update InvalidPackageDeclaration to report if rootPackage is not present

detekt-rules-naming/src/main/kotlin/io/gitlab/arturbosch/detekt/rules/naming/InvalidPackageDeclaration.kt

@@ -25,7 +25,7 @@ class InvalidPackageDeclaration(config: Config = Config.empty) : Rule(config) {
         debt = Debt.FIVE_MINS
     )
 
-    @Configuration("if specified this part of the package structure is ignored")
+    @Configuration("if specified the declaration must start with this root, but it is ignored in the file structure")
     private val rootPackage: String by config("")
 
     override fun visitPackageDirective(directive: KtPackageDirective) {
@@ -34,26 +34,28 @@ class InvalidPackageDeclaration(config: Config = Config.empty) : Rule(config) {
         if (declaredPath.isNotBlank()) {
             val normalizedFilePath = directive.containingKtFile.absolutePath().parent.toNormalizedForm()
             val normalizedRootPackage = packageNameToNormalizedForm(rootPackage)
-            val expectedPath =
-                if (normalizedRootPackage.isBlank()) {
-                    declaredPath
-                } else {
-                    declaredPath.substringAfter(normalizedRootPackage)
+            val expectedPath = when {
+                normalizedRootPackage.isBlank() -> declaredPath
+                !declaredPath.startsWith(normalizedRootPackage) -> {
+                    directive.reportInvalidPackageDeclaration("The package declaration is missing the root package")
+                    return
                 }
+                else -> declaredPath.substringAfter(normalizedRootPackage)
+            }
 
             val isInRootPackage = expectedPath.isBlank()
             if (!isInRootPackage && !normalizedFilePath.endsWith(expectedPath)) {
-                report(
-                    CodeSmell(
-                        issue,
-                        Entity.from(directive),
-                        "The package declaration does not match the actual file location.",
-                    )
+                directive.reportInvalidPackageDeclaration(
+                    "The package declaration does not match the actual file location."
                 )
             }
         }
     }
 
+    private fun KtElement.reportInvalidPackageDeclaration(message: String) {
+        report(CodeSmell(issue, Entity.from(this), message))
+    }
+
     private fun <T> Iterable<T>.toNormalizedForm() = joinToString("|")
 
     private fun packageNameToNormalizedForm(packageName: String) = packageName.split('.').toNormalizedForm()

detekt-rules-naming/src/test/kotlin/io/gitlab/arturbosch/detekt/rules/naming/InvalidPackageDeclarationSpec.kt

@@ -93,6 +93,20 @@ internal class InvalidPackageDeclarationSpec : Spek({
 
                 assertThat(findings).hasSize(1)
             }
+
+            it("should report if root package is missing") {
+                val source = """
+                    package foo.bar
+
+                    class C
+                """
+
+                val ktFile = compileContentForTest(source, createPath("src/foo/bar/File.kt"))
+                val findings = InvalidPackageDeclaration(config).lint(ktFile)
+
+                assertThat(findings).hasSize(1)
+            }
+
             it("should report if file path matches root package but package declaration differs") {
                 val source = """
                     package io.foo.bar