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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def number_of_mapped_reads(data, bam_file, keep_dups=True, bed_file=None, target_name=None):
 """Count mapped reads, allow adjustment for duplicates and BED regions. Since samtools view does not use indexes for BED files (https://github.com/samtools/samtools/issues/88) we loop over regions in a BED file and add the counts together. Uses a global cache file to store counts, making it possible to pass this single file for CWL runs. For parallel processes it can have concurrent append writes, so we have a simple file locking mechanism to avoid this. """

    # Flag explainer https://broadinstitute.github.io/picard/explain-flags.html
    callable_flags = ["not unmapped", "not mate_is_unmapped", "not secondary_alignment",
                      "not failed_quality_control"]
    if keep_dups:
        query_flags = callable_flags
        flag = 780  # not (read unmapped or mate unmapped or fails QC or secondary alignment)
    else:
        query_flags = callable_flags + ["not duplicate"]
        flag = 1804  # as above plus not duplicate

    # Back compatible cache
    oldcache_file = _backcompatible_cache_file(query_flags, bed_file, target_name, data)
    if oldcache_file:
        with open(oldcache_file) as f:
            return int(f.read().strip())

    # New cache
    key = json.dumps({"flags": sorted(query_flags),
                      "region": os.path.basename(bed_file) if bed_file else "",
                      "sample": dd.get_sample_name(data)},
                     separators=(",", ":"), sort_keys=True)
    cache_file = get_cache_file(data)
    if utils.file_exists(cache_file):
        with open(cache_file) as in_handle:
            for cur_key, cur_val in (l.strip().split("\t") for l in in_handle):
                if cur_key == key:
                    return int(cur_val)

    # Calculate stats
    count_dir = utils.safe_makedir(os.path.join(dd.get_work_dir(data), "coverage",
                                                dd.get_sample_name(data), "counts"))
    if not bed_file:
        bed_file = os.path.join(count_dir, "fullgenome.bed")
        if not utils.file_exists(bed_file):
            with file_transaction(data, bed_file) as tx_out_file:
                with open(tx_out_file, "w") as out_handle:
                    for c in ref.file_contigs(dd.get_ref_file(data), data["config"]):
                        out_handle.write("%s\t%s\t%s\n" % (c.name, 0, c.size))
    count_file = os.path.join(count_dir,
                              "%s-%s-counts.txt" % (os.path.splitext(os.path.basename(bed_file))[0], flag))
    if not utils.file_exists(count_file):
        bam.index(bam_file, data["config"], check_timestamp=False)
        num_cores = dd.get_num_cores(data)
        with file_transaction(data, count_file) as tx_out_file:
            cmd = ("hts_nim_tools count-reads -t {num_cores} -F {flag} {bed_file} {bam_file} > {tx_out_file}")
            do.run(cmd.format(**locals()), "Count mapped reads: %s" % (dd.get_sample_name(data)))
    count = 0
    with open(count_file) as in_handle:
        for line in in_handle:
            count += int(line.rstrip().split()[-1])

    with _simple_lock(cache_file):
        with open(cache_file, "a") as out_handle:
            out_handle.write("%s\t%s\n" % (key, count))
    return count