src/main/kotlin/filesystem/FSCreator.kt

@@ -1,21 +1,30 @@
 package filesystem
 
 import java.nio.file.FileAlreadyExistsException
-import java.nio.file.FileSystemException
 import java.nio.file.Files
 import java.nio.file.Path
 
 class FSCreator {
     /**
      * Create entry, leaving existing folders' contents, but overwriting existing files.
+     * @throws CyclicFolderException Cyclic folders cannot be created.
      */
-    @Throws(FileSystemException::class)
+    @Throws(CyclicFolderException::class)
     fun create(
         entryToCreate: FSEntry,
         destination: String,
     ) {
-        val queue = ArrayDeque<Pair<FSEntry, Path>>()
-        queue.add(entryToCreate to Path.of(destination))
+        // No point in running anything if we know the input is invalid.
+        if (entryToCreate is FSFolder && entryToCreate.isCyclic()) {
+            throw CyclicFolderException()
+        }
+
+        val queue =
+            ArrayDeque(
+                listOf(
+                    entryToCreate to Path.of(destination),
+                ),
+            )
 
         while (queue.isNotEmpty()) {
             val (entry, dest) = queue.removeFirst()
@@ -33,4 +42,6 @@ class FSCreator {
             }
         }
     }
-}
+}
+
+class CyclicFolderException : Exception("Cyclic FSFolders are not supported")

src/main/kotlin/filesystem/FSEntry.kt

@@ -6,4 +6,22 @@ sealed class FSEntry(val name: String)
 
 class FSFile(name: String, val content: String) : FSEntry(name)
 
-class FSFolder(name: String, val entries: List<FSEntry>) : FSEntry(name)
+class FSFolder(name: String, val entries: List<FSEntry>) : FSEntry(name) {
+    /**
+     * Check whether a folder is cyclic.
+     */
+    fun isCyclic(): Boolean {
+        val seen = listOf(this).toHashSet<FSEntry>()
+        val queue = ArrayDeque(entries)
+        while (queue.isNotEmpty()) {
+            val entry = queue.removeFirst()
+            if (!seen.add(entry)) {
+                return true
+            }
+            if (entry is FSFolder) {
+                queue.addAll(entry.entries)
+            }
+        }
+        return false
+    }
+}

src/test/kotlin/filesystem/FSCreatorTest.kt

@@ -1,7 +1,6 @@
 package filesystem
 
 import org.junit.jupiter.api.*
-import java.nio.file.FileSystemException
 import java.nio.file.Files
 import java.nio.file.Path
 import java.util.concurrent.TimeUnit
@@ -81,7 +80,7 @@ class FSCreatorTest {
                                 FSFile("hi", "hi"),
                             ),
                         ),
-                        FSFolder("another-folder", listOf()),
+                        FSFolder("folder", listOf()),
                         FSFile("1.txt", "One!"),
                         FSFile("2.txt", "Two!"),
                     ),
@@ -95,7 +94,7 @@ class FSCreatorTest {
                     "folder",
                     listOf(
                         FSFolder(
-                            "another-folder",
+                            "folder",
                             listOf(
                                 FSFolder(
                                     "secrets",
@@ -113,22 +112,36 @@ class FSCreatorTest {
             )
         }
         assertEquals("hi", Files.readString(Path.of("_tmp/folder/sub-folder/hi")))
-        assertEquals("P4ssW0rd", Files.readString(Path.of("_tmp/folder/another-folder/secrets/secret")))
+        assertEquals("P4ssW0rd", Files.readString(Path.of("_tmp/folder/folder/secrets/secret")))
         assertEquals("One is a good number", Files.readString(Path.of("_tmp/folder/1.txt")))
         assertEquals("Two!", Files.readString(Path.of("_tmp/folder/2.txt")))
         assertEquals("Three!", Files.readString(Path.of("_tmp/folder/3.txt")))
     }
 
     @Test
-    @Timeout(500, unit = TimeUnit.MILLISECONDS) // in case implementation starts trying to handle recursion
-    fun `create throws on recursive folder`() {
+    @Timeout(500, unit = TimeUnit.MILLISECONDS) // in case implementation starts trying to handle cyclic folders
+    fun `create throws on cyclic folder`() {
         val files = mutableListOf<FSEntry>()
         val folder = FSFolder("folder", files)
         files.add(folder)
-        assertThrows<FileSystemException> {
+        assertThrows<CyclicFolderException> {
             creator.create(folder, "_tmp")
         }
     }
+
+    @Test
+    @Timeout(500, unit = TimeUnit.MILLISECONDS)
+    fun `create throws on long cyclic folder`() {
+        val files = mutableListOf<FSEntry>()
+        val folder1 = FSFolder("folder", files)
+        val folder2 = FSFolder("folder2", listOf(folder1))
+        val folder3 = FSFolder("folder3", listOf(folder2))
+        val folder4 = FSFolder("folder4", listOf(folder3))
+        files.add(folder4)
+        assertThrows<CyclicFolderException> {
+            creator.create(folder4, "_tmp")
+        }
+    }
 }
 
 fun deleteRecursive(path: Path) {

src/test/kotlin/filesystem/FSEntryTest.kt

@@ -0,0 +1,42 @@
+package filesystem
+
+import org.junit.jupiter.api.Test
+import kotlin.test.assertFalse
+import kotlin.test.assertTrue
+
+class FSEntryTest {
+    @Test
+    fun `non-cyclic folder`() {
+        val folder =
+            FSFolder(
+                "folder",
+                listOf(
+                    FSFolder("folder", listOf()),
+                    FSFile("text.txt", "Hello!"),
+                ),
+            )
+        assertFalse(folder.isCyclic())
+    }
+
+    @Test
+    fun `cyclic folder`() {
+        val files = mutableListOf<FSEntry>()
+        val folder = FSFolder("folder", files)
+        files.add(folder)
+        assertTrue(folder.isCyclic())
+    }
+
+    @Test
+    fun `long cyclic folder`() {
+        val files = mutableListOf<FSEntry>()
+        val folder1 = FSFolder("folder", files)
+        val folder2 = FSFolder("folder2", listOf(folder1))
+        val folder3 = FSFolder("folder3", listOf(folder2))
+        val folder4 = FSFolder("folder4", listOf(folder3))
+        files.add(folder4)
+        assertTrue(folder1.isCyclic())
+        assertTrue(folder2.isCyclic())
+        assertTrue(folder3.isCyclic())
+        assertTrue(folder4.isCyclic())
+    }
+}