {
int n, k;
cout << "Select an integer n: \n";
cin >> n;
cout << "Select an integer k: \n";
cin >> k;
long double v = binomial(n,k);
cout << "The binomial coefficient is: " << v << "\n";
return 0;
}

An int variable can only hold numbers up to a certain size. This varies from compiler to compiler and platform to platform but a typical limit would be around 2 billion. Your program is using numbers bigger than that so you get errors.
If you want to compute with big integers the answer is to get a big integer library. GMP is a popular one.

If int is 32 bits long on your system (very common nowadays), then the factorial of 13 doesn't fit into it (6227020800 > 2147483647).
Either transition to something bigger (unsigned long long, anyone?), or use a bigint library, or come up with a better/more clever algorithm that doesn't involve computing large factorials, at least not directly.

One of the suggests would be to use some other type.
Here is a list of integer types, sizes and limits.
--------------------------------------------------------------------------------------
|type |size (B)|Limits |
--------------------------------------------------------------------------------------
|long long |8 |–9,223,372,036,854,775,808 to 9,223,372,036,854,775,807|
--------------------------------------------------------------------------------------
|unsigned long long |8 |0 to 18,446,744,073,709,551,615 |
--------------------------------------------------------------------------------------
|int |4 |–2,147,483,648 to 2,147,483,647 |
--------------------------------------------------------------------------------------
|unsigned int |4 |0 to 4,294,967,295 |
--------------------------------------------------------------------------------------
|short |2 |–32,768 to 32,767 |
--------------------------------------------------------------------------------------
|unsigned short |2 |0 to 65,535 |
--------------------------------------------------------------------------------------
|char |1 |–128 to 127 |
--------------------------------------------------------------------------------------
|unsigned char |1 |0 to 255 |
--------------------------------------------------------------------------------------
Note long and int usually are the same size.
Note those limits aren't the same on all architectures standart guarantee only two things about variable sizes:
1 = sizeof(char) = sizeof(unsigned char)
2 = sizeof(shor) = sizeof(unsigned short) <= sizeof(int) = sizeof(unsigned int) <= sizeof(long) = sizeof(unsigned long) <= sizeof(long long) = sizeof(unsigned long long)
Another option is to use bigint library, however in this case calculations will take more time but will fit.

Related

Why does not my code give the last two numbers?

I have written a C++ program to find all the automorphic numbers (numbers which are repeated in the final digits of their squares such as 5x5=25, 76x76=5776) from 1 to 111,111. The program runs fine except that it fails to give 90625 and 109376. The code is as follows:
#include <cstdlib>
#include <iostream>
using namespace std;
int main() {
long int square;
int a, sum = 0, result, b;
for (int i = 1; i < 111111; i++) {
result = 1;
b = i;
while (b > 0){
b = b / 10;
result = result * 10;
}
square = i * i;
a = square % result;
if(i == a){
sum = sum + i;
cout << i << endl;
}
}
cout << sum << endl;
return 0;
}
Long int has only 4 bytes
long 4 bytes -2,147,483,648 to 2,147,483,647
unsigned long 4 bytes 0 to 4,294,967,295
The square of 90625 and 109376 are 8,212,890,625 and 11,963,109,376 respectively. So as the values overflow, you won't be able to produce those two values in long int limit. You can use integer type long long.
long long 8 bytes –9,223,372,036,854,775,808 to 9,223,372,036,854,775,807
unsigned long long 8 bytes 0 to 18,446,744,073,709,551,615
And if you want to use bigger numbers you can handle them using Arrays or let libraries like GMP to handle larger numbers.

Overflow on Multiplication in C++

In my code I am trying to multiply two numbers. The algorithm is simple as (k)*(k-1)^n. I stored the product (k-1)^n in variable p1 and then I multiply it with k. For n=10, k=10 (k-1)^n-1 should be 387420489 and I got this in variable p1 but on multiplying it with k, I get a negative number. I used modulus but instead of 3874208490, I get some other large positive number. What is the correct approach?
#include <iostream>
using namespace std;
typedef long long ll;
ll big = 1000000000 + 7;
ll multiply(ll a, ll b)
{
ll ans = 1;
for (int i = 1; i <= b; i++)
ans = ans * a;
return ans % big;
}
int main()
{
int t;
scanf("%d", &t);
while (t--)
{
ll n, k;
cin >> n >> k;
ll p1 = multiply(k - 1, n - 1);
cout << p1 << endl; // this gives correct value
ll p2 = (k % big) * (p1 % big);
cout << ((p2 + big) % big) % big << endl;
}
}
What is ll type? If it is just int (and I pretty sure it is), it gets overflowed, because 32-bit signed type can't store values more than (2^31)-1, which approximately equals to 2 * 10^9. You can use long long int to make it work, then your code will work with the results less than 2^63.
It's not surprising you get an overflow. I plugged your equation into wolfram alpha, fixing n at 10 and iterating over k from 0 to 100.
The curve gets very vertical, very quickly at around k = 80.
10^21 requires 70 binary bits to represent it, and you only have 63 in a long long.
You're going to have to decide what the limits of this algorithm's parameters are and pick data types corresponding. Perhaps a double would be more suitable?
link to plot is here

How to reduce time complexity for large data set inputs in this program?

I have written this code to calculate the number of set bits between the range of numbers. My program gets compiled fine and giving proper output. It is taking too much time for large inputs and "Time limit exceeding".
#define forn(i, n) for(long int i = 0; i < (long int)(n); i++)
#define ford(i, n) for(long int i = (long int)(n) - 1; i >= 0; i--)
#define fore(i, a, n) for(long int i = (int)(a); i < (long int)(n); i++)
long int solve(long int i) {
i = i - ((i >> 1) & 0x55555555);
i = (i & 0x33333333) + ((i >> 2) & 0x33333333);
return (((i + (i >> 4)) & 0x0F0F0F0F) * 0x01010101) >> 24;
}
int main() {
freopen("C:/Projects/CodeChef/SetBits/input.txt", "rt", stdin);
freopen("C:/Projects/CodeChef/SetBits/output.txt", "wt", stdout);
int tt;
long long int num1;
long long int num2;
scanf("%d", &tt);
forn(ii, tt) {
unsigned long int bits = 0;
unsigned long long int total_bits = 0;
scanf("%lld",&num1);
scanf("%lld",&num2);
fore(jj, num1, num2+1) {
bits = solve(jj);
total_bits += bits;
}
printf("%lld\n",total_bits);
}
return 0;
}
Example test case:-
Sample Input:
3
-2 0
-3 4
-1 4
Sample Output:
63
99
37
For the first case, -2 contains 31 1's followed by a 0, -1 contains 32 1's and 0 contains 0 1's. Thus the total is 63.
For the second case, the answer is 31 + 31 + 32 + 0 + 1 + 1 + 2 + 1 = 99
Test case having large values:-
10
-1548535525 662630637
-1677484556 -399596060
-2111785037 1953091095
643110128 1917824721
-1807916951 491608908
-1536297104 1976838237
-1891897587 -736733635
-2088577104 353890389
-2081420990 819160807
-1585188028 2053582020
Any suggestions on how to optimize the code so that it will take less time. All helpful suggestions and answers will be appreciated with vote up. :)
I don't really have a clue what you are doing, but I do know you can clean up your code considerable, and you can inline you function.
Also I have taken the liberty of 'rephrasing' you code, you are using C++ like C and those defines are just grim and mapping the files onto stdio is even worse. I haven't tested or compiled the code but it is all there.
#include <fstream>
inline long int solve(long int i) {
i = i - ((i >> 1) & 0x55555555);
i = (i & 0x33333333) + ((i >> 2) & 0x33333333);
return (((i + (i >> 4)) & 0x0F0F0F0F) * 0x01010101) >> 24;
}
int main() {
long first, last;
unsigned count;
std::ifstream inf("C:/Projects/CodeChef/SetBits/input.txt");
std::ofstream off("C:/Projects/CodeChef/SetBits/output.txt");
inf >> count;
for(unsigned i=0u; i!=count; ++i) {
inf >> first >> last;
long total=0;
++last;
for(long t=first; t!=last; ++t) {
total+=solve(t);
}
off << total << '\n';
}
return 0;
}
A few ideas as to how you could be speed this up:
you could build a std::map of the computed values and if they have been previously processed then use them rather than recomputing.
do the same but store ranges rather than single values but that will be tricky.
you could see if a value exists in the map and increment through the map until there are no more preprocessed values in which case start processing them for the iteration.
check if there is a trivial sequence between on number and the next may be you could work out the first value then just increment it.
may there is a O(1) algorithm for such a sequence
Look at intel TBB and using something like tbb::parallel for to distribute the work over each core, because you are dealing with such a small about or memory then you should get a really good return with a large chunk size.

Bitwise shift operation on a 128-bit number

Lets say that I have an array of 4 32-bit integers which I use to store the 128-bit number
How can I perform left and right shift on this 128-bit number?
Thanks!
Working with uint128? If you can, use the x86 SSE instructions, which were designed for exactly that. (Then, when you've bitshifted your value, you're ready to do other 128-bit operations...)
SSE2 bit shifts take ~4 instructions on average, with one branch (a case statement). No issues with shifting more than 32 bits, either. The full code for doing this is, using gcc intrinsics rather than raw assembler, is in sseutil.c (github: "Unusual uses of SSE2") -- and it's a bit bigger than makes sense to paste here.
The hurdle for many people in using SSE2 is that shift ops take immediate (constant) shift counts. You can solve that with a bit of C preprocessor twiddling (wordpress: C preprocessor tricks). After that, you have op sequences like:
LeftShift(uint128 x, int n) = _mm_slli_epi64(_mm_slli_si128(x, n/8), n%8)
for n = 65..71, 73..79, … 121..127
... doing the whole shift in two instructions.
void shiftl128 (
unsigned int& a,
unsigned int& b,
unsigned int& c,
unsigned int& d,
size_t k)
{
assert (k <= 128);
if (k >= 32) // shifting a 32-bit integer by more than 31 bits is "undefined"
{
a=b;
b=c;
c=d;
d=0;
shiftl128(a,b,c,d,k-32);
}
else
{
a = (a << k) | (b >> (32-k));
b = (b << k) | (c >> (32-k));
c = (c << k) | (d >> (32-k));
d = (d << k);
}
}
void shiftr128 (
unsigned int& a,
unsigned int& b,
unsigned int& c,
unsigned int& d,
size_t k)
{
assert (k <= 128);
if (k >= 32) // shifting a 32-bit integer by more than 31 bits is "undefined"
{
d=c;
c=b;
b=a;
a=0;
shiftr128(a,b,c,d,k-32);
}
else
{
d = (c << (32-k)) | (d >> k); \
c = (b << (32-k)) | (c >> k); \
b = (a << (32-k)) | (b >> k); \
a = (a >> k);
}
}
Instead of using a 128 bit number why not use a bitset? Using a bitset, you can adjust how big you want it to be. Plus you can perform quite a few operations on it.
You can find more information on these here:
http://www.cppreference.com/wiki/utility/bitset/start?do=backlink
First, if you're shifting by n bits and n is greater than or equal to 32, divide by 32 and shift whole integers. This should be trivial. Now you're left with a remaining shift count from 0 to 31. If it's zero, return early, you're done.
For each integer you'll need to shift by the remaining n, then shift the adjacent integer by the same amount and combine the valid bits from each.
Since you mentioned you're storing your 128-bit value in an array of 4 integers, you could do the following:
void left_shift(unsigned int* array)
{
for (int i=3; i >= 0; i--)
{
array[i] = array[i] << 1;
if (i > 0)
{
unsigned int top_bit = (array[i-1] >> 31) & 0x1;
array[i] = array[i] | top_bit;
}
}
}
void right_shift(unsigned int* array)
{
for (int i=0; i < 4; i++)
{
array[i] = array[i] >> 1;
if (i < 3)
{
unsigned int bottom_bit = (array[i+1] & 0x1) << 31;
array[i] = array[i] | bottom_bit;
}
}
}

How to determine how many bytes an integer needs?

I'm looking for the most efficient way to calculate the minimum number of bytes needed to store an integer without losing precision.
e.g.
int: 10 = 1 byte
int: 257 = 2 bytes;
int: 18446744073709551615 (UINT64_MAX) = 8 bytes;
Thanks
P.S. This is for a hash functions which will be called many millions of times
Also the byte sizes don't have to be a power of two
The fastest solution seems to one based on tronics answer:
int bytes;
if (hash <= UINT32_MAX)
{
if (hash < 16777216U)
{
if (hash <= UINT16_MAX)
{
if (hash <= UINT8_MAX) bytes = 1;
else bytes = 2;
}
else bytes = 3;
}
else bytes = 4;
}
else if (hash <= UINT64_MAX)
{
if (hash < 72057594000000000ULL)
{
if (hash < 281474976710656ULL)
{
if (hash < 1099511627776ULL) bytes = 5;
else bytes = 6;
}
else bytes = 7;
}
else bytes = 8;
}
The speed difference using mostly 56 bit vals was minimal (but measurable) compared to Thomas Pornin answer. Also i didn't test the solution using __builtin_clzl which could be comparable.
Use this:
int n = 0;
while (x != 0) {
x >>= 8;
n ++;
}
This assumes that x contains your (positive) value.
Note that zero will be declared encodable as no byte at all. Also, most variable-size encodings need some length field or terminator to know where encoding stops in a file or stream (usually, when you encode an integer and mind about size, then there is more than one integer in your encoded object).
You need just two simple ifs if you are interested on the common sizes only. Consider this (assuming that you actually have unsigned values):
if (val < 0x10000) {
if (val < 0x100) // 8 bit
else // 16 bit
} else {
if (val < 0x100000000L) // 32 bit
else // 64 bit
}
Should you need to test for other sizes, choosing a middle point and then doing nested tests will keep the number of tests very low in any case. However, in that case making the testing a recursive function might be a better option, to keep the code simple. A decent compiler will optimize away the recursive calls so that the resulting code is still just as fast.
Assuming a byte is 8 bits, to represent an integer x you need [log2(x) / 8] + 1 bytes where [x] = floor(x).
Ok, I see now that the byte sizes aren't necessarily a power of two. Consider the byte sizes b. The formula is still [log2(x) / b] + 1.
Now, to calculate the log, either use lookup tables (best way speed-wise) or use binary search, which is also very fast for integers.
The function to find the position of the first '1' bit from the most significant side (clz or bsr) is usually a simple CPU instruction (no need to mess with log2), so you could divide that by 8 to get the number of bytes needed. In gcc, there's __builtin_clz for this task:
#include <limits.h>
int bytes_needed(unsigned long long x) {
int bits_needed = sizeof(x)*CHAR_BIT - __builtin_clzll(x);
if (bits_needed == 0)
return 1;
else
return (bits_needed + 7) / 8;
}
(On MSVC you would use the _BitScanReverse intrinsic.)
You may first get the highest bit set, which is the same as log2(N), and then get the bytes needed by ceil(log2(N) / 8).
Here are some bit hacks for getting the position of the highest bit set, which are copied from http://graphics.stanford.edu/~seander/bithacks.html#IntegerLogObvious, and you can click the URL for details of how these algorithms work.
Find the integer log base 2 of an integer with an 64-bit IEEE float
int v; // 32-bit integer to find the log base 2 of
int r; // result of log_2(v) goes here
union { unsigned int u[2]; double d; } t; // temp
t.u[__FLOAT_WORD_ORDER==LITTLE_ENDIAN] = 0x43300000;
t.u[__FLOAT_WORD_ORDER!=LITTLE_ENDIAN] = v;
t.d -= 4503599627370496.0;
r = (t.u[__FLOAT_WORD_ORDER==LITTLE_ENDIAN] >> 20) - 0x3FF;
Find the log base 2 of an integer with a lookup table
static const char LogTable256[256] =
{
#define LT(n) n, n, n, n, n, n, n, n, n, n, n, n, n, n, n, n
-1, 0, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3,
LT(4), LT(5), LT(5), LT(6), LT(6), LT(6), LT(6),
LT(7), LT(7), LT(7), LT(7), LT(7), LT(7), LT(7), LT(7)
};
unsigned int v; // 32-bit word to find the log of
unsigned r; // r will be lg(v)
register unsigned int t, tt; // temporaries
if (tt = v >> 16)
{
r = (t = tt >> 8) ? 24 + LogTable256[t] : 16 + LogTable256[tt];
}
else
{
r = (t = v >> 8) ? 8 + LogTable256[t] : LogTable256[v];
}
Find the log base 2 of an N-bit integer in O(lg(N)) operations
unsigned int v; // 32-bit value to find the log2 of
const unsigned int b[] = {0x2, 0xC, 0xF0, 0xFF00, 0xFFFF0000};
const unsigned int S[] = {1, 2, 4, 8, 16};
int i;
register unsigned int r = 0; // result of log2(v) will go here
for (i = 4; i >= 0; i--) // unroll for speed...
{
if (v & b[i])
{
v >>= S[i];
r |= S[i];
}
}
// OR (IF YOUR CPU BRANCHES SLOWLY):
unsigned int v; // 32-bit value to find the log2 of
register unsigned int r; // result of log2(v) will go here
register unsigned int shift;
r = (v > 0xFFFF) << 4; v >>= r;
shift = (v > 0xFF ) << 3; v >>= shift; r |= shift;
shift = (v > 0xF ) << 2; v >>= shift; r |= shift;
shift = (v > 0x3 ) << 1; v >>= shift; r |= shift;
r |= (v >> 1);
// OR (IF YOU KNOW v IS A POWER OF 2):
unsigned int v; // 32-bit value to find the log2 of
static const unsigned int b[] = {0xAAAAAAAA, 0xCCCCCCCC, 0xF0F0F0F0,
0xFF00FF00, 0xFFFF0000};
register unsigned int r = (v & b[0]) != 0;
for (i = 4; i > 0; i--) // unroll for speed...
{
r |= ((v & b[i]) != 0) << i;
}
Find the number of bits by taking the log2 of the number, then divide that by 8 to get the number of bytes.
You can find logn of x by the formula:
logn(x) = log(x) / log(n)
Update:
Since you need to do this really quickly, Bit Twiddling Hacks has several methods for quickly calculating log2(x). The look-up table approach seems like it would suit your needs.
This will get you the number of bytes. It's not strictly the most efficient, but unless you're programming a nanobot powered by the energy contained in a red blood cell, it won't matter.
int count = 0;
while (numbertotest > 0)
{
numbertotest >>= 8;
count++;
}
You could write a little template meta-programming code to figure it out at compile time if you need it for array sizes:
template<unsigned long long N> struct NBytes
{ static const size_t value = NBytes<N/256>::value+1; };
template<> struct NBytes<0>
{ static const size_t value = 0; };
int main()
{
std::cout << "short = " << NBytes<SHRT_MAX>::value << " bytes\n";
std::cout << "int = " << NBytes<INT_MAX>::value << " bytes\n";
std::cout << "long long = " << NBytes<ULLONG_MAX>::value << " bytes\n";
std::cout << "10 = " << NBytes<10>::value << " bytes\n";
std::cout << "257 = " << NBytes<257>::value << " bytes\n";
return 0;
}
output:
short = 2 bytes
int = 4 bytes
long long = 8 bytes
10 = 1 bytes
257 = 2 bytes
Note: I know this isn't answering the original question, but it answers a related question that people will be searching for when they land on this page.
Floor((log2(N) / 8) + 1) bytes
You need exactly the log function
nb_bytes = floor(log(x)/log(256))+1
if you use log2, log2(256) == 8 so
floor(log2(x)/8)+1
You need to raise 256 to successive powers until the result is larger than your value.
For example: (Tested in C#)
long long limit = 1;
int byteCount;
for (byteCount = 1; byteCount < 8; byteCount++) {
limit *= 256;
if (limit > value)
break;
}
If you only want byte sizes to be powers of two (If you don't want 65,537 to return 3), replace byteCount++ with byteCount *= 2.
I think this is a portable implementation of the straightforward formula:
#include <limits.h>
#include <math.h>
#include <stdio.h>
int main(void) {
int i;
unsigned int values[] = {10, 257, 67898, 140000, INT_MAX, INT_MIN};
for ( i = 0; i < sizeof(values)/sizeof(values[0]); ++i) {
printf("%d needs %.0f bytes\n",
values[i],
1.0 + floor(log(values[i]) / (M_LN2 * CHAR_BIT))
);
}
return 0;
}
Output:
10 needs 1 bytes
257 needs 2 bytes
67898 needs 3 bytes
140000 needs 3 bytes
2147483647 needs 4 bytes
-2147483648 needs 4 bytes
Whether and how much the lack of speed and the need to link floating point libraries depends on your needs.
I know this question didn't ask for this type of answer but for those looking for a solution using the smallest number of characters, this does the assignment to a length variable in 17 characters, or 25 including the declaration of the length variable.
//Assuming v is the value that is being counted...
int l=0;
for(;v>>l*8;l++);
This is based on SoapBox's idea of creating a solution that contains no jumps, branches etc... Unfortunately his solution was not quite correct. I have adopted the spirit and here's a 32bit version, the 64bit checks can be applied easily if desired.
The function returns number of bytes required to store the given integer.
unsigned short getBytesNeeded(unsigned int value)
{
unsigned short c = 0; // 0 => size 1
c |= !!(value & 0xFF00); // 1 => size 2
c |= (!!(value & 0xFF0000)) << 1; // 2 => size 3
c |= (!!(value & 0xFF000000)) << 2; // 4 => size 4
static const int size_table[] = { 1, 2, 3, 3, 4, 4, 4, 4 };
return size_table[c];
}
For each of eight times, shift the int eight bits to the right and see if there are still 1-bits left. The number of times you shift before you stop is the number of bytes you need.
More succinctly, the minimum number of bytes you need is ceil(min_bits/8), where min_bits is the index (i+1) of the highest set bit.
There are a multitude of ways to do this.
Option #1.
int numBytes = 0;
do {
numBytes++;
} while (i >>= 8);
return (numBytes);
In the above example, is the number you are testing, and generally works for any processor, any size of integer.
However, it might not be the fastest. Alternatively, you can try a series of if statements ...
For a 32 bit integers
if ((upper = (value >> 16)) == 0) {
/* Bit in lower 16 bits may be set. */
if ((high = (value >> 8)) == 0) {
return (1);
}
return (2);
}
/* Bit in upper 16 bits is set */
if ((high = (upper >> 8)) == 0) {
return (3);
}
return (4);
For 64 bit integers, Another level of if statements would be required.
If the speed of this routine is as critical as you say, it might be worthwhile to do this in assembler if you want it as a function call. That could allow you to avoid creating and destroying the stack frame, saving a few extra clock cycles if it is that critical.
A bit basic, but since there will be a limited number of outputs, can you not pre-compute the breakpoints and use a case statement? No need for calculations at run-time, only a limited number of comparisons.
Why not just use a 32-bit hash?
That will work at near-top-speed everywhere.
I'm rather confused as to why a large hash would even be wanted. If a 4-byte hash works, why not just use it always? Excepting cryptographic uses, who has hash tables with more then 232 buckets anyway?
there are lots of great recipes for stuff like this over at Sean Anderson's "Bit Twiddling Hacks" page.
This code has 0 branches, which could be faster on some systems. Also on some systems (GPGPU) its important for threads in the same warp to execute the same instructions. This code is always the same number of instructions no matter what the input value.
inline int get_num_bytes(unsigned long long value) // where unsigned long long is the largest integer value on this platform
{
int size = 1; // starts at 1 sot that 0 will return 1 byte
size += !!(value & 0xFF00);
size += !!(value & 0xFFFF0000);
if (sizeof(unsigned long long) > 4) // every sane compiler will optimize this out
{
size += !!(value & 0xFFFFFFFF00000000ull);
if (sizeof(unsigned long long) > 8)
{
size += !!(value & 0xFFFFFFFFFFFFFFFF0000000000000000ull);
}
}
static const int size_table[] = { 1, 2, 4, 8, 16 };
return size_table[size];
}
g++ -O3 produces the following (verifying that the ifs are optimized out):
xor %edx,%edx
test $0xff00,%edi
setne %dl
xor %eax,%eax
test $0xffff0000,%edi
setne %al
lea 0x1(%rdx,%rax,1),%eax
movabs $0xffffffff00000000,%rdx
test %rdx,%rdi
setne %dl
lea (%rdx,%rax,1),%rax
and $0xf,%eax
mov _ZZ13get_num_bytesyE10size_table(,%rax,4),%eax
retq
Why so complicated? Here's what I came up with:
bytesNeeded = (numBits/8)+((numBits%8) != 0);
Basically numBits divided by eight + 1 if there is a remainder.
There are already a lot of answers here, but if you know the number ahead of time, in c++ you can use a template to make use of the preprocessor.
template <unsigned long long N>
struct RequiredBytes {
enum : int { value = 1 + (N > 255 ? RequiredBits<(N >> 8)>::value : 0) };
};
template <>
struct RequiredBytes<0> {
enum : int { value = 1 };
};
const int REQUIRED_BYTES_18446744073709551615 = RequiredBytes<18446744073709551615>::value; // 8
or for a bits version:
template <unsigned long long N>
struct RequiredBits {
enum : int { value = 1 + RequiredBits<(N >> 1)>::value };
};
template <>
struct RequiredBits<1> {
enum : int { value = 1 };
};
template <>
struct RequiredBits<0> {
enum : int { value = 1 };
};
const int REQUIRED_BITS_42 = RequiredBits<42>::value; // 6