Question

I wanted a number that would remain unique for a day (24 hours). Following is the code that I came up with; I was wondering about its fallacies/possible risks; ‘I believe’ this guarantees a 12 digit unique number for a day atleast.

Logic is to get the current date/time (hhmmssmmm) and concat the first four bytes of query performance counter result.

__forceinline bool GetUniqueID(char caUID[MAX_STRING_LENGTH])
{
    //Logic: Add HHMMSSmmm with mid 3 bytes of performance counter.
    //Guarantees that in a single milli second band (0 to 999) the three bytes 
    //of performance counter would always be unique.
    //1. Get system time, and use
    bool bStatus = false;
    try
    {

        SYSTEMTIME localtime;
        GetLocalTime(&localtime);//Get local time, so that we may pull out HHMMSSmmm

        LARGE_INTEGER li;
        char cNT[MAX_STRING_LENGTH];//new time.
        memset(cNT, '\0', sizeof(cNT));
        try
        {
            //Try to get the performance counter,
            //if one is provided by the OEM.

            QueryPerformanceCounter(&li);//This function retrieves the current value of the 
                                         //high-resolution performance counter if one is provided by the OEM
                                         //We use the first four bytes only of it.
            sprintf(cNT, "%u", li.QuadPart);
        }
        catch(...)
        {
            //Not provided by OEM.
            //Lets go with the GetTickCounts();
            //ddHHMMSS + 4 bytes of dwTicks
            sprintf(cNT,"%04d", GetTickCount());
        }


        //Get the first four bytes.
        int iSkipTo     = 0;//This is incase we'd decide to pull out next four bytes, rather than first four bytes.
        int iGetChars   = 4;//Number of chars to get.
        char *pSub = (char*) malloc(iGetChars+1);//Clear memory
        strncpy(pSub, cNT + iSkipTo, iGetChars);//Get string
        pSub[iGetChars] = '\0'; //Mark end.

        //Prepare unique id
        sprintf(caUID, "%02d%02d%02d%3d%s", 
                                    localtime.wHour, 
                                    localtime.wMinute, 
                                    localtime.wSecond, 
                                    localtime.wMilliseconds, 
                                    pSub); //First four characters concat.

        bStatus = true;
    }
    catch(...)
    {
        //Couldnt prepare. There was some problem.
        bStatus = false;
    }

    return bStatus;
}

Following is the output that I get:

Unique:[125907 462224]
Unique:[125907 462225]
Unique:[125907 462226]
Unique:[125907 462227]
Unique:[125907 462228]
Unique:[125907 462230]
Unique:[125907 462231]
Unique:[125907 462232]
Unique:[125907 462233]
Unique:[125907 462234]
Unique:[125907 462235]
Unique:[125907 462237]
Unique:[125907 462238]
Unique:[125907 462239]
Unique:[125907 462240]
Unique:[125907 462241]
Unique:[125907 462243]
Unique:[125907 462244]
Unique:[125907 462245]
Unique:[125907 462246]
Unique:[125907 462247]
Unique:[125907 462248]
Unique:[125907 462249]
Unique:[125907 462251]
Unique:[125907 462252]
Unique:[125907 462253]
Unique:[125907 462254]
Unique:[125907 462255]
Unique:[125907 462256]
Unique:[125907 462257]
Unique:[125907 462258]
Millisecond changed, 46
Unique:[125907 622261]
Unique:[125907 622262]
Unique:[125907 622263]
Unique:[125907 622264]
Unique:[125907 622265]
Unique:[125907 622267]
Unique:[125907 622268]
Unique:[125907 622269]
Unique:[125907 622270]
Unique:[125907 622271]
Unique:[125907 622273]
Unique:[125907 622274]
Unique:[125907 622275]
Unique:[125907 622276]
Unique:[125907 622277]
Unique:[125907 622278]
Unique:[125907 622279]
Unique:[125907 622281]
Unique:[125907 622282]
Unique:[125907 622283]
Unique:[125907 622284]
Unique:[125907 622285]
Unique:[125907 622286]
Unique:[125907 622288]
Unique:[125907 622289]
Unique:[125907 622290]
Unique:[125907 622291]
Unique:[125907 622292]
Unique:[125907 622293]
Unique:[125907 622295]
Unique:[125907 622296]
Unique:[125907 622297]
Unique:[125907 622298]
Unique:[125907 622299]
Unique:[125907 622300]
Unique:[125907 622301]
Unique:[125907 622302]
Unique:[125907 622304]
Unique:[125907 622305]
Unique:[125907 622306]
Millisecond changed, 62
Unique:[125907 782308]
Unique:[125907 782310]
Unique:[125907 782311]
Unique:[125907 782312]
Unique:[125907 782313]
Unique:[125907 782314]
Unique:[125907 782316]
Unique:[125907 782317]
Unique:[125907 782318]
Unique:[125907 782319]
Millisecond changed, 125
Unique:[1259071402495]
Unique:[1259071402497]
Unique:[1259071402498]
Unique:[1259071402499]
Unique:[1259071402500]
Unique:[1259071402502]
Unique:[1259071402503]
Unique:[1259071402504]
Unique:[1259071402505]
Unique:[1259071402507]

Now I am thinking of keeping the generated IDs in a list, and compare the new generated one with the existing ones in the list. If it already exist in the list, then well I can certainly skip the number and generate another, but certainly and clearly this logic would fail.

Would appreciate your comments/suggestions/updates/etc.

Thanks
JT.

Answer 1

My solution was to get the system time and add a counter to that (pseudo code):

static int counter = 0;
static Time lastTime;

String getNextId() {
    Time now = System.getTime();
    if (lastTime == now)
        counter ++;
    else
        counter = 0;
    return now+counter;
}

This would guarantee that I’d get a new ID even when I called the method more often than getTime() changes.