#!/bin/sh
#shell to repair day/month/year, assuming that days are only incremented
# by 1 or decremented by 1.
#Wish: Fix to handle arbitrary jumps
#Robert Grumbine 
#7 October 1999 -- New variant: 
#       Use case statement for control (clarity)
#       Add in some 'early out' tests
#          -- combination gives about 25% speed up and improved readability
#       N.B.: Much of the time spent is in parsing the arguments rather than
#          in the logical tree
#3 January 2001 -- Correctly handle yy of 1-9.  
#7 July 2003    -- Fix up decrement cases
#  Should really be handled by something more portable, like python RG 2018
               

#set -x

#Handle the arguments.  Verify that they are reasonable, and parse
# Them in to 'century' (first 2 digits of a 4 digit year), year,
# month, and day.  cc, yy, mo, dy respectively.
if [ $# -eq 1 ] ; then
  if [ ${#1} -eq 6 ] ; then
    yy=`echo $1 | cut -c1-2`
    mo=`echo $1 | cut -c3-4`
    dy=`echo $1 | cut -c5-6`
  elif [ ${#1} -eq 8 ] ; then
    cc=`echo $1 | cut -c1-2`
    yy=`echo $1 | cut -c3-4`
    mo=`echo $1 | cut -c5-6`
    dy=`echo $1 | cut -c7-8`
  elif [ ${#1} -eq 3 ] ; then
    yy=00
    mo=0`echo $1 | cut -c1`
    dy=`echo $1 | cut -c2-3`
  elif [ ${#1} -eq 4 ] ; then
    yy=00
    mo=`echo $1 | cut -c1-2`
    dy=`echo $1 | cut -c3-4`
  elif [ ${#1} -eq 5 ] ; then
    yy=0`echo $1 | cut -c1`
    mo=`echo $1 | cut -c2-3`
    dy=`echo $1 | cut -c4-5`
  else
    echo "Don't know how to handle a date that isn't 6 or 8 chars long!"
    echo Input date was $1
    exit -1
  fi
else
  if [ $# -eq 3 ] ; then
    cc=19
    if [ ${#1} -eq 4 ] ; then
      cc=`echo $1 | cut -c1-2`
      yy=`echo $1 | cut -c3-4`
     else
      yy=$1
      if [ $yy -lt 10 ] ; then
        yy=0$yy
      fi
    fi
    mo=$2
    dy=$3
  elif [ $# -eq 4 ] ; then
    cc=$1
    yy=$2
      if [ $yy -lt 10 ] ; then
        yy=0$yy
      fi
    mo=$3
    dy=$4
  else
    echo "Don't know how to handle other than 1, 3, or 4 arguments." 
    echo You entered $#
    exit -1
  fi
fi

#Pre-emptive check to see if there's any need to do work:
if [ $dy -gt 01 -a $dy -lt 29 ] ; then
  if [ ${#1} -eq 6 ] ; then
    echo $yy$mo$dy
  else
    echo $cc$yy$mo$dy
  fi
  exit
fi

#Have the dates, now work on adjusting for date increments
case $mo in
  01|03|05|07|08) 
    if [ $dy -eq 32 ] ; then
      mo=0`expr $mo + 1`
      dy=01
    fi
    ;;
  10)
    if [ $dy -eq 32 ] ; then
      mo=11; dy=01;
    fi
    ;;
  04|06)
    if [ $dy -eq 31 ] ; then
      mo=0`expr $mo + 1`
      dy=01
    fi
    ;;
  09|11) 
    if [ $dy -eq 31 ] ; then
      mo=`expr $mo + 1`
      dy=01
    fi
    ;;
  12) 
    if [ $dy -eq 32 ] ; then
      mo=01
      dy=01
      if [ $yy -eq 99 ] ; then
        cc=`expr $cc + 1`
        yy=00
      else 
        yy=`expr $yy + 1`
        if [ $yy -lt 10 ] ; then
          yy=0$yy
        fi
      fi
    fi
    ;;
  02) 
    if [ $dy -eq 29 ] ; then
    nn=`expr $yy / 4 `
    if [ ` expr $nn \* 4 ` -ne $yy ] ; then #The rule: If year is divisible by
                                            # 4, then it is a leap year.
      #echo year not divisible by 4, not a leap year
        mo=03
        dy=01
     else 
      #echo $cc $nn $yy else case 
      if [ $yy -ne 00 ] ; then   #Don't need century rules
        mo=$mo
        dy=$dy
       else
        if [ $yy -eq 00 -a `expr $cc \/ 4 \* 4` -eq $cc  ] ; then 
                #If it is a century year, and not a century year divisible 
                #by 400, then it really is not a leap year.  
          mo=$mo
          dy=$dy
         else
          mo=03
          dy=01
        fi
      fi
    fi
  fi
    if [ $dy -eq 30 ] ; then
      mo=03
      dy=01
    fi
    ;;
 *) 
  echo Should not have reached this default case!
  ;;

esac


# Now manage decrementing of days:
if [ $dy -ne 00 ] ; then
  if [ ${#1} -eq 6 ] ; then
    echo $yy$mo$dy
  else
    echo $cc$yy$mo$dy
  fi
  exit
fi

#only reach this point if dy is 00
case $mo in 
  01)
    mo=12
    dy=31
    yy=`expr $yy - 1`
    if [ $yy -lt 10 ] ; then
      yy=0$yy
    fi
    if [ $yy -lt 0 ] ; then
      cc=`expr $cc - 1`
      yy=`expr $yy + 100`
      if [ $yy -lt 10 ] ; then
        yy=0$yy
      fi
    fi
    ;;
  02|04|06|08|09)
    mo=0`expr $mo - 1`
    dy=31
    ;;
  05|07|10)
    mo=0`expr $mo - 1`
    dy=30
    ;;
  11)
    mo=`expr $mo - 1`
    dy=31
    ;;
  12)
    mo=`expr $mo - 1`
    dy=30
    ;;
  03)
    nn=`expr $yy / 4 `
    if [ ` expr $nn \* 4 ` -eq $yy ] ; then
      if [ $yy -ne 0 ] ; then
        mo=02
        dy=29
      else 
        if [ $yy -eq 00 -a `expr $cc \/ 4 \* 4` -eq $cc  ] ; then 
          mo=02
          dy=29
        else
          mo=02
          dy=28
        fi
      fi
     else
      mo=02
      dy=28
    fi
    ;;
  *)
    echo should never have reached this point
    exit
    ;;
esac

if [ ${#1} -eq 6 ] ; then
  echo $yy$mo$dy
else
  echo $cc$yy$mo$dy
fi