Initial sketch of the task. Needs cleanup

.gitignore

+myfilesystem.fs
+

myfs.py

+import sys
+
+if len(sys.argv) < 2:
+    # Missing filesystem name. Assuming default myfilesystem.fs
+    backingfile = 'myfilesystem.fs'
+else:
+    backingfile = sys.argv[1]
+    # one could actually use a usb stick raw at "/dev/something"
+    # Data loss, and it'll need reformatting afterwards!
+
+
+# create backingfile if needed
+try: 
+    with open(backingfile, 'x'): pass 
+except FileExistsError: pass
+
+##########################################
+##########################################
+
+# file system parameters
+TOTAL_SIZE = 0x800 # 2kB
+
+HEADER_START = 0x0
+COUNTER_BYTES = 0x1
+FILE_ENTRY_SIZE = 0x10
+
+FILE_BLOCK_SIZE = 0x100
+
+# file block 0 is the metainfo block
+# line 0 of metainfo contains the number of files
+# lines 1.. contain the filenames. 
+#     line N has the filename of the file in Block N
+
+# file blocks 1.. contain just the file contents
+
+# use "xxd myfilesystem.fs" to look at the raw contents
+
+# internal exception to signal no file
+class NoFile(Exception):
+    pass
+
+
+def format(f):
+    # 2kB filesystem
+    f.write(b'\0' * TOTAL_SIZE)
+
+
+def _get_num_files(f):
+    f.seek(HEADER_START)
+    bytes = f.read(COUNTER_BYTES)
+    num = int.from_bytes(bytes, "little", signed=False)
+    return num
+
+
+def _set_num_files(f, n):
+    f.seek(HEADER_START)
+    bytes = n.to_bytes(COUNTER_BYTES, "little", signed=False)
+    f.write(bytes)
+   
+
+def save(f, filename, content):
+    num_existing = _get_num_files(f)
+    new_fileno = num_existing + 1
+    _set_num_files(f, new_fileno)
+
+    # write filename in ftable next free entry, truncate if needed
+    f.seek(HEADER_START + FILE_ENTRY_SIZE * new_fileno)
+    f.write(filename.encode('ascii')[:FILE_ENTRY_SIZE])
+    
+    # write content to file block, truncate if needed
+    f.seek(HEADER_START + FILE_BLOCK_SIZE * new_fileno)
+    f.write(content.encode('ascii')[:FILE_BLOCK_SIZE])
+
+
+
+def find_fileno(f, filename):
+    N = _get_num_files(f)
+    for pos in range(0, N):
+        f.seek(HEADER_START + FILE_ENTRY_SIZE * (pos+1))
+        name = f.read(FILE_ENTRY_SIZE)
+        name = name.decode("ascii").strip('\0')
+        if name == filename:
+            return pos
+    else:
+        print(f"File {filename} not found", file=sys.stderr)
+        raise NoFile
+    
+
+def load(f, filename):
+    fileno = find_fileno(f, filename)
+    f.seek(HEADER_START + FILE_BLOCK_SIZE * (fileno+1))
+    content = f.read(FILE_BLOCK_SIZE)
+    content = content.decode("ascii").strip('\0')
+    # what if \0 is a valid part of the content?
+    return content
+
+def copy(f, infile, outfile):
+    try:
+        content = load(f, infile)
+    except NoFile:
+        return
+    
+    save(f, outfile, content)
+
+
+def remove(f, filename):
+    fileno = find_fileno(f, filename)
+    f.seek(HEADER_START + FILE_BLOCK_SIZE * (fileno+1))
+    f.write(b'\0' * FILE_BLOCK_SIZE)
+
+    f.seek(HEADER_START + FILE_ENTRY_SIZE * (fileno+1))
+    f.write(b'\0' * FILE_ENTRY_SIZE)
+
+
+##########################
+##########################
+
+alice_sample = """Alice was beginning to get very tired of sitting by her sister on the
+bank, and of having nothing to do: once or twice she had peeped into the
+book her sister was reading, but it had no pictures or conversations in
+it, 'and what is the use of a book,' thought Alice 'without pictures or
+conversations?'
+"""
+
+
+if __name__ == "__main__":
+    print()
+    print("*" * 60)
+    print(f"* Use xxd {backingfile} to inspect contents")
+    print("*" * 60)
+    print()
+
+    with open(backingfile, 'r+b') as f:
+        format(f)
+        save(f, "hello.txt", "Hello")
+        save(f, "alice.txt", alice_sample)
+        alice_back = load(f, "alice.txt")
+
+        print(alice_back)
+
+        copy(f, "hello.txt", "clone.txt")
+        save(f, "bye.txt", "Bye Bye")
+        copy(f, "hello.txt", "clone2.txt")
+        remove(f, "bye.txt")
+
+
+# ls(f)
+
+# save(f, A)
+# save(f, B)
+# remove(f,A)
+# save(f,C)
+#  C should re-use the free slot
+
+
+
+        # ls(f)
+
+        # error if FS full, filetable full, file too big, filename too big, etc.
+
+        # how to use the free file block after remove()?
+
+        # keep track of file sizes?
+
+        # binary file support? 
+        # 
+        # by file ending?
+
+        # make a simple REPL that understands the above commands
+        # NoFile should be handled in the REPL, REPL itself not crashing
+
+        # files bigger than 1 block
+