Fix CPU and backend sampling mismatches

src/llama-sampler.cpp

@@ -1114,7 +1114,11 @@ static void llama_sampler_dist_apply(struct llama_sampler * smpl, llama_token_da
 
     cur_p->selected = 0;
 
+    std::uniform_real_distribution<double> dist(0.0f, 1.0f);
+
     if (cur_p->size == 1) {
+        // keep the RNG state aligned with backend sampling, which draws once per output
+        dist(ctx->rng);
         cur_p->data[0].p = 1.0f;
         return;
     }
@@ -1139,7 +1143,6 @@ static void llama_sampler_dist_apply(struct llama_sampler * smpl, llama_token_da
     // sample from the obtained probabilities and normalize the probs in a single pass
     // this is ~3x faster on Mac with full gpt-oss vocab than the version below
     //
-    std::uniform_real_distribution<double> dist(0.0f, 1.0f);
     const double rnd = dist(ctx->rng);
 
           double sum_run = 0.0f;

tests/test-sampling.cpp

@@ -61,6 +61,31 @@ private:
     std::vector<llama_token_data> cur;
 };
 
+static llama_token sample_dist(llama_sampler * sampler, const std::vector<float> & logits) {
+    std::vector<llama_token_data> cur;
+    for (llama_token token_id = 0; token_id < (llama_token) logits.size(); ++token_id) {
+        cur.push_back({ token_id, logits[token_id], 0.0f });
+    }
+
+    llama_token_data_array cur_p = { cur.data(), cur.size(), -1, false };
+    llama_sampler_apply(sampler, &cur_p);
+    GGML_ASSERT(cur_p.selected >= 0);
+    return cur_p.data[cur_p.selected].id;
+}
+
+static void test_dist_singleton_rng() {
+    llama_sampler * singleton = llama_sampler_init_dist(4242);
+    llama_sampler * control   = llama_sampler_init_dist(4242);
+
+    sample_dist(singleton, { 0.0f });
+    sample_dist(control,   { 0.0f, 0.0f });
+
+    GGML_ASSERT(sample_dist(singleton, { 0.0f, 0.0f }) == sample_dist(control, { 0.0f, 0.0f }));
+
+    llama_sampler_free(singleton);
+    llama_sampler_free(control);
+}
+
 static void test_temp(const std::vector<float> & probs, const std::vector<float> & probs_expected, float temp) {
     sampler_tester tester(probs, probs_expected);
 
@@ -308,6 +333,8 @@ static void test_perf() {
 int main(void) {
     ggml_time_init();
 
+    test_dist_singleton_rng();
+
     test_temp({0.1f, 0.2f, 0.3f, 0.4f}, {0.1f, 0.2f, 0.3f, 0.4f}, 1.0f);
     test_temp({0.1f, 0.2f, 0.3f, 0.4f}, {0.0f, 0.0f, 0.0f, 1.0f}, 0.0f);