LSP-390 EquationsExternalizer: make a singleton to prevent externalizer hashcode mismatch errors in LSP

LSP creates "a new copy of the world" for each request, and this caused EquationsExternalizer to be recreated too.

GitOrigin-RevId: 3f56d2a933d3d96533f5afd4c206fcecde04a716

java/java-analysis-impl/src/com/intellij/codeInspection/bytecodeAnalysis/BytecodeAnalysisGist.java

@@ -24,7 +24,7 @@ public final class BytecodeAnalysisGist {
     gist = GistManager.getInstance().newVirtualFileGist(
       "BytecodeAnalysisIndex",
       ClassDataIndexer.FINAL_VERSION,
-      new BytecodeAnalysisIndex.EquationsExternalizer(),
+      BytecodeAnalysisIndex.EquationsExternalizer.INSTANCE,
       new ClassDataIndexer()
     );
   }

java/java-analysis-impl/src/com/intellij/codeInspection/bytecodeAnalysis/BytecodeAnalysisIndex.java

@@ -138,6 +138,8 @@ public final class BytecodeAnalysisIndex extends ScalarIndexExtension<HMember> {
    * Externalizer for compressed equations.
    */
   public static class EquationsExternalizer implements DataExternalizer<Map<HMember, Equations>> {
+    public static final EquationsExternalizer INSTANCE = new EquationsExternalizer();
+
     @Override
     public void save(@NotNull DataOutput out, Map<HMember, Equations> value) throws IOException {
       DataInputOutputUtilRt.writeSeq(out, value.entrySet(), entry -> {

java/java-analysis-impl/src/com/intellij/codeInspection/bytecodeAnalysis/ClassDataIndexer.java

@@ -284,7 +284,7 @@ public final class ClassDataIndexer implements VirtualFileGist.GistCalculator<Ma
     public void accept(Map<HMember, Equations> map) {
       try {
         UnsyncByteArrayOutputStream stream = new UnsyncByteArrayOutputStream();
-        new BytecodeAnalysisIndex.EquationsExternalizer().save(new DataOutputStream(stream), map);
+        BytecodeAnalysisIndex.EquationsExternalizer.INSTANCE.save(new DataOutputStream(stream), map);
         ourTotalSize.addAndGet(stream.size());
         ourTotalCount.incrementAndGet();
       }