Add Intention for single character substring #KT-22171 Fixed

idea/src/META-INF/plugin.xml

@@ -1526,6 +1526,11 @@
       <category>Kotlin</category>
     </intentionAction>
 
+    <intentionAction>
+      <className>org.jetbrains.kotlin.idea.intentions.ReplaceSubstringWithIndexingOperationIntention</className>
+      <category>Kotlin</category>
+    </intentionAction>
+
     <localInspection implementationClass="org.jetbrains.kotlin.idea.intentions.ObjectLiteralToLambdaInspection"
                      displayName="Object literal can be converted to lambda"
                      groupPath="Kotlin"

idea/src/org/jetbrains/kotlin/idea/intentions/ReplaceSubstringWithIndexingOperationIntention.kt

@@ -0,0 +1,55 @@
+/*
+ * Copyright 2000-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.idea.intentions
+
+import com.intellij.openapi.editor.Editor
+import com.intellij.openapi.util.TextRange
+import org.jetbrains.kotlin.idea.caches.resolve.analyze
+import org.jetbrains.kotlin.psi.*
+import org.jetbrains.kotlin.resolve.calls.callUtil.getResolvedCall
+import org.jetbrains.kotlin.resolve.constants.evaluate.ConstantExpressionEvaluator
+import org.jetbrains.kotlin.resolve.descriptorUtil.fqNameUnsafe
+import org.jetbrains.kotlin.resolve.lazy.BodyResolveMode
+
+class ReplaceSubstringWithIndexingOperationIntention : ReplaceSubstringIntention("Replace 'substring' call with indexing operation call") {
+    override fun applicabilityRange(element: KtDotQualifiedExpression): TextRange? {
+        if (element.isSubstringMethod()) {
+            return applicabilityRangeInner(element)
+        }
+
+        return null
+    }
+
+    override fun applicabilityRangeInner(element: KtDotQualifiedExpression): TextRange? {
+        val arguments = element.callExpression?.valueArguments ?: return null
+        if (arguments.size != 2) return null
+
+        val arg1 = element.getValueArgument(0) ?: return null
+        val arg2 = element.getValueArgument(1) ?: return null
+        if (arg1 + 1 != arg2) return null
+
+        return getTextRange(element)
+    }
+
+    override fun applyTo(element: KtDotQualifiedExpression, editor: Editor?) {
+        val expression = element.callExpression?.valueArguments?.firstOrNull()?.getArgumentExpression() ?: return
+        element.replaceWith("$0[$1]", expression)
+    }
+
+    private fun KtDotQualifiedExpression.isSubstringMethod(): Boolean {
+        val resolvedCall = toResolvedCall(BodyResolveMode.PARTIAL) ?: return false
+        return (resolvedCall.resultingDescriptor.fqNameUnsafe.asString() == "kotlin.text.substring")
+    }
+
+    private fun KtDotQualifiedExpression.getValueArgument(index: Int): Int? {
+        val bindingContext = analyze()
+        val resolvedCall = callExpression.getResolvedCall(bindingContext) ?: return null
+        val expression = resolvedCall.call.valueArguments[index].getArgumentExpression() as? KtConstantExpression ?: return null
+        val constant = ConstantExpressionEvaluator.getConstant(expression, bindingContext) ?: return null
+        val constantType = bindingContext.getType(expression) ?: return null
+        return constant.getValue(constantType) as? Int
+    }
+}