Repository: cqfn/diKTat (kotlin)

Issue:
Improve autofix in AVOID_NULL_CHECKS rule
## Describe the bug
* `val x = if (y != null) f(y) else g(y)` should be simplified to `val x = y?.let { f(y) } ?: g(y)` without an additional `run`
* `val x = if (y != null) y else z` should be simplified to `val x = y ?: z`, without any `let` or `run`

## Environment information
* diktat version: 0.5.3

Pull request description:
This pull request closes #899 

## Actions checklist
* [x] improved autofix: now we don't use `let` and `run` in some cases
* [x] added tests on fixer

Patch:
diff --git a/diktat-gradle-plugin/src/main/kotlin/org/cqfn/diktat/plugin/gradle/DiktatJavaExecTaskBase.kt b/diktat-gradle-plugin/src/main/kotlin/org/cqfn/diktat/plugin/gradle/DiktatJavaExecTaskBase.kt
index 6ce190e1c9..0c21a4d566 100644
--- a/diktat-gradle-plugin/src/main/kotlin/org/cqfn/diktat/plugin/gradle/DiktatJavaExecTaskBase.kt
+++ b/diktat-gradle-plugin/src/main/kotlin/org/cqfn/diktat/plugin/gradle/DiktatJavaExecTaskBase.kt
@@ -65,7 +65,7 @@ open class DiktatJavaExecTaskBase @Inject constructor(
             // validate configuration
             require(inputs == null && excludes == null) {
                 "`inputs` and `excludes` arguments for diktat task are deprecated and now should be changed for `inputs {}` " +
-                        "with configuration for PatternFilterable. Please check https://github.com/diktat-static-analysis/diktat/README.md for more info."
+                        "with configuration for PatternFilterable. Please check https://github.com/analysis-dev/diktat/README.md for more info."
             }
         }
 
diff --git a/diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/rules/chapter4/NullChecksRule.kt b/diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/rules/chapter4/NullChecksRule.kt
index da03cc778c..8ce43efab3 100644
--- a/diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/rules/chapter4/NullChecksRule.kt
+++ b/diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/rules/chapter4/NullChecksRule.kt
@@ -24,6 +24,7 @@ import org.jetbrains.kotlin.com.intellij.psi.tree.IElementType
 import org.jetbrains.kotlin.psi.KtBinaryExpression
 import org.jetbrains.kotlin.psi.KtBlockExpression
 import org.jetbrains.kotlin.psi.KtIfExpression
+import org.jetbrains.kotlin.psi.psiUtil.blockExpressionsOrSingle
 
 /**
  * This rule check and fixes explicit null checks (explicit comparison with `null`)
@@ -120,56 +121,66 @@ class NullChecksRule(configRules: List<RulesConfig>) : DiktatRule(
                                      isEqualToNull: Boolean
     ) {
         val variableName = binaryExpression.left!!.text
-        val thenCodeLines = condition.extractLinesFromBlock(THEN)
-        val elseCodeLines = condition.extractLinesFromBlock(ELSE)
-        val text = if (isEqualToNull) {
-            when {
-                elseCodeLines.isNullOrEmpty() ->
-                    if (condition.getBreakNodeFromIf(THEN)) {
-                        "$variableName ?: break"
-                    } else {
-                        "$variableName ?: run {${thenCodeLines?.joinToString(prefix = "\n", postfix = "\n", separator = "\n")}}"
-                    }
-                thenCodeLines!!.singleOrNull() == "null" -> """
-                        |$variableName?.let {
-                        |${elseCodeLines.joinToString(separator = "\n")}
-                        |}
-                    """.trimMargin()
-                thenCodeLines.singleOrNull() == "break" -> """
-                        |$variableName?.let {
-                        |${elseCodeLines.joinToString(separator = "\n")}
-                        |} ?: break
-                        """.trimMargin()
-                else -> """
-                        |$variableName?.let {
-                        |${elseCodeLines.joinToString(separator = "\n")}
-                        |}
-                        |?: run {
-                        |${thenCodeLines.joinToString(separator = "\n")}
-                        |}
-                    """.trimMargin()
-            }
+        val thenFromExistingCode = condition.extractLinesFromBlock(THEN)
+        val elseFromExistingCode = condition.extractLinesFromBlock(ELSE)
+
+        // if (a == null) { foo() } else { bar() } -> if (a != null) { bar() } else { foo() }
+        val thenCodeLines = if (isEqualToNull) {
+            elseFromExistingCode
         } else {
-            when {
-                elseCodeLines.isNullOrEmpty() || (elseCodeLines.singleOrNull() == "null") ->
-                    "$variableName?.let {${thenCodeLines?.joinToString(prefix = "\n", postfix = "\n", separator = "\n")}}"
-                elseCodeLines.singleOrNull() == "break" ->
-                    "$variableName?.let {${thenCodeLines?.joinToString(prefix = "\n", postfix = "\n", separator = "\n")}} ?: break"
-                else -> """
-                        |$variableName?.let {
-                        |${thenCodeLines?.joinToString(separator = "\n")}
-                        |}
-                        |?: run {
-                        |${elseCodeLines.joinToString(separator = "\n")}
-                        |}
-                    """.trimMargin()
-            }
+            thenFromExistingCode
         }
+        val elseCodeLines = if (isEqualToNull) {
+            thenFromExistingCode
+        } else {
+            elseFromExistingCode
+        }
+        val numberOfStatementsInElseBlock = if (isEqualToNull) {
+            (condition.treeParent.psi as KtIfExpression).then?.blockExpressionsOrSingle()?.count() ?: 0
+        } else {
+            (condition.treeParent.psi as KtIfExpression).`else`?.blockExpressionsOrSingle()?.count() ?: 0
+        }
+
+        val elseEditedCodeLines = getEditedElseCodeLines(elseCodeLines, numberOfStatementsInElseBlock)
+        val thenEditedCodeLines = getEditedThenCodeLines(variableName, thenCodeLines, elseEditedCodeLines)
+
+        val text = "$thenEditedCodeLines $elseEditedCodeLines"
         val tree = KotlinParser().createNode(text)
         condition.treeParent.treeParent.addChild(tree, condition.treeParent)
         condition.treeParent.treeParent.removeChild(condition.treeParent)
     }
 
+    private fun getEditedElseCodeLines(elseCodeLines: List<String>?, numberOfStatementsInElseBlock: Int): String = when {
+        // else { "null"/empty } -> ""
+        elseCodeLines == null || elseCodeLines.singleOrNull() == "null" -> ""
+        // else { bar() } -> ?: bar()
+        numberOfStatementsInElseBlock == 1 -> "?: ${elseCodeLines.joinToString(postfix = "\n", separator = "\n")}"
+        // else { ... } -> ?: run { ... }
+        else -> getDefaultCaseElseCodeLines(elseCodeLines)
+    }
+
+    @Suppress("UnsafeCallOnNullableType")
+    private fun getEditedThenCodeLines(
+        variableName: String,
+        thenCodeLines: List<String>?,
+        elseEditedCodeLines: String
+    ): String = when {
+        // if (a != null) {  } -> a ?: editedElse
+        (thenCodeLines.isNullOrEmpty() && elseEditedCodeLines.isNotEmpty()) ||
+                // if (a != null) { a } else { ... } -> a ?: editedElse
+                (thenCodeLines?.singleOrNull() == variableName && elseEditedCodeLines.isNotEmpty()) -> variableName
+        // if (a != null) { a.foo() } -> a?.foo()
+        thenCodeLines?.singleOrNull()?.startsWith("$variableName.") ?: false -> "$variableName?${thenCodeLines?.firstOrNull()!!.removePrefix(variableName)}"
+        // if (a != null) { break } -> a?.let { ... }
+        // if (a != null) { foo() } -> a?.let { ... }
+        else -> getDefaultCaseThenCodeLines(variableName, thenCodeLines)
+    }
+
+    private fun getDefaultCaseThenCodeLines(variableName: String, thenCodeLines: List<String>?): String =
+            "$variableName?.let {${thenCodeLines?.joinToString(prefix = "\n", postfix = "\n", separator = "\n")}}"
+
+    private fun getDefaultCaseElseCodeLines(elseCodeLines: List<String>): String = "?: run {${elseCodeLines.joinToString(prefix = "\n", postfix = "\n", separator = "\n")}}"
+
     @Suppress("COMMENT_WHITE_SPACE", "UnsafeCallOnNullableType")
     private fun nullCheckInOtherStatements(binaryExprNode: ASTNode) {
         val condition = (binaryExprNode.psi as KtBinaryExpression)


Test patch:
diff --git a/diktat-rules/src/test/kotlin/org/cqfn/diktat/ruleset/chapter4/NullChecksRuleFixTest.kt b/diktat-rules/src/test/kotlin/org/cqfn/diktat/ruleset/chapter4/NullChecksRuleFixTest.kt
index 088b705ff8..f97bf3ebdb 100644
--- a/diktat-rules/src/test/kotlin/org/cqfn/diktat/ruleset/chapter4/NullChecksRuleFixTest.kt
+++ b/diktat-rules/src/test/kotlin/org/cqfn/diktat/ruleset/chapter4/NullChecksRuleFixTest.kt
@@ -25,4 +25,10 @@ class NullChecksRuleFixTest : FixTestBase("test/paragraph4/null_checks", ::NullC
     fun `should fix require function`() {
         fixAndCompare("RequireFunctionExpected.kt", "RequireFunctionTest.kt")
     }
+
+    @Test
+    @Tag(WarningNames.AVOID_NULL_CHECKS)
+    fun `should fix if conditions when assigned`() {
+        fixAndCompare("IfConditionAssignCheckExpected.kt", "IfConditionAssignCheckTest.kt")
+    }
 }
diff --git a/diktat-rules/src/test/resources/test/paragraph4/null_checks/IfConditionAssignCheckExpected.kt b/diktat-rules/src/test/resources/test/paragraph4/null_checks/IfConditionAssignCheckExpected.kt
new file mode 100644
index 0000000000..c60654544d
--- /dev/null
+++ b/diktat-rules/src/test/resources/test/paragraph4/null_checks/IfConditionAssignCheckExpected.kt
@@ -0,0 +1,45 @@
+package test.paragraph4.null_checks
+
+fun foo() {
+    val x = a?.let {
+f(a)
+} ?: g(a)
+
+    val y = a ?: 0
+
+    x ?: println("NULL")
+
+    val z = x ?: run {
+println("NULL")
+0
+}
+
+    x?.let {
+f(x)
+} ?: run {
+println("NULL")
+g(x)
+}
+}
+
+fun bar() {
+    val x = a?.let {
+f(a)
+} ?: g(a)
+
+    val y = a ?: 0
+
+    x ?: println("NULL")
+
+    val z = x ?: run {
+println("NULL")
+0
+}
+
+    x?.let {
+f(x)
+} ?: run {
+println("NULL")
+g(x)
+}
+}
diff --git a/diktat-rules/src/test/resources/test/paragraph4/null_checks/IfConditionAssignCheckTest.kt b/diktat-rules/src/test/resources/test/paragraph4/null_checks/IfConditionAssignCheckTest.kt
new file mode 100644
index 0000000000..b7527a7c62
--- /dev/null
+++ b/diktat-rules/src/test/resources/test/paragraph4/null_checks/IfConditionAssignCheckTest.kt
@@ -0,0 +1,53 @@
+package test.paragraph4.null_checks
+
+fun foo() {
+    val x = if (a != null) f(a) else g(a)
+
+    val y = if (a != null) a else 0
+
+    if (x != null) {
+        x
+    } else {
+        println("NULL")
+    }
+
+    val z = if (x != null) {
+        x
+    } else {
+        println("NULL")
+        0
+    }
+
+    if (x != null) {
+        f(x)
+    } else {
+        println("NULL")
+        g(x)
+    }
+}
+
+fun bar() {
+    val x = if (a == null) g(a) else f(a)
+
+    val y = if (a == null) 0 else a
+
+    if (x == null) {
+        println("NULL")
+    } else {
+        x
+    }
+
+    val z = if (x == null) {
+        println("NULL")
+        0
+    } else {
+        x
+    }
+
+    if (x == null) {
+        println("NULL")
+        g(x)
+    } else {
+        f(x)
+    }
+}
diff --git a/diktat-rules/src/test/resources/test/paragraph4/null_checks/IfConditionNullCheckExpected.kt b/diktat-rules/src/test/resources/test/paragraph4/null_checks/IfConditionNullCheckExpected.kt
index 255982c5e8..47af964fc1 100644
--- a/diktat-rules/src/test/resources/test/paragraph4/null_checks/IfConditionNullCheckExpected.kt
+++ b/diktat-rules/src/test/resources/test/paragraph4/null_checks/IfConditionNullCheckExpected.kt
@@ -18,10 +18,7 @@ bar()
 
     some?.let {
 print("qwe")
-}
-?: run {
-print("asd")
-}
+} ?: print("asd")
 
     some?.let {
 print("qweqwe")
@@ -29,10 +26,7 @@ print("qweqwe")
 
     some?.let {
 print("qqq")
-}
-?: run {
-print("www")
-}
+} ?: print("www")
 
     some?.let {
 print("ttt")
@@ -40,8 +34,7 @@ print("ttt")
 
     some?.let {
 print("ttt")
-}
-?: run {
+} ?: run {
 null
 value
 }
@@ -52,11 +45,8 @@ fun foo() {
     while (result != 0 ) {
         result?.let {
 goo()
-}
-?: run {
-for(i in 1..10)
+} ?: for(i in 1..10)
 break
-}
     }
     while (result != 0) {
         result = goo()
@@ -69,3 +59,23 @@ break
     }
 }
 
+fun checkSmartCases() {
+    val x = a?.toString() ?: "Null"
+    val y = a.b.c?.toString() ?: a.b.toString()
+    a?.let {
+print()
+}
+    a?.let {
+foo()
+} ?: boo()
+}
+
+fun reversedCheckSmartCases() {
+    val x = a?.toString() ?: "Null"
+    val y = a.b.c?.toString() ?: a.b.toString()
+    a ?: print()
+    a?.let {
+foo()
+} ?: boo()
+}
+
diff --git a/diktat-rules/src/test/resources/test/paragraph4/null_checks/IfConditionNullCheckTest.kt b/diktat-rules/src/test/resources/test/paragraph4/null_checks/IfConditionNullCheckTest.kt
index f95a7d34f0..135c389548 100644
--- a/diktat-rules/src/test/resources/test/paragraph4/null_checks/IfConditionNullCheckTest.kt
+++ b/diktat-rules/src/test/resources/test/paragraph4/null_checks/IfConditionNullCheckTest.kt
@@ -69,3 +69,45 @@ fun foo() {
     }
 }
 
+fun checkSmartCases() {
+    val x = if (a != null) {
+        a.toString()
+    } else {
+        "Null"
+    }
+    val y = if (a.b.c != null) {
+        a.b.c.toString()
+    } else {
+        a.b.toString()
+    }
+    if (a != null) {
+        print()
+    }
+    if (a != null) {
+        foo()
+    } else {
+        boo()
+    }
+}
+
+fun reversedCheckSmartCases() {
+    val x = if (a == null) {
+        "Null"
+    } else {
+        a.toString()
+    }
+    val y = if (a.b.c == null) {
+        a.b.toString()
+    } else {
+        a.b.c.toString()
+    }
+    if (a == null) {
+        print()
+    }
+    if (a == null) {
+        boo()
+    } else {
+        foo()
+    }
+}
+
diff --git a/diktat-rules/src/test/resources/test/smoke/src/main/kotlin/Example7Expected.kt b/diktat-rules/src/test/resources/test/smoke/src/main/kotlin/Example7Expected.kt
index 7887176225..6de69b3349 100644
--- a/diktat-rules/src/test/resources/test/smoke/src/main/kotlin/Example7Expected.kt
+++ b/diktat-rules/src/test/resources/test/smoke/src/main/kotlin/Example7Expected.kt
@@ -15,10 +15,7 @@ fun foo() {
 
     prop?.let {
         doAnotherSmth()
-    }
-        ?: run {
-            doSmth()
-        }
+    } ?: doSmth()
 }
 
 fun fooo() {