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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/>. 

# 

 

__all__ = ["ExposureIdInfo"] 

 

 

class ExposureIdInfo(object): 

"""Exposure ID and number of bits used. 

 

Attributes include: 

 

expId 

exposure ID as an 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 = int(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