python: Add initial prototype

python/passgeny/bhash.py

@@ -0,0 +1,40 @@
+import math
+
+class BhashException(Exception): pass
+
+class Bhash:
+    """
+    Hash manipulation class.
+
+    The hash is internally treated as a big-endinan integer. This allows for
+    easy implementation of operations such as modulo.
+
+    Note: The consumed bits are calculated using math.log() so the exact number
+    may be inprecise.
+    """
+    def __init__(self):
+        self.bhash = None
+        self.bits_avail = 0
+        self.bits_used = 0.0
+
+    def from_bytes(self, buf: bytes):
+        """
+        Initialize a Bhash from a bytes object
+        """
+        self.bhash = int.from_bytes(buf, byteorder='big')
+        self.bits_avail = len(buf) * 8
+        self.bits_used = 0.0
+
+    def modulo(self, mod: int) -> int:
+        """
+        Treat the hash as a bigint and divide it by mod. The hash is updated
+        with the division result while the module value is returned.
+        """
+        self.bits_used += math.log(mod) / math.log(2)
+        if self.bits_used > self.bits_avail:
+            raise BhashException("Consumed all bits in hash")
+
+        r = self.bhash % mod
+        self.bhash //= mod
+
+        return r