<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _get_purecn_files(paired, work_dir, require_exist=False):
 """Retrieve organized structure of PureCN output files. """

    out_base = os.path.join(work_dir, "%s-purecn" % (dd.get_sample_name(paired.tumor_data)))
    out = {"plot": {}}
    all_files = []
    for plot in ["chromosomes", "local_optima", "segmentation", "summary"]:
        if plot == "summary":
            cur_file = "%s.pdf" % out_base
        else:
            cur_file = "%s_%s.pdf" % (out_base, plot)
        if not require_exist or os.path.exists(cur_file):
            out["plot"][plot] = cur_file
            all_files.append(os.path.basename(cur_file))
    for key, ext in [["hetsummary", ".csv"], ["dnacopy", "_dnacopy.seg"], ["genes", "_genes.csv"],
                     ["log", ".log"], ["loh", "_loh.csv"], ["rds", ".rds"],
                     ["variants", "_variants.csv"]]:
        cur_file = "%s%s" % (out_base, ext)
        if not require_exist or os.path.exists(cur_file):
            out[key] = cur_file
            all_files.append(os.path.basename(cur_file))
    return out_base, out, all_files