Fastest way to separate the digits of an int into an array in .NET?
.net, arrays, c#, numbers, performance
Solution
How about:
public static int[] ConvertToArrayOfDigits(int value)
{
int size = DetermineDigitCount(value);
int[] digits = new int[size];
for (int index = size - 1; index >= 0; index--)
{
digits[index] = value % 10;
value = value / 10;
}
return digits;
}
private static int DetermineDigitCount(int x)
{
// This bit could be optimised with a binary search
return x < 10 ? 1
: x < 100 ? 2
: x < 1000 ? 3
: x < 10000 ? 4
: x < 100000 ? 5
: x < 1000000 ? 6
: x < 10000000 ? 7
: x < 100000000 ? 8
: x < 1000000000 ? 9
: 10;
}
Note that this won't cope with negative numbers... do you need it to?
EDIT: Here's a version which memoizes the results for under 10000, as suggested by Eric. If you can absolutely guarantee that you won't change the contents of the returned array, you could remove the `Clone` call. It also has the handy property of reducing the number of checks to determine the length of "large" numbers - and small numbers will only go through that code once anyway :)
private static readonly int[][] memoizedResults = new int[10000][];
public static int[] ConvertToArrayOfDigits(int value)
{
if (value < 10000)
{
int[] memoized = memoizedResults[value];
if (memoized == null) {
memoized = ConvertSmall(value);
memoizedResults[value] = memoized;
}
return (int[]) memoized.Clone();
}
// We know that value >= 10000
int size = value < 100000 ? 5
: value < 1000000 ? 6
: value < 10000000 ? 7
: value < 100000000 ? 8
: value < 1000000000 ? 9
: 10;
return ConvertWithSize(value, size);
}
private static int[] ConvertSmall(int value)
{
// We know that value < 10000
int size = value < 10 ? 1
: value < 100 ? 2
: value < 1000 ? 3 : 4;
return ConvertWithSize(value, size);
}
private static int[] ConvertWithSize(int value, int size)
{
int[] digits = new int[size];
for (int index = size - 1; index >= 0; index--)
{
digits[index] = value % 10;
value = value / 10;
}
return digits;
}
Note that this doesn't try to be thread-safe at the moment. You may need to add a memory barrier to make sure that the write to the memoized results isn't visible until the writes within the individual result are visible. I've given up trying to reason about these things unless I absolutely have to. I'm sure you can make it lock-free with effort, but you should really get someone very smart to do so if you really need to.
EDIT: I've just realised that the "large" case can make use of the "small" case - split the large number into two small ones and use the memoised results. I'll give that a go after dinner and write a benchmark...
EDIT: Okay, ready for a giant amount of code? I realised that for uniformly random numbers at least, you'll get "big" numbers much more often than small ones - so you need to optimise for that. Of course, that might not be the case for real data, but anyway... it means I now do my size tests in the opposite order, hoping for big numbers first.
I've got a benchmark for the original code, the simple memoization, and then the extremely-unrolled code.
Results (in ms):
Simple: 3168
SimpleMemo: 3061
UnrolledMemo: 1204
Code:
using System;
using System.Diagnostics;
class DigitSplitting
{
static void Main()
{
Test(Simple);
Test(SimpleMemo);
Test(UnrolledMemo);
}
const int Iterations = 10000000;
static void Test(Func<int, int[]> candidate)
{
Random rng = new Random(0);
Stopwatch sw = Stopwatch.StartNew();
for (int i = 0; i < Iterations; i++)
{
candidate(rng.Next());
}
sw.Stop();
Console.WriteLine("{0}: {1}",
candidate.Method.Name, (int) sw.ElapsedMilliseconds);
}
#region Simple
static int[] Simple(int value)
{
int size = DetermineDigitCount(value);
int[] digits = new int[size];
for (int index = size - 1; index >= 0; index--)
{
digits[index] = value % 10;
value = value / 10;
}
return digits;
}
private static int DetermineDigitCount(int x)
{
// This bit could be optimised with a binary search
return x < 10 ? 1
: x < 100 ? 2
: x < 1000 ? 3
: x < 10000 ? 4
: x < 100000 ? 5
: x < 1000000 ? 6
: x < 10000000 ? 7
: x < 100000000 ? 8
: x < 1000000000 ? 9
: 10;
}
#endregion Simple
#region SimpleMemo
private static readonly int[][] memoizedResults = new int[10000][];
public static int[] SimpleMemo(int value)
{
if (value < 10000)
{
int[] memoized = memoizedResults[value];
if (memoized == null) {
memoized = ConvertSmall(value);
memoizedResults[value] = memoized;
}
return (int[]) memoized.Clone();
}
// We know that value >= 10000
int size = value >= 1000000000 ? 10
: value >= 100000000 ? 9
: value >= 10000000 ? 8
: value >= 1000000 ? 7
: value >= 100000 ? 6
: 5;
return ConvertWithSize(value, size);
}
private static int[] ConvertSmall(int value)
{
// We know that value < 10000
return value >= 1000 ? new[] { value / 1000, (value / 100) % 10,
(value / 10) % 10, value % 10 }
: value >= 100 ? new[] { value / 100, (value / 10) % 10,
value % 10 }
: value >= 10 ? new[] { value / 10, value % 10 }
: new int[] { value };
}
private static int[] ConvertWithSize(int value, int size)
{
int[] digits = new int[size];
for (int index = size - 1; index >= 0; index--)
{
digits[index] = value % 10;
value = value / 10;
}
return digits;
}
#endregion
#region UnrolledMemo
private static readonly int[][] memoizedResults2 = new int[10000][];
private static readonly int[][] memoizedResults3 = new int[10000][];
static int[] UnrolledMemo(int value)
{
if (value < 10000)
{
return (int[]) UnclonedConvertSmall(value).Clone();
}
if (value >= 1000000000)
{
int[] ret = new int[10];
int firstChunk = value / 100000000;
ret[0] = firstChunk / 10;
ret[1] = firstChunk % 10;
value -= firstChunk * 100000000;
int[] secondChunk = ConvertSize4(value / 10000);
int[] thirdChunk = ConvertSize4(value % 10000);
ret[2] = secondChunk[0];
ret[3] = secondChunk[1];
ret[4] = secondChunk[2];
ret[5] = secondChunk[3];
ret[6] = thirdChunk[0];
ret[7] = thirdChunk[1];
ret[8] = thirdChunk[2];
ret[9] = thirdChunk[3];
return ret;
}
else if (value >= 100000000)
{
int[] ret = new int[9];
int firstChunk = value / 100000000;
ret[0] = firstChunk;
value -= firstChunk * 100000000;
int[] secondChunk = ConvertSize4(value / 10000);
int[] thirdChunk = ConvertSize4(value % 10000);
ret[1] = secondChunk[0];
ret[2] = secondChunk[1];
ret[3] = secondChunk[2];
ret[4] = secondChunk[3];
ret[5] = thirdChunk[0];
ret[6] = thirdChunk[1];
ret[7] = thirdChunk[2];
ret[8] = thirdChunk[3];
return ret;
}
else if (value >= 10000000)
{
int[] ret = new int[8];
int[] firstChunk = ConvertSize4(value / 10000);
int[] secondChunk = ConvertSize4(value % 10000);
ret[0] = firstChunk[0];
ret[1] = firstChunk[0];
ret[2] = firstChunk[0];
ret[3] = firstChunk[0];
ret[4] = secondChunk[0];
ret[5] = secondChunk[1];
ret[6] = secondChunk[2];
ret[7] = secondChunk[3];
return ret;
}
else if (value >= 1000000)
{
int[] ret = new int[7];
int[] firstChunk = ConvertSize4(value / 10000);
int[] secondChunk = ConvertSize4(value % 10000);
ret[0] = firstChunk[1];
ret[1] = firstChunk[2];
ret[2] = firstChunk[3];
ret[3] = secondChunk[0];
ret[4] = secondChunk[1];
ret[5] = secondChunk[2];
ret[6] = secondChunk[3];
return ret;
}
else if (value >= 100000)
{
int[] ret = new int[6];
int[] firstChunk = ConvertSize4(value / 10000);
int[] secondChunk = ConvertSize4(value % 10000);
ret[0] = firstChunk[2];
ret[1] = firstChunk[3];
ret[2] = secondChunk[0];
ret[3] = secondChunk[1];
ret[4] = secondChunk[2];
ret[5] = secondChunk[3];
return ret;
}
else
{
int[] ret = new int[5];
int[] chunk = ConvertSize4(value % 10000);
ret[0] = value / 10000;
ret[1] = chunk[0];
ret[2] = chunk[1];
ret[3] = chunk[2];
ret[4] = chunk[3];
return ret;
}
}
private static int[] UnclonedConvertSmall(int value)
{
int[] ret = memoizedResults2[value];
if (ret == null)
{
ret = value >= 1000 ? new[] { value / 1000, (value / 100) % 10,
(value / 10) % 10, value % 10 }
: value >= 100 ? new[] { value / 100, (value / 10) % 10,
value % 10 }
: value >= 10 ? new[] { value / 10, value % 10 }
: new int[] { value };
memoizedResults2[value] = ret;
}
return ret;
}
private static int[] ConvertSize4(int value)
{
int[] ret = memoizedResults3[value];
if (ret == null)
{
ret = new[] { value / 1000, (value / 100) % 10,
(value / 10) % 10, value % 10 };
memoizedResults3[value] = ret;
}
return ret;
}
#endregion UnrolledMemo
}
Problem
I want to separate the digits of an integer, say 12345, into an array of bytes {1,2,3,4,5}, but I want the most performance effective way to do that, because my program does that millions of times. Any suggestions? Thanks.