diff --git a/resizehfs.py b/resizehfs.py
new file mode 100755
index 0000000..8a514f7
--- /dev/null
+++ b/resizehfs.py
@@ -0,0 +1,127 @@
+#!/usr/bin/env python3
+
+# MIT License
+
+# Copyright (c) 2018 Elliot Nunn
+
+# Permission is hereby granted, free of charge, to any person obtaining a copy
+# of this software and associated documentation files (the "Software"), to deal
+# in the Software without restriction, including without limitation the rights
+# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+# copies of the Software, and to permit persons to whom the Software is
+# furnished to do so, subject to the following conditions:
+
+# The above copyright notice and this permission notice shall be included in all
+# copies or substantial portions of the Software.
+
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+# SOFTWARE.
+
+
+import struct
+
+
+def alignment(num):
+    # count trailing zero bits to a max of 32
+    for i in reversed(range(33)):
+        if num & ((1 << i) - 1) == 0: return i
+
+
+def bitmap_len(drNmAlBlks):
+    # how many bytes occupied by the 512b blocks required to make up n bits?
+    num_512s = (drNmAlBlks + 4095) // 4096
+    return num_512s * 512
+
+
+def resize(img, freespace):
+    drVBMSt, = struct.unpack_from('>H', img, 0x40E)
+    drNmAlBlks, = struct.unpack_from('>H', img, 0x412)
+    drAlBlkSiz, = struct.unpack_from('>L', img, 0x414)
+    drAlBlSt, = struct.unpack_from('>H', img, 0x41C)
+    drFreeBks, = struct.unpack_from('>H', img, 0x422)
+
+    align = alignment(drAlBlSt)
+
+    header = bytearray(img[:0x600])
+    bitmap = img[drVBMSt*512:drVBMSt*512+bitmap_len(drNmAlBlks)]
+    guts = img[drAlBlSt*512:drAlBlSt*512+drNmAlBlks*drAlBlkSiz]
+
+    # Decide how much to shrink
+    drFreeBks -= drNmAlBlks
+    for drNmAlBlks in range(drNmAlBlks, 0, -1):
+        byte = bitmap[(drNmAlBlks-1) >> 3]
+        mask = 0x80 >> ((drNmAlBlks-1) & 7)
+        if byte & mask: break
+    drFreeBks += drNmAlBlks
+
+    # Decide how much to expand
+    while drNmAlBlks < 0xffff and drFreeBks * drAlBlkSiz < freespace:
+        drFreeBks += 1
+        drNmAlBlks += 1
+
+    # Resize components
+    bitmap = bitmap[:bitmap_len(drNmAlBlks)].ljust(bitmap_len(drNmAlBlks), b'\0')
+    guts = guts[:drNmAlBlks*drAlBlkSiz].ljust(drNmAlBlks*drAlBlkSiz, b'\0')
+
+    # Reposition the guts after the bitmap, preserving original alignment
+    drAlBlSt = 3 + len(bitmap)//512
+    while alignment(drAlBlSt) < align: drAlBlSt += 1
+
+    struct.pack_into('>H', header, 0x40E, len(header)//512) # drVBMSt
+    struct.pack_into('>H', header, 0x412, drNmAlBlks)
+    struct.pack_into('>H', header, 0x41C, drAlBlSt)
+    struct.pack_into('>H', header, 0x422, drFreeBks)
+
+    accum = bytearray()
+    accum.extend(header)
+    accum.extend(bitmap)
+    accum.extend(bytes(drAlBlSt*512 - len(accum)))
+    accum.extend(guts)
+    accum.extend(header[0x400:0x800])
+    accum.extend(bytes(512))
+
+    return accum
+
+
+if __name__ == '__main__':
+    import sys
+    import argparse
+
+    parser = argparse.ArgumentParser(description='''
+        Resize an HFS disk image by adding/removing allocation blocks.
+        Warning: only images from `machfs` can be shrunk, because the
+        real MacOS uses the allocation blocks at the end of the disk.
+    ''')
+
+    parser.add_argument('diskimage', metavar='PATH', action='store', help='disk image')
+    parser.add_argument('--freespace', metavar='BYTES', action='store', type=int, default=0, help='free space target (0 means shrink)')
+    parser.add_argument('--verbose', '-v', action='store_true', help='print stats')
+
+    args = parser.parse_args()
+
+    with open(args.diskimage, 'rb') as f:
+        oldimage = f.read()
+
+    if oldimage[1024:1026] != b'BD': sys.exit('Not an HFS volume')
+
+    newimage = resize(oldimage, args.freespace)
+
+    if args.verbose:
+        for kind, img in (('old', oldimage), ('new', newimage)):
+            drNmAlBlks, = struct.unpack_from('>H', img, 0x412)
+            drFreeBks, = struct.unpack_from('>H', img, 0x422)
+            print(kind, 'bytes:      ', len(img))
+            print(kind, 'drNmAlBlks: ', drNmAlBlks)
+            print(kind, 'drFreeBks:  ', drFreeBks)
+
+        print('percentage size:', 100 * len(newimage) // len(oldimage))
+        print('changed:        ', ('no' if newimage == oldimage else 'yes'))
+
+    if newimage != oldimage:
+        with open(args.diskimage, 'wb') as f:
+            f.write(newimage)