Unbiased random (espressif#2468)

* An example to read high frequency analog data using i2s_adc * Changed random() to use Lemire's method for bounding

Author: lbernstone

cores/esp32/WMath.cpp

@@ -37,10 +37,23 @@ void randomSeed(unsigned long seed)
 
 long random(long howbig)
 {
- if(howbig == 0) {
- return 0;
+ uint32_t x = esp_random();
+ uint64_t m = uint64_t(x) * uint64_t(howbig);
+ uint32_t l = uint32_t(m);
+ if (l < howbig) {
+ uint32_t t = -howbig;
+ if (t >= howbig) {
+ t -= howbig;
+ if (t >= howbig) 
+ t %= howbig;
+ }
+ while (l < t) {
+ x = esp_random();
+ m = uint64_t(x) * uint64_t(howbig);
+ l = uint32_t(m);
+ }
  }
- return esp_random() % howbig;
+ return m >> 32;
 }
 
 long random(long howsmall, long howbig)