Add function to calculate a unique base36 string for each item in a list.

The base36 string is either 3 or 4 characters large.

topydo/lib/HashListValues.py

+# Topydo - A todo.txt client written in Python.
+# Copyright (C) 2014 Bram Schoenmakers <me@bramschoenmakers.nl>
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+from math import ceil, log
+
+_table_sizes = {
+    # we choose a large table size reduce the change of collisions
+    3: 46649, # largest prime under zzz_36
+    4: 1679609 # largest prime under zzzz_36
+}
+
+def _to_base36(p_value):
+    """
+    Converts integer to base36 string.
+
+    Based on answer on
+    https://stackoverflow.com/questions/1181919/python-base-36-encoding
+    """
+    alphabet = '0123456789abcdefghijklmnopqrstuvwxyz'
+
+    base36 = ''
+    while p_value:
+        p_value, i = divmod(p_value, 36)
+        base36 = alphabet[i] + base36
+
+    return base36 or alphabet[0]
+
+def hash_list_values(p_list, p_hash=hash):
+    """
+    Calculates a unique value for each item in the list, these can be used as
+    identifiers.
+
+    The value is based on hashing an item using the p_hash function.
+
+    Suitable for lists not larger than approx. 16K items
+
+    Returns a tuple with the status and a list of tuples where each item is
+    combined with the ID.
+    """
+    result = []
+    used = set()
+
+    # choose a larger key size if there's >1% chance of collision
+    size = _table_sizes[3] if len(p_list) < _table_sizes[3] * 0.01 else table_sizes[4]
+
+    for item in p_list:
+        hash_value = p_hash(item) % size
+
+        while hash_value in used:
+            hash_value = (hash_value + 1) % size
+
+        used.add(hash_value)
+        result.append((item, _to_base36(hash_value)))
+
+    return result
+