sampling : implement disabled branch to support blue noise

src/llama-sampler.cpp

@@ -448,10 +448,12 @@ struct blue_noise_rng {
 struct llama_dist_rng {
     virtual ~llama_dist_rng() = default;
 
-    virtual bool                            requires_sorted()  = 0; // whether the RNG requires sorted input for proper properties
+    // whether the RNG requires sorted input for proper properties
+    // this also indicates whether the RNG output itself must be consumed in a coherent order
+    virtual bool                            requires_sorted()  = 0;
 
-    // for compatilibility with std::discrete_distribution
-    // nly used in a disabled branch of llama_sampler_dist_apply
+    // for compatibility with std::discrete_distribution
+    // only used in a disabled branch of llama_sampler_dist_apply
     virtual uint32_t                        rng_min()          = 0;
     virtual uint32_t                        rng_max()          = 0;
     virtual uint32_t                        next()             = 0; // uniform bits in [rng_min(), rng_max()]
@@ -474,6 +476,48 @@ struct llama_dist_urbg {
     result_type operator()() { return rng.next(); }
 };
 
+// wrapper to use existing llama_sample_dist for mt19937, otherwise implements CDF walk directly
+// this is currently only used in a disabled branch of llama_sampler_dist_apply, added for compatibility and potential use by other samplers
+// flag normalized to skip recomputing the probability sum when probs already sum to 1
+static int llama_sample_dist_rng(llama_token_data_array * cur_p, llama_dist_rng & rng, bool normalized = false) {
+    if (!rng.requires_sorted()) {
+        llama_dist_urbg urbg{rng};
+        return llama_sample_dist(cur_p, urbg);
+    }
+
+    if (!cur_p->sorted) {
+        llama_token_data_array_partial_sort_inplace(cur_p, cur_p->size);
+    }
+    const double rnd = rng.nextf();
+
+    double sum_run = 0.0;
+
+    if (normalized) {
+        for (size_t i = 0; i < cur_p->size; ++i) {
+            sum_run += cur_p->data[i].p;
+            if (sum_run >= rnd) {
+                return i;
+            }
+        }
+    } else {
+        double sum_cum = 0.0;
+        for (size_t i = 0; i < cur_p->size; ++i) {
+            sum_cum += cur_p->data[i].p;
+        }
+
+        const double sum_tgt = sum_cum * rnd;
+
+        for (size_t i = 0; i < cur_p->size; ++i) {
+            sum_run += cur_p->data[i].p;
+            if (sum_run >= sum_tgt) {
+                return i;
+            }
+        }
+    }
+
+    return (int)(cur_p->size - 1);
+}
+
 struct llama_dist_rng_mt19937 : llama_dist_rng {
     std::mt19937 rng;
 
@@ -1301,9 +1345,7 @@ static void llama_sampler_dist_apply(struct llama_sampler * smpl, llama_token_da
         cur_p->data[i].p /= sum_cum;
     }
 
-    // this implementation is not guaranteed to preserve blue noise properties
-    llama_dist_urbg urbg{*ctx->rng};
-    cur_p->selected = llama_sample_dist(cur_p, urbg);
+    cur_p->selected = llama_sample_dist_rng(cur_p, *ctx->rng, true);
 #endif
 }