probdist.h

   1 #ifndef ZZGO_PROBDIST_H
   2 #define ZZGO_PROBDIST_H
   3
   4 /* Tools for picking an item according to a probability distribution. */
   5
   6 /* The probability distribution structure is designed to be once
   7  * initialized, then each item sequentially assigned a value one,
   8  * then multiple times an item picked randomly. */
   9
  10 #include "move.h"
  11 #include "util.h"
  12
  13 struct probdist {
  14         int n;
  15         float *items; // [n], probability pick<=i
  16 };
  17 #define probdist_total(pd) ((pd)->items[(pd)->n - 1])
  18 #define probdist_one(pd, i) ((pd)->items[i] - (likely(i > 0) ? (pd)->items[i - 1] : 0))
  19
  20 /* You must call this for all items, *in sequence* (0, 1, ...).
  21  * @val is probability of item @i (as opposed to items[i], which
  22  * is probability of item <=i, thus includes the sum of predecessors
  23  * as well). */
  24 static void probdist_set(struct probdist *pd, int i, float val);
  25
  26 int probdist_pick(struct probdist *pd);
  27
  28
  29 /* We disable the assertions here since this is quite time-critical
  30  * part of code, and also the compiler is reluctant to inline the
  31  * functions otherwise. */
  32 static inline void
  33 probdist_set(struct probdist *pd, int i, float val)
  34 {
  35 #if 0
  36         assert(i >= 0 && i < pd->n);
  37         assert(val >= 0);
  38 #endif
  39         pd->items[i] = (likely(i > 0) ? pd->items[i - 1] : 0) + val;
  40 }
  41
  42 #endif