uct/policy/ucb1.c

   1 #include <assert.h>
   2 #include <math.h>
   3 #include <stdio.h>
   4 #include <stdlib.h>
   5 #include <string.h>
   6
   7 #include "board.h"
   8 #include "debug.h"
   9 #include "move.h"
  10 #include "random.h"
  11 #include "uct/internal.h"
  12 #include "uct/tree.h"
  13 #include "uct/policy/generic.h"
  14
  15 /* This implements the basic UCB1 policy. */
  16
  17 struct ucb1_policy {
  18         /* This is what the Modification of UCT with Patterns in Monte Carlo Go
  19          * paper calls 'p'. Original UCB has this on 2, but this seems to
  20          * produce way too wide searches; reduce this to get deeper and
  21          * narrower readouts - try 0.2. */
  22         float explore_p;
  23         /* First Play Urgency - if set to less than infinity (the MoGo paper
  24          * above reports 1.0 as the best), new branches are explored only
  25          * if none of the existing ones has higher urgency than fpu. */
  26         float fpu;
  27 };
  28
  29
  30 struct tree_node *
  31 ucb1_descend(struct uct_policy *p, void **state, struct tree *tree, struct tree_node *node, int parity, bool allow_pass)
  32 {
  33         /* We want to count in the prior stats here after all. Otherwise,
  34          * nodes with positive prior will get explored _LESS_ since the
  35          * urgency will be always higher; even with normal FPU because
  36          * of the explore coefficient. */
  37
  38         struct ucb1_policy *b = p->data;
  39         float xpl = log(node->u.playouts + node->prior.playouts);
  40
  41         uctd_try_node_children(node, allow_pass, ni, urgency) {
  42                 int uct_playouts = ni->u.playouts + ni->prior.playouts;
  43
  44                 if (uct_playouts) {
  45                         urgency = (ni->u.playouts * tree_node_get_value(tree, parity, ni->u.value)
  46                                    + ni->prior.playouts * tree_node_get_value(tree, parity, ni->prior.value))
  47                                   / uct_playouts;
  48                         urgency += b->explore_p * sqrt(xpl / uct_playouts);
  49                 } else {
  50                         urgency = b->fpu;
  51                 }
  52         } uctd_set_best_child(ni, urgency);
  53         return uctd_get_best_child();
  54 }
  55
  56 void
  57 ucb1_update(struct uct_policy *p, struct tree *tree, struct tree_node *node, enum stone node_color, enum stone player_color, struct playout_amafmap *map, float result)
  58 {
  59         /* It is enough to iterate by a single chain; we will
  60          * update all the preceding positions properly since
  61          * they had to all occur in all branches, only in
  62          * different order. */
  63         for (; node; node = node->parent) {
  64                 stats_add_result(&node->u, result, 1);
  65         }
  66 }
  67
  68
  69 struct uct_policy *
  70 policy_ucb1_init(struct uct *u, char *arg)
  71 {
  72         struct uct_policy *p = calloc(1, sizeof(*p));
  73         struct ucb1_policy *b = calloc(1, sizeof(*b));
  74         p->uct = u;
  75         p->data = b;
  76         p->descend = ucb1_descend;
  77         p->choose = uctp_generic_choose;
  78         p->update = ucb1_update;
  79
  80         b->explore_p = 0.2;
  81         b->fpu = 1.1; //INFINITY;
  82
  83         if (arg) {
  84                 char *optspec, *next = arg;
  85                 while (*next) {
  86                         optspec = next;
  87                         next += strcspn(next, ":");
  88                         if (*next) { *next++ = 0; } else { *next = 0; }
  89
  90                         char *optname = optspec;
  91                         char *optval = strchr(optspec, '=');
  92                         if (optval) *optval++ = 0;
  93
  94                         if (!strcasecmp(optname, "explore_p") && optval) {
  95                                 b->explore_p = atof(optval);
  96                         } else if (!strcasecmp(optname, "fpu") && optval) {
  97                                 b->fpu = atof(optval);
  98                         } else {
  99                                 fprintf(stderr, "ucb1: Invalid policy argument %s or missing value\n",
 100                                         optname);
 101                                 exit(1);
 102                         }
 103                 }
 104         }
 105
 106         return p;
 107 }