Fixed KT-4318 Keyword should be selected in completion if there's exact match

#KT-4318 Fixed

idea/src/org/jetbrains/jet/plugin/completion/weigher/JetCompletionSorting.kt

@@ -20,6 +20,8 @@ import com.intellij.codeInsight.completion.CompletionResultSet
 import com.intellij.codeInsight.completion.CompletionParameters
 import com.intellij.codeInsight.completion.CompletionSorter
 import org.jetbrains.jet.lang.psi.JetFile
+import com.intellij.codeInsight.lookup.LookupElementWeigher
+import com.intellij.codeInsight.lookup.LookupElement
 
 public fun CompletionResultSet.addJetSorting(parameters : CompletionParameters) : CompletionResultSet {
     var sorter = CompletionSorter.defaultSorter(parameters, getPrefixMatcher())!!
@@ -31,5 +33,14 @@ public fun CompletionResultSet.addJetSorting(parameters : CompletionParameters)
             JetDeclarationRemotenessWeigher(parameters.getOriginalFile() as JetFile),
             JetAccessibleWeigher())
 
+    sorter = sorter.weighBefore("middleMatching", PreferMatchingItemWeigher(parameters))
+
     return withRelevanceSorter(sorter)
+}
+
+class PreferMatchingItemWeigher(private val parameters: CompletionParameters) : LookupElementWeigher("preferMatching", false, true){
+    override fun weigh(element: LookupElement): Comparable<Int>? {
+        val prefix = parameters.getLookup().itemPattern(element)
+        return if (element.getLookupString() == prefix) 0 else 1
+    }
 }

idea/testData/completion/weighers/ExactMatchForKeyword.kt

@@ -0,0 +1,11 @@
+fun test() {
+    val ifMore = true
+    val isFirst = true
+    if<caret>
+}
+
+// ORDER: if
+// ORDER: ifMore
+// ORDER: isFirst
+// ORDER: ifn
+// SELECTED: 0

idea/testData/completion/weighers/LocalFileBeforeImported.kt

@@ -9,4 +9,4 @@ val some = foo<caret>
 
 // ORDER: foo, fooVar, fooLocal, fooImported, fooNotImported
 // INVOCATION_COUNT: 2
-// SELECTED: 1
+// SELECTED: 0

idea/testData/completion/weighers/PropertiesBeforeKeywords.kt

@@ -16,4 +16,4 @@ class Test(val br2 = 12) {
 // "br" function is before other elements because of exact prefix match
 
 // ORDER: br, br4, br5, br1, br2, br3, break, brf
-// SELECTED: 1
+// SELECTED: 0

idea/tests/org/jetbrains/jet/completion/weighers/CompletionWeigherTestGenerated.java

@@ -41,6 +41,11 @@ public class CompletionWeigherTestGenerated extends AbstractCompletionWeigherTes
         doTest("idea/testData/completion/weighers/DeprecatedFun.kt");
     }
     
+    @TestMetadata("ExactMatchForKeyword.kt")
+    public void testExactMatchForKeyword() throws Exception {
+        doTest("idea/testData/completion/weighers/ExactMatchForKeyword.kt");
+    }
+    
     @TestMetadata("KeywordsLast.kt")
     public void testKeywordsLast() throws Exception {
         doTest("idea/testData/completion/weighers/KeywordsLast.kt");