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Merge branch 'fix-random' of https://github.com/remay/microbit-dal into remay-fix-random

master
Joe Finney 2015-11-29 23:43:46 +00:00
parent d5dfdbcc5e 5333729644
commit 958136673f
1 changed files with 36 additions and 7 deletions
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43
source/MicroBit.cpp
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@ -303,17 +303,46 @@ int MicroBit::sleep(int milliseconds)
*/
int MicroBit::random(int max)
{
uint32_t m, result;
//return MICROBIT_INVALID_VALUE if max is <= 0...
if(max <= 0)
return MICROBIT_INVALID_PARAMETER;
// Our maximum return value is actually one less than passed
max--;
do {
m = (uint32_t)max;
result = 0;
while(m >>= 1) {
// Cycle the LFSR (Linear Feedback Shift Register).
// We use an optimal sequence with a period of 2^32-1, as defined by Bruce Schneider here (a true legend in the field!),
// For those interested, it's documented in his paper:
// "Pseudo-Random Sequence Generator for 32-Bit CPUs: A fast, machine-independent generator for 32-bit Microprocessors"
// https://www.schneier.com/paper-pseudorandom-sequence.html
// Avoid interupts (and hence fibre context switch) while we are doing this
// Cycle the LFSR (Linear Feedback Shift Register).
// We use an optimal sequence with a period of 2^32-1, as defined by Bruce Schneider here (a true legend in the field!),
// For those interested, it's documented in his paper:
// "Pseudo-Random Sequence Generator for 32-Bit CPUs: A fast, machine-independent generator for 32-bit Microprocessors"
randomValue = ((((randomValue >> 31) ^ (randomValue >> 6) ^ (randomValue >> 4) ^ (randomValue >> 2) ^ (randomValue >> 1) ^ randomValue) & 0x0000001) << 31 ) | (randomValue >> 1);
return randomValue % max;
__disable_irq();
randomValue = ((((randomValue >> 31)
^ (randomValue >> 6)
^ (randomValue >> 4)
^ (randomValue >> 2)
^ (randomValue >> 1)
^ randomValue)
& 0x0000001)
<< 31 )
| (randomValue >> 1);
__enable_irq();
result = ((result << 1) | (randomValue & 0x00000001));
}
} while (result > (uint32_t)max);
return result;
}