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Revised Compression Method #2

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merged 5 commits into from
May 11, 2017
Merged

Revised Compression Method #2

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fc1212b
compress may return less than target bytes
Apr 5, 2013
26b3692
Revised compression method
Apr 5, 2013
6ad5c71
Compression method comments
lafncow May 9, 2017
7c850d0
Merge branch 'master' into PyPI
lafncow May 10, 2017
de4fe43
Updated tests
lafncow May 10, 2017
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52 changes: 30 additions & 22 deletions humanhash.py
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Original file line number Diff line number Diff line change
Expand Up @@ -7,6 +7,7 @@

import operator
import uuid as uuidlib
import math
import sys

if sys.version_info.major == 3:
Expand Down Expand Up @@ -103,7 +104,7 @@ def humanize_list(self, hexdigest, words=4):

>>> digest = '60ad8d0d871b6095808297'
>>> HumanHasher().humanize_list(digest)
['sodium', 'magnesium', 'nineteen', 'hydrogen']
['equal', 'monkey', 'lake', 'beryllium']
"""
# Gets a list of byte values between 0-255.
bytes_ = map(lambda x: int(x, 16),
Expand All @@ -122,11 +123,11 @@ def humanize(self, hexdigest, words=4, separator='-'):

>>> digest = '60ad8d0d871b6095808297'
>>> HumanHasher().humanize(digest)
'sodium-magnesium-nineteen-hydrogen'
'equal-monkey-lake-beryllium'
>>> HumanHasher().humanize(digest, words=6)
'hydrogen-pasta-mississippi-august-may-lithium'
'sodium-magnesium-nineteen-william-alanine-nebraska'
>>> HumanHasher().humanize(digest, separator='*')
'sodium*magnesium*nineteen*hydrogen'
'equal*monkey*lake*beryllium'
"""
# Map the compressed byte values through the word list.
return separator.join(self.humanize_list(hexdigest, words))
Expand All @@ -139,31 +140,38 @@ def compress(bytes_, target):

>>> bytes_ = [96, 173, 141, 13, 135, 27, 96, 149, 128, 130, 151]
>>> list(HumanHasher.compress(bytes_, 4))
[205, 128, 156, 96]
[64, 145, 117, 21]

Attempting to compress a smaller number of bytes to a larger number is
an error:
If there are less than the target number bytes, return input bytes

>>> HumanHasher.compress(bytes_, 15) # doctest: +ELLIPSIS
Traceback (most recent call last):
...
ValueError: Fewer input bytes than requested output
>>> list(HumanHasher.compress(bytes_, 15)) # doctest: +ELLIPSIS
[96, 173, 141, 13, 135, 27, 96, 149, 128, 130, 151]
"""

bytes_list = list(bytes_)

length = len(bytes_list)
if target > length:
raise ValueError("Fewer input bytes than requested output")
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I'm curious, why did you remove this check?

I might flip this check back in the next version.

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I removed this check to make the function more generous in what it accepts. I changed the comments to reflect the change. My thinking is that the purpose of compression is to ensure that there are no more than the target number of bytes, which is satisfied if target > length. So now if target > length, the original bytes are returned since there is no compression needed.

I do understand that this is a different behavior. The impact on end users is that hashing small inputs will not cause an error, which I think is more intuitive. Another solution would be to pad input bytes up to the target number, but this would lead to padded final outputs.

For example:

digest = '010203'

# Old method
humanhash.humanize(digest, 6)
Traceback (most recent call last):
...
ValueError: Fewer input bytes than requested output

# New method
humanhash.humanize(digest, 6)
'alabama-alanine-alaska'

# Padding method
humanhash.humanize(digest, 6)
'alabama-alanine-alaska-ack-ack-ack'

It's your call, I'm happy to help if I can. :)


# Split `bytes` into `target` segments.
seg_size = length // target
segments = [bytes_list[i * seg_size:(i + 1) * seg_size]
for i in range(target)]
# Catch any left-over bytes in the last segment.
segments[-1].extend(bytes_list[target * seg_size:])

return map(checksum, segments)
# If there are less than the target number bytes, return input bytes
if target >= length:
return bytes_

# Split `bytes` evenly into `target` segments
# Each segment hashes `seg_size` bytes, rounded down for some
seg_size = float(length) / float(target)
# Initialize `target` number of segments
segments = [0] * target
seg_num = 0

# Use a simple XOR checksum-like function for compression
for i, byte in enumerate(bytes_list):
# Divide the byte index by the segment size to assign its segment
# Floor to create a valid segment index
# Min to ensure the index is within `target`
seg_num = min(int(math.floor(i / seg_size)), target-1)
# Apply XOR to the existing segment and the byte
segments[seg_num] = operator.xor(segments[seg_num], byte)

return segments

def uuid(self, **params):

Expand Down