Refactor the core of choosing by weights into a function

This eliminates duplicated code, and lets us test a hairy piece of
functionality.

src/test/test.h

@@ -65,6 +65,10 @@
 
 #define test_memeq_hex(expr1, hex) test_mem_op_hex(expr1, ==, hex)
 
+#define tt_double_op(a,op,b)                                            \
+  tt_assert_test_type(a,b,#a" "#op" "#b,double,(val1_ op val2_),"%f",   \
+                      TT_EXIT_TEST_FUNCTION)
+
 const char *get_fname(const char *name);
 crypto_pk_t *pk_generate(int idx);
 

src/test/test_dir.c

@@ -7,6 +7,7 @@
 #define DIRSERV_PRIVATE
 #define DIRVOTE_PRIVATE
 #define ROUTER_PRIVATE
+#define ROUTERLIST_PRIVATE
 #define HIBERNATE_PRIVATE
 #include "or.h"
 #include "directory.h"
@@ -1381,6 +1382,85 @@ test_dir_v3_networkstatus(void)
     ns_detached_signatures_free(dsig2);
 }
 
+static void
+test_dir_random_weighted(void *testdata)
+{
+  int histogram[10];
+  uint64_t vals[10] = {3,1,2,4,6,0,7,5,8,9}, total=0;
+  uint64_t zeros[5] = {0,0,0,0,0};
+  int i, choice;
+  const int n = 50000;
+  double max_sq_error;
+  (void) testdata;
+
+  /* Try a ten-element array with values from 0 through 10. The values are
+   * in a scrambled order to make sure we don't depend on order. */
+  memset(histogram,0,sizeof(histogram));
+  for (i=0; i<10; ++i)
+    total += vals[i];
+  tt_int_op(total, ==, 45);
+  for (i=0; i<n; ++i) {
+    uint64_t t;
+    choice = choose_array_element_by_weight(vals, 10, &t);
+    tt_int_op(t, ==, total);
+    tt_int_op(choice, >=, 0);
+    tt_int_op(choice, <, 10);
+    histogram[choice]++;
+  }
+
+  /* Now see if we chose things about frequently enough. */
+  max_sq_error = 0;
+  for (i=0; i<10; ++i) {
+    int expected = (int)(n*vals[i]/total);
+    double frac_diff = 0, sq;
+    TT_BLATHER(("  %d : %5d vs %5d\n", (int)vals[i], histogram[i], expected));
+    if (expected)
+      frac_diff = (histogram[i] - expected) / ((double)expected);
+    else
+      tt_int_op(histogram[i], ==, 0);
+
+    sq = frac_diff * frac_diff;
+    if (sq > max_sq_error)
+      max_sq_error = sq;
+  }
+  /* It should almost always be much much less than this.  If you want to
+   * figure out the odds, please feel free. */
+  tt_double_op(max_sq_error, <, .05);
+
+  /* Now try a singleton; do we choose it? */
+  for (i = 0; i < 100; ++i) {
+    choice = choose_array_element_by_weight(vals, 1, NULL);
+    tt_int_op(choice, ==, 0);
+  }
+
+  /* Now try an array of zeros.  We should choose randomly. */
+  memset(histogram,0,sizeof(histogram));
+  for (i = 0; i < n; ++i) {
+    uint64_t t;
+    choice = choose_array_element_by_weight(zeros, 5, &t);
+    tt_int_op(t, ==, 0);
+    tt_int_op(choice, >=, 0);
+    tt_int_op(choice, <, 5);
+    histogram[choice]++;
+  }
+  /* Now see if we chose things about frequently enough. */
+  max_sq_error = 0;
+  for (i=0; i<5; ++i) {
+    int expected = n/5;
+    double frac_diff = 0, sq;
+    TT_BLATHER(("  %d : %5d vs %5d\n", (int)vals[i], histogram[i], expected));
+    frac_diff = (histogram[i] - expected) / ((double)expected);
+    sq = frac_diff * frac_diff;
+    if (sq > max_sq_error)
+      max_sq_error = sq;
+  }
+  /* It should almost always be much much less than this.  If you want to
+   * figure out the odds, please feel free. */
+  tt_double_op(max_sq_error, <, .05);
+ done:
+  ;
+}
+
 #define DIR_LEGACY(name)                                                   \
   { #name, legacy_test_helper, TT_FORK, &legacy_setup, test_dir_ ## name }
 
@@ -1396,6 +1476,7 @@ struct testcase_t dir_tests[] = {
   DIR_LEGACY(measured_bw),
   DIR_LEGACY(param_voting),
   DIR_LEGACY(v3_networkstatus),
+  DIR(random_weighted),
   END_OF_TESTCASES
 };