Add tests for SampleTopK that highlight existing problems and fix those:

- Sampling was not correct for k>1 and temperature=0.
- Sampling was not correct for only negative logits.

Also restructure the code a bit for better readability and add some asserts for things that shouldn't happen.

PiperOrigin-RevId: 676043267

ops/ops-inl.h

@@ -23,6 +23,7 @@
 
 #include <array>
 #include <cmath>
+#include <limits>
 #include <random>
 #include <type_traits>  // std::enable_if_t
 
@@ -593,6 +594,12 @@ SampleArgmax(const float* probabilities, size_t vocab_size) {
 template <size_t k>
 HWY_NOINLINE HWY_MAYBE_UNUSED std::discrete_distribution<int>
 create_distribution(std::array<float, k>& top_k, float temperature) {
+  HWY_ASSERT(temperature >= 0.0f);
+  if (temperature == 0.0f) {
+    // Temperature == 0 is a special case which always returns the argmax (0).
+    // We also want to avoid dividing by zero in the code below.
+    return std::discrete_distribution<int>();
+  }
   namespace hn = hwy::HWY_NAMESPACE;
   using D = hn::ScalableTag<float>;
 
@@ -609,32 +616,35 @@ create_distribution(std::array<float, k>& top_k, float temperature) {
 
 template <size_t k, typename TAcceptToken>
 HWY_NOINLINE HWY_MAYBE_UNUSED int SampleTopK(
-    const float* HWY_RESTRICT probabilities, size_t vocab_size,
+    const float* HWY_RESTRICT logits, size_t vocab_size,
     std::mt19937& gen, float temperature, TAcceptToken& accept_token) {
   static_assert(k != 0, "");
+  HWY_ASSERT(k <= vocab_size);
   // TODO: Optimize, potentially using new VQSort PartialSort.
   std::array<float, k> top_k{};  // sorted from highest [0], to lowest [k-1]
+  top_k.fill(-std::numeric_limits<float>::infinity());
   std::array<int, k> indices{};
+  size_t num_accepted = 0;
   for (size_t i = 0; i < vocab_size; ++i) {
-    if (probabilities[i] < top_k[k - 1] &&
-        (!accept_token || accept_token(StaticCast<int>(i), probabilities[i]))) {
-      continue;
-    }
+    if (logits[i] < top_k[k - 1]) continue;
+    bool accepted =
+        !accept_token || accept_token(StaticCast<int>(i), logits[i]);
+    if (!accepted) continue;
+    num_accepted++;
     for (size_t j = 0; j < k; ++j) {
-      if (probabilities[i] > top_k[j] &&
-          (!accept_token ||
-           accept_token(StaticCast<int>(i), probabilities[i]))) {
+      if (logits[i] > top_k[j]) {
         // shift elements by 1, insert the new value, move on to next value
         for (size_t idx = k - 1; idx > j; --idx) {
           top_k[idx] = top_k[idx - 1];
           indices[idx] = indices[idx - 1];
         }
-        top_k[j] = probabilities[i];
+        top_k[j] = logits[i];
         indices[j] = StaticCast<int>(i);
         break;
       }
     }
   }
+  HWY_ASSERT(k <= num_accepted);
   return indices[create_distribution<k>(top_k, temperature)(gen)];
 }
 

ops/ops_test.cc

@@ -24,6 +24,8 @@
 #include <algorithm>
 #include <array>
 #include <cmath>
+#include <functional>
+#include <numeric>
 #include <random>
 #include <vector>
 
@@ -559,6 +561,43 @@ void TestAllLayerNorm() {
   TestLayerNorm<float, BF16, BF16>(rng);
 }
 
+void TestSampleTopK() {
+  const size_t kSize = 52;
+  std::vector<float> logits(kSize);
+  // SampleTopK is typically used on logits, which can be negative.
+  // Create a vector going from -100 to -100+51=49.
+  std::iota(logits.begin(), logits.end(), -100.0f);
+  std::mt19937 gen;
+  gen.seed(0x12345678);
+  float temperature = 1.0f;
+  // SampleTopK<1> should return the argmax.
+  std::function<bool(int, float)> accept_token;
+  int sample = SampleTopK<1>(logits.data(), kSize, gen, temperature,
+                             accept_token);
+  EXPECT_EQ(sample, 51);  // Last is largest.
+  // Only accept even tokens, expect the last (largest) even index.
+  accept_token = [](int i, float) { return i % 2 == 0; };
+  sample = SampleTopK<1>(logits.data(), kSize, gen, temperature,
+                         accept_token);
+  EXPECT_EQ(sample, 50);  // Last even index.
+  // Reset the logits to a positive, increasing sequence.
+  std::iota(logits.begin(), logits.end(), 1.0f);
+  // Sample from the top 3, expect one of the top 3 even indices.
+  for (int i = 0; i < 100; ++i) {
+    sample = SampleTopK<3>(logits.data(), kSize, gen, temperature,
+                          accept_token);
+    EXPECT_TRUE(sample == 50 || sample == 48 || sample == 46);
+  }
+  // Now set the temperature to 0.0f, which should always return the argmax,
+  // even for k=3.
+  temperature = 0.0f;
+  for (int i = 0; i < 100; ++i) {
+    sample = SampleTopK<3>(logits.data(), kSize, gen, temperature,
+                          accept_token);
+    EXPECT_EQ(sample, 50);
+  }
+}
+
 // NOLINTNEXTLINE(google-readability-namespace-comments)
 }  // namespace HWY_NAMESPACE
 }  // namespace gcpp
@@ -579,6 +618,7 @@ HWY_EXPORT_AND_TEST_P(OpsTest, TestRopeAndMulBy);
 HWY_EXPORT_AND_TEST_P(OpsTest, TestAllRMSNorm);
 HWY_EXPORT_AND_TEST_P(OpsTest, TestAllLayerNorm);
 HWY_EXPORT_AND_TEST_P(OpsTest, TestLayerNormSimple);
+HWY_EXPORT_AND_TEST_P(OpsTest, TestSampleTopK);
 HWY_AFTER_TEST();
 
 }  // namespace gcpp