Factor out bin numbers calculations into a helper

Factor out code to calculate the prefix length, total number of prefixes
and number of prefixes to be allocated to each bin into a helper
function _get_bin_numbers() so that we a) do not duplicate logic,
b) can document the logic in one place and c) ensure the logic is
consistent.

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

tuf/repository_tool.py

@@ -2494,35 +2494,19 @@ def delegate_hashed_bins(self, list_of_targets, keys_of_hashed_bins,
  securesystemslib.formats.ANYKEYLIST_SCHEMA.check_match(keys_of_hashed_bins)
  tuf.formats.NUMBINS_SCHEMA.check_match(number_of_bins)
 
- # Convert 'number_of_bins' to hexadecimal and determine the number of
- # hexadecimal digits needed by each hash prefix. Calculate the total
- # number of hash prefixes (e.g., 000 - FFF total values) to be spread over
- # 'number_of_bins'. Example: number_of_bins = 32,
- # total_hash_prefixes = 256, and each hashed bin is responsible for 8 hash
- # prefixes. Hashed bin roles created = 00-07.json, 08-0f.json, ...,
- # f8-ff.json.
- prefix_length = len("{:x}".format(number_of_bins - 1))
- total_hash_prefixes = 16 ** prefix_length
- bin_size = total_hash_prefixes // number_of_bins
-
- # For simplicity, ensure that 'total_hash_prefixes' (16 ^ n) can be evenly
- # distributed over 'number_of_bins' (must be 2 ^ n). Each bin will contain
- # (total_hash_prefixes / number_of_bins) hash prefixes.
- if total_hash_prefixes % number_of_bins != 0:
- raise securesystemslib.exceptions.Error('The "number_of_bins" argument'
- ' must be a power of 2.')
+ prefix_length, prefix_count, bin_size = _get_bin_numbers(number_of_bins)
 
  logger.info('Creating hashed bin delegations.\n' +
  repr(len(list_of_targets)) + ' total targets.\n' +
  repr(number_of_bins) + ' hashed bins.\n' +
- repr(total_hash_prefixes) + ' total hash prefixes.\n' +
+ repr(prefix_count) + ' total hash prefixes.\n' +
  'Each bin ranges over ' + repr(bin_size) + ' hash prefixes.')
 
  # Generate a list of bin names, the range of prefixes to be delegated to
  # that bin, along with the corresponding full list of target prefixes
  # to be delegated to that bin
  ordered_roles = []
- for idx in range(0, total_hash_prefixes, bin_size):
+ for idx in range(0, prefix_count, bin_size):
  high = idx + bin_size - 1
  name = _create_bin_name(idx, high, prefix_length)
  if bin_size == 1:
@@ -2747,6 +2731,38 @@ def _create_bin_name(low, high, prefix_len):
 
 
 
+def _get_bin_numbers(number_of_bins):
+ """
+ Given the desired number of bins (number_of_bins) calculate the prefix length
+ (prefix_length), total number of prefixes (prefix_count) and the number of
+ prefixes to be stored in each bin (bin_size).
+ Example: number_of_bins = 32
+ prefix_length = 2
+ prefix_count = 256
+ bin_size = 8
+ That is, each of the 32 hashed bins are responsible for 8 hash prefixes, i.e.
+ 00-07, 08-0f, ..., f8-ff.
+ """
+ # Convert 'number_of_bins' to hexadecimal and determine the number of
+ # hexadecimal digits needed by each hash prefix
+ prefix_length = len("{:x}".format(number_of_bins - 1))
+ # Calculate the total number of hash prefixes (e.g., 000 - FFF total values)
+ prefix_count = 16 ** prefix_length
+ # Determine how many prefixes to assign to each bin
+ bin_size = prefix_count // number_of_bins
+
+ # For simplicity, ensure that 'prefix_count' (16 ^ n) can be evenly
+ # distributed over 'number_of_bins' (must be 2 ^ n). Each bin will contain
+ # (prefix_count / number_of_bins) hash prefixes.
+ if prefix_count % number_of_bins != 0:
+ raise securesystemslib.exceptions.Error('The "number_of_bins" argument'
+ ' must be a power of 2.')
+
+ return prefix_length, prefix_count, bin_size
+
+
+
+
 def _find_bin_for_hash(path_hash, number_of_bins):
  """
  <Purpose>
@@ -2765,22 +2781,14 @@ def _find_bin_for_hash(path_hash, number_of_bins):
  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:
- # Note: doesn't guarantee a power of two for any x and y, but does work
- # due to the relationship between nuber_of_bins and prefix_count
- raise securesystemslib.exceptions.Error('The "number_of_bins" argument'
- ' must be a power of 2.')
+ prefix_length, _, bin_size = _get_bin_numbers(number_of_bins)
 
- bin_size = prefix_count // number_of_bins
- prefix = int(path_hash[:prefix_len], 16)
+ prefix = int(path_hash[:prefix_length], 16)
 
  low = prefix - (prefix % bin_size)
  high = (low + bin_size - 1)
 
- return _create_bin_name(low, high, prefix_len)
+ return _create_bin_name(low, high, prefix_length)