45 lines
1.3 KiB
C
45 lines
1.3 KiB
C
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// gcc nomod.c -o nomod; ./nomod
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/*
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Explanation:
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* rand is treated as a 32-bit fixed-point number in [0, 1) (implicitly scaled by 2³²).
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* Multiplying by len scales it to [0, len).
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* Shifting right by 32 bits extracts the integer part (0..len-1).
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I guess I will go with decimal system for understanding this...
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So lets say I have two-digit decimal numbers from 00..99 this
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means that multiplying two of these should always fit in a
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four-digit decimal number isn't it? from 0000..9999 because
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99*99 is 9801 so it fits in and its the biggest possible pairs
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to multiply.
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Now lets have a "len" number with also at most two digits, let
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it be 20 - this will be our "modulus". Lets multiply a random
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two-digit number with this. We get lets say 71*20 which is 1400.
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Shifting two digits to the right it becomes 14. If I do 99*20
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(which is biggest) I get 1980 so when shiften decimally it
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becomes 19 (which is again smaller than 20).
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It looks like the random numbers from 00..99 indeed start to map
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onto 00..19 in this case. Very fascinating! Just avoid cryptin'
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*/
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#include <stdio.h>
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#include <stdint.h>
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#include "fastrand.h"
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int main() {
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uint32_t from = 12;
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uint32_t to = 32;
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int n = 30;
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rand_state rs = init_rand();
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for(int i = 0; i < n; ++i) {
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uint32_t choice = rand_between(&rs, from, to);
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printf("rand [%u, %u): %u\n", from, to, choice);
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}
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return 0;
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}
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