diff --git a/2024/09/main.py b/2024/09/main.py
new file mode 100644
index 0000000..4e5d8fc
--- /dev/null
+++ b/2024/09/main.py
@@ -0,0 +1,123 @@
+#!/usr/bin/env python
+# -*- encoding: utf-8; py-indent-offset: 4 -*-
+
+# Author: Sebastian Mark
+# CC-BY-SA (https://creativecommons.org/licenses/by-sa/4.0/deed.de)
+
+# pylint: disable=missing-module-docstring,missing-function-docstring,consider-using-f-string
+
+
+def readinput():
+    with open("input", "r", encoding="utf-8") as file:
+        lines = [int(digit) for digit in file.read().rstrip("\n")]
+
+    return lines
+
+
+def expand_map(diskmap: list) -> list:
+    fileblocks = []
+    fileid = 0
+
+    for i, filesize in enumerate(diskmap):
+        # when processing file, use fileid as block, otherwise use free space indicator
+        block = str(fileid) if i % 2 == 0 else "."
+        # when processing file, count fileid up
+        fileid += i % 2 == 0
+        # create as many new blocks as the file needs
+        fileblocks.extend([block] * filesize)
+
+    return fileblocks
+
+
+def defragment_blocks(fileblocks: list) -> list:
+    defragmented = fileblocks.copy()
+    freespace = 0
+
+    for pos in range(len(defragmented) - 1, 0, -1):
+        # search next free space, start from prev free space
+        freespace = defragmented.index(".", freespace)
+        # abort if no more free space before current pos
+        if pos < freespace:
+            break
+        # swap current fileblock with free space
+        defragmented[freespace], defragmented[pos] = (
+            defragmented[pos],
+            defragmented[freespace],
+        )
+
+    return defragmented
+
+
+def defragment_files(fileblocks: list) -> list:
+    defragmented = fileblocks.copy()
+
+    # find all fileblocks and try to fit them into a free space
+    file_end = len(fileblocks) - 1
+    while file_end > 0:
+        if fileblocks[file_end] == ".":
+            file_end -= 1
+            continue
+
+        # get blocks for file
+        file_length = 1
+        while fileblocks[file_end] == fileblocks[file_end - file_length]:
+            file_length += 1
+        file_start = file_end - file_length + 1
+        file = fileblocks[file_start : file_end + 1]
+
+        # search free space and try to fit the file in it
+        free_start = 0
+        while free_start < len(defragmented) - 1:
+            # find and measure size of next free block
+            # (that is to the left of file)
+            free_length = 0
+            while (
+                defragmented[free_start + free_length] == "."
+                and free_start < file_start
+            ):
+                free_length += 1
+
+            # if file fits into free block, move it
+            # and skip to next file
+            if free_length >= file_length:
+                # move block to free space
+                defragmented[free_start : free_start + file_length] = file
+                # mark block as free
+                defragmented[file_start : file_end + 1] = ["."] * file_length
+                break
+
+            # file does not fit,
+            # continue search after this block
+            free_start += 1 + free_length
+
+        file_end -= file_length
+
+    return defragmented
+
+
+def calc_checksum(fileblocks: list) -> list:
+    checksum = 0
+    for i, block in enumerate(fileblocks):
+        if block == ".":  # skip free space
+            continue
+        checksum += i * int(block)
+    return checksum
+
+
+def main():
+    diskmap = readinput()
+    fileblocks = expand_map(diskmap)
+
+    # part1
+    defragmented_blocks = defragment_blocks(fileblocks)
+    checksum = calc_checksum(defragmented_blocks)
+    print("Filesystem checksum (part 1): %d" % checksum)
+
+    # part2
+    defragmented_blocks = defragment_files(fileblocks)
+    checksum = calc_checksum(defragmented_blocks)
+    print("Filesystem checksum (part 2): %d" % checksum)
+
+
+if __name__ == "__main__":
+    main()