Resolves #231 - Speedup UnitDependencyOptionValue completion

It's not necessary to get all VirtualFiles with given extension since we only need file names.
Also, it's faster to process all files and extensions at once.

Author: VladRassokhin

src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/semanticdata/optionvalues/UnitDependencyOptionValue.kt

@@ -2,16 +2,20 @@ package net.sjrx.intellij.plugins.systemdunitfiles.semanticdata.optionvalues
 
 import com.intellij.codeInspection.ProblemsHolder
 import com.intellij.openapi.project.Project
+import com.intellij.openapi.util.ModificationTracker
 import com.intellij.openapi.util.TextRange
 import com.intellij.openapi.util.text.Strings
-import com.intellij.psi.search.FilenameIndex
+import com.intellij.openapi.vfs.newvfs.persistent.FSRecords
+import com.intellij.psi.search.FilenameIndex.processAllFileNames
+import com.intellij.psi.search.GlobalSearchScope
+import com.intellij.psi.util.CachedValueProvider
+import com.intellij.psi.util.CachedValuesManager
+import com.intellij.util.containers.CollectionFactory
 import net.sjrx.intellij.plugins.systemdunitfiles.psi.UnitFilePropertyType
 import net.sjrx.intellij.plugins.systemdunitfiles.semanticdata.SemanticDataRepository
 import net.sjrx.intellij.plugins.systemdunitfiles.semanticdata.Validator
-import java.io.BufferedReader
 import java.io.IOException
 import java.io.InputStream
-import java.io.InputStreamReader
 import java.util.*
 
 /**
@@ -44,12 +48,8 @@ class UnitDependencyOptionValue : OptionValueInformation {
   }
 
   override fun getAutoCompleteOptions(project: Project): Set<String> {
-    val autoCompletionOptions: MutableSet<String> = HashSet(unitNames)
-    for (unitTypes in validUnitTypes) {
-      for (file in FilenameIndex.getAllFilesByExt(project, unitTypes)) {
-        autoCompletionOptions.add(file.name)
-      }
-    }
+    val autoCompletionOptions = CollectionFactory.createSmallMemoryFootprintSet(unitNames ?: emptySet())
+    autoCompletionOptions.addAll(getAllProjectUnitFileNames(project))
     return autoCompletionOptions
   }
 
@@ -82,8 +82,8 @@ class UnitDependencyOptionValue : OptionValueInformation {
         val unitType = word.substring(lastDot + 1)
         if (!validUnitTypes.contains(unitType)) {
           holder.registerProblem(
-            property.valueNode.psi, range,
-            "Unit type " + unitType + " is unsupported, valid types are: " + Strings.join(validUnitTypes, ", ")
+                  property.valueNode.psi, range,
+                  "Unit type " + unitType + " is unsupported, valid types are: " + Strings.join(validUnitTypes, ", ")
           )
         }
       } else {
@@ -97,6 +97,51 @@ class UnitDependencyOptionValue : OptionValueInformation {
   override val validatorName: String
     get() = VALIDATOR_NAME
 
+  /**
+   * That's a simplified version of `com.intellij.psi.search.FilenameIndex.getAllFilesByExt()`
+   * since we need only file names, but not whole VirtualFile's
+   */
+  private fun getAllProjectUnitFileNames(project: Project): Set<String> {
+    return CachedValuesManager.getManager(project).getCachedValue(project, CachedValueProvider {
+      return@CachedValueProvider CachedValueProvider.Result<Set<String>>(
+              getAllProjectUnitFileNamesImpl(project),
+              getFilenameIndexModificationTracker(),
+      )
+    })
+  }
+
+  /*
+   * Simplified copy of com.intellij.openapi.fileEditor.impl.UniqueVFilePathBuilderImpl.getFilenameIndexModificationTracker
+   * Assuming FileBasedIndexExtension.USE_VFS_FOR_FILENAME_INDEX == true (it was set so at least since 2022.2)
+   */
+  private fun getFilenameIndexModificationTracker(): ModificationTracker {
+    return ModificationTracker {
+      @Suppress("UnstableApiUsage")
+      FSRecords.getNamesIndexModCount()
+    }
+  }
+
+  private fun getAllProjectUnitFileNamesImpl(project: Project): Set<String> {
+    val names = CollectionFactory.createSmallMemoryFootprintSet<String>()
+    val minNameLength = validUnitTypes.minOf { it.length } + 2 /* dot and at least on character in name */
+    val maxExtLength = validUnitTypes.maxOf { it.length }
+
+    processAllFileNames({ name: String ->
+      val length = name.length
+      if (length >= minNameLength) {
+        val dot = name.lastIndexOf('.')
+        if (dot != -1 && (length - (dot + 1) <= maxExtLength)) {
+          val ext = name.substring(dot + 1)
+          if (ext in validUnitTypes || ext.lowercase() in validUnitTypes) {
+            names.add(name)
+          }
+        }
+      }
+      true
+    }, GlobalSearchScope.allScope(project), null)
+    return names
+  }
+
   companion object {
     @Volatile
     private var unitNames: Set<String>? = null