exposureIdInfo.py

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#
# LSST Data Management System
# Copyright 2016 LSST Corporation.
#
# This product includes software developed by the
# LSST Project (http://www.lsst.org/).
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.    See the
# GNU General Public License for more details.
#
# You should have received a copy of the LSST License Statement and
# the GNU General Public License along with this program.  If not,
# see <http://www.lsstcorp.org/LegalNotices/>.
#

from past.builtins import long

__all__ = ["ExposureIdInfo"]


class ExposureIdInfo(object):
    """Exposure ID and number of bits used.

    Attributes include:

    expId
        exposure ID as a long int
    expBits
        maximum number of bits allowed for exposure IDs
    maxBits
        maximum number of bits available for values that combine exposure ID
        with other information, such as source ID
    unusedBits
        maximum number of bits available for non-exposure info (maxBits - expBits)

    One common use is creating an ID factory for making a source table.
    For example, given a data butler `butler` and a data ID `dataId`::

        from lsst.afw.table import IdFactory, SourceTable
        exposureIdInfo = butler.get("expIdInfo", dataId)
        sourceIdFactory = IdFactory.makeSource(exposureIdInfo.expId, exposureIdInfo.unusedBits)
        schema = SourceTable.makeMinimalSchema()
        #...add fields to schema as desired, then...
        sourceTable = SourceTable.make(self.schema, sourceIdFactory)

    At least one bit must be reserved, even if there is no exposure ID, for reasons
    that are not entirely clear (this is DM-6664).
    """

    def __init__(self, expId=0, expBits=1, maxBits=64):
        """Construct an ExposureIdInfo

        See the class doc string for an explanation of the arguments.
        """
        expId = long(expId)
        expBits = int(expBits)
        maxBits = int(maxBits)

        if expId.bit_length() > expBits:
            raise RuntimeError("expId=%s uses %s bits > expBits=%s" % (expId, expId.bit_length(), expBits))
        if maxBits < expBits:
            raise RuntimeError("expBits=%s > maxBits=%s" % (expBits, maxBits))

        self.expId = expId
        self.expBits = expBits
        self.maxBits = maxBits

    @property
    def unusedBits(self):
        return self.maxBits - self.expBits