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21.6. YearCollection.__findPrev(): Find predecessor month

For the algorithm, see Section 21.5, “YearCollection.neighbors(): Find previous and next month”.

noteweb
# - - -   Y e a r C o l l e c t i o n . _ _ f i n d P r e v

    def __findPrev ( self, yyyy_mm ):
        '''Find the month preceding yyyy_mm.

          [ yyyy_mm is a month in self with key string 'yyyy-mm' ->
              if self has any months before yyyy_mm ->
                return the month key of the immediate predecessor
              else -> return None ]
        '''

First we find the YearRow containing yyyy_mm, and ask that instance if the month has a predecessor in the same year.

noteweb
        #-- 1 --
        # [ yyyy  :=  year part of yyyy_mm
        #   mm    :=  month part of yyyy_mm ]
        yyyy, mm  =  yyyy_mm.split('-')

        #-- 2 --
        # [ yyyy is a year in self ->
        #     yearRow  :=  YearRow instance for year yyyy ]
        yearRow  =  self[yyyy]

        #-- 3 --
        # [ if yearRow has a predecessor to month mm ->
        #     return that month's key as a string 'mm'
        #   else -> I ]
        try:
            prev  =  yearRow.predecessor ( mm )
            return '%s-%s' % (yyyy, prev)
        except KeyError:
            pass

If there is a predecessor, it must be in a previous year. First we form a list of the year numbers in descending order. Then we find the position of yyyy in that list, and step through the remaining elements (if any) until we find a year that has at least one month in it, and return the last month.

noteweb
        #-- 4 --
        # [ yyyyList  :=  all year key strings in self, sorted
        #                 in descending order ]
        yyyyList  =  self.__yearMap.keys()
        yyyyList.sort()
        yyyyList.reverse()

        #-- 5 --
        # [ yyyy is an element of yyyyList ->
        #     pos  :=  yyyy's position in yyyyList ]
        pos  =  yyyyList.index ( yyyy )

For the method that looks for the last month of a year, see Section 22.5, “YearRow.lastMonth(): Return the last month”.

noteweb
        #-- 6 --
        # [ if any year with a key in yyyyList[pos+1:] has at
        #   least one month in it ->
        #     return the yyyy-mm key of the last month in the
        #     first such year
        #   else -> I ]
        for prevYear in yyyyList[pos+1:]:
            prevRow  =  self[prevYear]
            if len(prevRow) > 0:
                lastMM  =  prevRow.lastMonth()
                return '%s-%s' % (prevYear, lastMM)

        #-- 7 --
        return None