From 31eaa23bb8d818c0f6f9d514e41f7da85029d533 Mon Sep 17 00:00:00 2001
From: "Sybren A. St?vel" <sybren@stuvel.eu>
Date: Sun, 10 Jul 2011 01:35:46 +0200
Subject: Reinstated some of the backed out changeset 812d745b6bef:     - added
 support for block size     - improved some exceptions

---
 rsa/transform.py | 41 ++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 40 insertions(+), 1 deletion(-)

(limited to 'rsa/transform.py')

diff --git a/rsa/transform.py b/rsa/transform.py
index 35b7461..076c385 100755
--- a/rsa/transform.py
+++ b/rsa/transform.py
@@ -9,8 +9,22 @@ import types
 def bit_size(number):
     """Returns the number of bits required to hold a specific long number"""
 
+    if number < 0:
+        raise ValueError('Only nonnegative numbers possible: %s' % number)
+
+    if number == 0:
+        return 1
+    
     return int(math.ceil(math.log(number, 2)))
 
+def byte_size(number):
+    """Returns the number of bytes required to hold a specific long number.
+    
+    The number of bytes is rounded up.
+    """
+
+    return int(math.ceil(bit_size(number) / 8.0))
+
 def bytes2int(bytes):
     """Converts a list of bytes or an 8-bit string to an integer.
 
@@ -72,7 +86,7 @@ def int2bytes(number, block_size=None):
 
     # Do some bounds checking
     if block_size is not None:
-        needed_bytes = int(math.ceil(bit_size(number) / 8.0))
+        needed_bytes = byte_size(number)
         if needed_bytes > block_size:
             raise OverflowError('Needed %i bytes for number, but block size '
                 'is %i' % (needed_bytes, block_size))
@@ -92,6 +106,31 @@ def int2bytes(number, block_size=None):
     return padding + ''.join(bytes)
 
 
+def block_op(block_provider, block_size, operation):
+    r'''Generator, applies the operation on each block and yields the result
+    
+    Each block is converted to a number, the given operation is applied and then
+    the resulting number is converted back to a block of data. The resulting
+    block is yielded.
+    
+    @param block_provider: an iterable that iterates over the data blocks.
+    @param block_size: the used block size
+    @param operation: a function that accepts an integer and returns an integer 
+    
+    >>> blocks = ['\x00\x01\x02', '\x03\x04\x05']
+    >>> list(block_op(blocks, 3, lambda x: (x + 6)))
+    ['\x00\x01\x08', '\x03\x04\x0b']
+    
+    '''
+
+    for block in block_provider:
+        number = bytes2int(block)
+        print 'In : %i (%i bytes)' % (number, byte_size(number))
+        after_op = operation(number)
+        print 'Out: %i (%i bytes)' % (after_op, byte_size(after_op))
+        yield int2bytes(after_op, block_size)
+
+
 def to64(number):
     """Converts a number in the range of 0 to 63 into base 64 digit
     character in the range of '0'-'9', 'A'-'Z', 'a'-'z','-','_'.
-- 
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