Deny cyclic FSFolders (#3)

Contributes to #2 .

Handle cyclic folders explicitly, instead of relying on the filesystem.

Reviewed-on: #3

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
+    }
+}