get N random headers from a fastq file without reading the
    whole thing into memory
    modified from: http://www.biostars.org/p/6544/
    quick=True will just grab the first N reads rather than do a true
    downsampling

def downsample(f1, f2, data, N, quick=False):
    """ get N random headers from a fastq file without reading the
    whole thing into memory
    modified from: http://www.biostars.org/p/6544/
    quick=True will just grab the first N reads rather than do a true
    downsampling
    """
    if quick:
        rand_records = range(N)
    else:
        records = sum(1 for _ in open(f1)) / 4
        N = records if N > records else N
        rand_records = sorted(random.sample(xrange(records), N))

    fh1 = open_possible_gzip(f1)
    fh2 = open_possible_gzip(f2) if f2 else None
    outf1 = os.path.splitext(f1)[0] + ".subset" + os.path.splitext(f1)[1]
    outf2 = os.path.splitext(f2)[0] + ".subset" + os.path.splitext(f2)[1] if f2 else None

    if utils.file_exists(outf1):
        if not outf2:
            return outf1, outf2
        elif utils.file_exists(outf2):
            return outf1, outf2

    out_files = (outf1, outf2) if outf2 else (outf1)

    with file_transaction(out_files) as tx_out_files:
        if isinstance(tx_out_files, six.string_types):
            tx_out_f1 = tx_out_files
        else:
            tx_out_f1, tx_out_f2 = tx_out_files
        sub1 = open_possible_gzip(tx_out_f1, "w")
        sub2 = open_possible_gzip(tx_out_f2, "w") if outf2 else None
        rec_no = - 1
        for rr in rand_records:
            while rec_no < rr:
                rec_no += 1
                for i in range(4): fh1.readline()
                if fh2:
                    for i in range(4): fh2.readline()
            for i in range(4):
                sub1.write(fh1.readline())
                if sub2:
                    sub2.write(fh2.readline())
            rec_no += 1
        fh1.close()
        sub1.close()
        if f2:
            fh2.close()
            sub2.close()

    return outf1, outf2