FilesExpand file tree

 master

/

arithmetic-compress.py

Copy path

Blame

More file actions

Blame

More file actions

Latest commit

 

History

History

History

75 lines (60 loc) · 2.27 KB

 master

/

arithmetic-compress.py

Copy path

Top

File metadata and controls

• Code
• Blame

75 lines (60 loc) · 2.27 KB

Raw

Copy raw file

Download raw file

Open symbols panel

Edit and raw actions

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

61

62

63

64

65

66

67

68

69

70

71

72

73

74

75

#

# Compression application using static arithmetic coding

#

# Usage: python arithmetic-compress.py InputFile OutputFile

# Then use the corresponding arithmetic-decompress.py application to recreate the original input file.

# Note that the application uses an alphabet of 257 symbols - 256 symbols for the byte

# values and 1 symbol for the EOF marker. The compressed file format starts with a list

# of 256 symbol frequencies, and then followed by the arithmetic-coded data.

#

# Copyright (c) Project Nayuki

#

# https://www.nayuki.io/page/reference-arithmetic-coding

# https://github.com/nayuki/Reference-arithmetic-coding

#

import contextlib, sys

import arithmeticcoding

# Command line main application function.

def main(args):

# Handle command line arguments

if len(args) != 2:

sys.exit("Usage: python arithmetic-compress.py InputFile OutputFile")

inputfile, outputfile = args

# Read input file once to compute symbol frequencies

freqs = get_frequencies(inputfile)

freqs.increment(256) # EOF symbol gets a frequency of 1

# Read input file again, compress with arithmetic coding, and write output file

with open(inputfile, "rb") as inp, \

contextlib.closing(arithmeticcoding.BitOutputStream(open(outputfile, "wb"))) as bitout:

write_frequencies(bitout, freqs)

compress(freqs, inp, bitout)

# Returns a frequency table based on the bytes in the given file.

# Also contains an extra entry for symbol 256, whose frequency is set to 0.

def get_frequencies(filepath):

freqs = arithmeticcoding.SimpleFrequencyTable([0] * 257)

with open(filepath, "rb") as input:

while True:

b = input.read(1)

if len(b) == 0:

break

freqs.increment(b[0])

return freqs

def write_frequencies(bitout, freqs):

for i in range(256):

write_int(bitout, 32, freqs.get(i))

def compress(freqs, inp, bitout):

enc = arithmeticcoding.ArithmeticEncoder(32, bitout)

while True:

symbol = inp.read(1)

if len(symbol) == 0:

break

enc.write(freqs, symbol[0])

enc.write(freqs, 256) # EOF

enc.finish() # Flush remaining code bits

# Writes an unsigned integer of the given bit width to the given stream.

def write_int(bitout, numbits, value):

for i in reversed(range(numbits)):

bitout.write((value >> i) & 1) # Big endian

# Main launcher

if __name__ == "__main__":

main(sys.argv[1 : ])