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@@ -1681,6 +1681,20 @@ scale_array_elements_to_u64(u64_dbl_t *entries, int n_entries,
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#undef SCALE_TO_U64_MAX
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}
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+/** Time-invariant 64-bit greater-than; works on two integers in the range
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+ * (0,INT64_MAX). */
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+#if SIZEOF_VOID_P == 8
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+#define gt_i64_timei(a,b) ((a) > (b))
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+#else
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+static INLINE int
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+gt_i64_timei(uint64_t a, uint64_t b)
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+{
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+ int64_t diff = (int64_t) (b - a);
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+ int res = diff >> 63;
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+ return res & 1;
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+}
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+#endif
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+
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/** Pick a random element of <b>n_entries</b>-element array <b>entries</b>,
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* choosing each element with a probability proportional to its (uint64_t)
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* value, and return the index of that element. If all elements are 0, choose
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@@ -1703,16 +1717,18 @@ choose_array_element_by_weight(const u64_dbl_t *entries, int n_entries)
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if (total == 0)
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return crypto_rand_int(n_entries);
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+ tor_assert(total < INT64_MAX);
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+
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rand_val = crypto_rand_uint64(total);
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for (i = 0; i < n_entries; ++i) {
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total_so_far += entries[i].u64;
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- if (total_so_far > rand_val) {
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+ if (gt_i64_timei(total_so_far, rand_val)) {
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i_chosen = i;
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n_chosen++;
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- /* Set rand_val to UINT_MAX rather than stopping the loop. This way,
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+ /* Set rand_val to INT64_MAX rather than stopping the loop. This way,
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* the time we spend in the loop does not leak which element we chose. */
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- rand_val = UINT64_MAX;
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+ rand_val = INT64_MAX;
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}
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}
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tor_assert(total_so_far == total);
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