#!/bin/bash
# days-between.sh:    Number of days between two dates.
# Usage: ./days-between.sh [M]M/[D]D/YYYY [M]M/[D]D/YYYY

ARGS=2                # Two command line parameters expected.
E_PARAM_ERR=65        # Param error.

REFYR=1600            # Reference year.
CENTURY=100
DIY=365
ADJ_DIY=367           # Adjusted for leap year + fraction.
MIY=12
DIM=31
LEAPCYCLE=4

MAXRETVAL=256         # Largest permissable
                      # positive return value from a function.

diff=		      # Declare global variable for date difference.
value=                # Declare global variable for absolute value.
day=                  # Declare globals for day, month, year.
month=
year=


Param_Error ()        # Command line parameters wrong.
{
  echo "Usage: `basename $0` [M]M/[D]D/YYYY [M]M/[D]D/YYYY"
  echo "       (date must be after 1/3/1600)"
  exit $E_PARAM_ERR
}  


Parse_Date ()                 # Parse date from command line params.
{
  month=${1%%/**}
  dm=${1%/**}                 # Day and month.
  day=${dm#*/}
  let "year = `basename $1`"  # Not a filename, but works just the same.
}  


check_date ()                 # Checks for invalid date(s) passed.
{
  [ "$day" -gt "$DIM" ] || [ "$month" -gt "$MIY" ] || [ "$year" -lt "$REFYR" ] && Param_Error
  # Exit script on bad value(s).
  # Uses "or-list / and-list".
  #
  # Exercise: Implement more rigorous date checking.
}


strip_leading_zero () # Better to strip possible leading zero(s)
{                     # from day and/or month
  val=${1#0}          # since otherwise Bash will interpret them
  return $val         # as octal values (POSIX.2, sect 2.9.2.1).
}


day_index ()          # Gauss' Formula:
{                     # Days from Jan. 3, 1600 to date passed as param.

  day=$1
  month=$2
  year=$3

  let "month = $month - 2"
  if [ "$month" -le 0 ]
  then
    let "month += 12"
    let "year -= 1"
  fi  

  let "year -= $REFYR"
  let "indexyr = $year / $CENTURY"


  let "Days = $DIY*$year + $year/$LEAPCYCLE - $indexyr + $indexyr/$LEAPCYCLE + $ADJ_DIY*$month/$MIY + $day - $DIM"
  # For an in-depth explanation of this algorithm, see
  # http://home.t-online.de/home/berndt.schwerdtfeger/cal.htm


  if [ "$Days" -gt "$MAXRETVAL" ]  # If greater than 256,
  then                             # then change to negative value
    let "dindex = 0 - $Days"       # which can be returned from function.
  else let "dindex = $Days"
  fi

  return $dindex

}  


calculate_difference ()            # Difference between to day indices.
{
  let "diff = $1 - $2"             # Global variable.
}  


abs ()                             # Absolute value
{                                  # Uses global "value" variable.
  if [ "$1" -lt 0 ]                # If negative
  then                             # then
    let "value = 0 - $1"           # change sign,
  else                             # else
    let "value = $1"               # leave it alone.
  fi
}



if [ $# -ne "$ARGS" ]              # Require two command line params.
then
  Param_Error
fi  

Parse_Date $1
check_date $day $month $year      # See if valid date.

strip_leading_zero $day           # Remove any leading zeroes
day=$?                            # on day and/or month.
strip_leading_zero $month
month=$?

day_index $day $month $year
date1=$?

abs $date1                         # Make sure it's positive
date1=$value                       # by getting absolute value.

Parse_Date $2
check_date $day $month $year

strip_leading_zero $day
day=$?
strip_leading_zero $month
month=$?

day_index $day $month $year
date2=$?

abs $date2                         # Make sure it's positive.
date2=$value

calculate_difference $date1 $date2

abs $diff                          # Make sure it's positive.
diff=$value

echo $diff

exit 0
# Compare this script with the implementation of Gauss' Formula in C at
# http://buschencrew.hypermart.net/software/datedif