Thread: Psuedo random generator with min max

  1. #1
    Registered User awsdert's Avatar
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    Psuedo random generator with min max

    Can't find my old thread on min max so asking again, I made this function which should be as close to random numbers getting on a computer without internet connection for that random.com or whatever it was, only I forgot how to use the 1 or 0 to produce a larger (or smaller) number within a range.
    Code:
    long mcc_rnd( time_t *ctx, long min, long max ) {
    	/* With bit unitialised it should be much harder to predict if 1 or 0 will recieved */
    	time_t bit, seed = time(NULL);
    	long val;
    	if ( ctx ) bit = *ctx;
    	if ( !bit ) bit = 1;
    	val = (seed & bit) ? 1 : 0;
    	bit <<= 1;
    	if ( ctx ) *ctx = bit;
    	if ( min > max ) min = max;
    	if ( min != max ) return val ? min;
    	if ( max ) return val ? max : 0;
    }
    Would appreciate being reminded of how to do so.
    Last edited by awsdert; 12-02-2019 at 06:32 AM. Reason: Forgot to correct a variable name

  2. #2
    Registered User awsdert's Avatar
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    After a google search I found a copy of what I used (forgot I had put one in the mitsy code) but that wound up crashing so now I went with something different:
    Code:
    typedef clock_t mcc_rnd_t;
    long mcc__rnd( mcc_rnd_t *ctx, long min, long max ) {
    	/* With bit unitialised it should be much harder to predict
    	 * if 1 or 0 will be recieved */
    	mcc_rnd_t bit, seed = clock();
    	long val;
    	if ( ctx ) bit = *ctx;
    	if ( !bit || bit > seed ) bit = 1;
    	val = (seed & bit) ? 1 : 0;
    	bit <<= 1;
    	if ( ctx ) *ctx = bit;
    	if ( min > max ) min = max;
    	if ( min != max ) {
    		val *= bit;
    		if ( val > max ) val = max;
    		val -= min;
    		return ( val < min ) ? min : val;
    	}
    	return val ? max : min;
    }
    #define mcc_rnd( ctx ) mcc__rnd( ctx, LONG_MIN, LONG_MAX )
    Not as flexibile as I'd like but it'll do, any suggestions for more flexible results than the below are welcome
    Code:
    gcc -Wall -o "bimath" "bimath.c" && "./bimath"
    num = 4, val = 0
    num = 16, val = 8
    num = 0, val = 32
    num = 0, val = 128
    num = 1024, val = 0
    num = 4096, val = 0
    num = 16384, val = 8192
    num = 0, val = 0
    num = 16, val = 0
    num = 0, val = 0
    num = 256, val = 128
    num = 1024, val = 0
    num = 4096, val = 0
    num = 16384, val = 8192
    num = 4, val = 0
    num = 16, val = 0
    num = 64, val = 0
    Compilation finished successfully.
    And yes those results do change upon every run of the test code

  3. #3
    Registered User awsdert's Avatar
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    Managed to make it a bit more random:
    Code:
    long mcc__rnd( mcc_rnd_t *ctx, long min, long max ) {
    	/* With bit unitialised it should be much harder to predict
    	 * if 1 or 0 will be recieved */
    	mcc_rnd_t bit = 1, seed = clock(), mov = 0;
    	long val;
    	if ( ctx ) mov = *ctx;
    	bit <<= mov++;
    	if ( !bit || bit > seed ) {
    		bit = 1;
    		mov = 0;
    	}
    	val = (seed & bit) ? 1 : 0;
    	bit <<= 1;
    	if ( ctx ) *ctx = mov;
    	if ( min > max ) min = max;
    	if ( min != max ) {
    		bit = ~((~0u) << mov);
    		val *= (clock() & bit);
    		if ( val > max ) val = max;
    		val -= (min >= 0) ? min : -min;
    		return ( val < min ) ? min : val;
    	}
    	return val ? max : min;
    }
    Code:
    gcc -Wall -o "bimath" "bimath.c" && "./bimath"
    num = 0, val = 0
    num = 7, val = 1
    num = 0, val = 0
    num = 0, val = 58
    num = 335, val = 0
    num = 1378, val = 0
    num = 0, val = 0
    num = 0, val = 9604
    num = 0, val = 0
    num = 9, val = 7
    num = 55, val = 21
    num = 198, val = 67
    num = 0, val = 465
    num = 0, val = 1504
    num = 9746, val = 0
    num = 1, val = 0
    num = 6, val = 0
    Compilation finished successfully.

  4. #4
    Registered User awsdert's Avatar
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    This is the closest to unpredictable as I can get it, while one can predict to an extent what range a number will fall in, those numbers can be wildly different in magnitude so trying to brute force in a real world scenario would be quite difficult I imagine.
    Code:
    long mcc__rnd( mcc_rnd_t *ctx, long min, long max ) {
    	/* With bit unitialised it should be much harder to predict
    	 * if 1 or 0 will be recieved */
    	mcc_rnd_t bit = 1, seed = time(NULL), mov = 0;
    	long val;
    	if ( ctx ) mov = *ctx;
    	bit <<= mov++;
    	if ( !bit || bit > seed ) {
    		bit = 1;
    		mov = 0;
    	}
    	val = (seed & bit) ? 1 : 0;
    	bit <<= 1;
    	if ( ctx ) *ctx = mov;
    	if ( min > max ) min = max;
    	if ( min != max ) {
    		seed *= clock();
    		bit = ~((~0u) << mov);
    		val = (seed & bit);
    		if ( val > max ) val = max;
    		val -= (min >= 0) ? min : -min;
    		return ( val < min ) ? min : val;
    	}
    	return val ? max : min;
    }
    Code:
    gcc -Wall -o "bimath" "bimath.c" && "./bimath"
    num = 1, val = 1
    num = 12, val = 6
    num = 17, val = 10
    num = 168, val = 1
    num = 102, val = 191
    num = 2596, val = 726
    num = 1083, val = 1172
    num = 32249, val = 16043
    num = 194309, val = 47031
    num = 733379, val = 397596
    num = 2288613, val = 1617047
    num = 4556640, val = 6318009
    num = 31990491, val = 197428
    num = 141549053, val = 42647126
    num = 339883433, val = 410515035
    num = 0, val = 989835645
    num = 2, val = 1
    num = 14, val = 7
    num = 51, val = 5
    num = 10, val = 99
    num = 122, val = 211
    num = 2616, val = 657
    num = 9295, val = 1192
    num = 15974, val = 32536
    num = 219261, val = 71983
    num = 758331, val = 422548
    num = 1559545, val = 1223762
    num = 16410484, val = 1394637
    num = 27067119, val = 28828488
    num = 37723754, val = 73039555
    num = 80173452, val = 115489253
    num = 0, val = 780062943
    num = 0, val = 0
    num = 12, val = 6
    num = 29, val = 15
    num = 52, val = 13
    num = 498, val = 75
    num = 3170, val = 1211
    num = 1657, val = 1746
    num = 24720, val = 16617
    num = 170574, val = 23296
    num = 824510, val = 488727
    num = 3010083, val = 577148
    num = 15009850, val = 8382611
    num = 25666485, val = 27427854
    num = 237649712, val = 4530057
    num = 583850667, val = 82295556
    num = 0, val = 1233802879
    num = 0, val = 1
    num = 7, val = 0
    num = 11, val = 29
    num = 212, val = 45
    num = 914, val = 68
    num = 425, val = 514
    num = 3097, val = 3275
    num = 1495, val = 18057
    num = 139246, val = 123040
    num = 531038, val = 383760
    num = 660686, val = 324903
    num = 2510476, val = 4271845
    num = 37914698, val = 6121635
    num = 48571333, val = 83887134
    num = 896327399, val = 430088089
    num = 0, val = 1546279611
    num = 1, val = 0
    num = 15, val = 0
    num = 58, val = 12
    num = 227, val = 60
    num = 577, val = 154
    num = 2136, val = 177
    num = 4630, val = 4808
    num = 27871, val = 19768
    num = 108189, val = 91983
    num = 762125, val = 426342
    num = 2276132, val = 1604566
    num = 16037268, val = 2782790
    num = 1109842, val = 2871211
    num = 170731792, val = 107145666
    num = 248497291, val = 319128893
    num = 0, val = 898449503
    num = 0, val = 0
    num = 1, val = 2
    num = 25, val = 4
    num = 20, val = 109
    num = 989, val = 54
    num = 3398, val = 1439
    num = 5892, val = 6070
    num = 37058, val = 28955
    num = 35634, val = 19428
    num = 427426, val = 91643
    num = 1941433, val = 1269867
    num = 11508265, val = 4881026
    num = 16880793, val = 18642162
    num = 52285015, val = 87600816
    num = 1062529134, val = 24103111
    num = 0, val = 638739522
    num = 3, val = 1
    num = 15, val = 0
    num = 26, val = 19
    num = 236, val = 69
    num = 871, val = 448
    num = 1009, val = 1098
    num = 7688, val = 7866
    num = 55060, val = 6086
    num = 77945, val = 4306
    num = 92727, val = 281232
    num = 2278300, val = 1606734
    num = 14949633, val = 1695155
    num = 46831117, val = 16799423
    num = 188182742, val = 89280815
    num = 590924347, val = 89369236
    num = 0, val = 1099613355
    num = 1, val = 0
    num = 8, val = 1
    num = 3, val = 21
    num = 151, val = 112
    num = 620, val = 197
    num = 487, val = 576
    num = 7433, val = 7522
    num = 55161, val = 14290
    num = 258448, val = 111170
    num = 912384, val = 52313
    num = 1754825, val = 1419042
    num = 1171680, val = 4694418
    num = 58637225, val = 26844162
    num = 97564185, val = 132879986
    num = 641568994, val = 140013883
    num = 0, val = 1651813113
    num = 3, val = 1
    num = 11, val = 4
    num = 6, val = 31
    num = 218, val = 51
    num = 853, val = 430
    num = 4061, val = 54
    num = 7000, val = 7178
    num = 22138, val = 14035
    num = 227120, val = 22409
    num = 954695, val = 283129
    num = 3181495, val = 748560
    num = 1926784, val = 3688153
    num = 7299312, val = 10822050
    num = 14433209, val = 49749010
    num = 523122217, val = 21567106
    num = 0, val = 530256114
    num = 3, val = 1
    num = 0, val = 2
    num = 50, val = 4
    num = 134, val = 95
    num = 513, val = 90
    num = 3541, val = 1582
    num = 6482, val = 6571
    num = 29901, val = 21798
    num = 234171, val = 29460
    num = 225627, val = 414132
    num = 691058, val = 19492
    num = 10257890, val = 3630651
    num = 34169003, val = 2375940
    num = 108411764, val = 9509837
    num = 885536228, val = 419296918
    num = 0, val = 2037043551
    num = 1, val = 0
    num = 8, val = 2
    num = 8, val = 1
    num = 220, val = 53
    num = 87, val = 176
    num = 1067, val = 1156
    num = 4006, val = 4095
    num = 20305, val = 12202
    num = 238374, val = 91096
    num = 574428, val = 91367
    Compilation finished successfully.

  5. #5
    and the hat of int overfl Salem's Avatar
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    Why are you basing your rng() on time() and clock().

    Which, depending on how often you call them, return
    - the same as last time
    - +1 from last time
    - always some number greater than last time.

    If you want crude-but-effective (for student homework), then use a LCG
    Linear congruential generator - Wikipedia

    If you want statistically sound, or cryptographically sound randomness, then that takes more work.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  6. #6
    Registered User awsdert's Avatar
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    Quote Originally Posted by Salem View Post
    Why are you basing your rng() on time() and clock().

    Which, depending on how often you call them, return
    - the same as last time
    - +1 from last time
    - always some number greater than last time.

    If you want crude-but-effective (for student homework), then use a LCG
    Linear congruential generator - Wikipedia

    If you want statistically sound, or cryptographically sound randomness, then that takes more work.
    I wanted a thread safe version that was not easy to predict, I don't need impossible just hard enough that outside a test scenario it would be pretty hard to predict without first finding out when the function is called in the 1st place, for games this would be sufficient and for testing my bignum functions it is also sufficient (those numbers were actually passed through my checker function to see if they matched up, as you'll note by the lack of further output they did match).Since it always changes the seed the randomness can only be predicted after a few rounds of it being used. Feel free to stress test it though

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