#!/bin/bash
# daysuntil -- Basically this is the daysago script backwards, where the
#     desired date is set as the current date and the current date is used as
#     the basis of the daysago calculation. 

# Like the previous script, use `which gdate` if you are on OS X
# If you are on Linux, use `which date`
setglobal date = $[which date]

proc daysInMonth
{
  match $1 {
    with 1|3|5|7|8|10|12  setglobal dim = '31'  # most common value
    with 4|6|9|11         setglobal dim = '30' 
    with 2                setglobal dim = '29'   # depending on if it's a leap year
    with *                setglobal dim = '-1'   # unknown month
  }
}

proc isleap
{
  # If specified year is a leap year, returns non-zero value for $leapyear

    setglobal leapyear = $[$date -d 12/31/$1 +%j | grep 366]
}

#######################
#### MAIN BLOCK

if test $Argc -ne 3  {
  echo "Usage: $[basename $0] mon day year"
  echo "  with just numerical values (ex: 1 1 2020)"
  exit 1
}

$date --version > /dev/null !2 > !1         # discard error, if any

if test $Status -ne 0  {
  echo "Sorry, but $[basename $0] can't run without GNU date." > !2
  exit 1
}

eval $[$date "+thismon=%m;thisday=%d;thisyear=%Y;dayofyear=%j]

setglobal endmon = $1; setglobal endday = $2; setglobal endyear = $3

# lots of parameter checks needed...

daysInMonth $endmon	# sets $dim variable
if test $endday -lt 0 -o $endday -gt $dim  {
  echo "Invalid: Month #$endmon only has $dim days." > !2 ; exit 1
}

if test $endmon -eq 2 -a $endday -eq 29  {
  isleap $endyear
  if test -z $leapyear  {
    echo "Invalid: $endyear wasn't a leapyear, so February only had 28 days." > !2
    exit 1
  }
}

if test $endyear -lt $thisyear  {
  echo "Invalid: $endmon/$endday/$endyear is prior to the current year." > !2 ; exit 1
}

if test $endyear -eq $thisyear -a $endmon -lt $thismon  {
  echo "Invalid: $endmon/$endday/$endyear is prior to the current month." > !2 ; exit 1
}

if test $endyear -eq $thisyear -a $endmon -eq $thismon -a $endday -lt $thisday  {
  echo "Invalid: $endmon/$endday/$endyear is prior to the current date." > !2 ; exit 1
}

if test $endyear -eq $thisyear -a $endmon -eq $thismon -a $endday -eq $thisday  {
  echo "There are zero days between $endmon/$endday/$endyear and today." > !2 ; exit 0
}

## if we're working with the same year, the calculation's a bit different

if test $endyear -eq $thisyear  {

  setglobal totaldays = $shExpr(' $($date -d "$endmon/$endday/$endyear" +%j) - $($date +%j) ')
  
} else {

  ## calculate this in chunks: starting with days left in this year

  ###  DAYS LEFT IN START YEAR

  # calculate the date string format for the specified starting date

  setglobal thisdatefmt = ""$thismon/$thisday/$thisyear""

  setglobal calculate = ""$[$date -d "12/31/$thisyear" +%j] - $[$date -d $thisdatefmt +%j]""

  setglobal daysleftinyear = $shExpr(' $calculate ')

  ###  DAYS IN INTERVENING YEARS

  setglobal daysbetweenyears = '0'
  setglobal tempyear = $shExpr(' $thisyear + 1 ')

  while test $tempyear -lt $endyear {
    setglobal daysbetweenyears = $shExpr(' $daysbetweenyears + $($date -d "12/31/$tempyear" +%j) ')
    setglobal tempyear = $shExpr(' $tempyear + 1 ')
  }

  ###   DAYS IN END YEAR

  setglobal dayofyear = $[$date --date $endmon/$endday/$endyear +%j]	# that's easy!

  ###   NOW ADD IT ALL UP

  setglobal totaldays = $shExpr(' $daysleftinyear + $daysbetweenyears + $dayofyear ')
}

echo "There are $totaldays days until the date $endmon/$endday/$endyear."
exit 0