[clangd] Use a sigmoid style function for #usages boost in symbol quality.

Summary:
This has a shape to similar logarithm function but grows much slower for
large #usages.

Metrics: https://reviews.llvm.org/P8096

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, cfe-commits, sammccall

Differential Revision: https://reviews.llvm.org/D49780

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@337907 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 17 insertions, 6 deletions

diff --git a/clangd/Quality.cpp b/clangd/Quality.cpp
index 1b64f9a9..afbcdec6 100644
--- a/clangd/Quality.cpp
+++ b/clangd/Quality.cpp
@@ -190,8 +190,18 @@ float SymbolQualitySignals::evaluate() const {
 
   // This avoids a sharp gradient for tail symbols, and also neatly avoids the
   // question of whether 0 references means a bad symbol or missing data.
-  if (References >= 10)
-    Score *= std::log10(References);
+  if (References >= 10) {
+    // Use a sigmoid style boosting function, which flats out nicely for large
+    // numbers (e.g. 2.58 for 1M refererences).
+    // The following boosting function is equivalent to:
+    //   m = 0.06
+    //   f = 12.0
+    //   boost = f * sigmoid(m * std::log(References)) - 0.5 * f + 0.59
+    // Sample data points: (10, 1.00), (100, 1.41), (1000, 1.82),
+    //                     (10K, 2.21), (100K, 2.58), (1M, 2.94)
+    float s = std::pow(References, -0.06);
+    Score *= 6.0 * (1 - s) / (1 + s) + 0.59;
+  }
 
   if (Deprecated)
     Score *= 0.1f;
diff --git a/unittests/clangd/CodeCompleteTests.cpp b/unittests/clangd/CodeCompleteTests.cpp
index 1949d70f..2f04bef2 100644
--- a/unittests/clangd/CodeCompleteTests.cpp
+++ b/unittests/clangd/CodeCompleteTests.cpp
@@ -476,11 +476,12 @@ TEST(CompletionTest, ScopedWithFilter) {
 }
 
 TEST(CompletionTest, ReferencesAffectRanking) {
-  auto Results = completions("int main() { abs^ }", {ns("absl"), func("abs")});
-  EXPECT_THAT(Results.Completions, HasSubsequence(Named("abs"), Named("absl")));
+  auto Results = completions("int main() { abs^ }", {ns("absl"), func("absb")});
+  EXPECT_THAT(Results.Completions, HasSubsequence(Named("absb"), Named("absl")));
   Results = completions("int main() { abs^ }",
-                        {withReferences(10000, ns("absl")), func("abs")});
-  EXPECT_THAT(Results.Completions, HasSubsequence(Named("absl"), Named("abs")));
+                        {withReferences(10000, ns("absl")), func("absb")});
+  EXPECT_THAT(Results.Completions,
+              HasSubsequence(Named("absl"), Named("absb")));
 }
 
 TEST(CompletionTest, GlobalQualified) {