#!/usr/bin/env python3
# kcs_decode.py
#
# Author : David Beazley (http://www.dabeaz.com)
# Copyright (C) 2010
#
# Requires Python 3

"""
Converts a WAV file containing Kansas City Standard data and
extracts text data from it. See:

http://en.wikipedia.org/wiki/Kansas_City_standard
"""

from collections import deque
from itertools import islice

# Generate a sequence representing sign bits
def generate_wav_sign_change_bits(wavefile):
    samplewidth = wavefile.getsampwidth()
    nchannels = wavefile.getnchannels()
    previous = 0
    while True:
        frames = wavefile.readframes(8192)
        if not frames:
            break

        # Extract most significant bytes from left-most audio channel
        msbytes = bytearray(frames[samplewidth-1::samplewidth*nchannels])

        # Emit a stream of sign-change bits
        for byte in msbytes:
            signbit = byte & 0x80
            yield 1 if (signbit ^ previous) else 0
            previous = signbit

# Base frequency (representing a 1)
BASE_FREQ = 2400

# Generate a sequence of data bytes by sampling the stream of sign change bits
def generate_bytes(bitstream,framerate):
    bitmasks = [0x1,0x2,0x4,0x8,0x10,0x20,0x40,0x80]

    # Compute the number of audio frames used to encode a single data bit
    frames_per_bit = int(round(float(framerate)*8/BASE_FREQ))

    # Queue of sampled sign bits
    sample = deque(maxlen=frames_per_bit)     

    # Fill the sample buffer with an initial set of data
    sample.extend(islice(bitstream,frames_per_bit-1))
    sign_changes = sum(sample)

    # Look for the start bit
    for val in bitstream:
        if val:
            sign_changes += 1
        if sample.popleft():
            sign_changes -= 1
        sample.append(val)

        # If a start bit detected, sample the next 8 data bits
        if sign_changes <= 9:
            byteval = 0
            for mask in bitmasks:
                if sum(islice(bitstream,frames_per_bit)) >= 12:
                    byteval |= mask
            yield byteval
            # Skip the final two stop bits and refill the sample buffer 
            sample.extend(islice(bitstream,2*frames_per_bit,3*frames_per_bit-1))
            sign_changes = sum(sample)

if __name__ == '__main__':
    import wave
    import sys
    if len(sys.argv) != 2:
        print("Usage: %s infile" % sys.argv[0],file=sys.stderr)
        raise SystemExit(1)

    wf = wave.open(sys.argv[1])
    sign_changes = generate_wav_sign_change_bits(wf)
    byte_stream  = generate_bytes(sign_changes, wf.getframerate())

    # Output the byte stream in 80-byte chunks
    outf = sys.stdout.buffer.raw
    while True:
        buffer = bytes(islice(byte_stream,80))
        if not buffer:
            break
        outf.write(buffer)