Add _find_bin_for_hash() helper to repository_tool

Add a helper function to determine the name of a bin that a hashed
targetfile will be delegated to.

Based sketches by Lukas Puehringer in issues theupdateframework#994 & theupdateframework#995

Signed-off-by: Joshua Lock <jlock@vmware.com>

tuf/repository_tool.py

@@ -2824,6 +2824,59 @@ def _get_hash(target_filepath):
 
 
 
+def _create_bin_name(low, high, prefix_len):
+ """
+ <Purpose>
+ Create a string name of a delegated hash bin, where name will be a range of
+ zero-padded (up to prefix_len) strings i.e. for low=00, high=07,
+ prefix_len=3 the returned name would be '000-007'.
+
+ """
+ if low == high:
+ return "{low:0{len}x}".format(low=low, len=prefix_len)
+
+ return "{low:0{len}x}-{high:0{len}x}".format(low=low, high=high,
+ len=prefix_len)
+
+
+
+
+
+def _find_bin_for_hash(path_hash, number_of_bins):
+ """
+ <Purpose>
+ For a given hashed filename, path_hash, calculate the name of a hashed bin
+ into which this file would be delegated given number_of_bins bins are in
+ use.
+
+ <Arguments>
+ path_hash:
+ The hash of the target file's path
+
+ number_of_bins:
+ The number of hashed_bins in use
+
+ <Returns>
+ The name of the hashed bin path_hash would be binned into.
+ """
+
+ prefix_len = len("{:x}".format(number_of_bins - 1))
+ prefix_count = 16 ** prefix_len
+
+ if prefix_count % number_of_bins != 0:
+ raise securesystemslib.exceptions.Error('The "number_of_bins" argument'
+ ' must be a power of 2.')
+
+ bin_size = prefix_count // number_of_bins
+ prefix = int(path_hash[:prefix_len], 16)
+
+ low = prefix - (prefix % bin_size)
+ high = (low + bin_size - 1)
+
+ return _create_bin_name(low, high, prefix_len)
+
+
+
 
 
 def create_new_repository(repository_directory, repository_name='default'):