wgtmac commented on code in PR #55449:
URL: https://github.com/apache/arrow/pull/55449#discussion_r4545536315
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cpp/src/arrow/util/alp/alp_test.cc:
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@@ -776,11 +774,34 @@ TYPED_TEST(AlpSamplerTest, PresetGenerationDecimalData) {
output, std::vector<TypeParam>(data.begin(), data.begin() +
kNumValues)));
}
+TYPED_TEST(AlpSamplerTest, SamplingWindowSize) {
+ constexpr int64_t kChunkSize = AlpSamplingOptions::kChunkSize;
+ const std::vector<TypeParam> nan_chunk(kChunkSize,
+
std::numeric_limits<TypeParam>::quiet_NaN());
+ const std::vector<TypeParam> decimal_chunk(kChunkSize,
static_cast<TypeParam>(1.25));
+ const auto decimal_preset =
AlpCompression<TypeParam>::MakePreset({decimal_chunk});
+
+ // The sampler targets eight chunks per window. An eight-chunk window samples
+ // both inputs, but only decimal_chunk can contribute usable parameters.
+ AlpSampler<TypeParam> every_chunk(/*sampling_window_size=*/8 * kChunkSize);
+ every_chunk.AddSample(nan_chunk);
+ every_chunk.AddSample(decimal_chunk);
+ EXPECT_EQ(decimal_preset.combinations,
every_chunk.MakePreset().combinations);
+
+ // A sixteen-chunk window samples every other chunk, skipping decimal_chunk.
+ // With only NaNs sampled, the preset falls back to identity.
+ AlpSampler<TypeParam> every_other_chunk(/*sampling_window_size=*/16 *
kChunkSize);
Review Comment:
Yes, let me make it explicit.
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