diff --git a/PFERD/report.py b/PFERD/report.py
new file mode 100644
index 0000000..382b82d
--- /dev/null
+++ b/PFERD/report.py
@@ -0,0 +1,80 @@
+from dataclasses import dataclass
+from pathlib import PurePath
+from typing import Set
+
+
+@dataclass
+class MarkDuplicateException(Exception):
+    """
+    Tried to mark a file that was already marked.
+    """
+
+    path: PurePath
+
+
+@dataclass
+class MarkConflictException(Exception):
+    """
+    Marking the path would have caused a conflict.
+
+    A conflict can have two reasons: Either the new file has the same path as
+    the parent directory of a known file, or a parent directory of the new file
+    has the same path as a known file. In either case, adding the new file
+    would require a file and a directory to share the same path, which is
+    usually not possible.
+    """
+
+    path: PurePath
+    collides_with: PurePath
+
+
+class Report:
+    """
+    A report of a synchronization. Includes all files found by the crawler, as
+    well as the set of changes made to local files.
+    """
+
+    def __init__(self):
+        self.known_files: Set[PurePath] = set()
+
+        self.new_files: Set[PurePath] = set()
+        self.changed_files: Set[PurePath] = set()
+        self.deleted_files: Set[PurePath] = set()
+
+    def mark(self, path: PurePath):
+        """
+        Mark a previously unknown file as known.
+
+        May throw a MarkDuplicateException or a MarkConflictException. For more
+        detail, see the respective exception's docstring.
+        """
+
+        for known_path in self.known_files:
+            if path == known_path:
+                raise MarkDuplicateException(path)
+
+            if path.relative_to(known_path) or known_path.relative_to(path):
+                raise MarkConflictException(path, known_path)
+
+        self.known_files.add(path)
+
+    def add_file(self, path: PurePath):
+        """
+        Unlike mark(), this function accepts any paths.
+        """
+
+        self.new_files.add(path)
+
+    def change_file(self, path: PurePath):
+        """
+        Unlike mark(), this function accepts any paths.
+        """
+
+        self.changed_files.add(path)
+
+    def delete_file(self, path: PurePath):
+        """
+        Unlike mark(), this function accepts any paths.
+        """
+
+        self.deleted_files.add(path)