Fix some undefined behaviors

- RareErrorIterator was passing probabilities 0 and 1 into std::geometric_distribution>-
- command_sample.test.cc was checking for '\0' past the end of a std::string_view

Author: Strilanc

src/stim/cmd/command_sample.test.cc

@@ -26,7 +26,7 @@ std::unordered_map<std::string_view, size_t> line_freq_with_lifetime_matching_ar
     std::unordered_map<std::string_view, size_t> result{};
     size_t start = 0;
     for (size_t k = 0; k <= data.size(); k++) {
-        if (data[k] == '\n' || data[k] == '\0') {
+        if (k == data.size() || data[k] == '\n' || data[k] == '\0') {
             result[data.substr(start, k - start)]++;
             start = k + 1;
         }

src/stim/util_bot/probability_util.cc

@@ -21,16 +21,25 @@
 using namespace stim;
 
 RareErrorIterator::RareErrorIterator(float probability)
-    : next_candidate(0), is_one(probability == 1), dist(probability) {
+    : next_candidate(0), probability(probability) {
     if (!(probability >= 0 && probability <= 1)) {
         throw std::out_of_range("Invalid probability: " + std::to_string(probability));
     }
+    if (0 < probability && probability < 1) {
+        dist = std::geometric_distribution<size_t>(probability);
+    }
 }
 
 size_t RareErrorIterator::next(std::mt19937_64 &rng) {
-    size_t result = next_candidate + (is_one ? 0 : dist(rng));
-    next_candidate = result + 1;
-    return result;
+    if (probability == 0) {
+        return SIZE_MAX;
+    } else if (probability == 1) {
+        return next_candidate++;
+    } else {
+        size_t result = next_candidate + dist(rng);
+        next_candidate = result + 1;
+        return result;
+    }
 }
 
 std::vector<size_t> stim::sample_hit_indices(float probability, size_t attempts, std::mt19937_64 &rng) {

src/stim/util_bot/probability_util.h

@@ -32,8 +32,10 @@ constexpr uint64_t INTENTIONAL_VERSION_SEED_INCOMPATIBILITY = 0xDEADBEEF124CULL;
 /// Gets more efficient as the hit probability drops.
 struct RareErrorIterator {
     size_t next_candidate;
-    bool is_one = false;
+    float probability;
     std::geometric_distribution<size_t> dist;
+    RareErrorIterator() = delete;
+    RareErrorIterator(const RareErrorIterator &) = delete;
     RareErrorIterator(float probability);
     size_t next(std::mt19937_64 &rng);