#!/bin/bash

# Scakko

function math {
  BASE=$1
  ARG=$2
  FORMAT="+%a %b %d %Y"

  OP=`echo $ARG | awk 'BEGIN{FS="[0-9]+"}{print $1}'`
  UN=`echo $ARG | awk 'BEGIN{FS="[0-9]+"}{print $2}'`
  NUM=`echo $ARG | awk 'BEGIN{FS="([\+\-]+)|([DWMY]+)"}{print $2}'`

  if [[ (( $OP == "" )) || (( $UN == "" )) || (( $NUM == "" )) ]]; then
    echo "Error: \"$ARG\" is not a valid operation string"
    exit 1
  fi

  FACT=`expr $NUM \* $[$UN]`

  RES=`expr $BASE $OP $FACT`
  date -d @$RES "$FORMAT"
}

E_BADARGS=65

s=1
m=`expr 60 \* $s`
h=`expr 60 \* $m`
D=`expr 24 \* $h`
W=`expr 7 \* $D`
M=`expr 30 \* $D`
Y=`expr 365 \* $D`

today_=`date +%D`
today=`date +%s -d $today_`
yesterday=`expr $today - $D`
tomorrow=`expr $today + $D`

case "$#" in
1)
  BASE=$today
  math $BASE $1
;;
2)
  case "$1" in
  today)
    BASE=$today
  ;;
  yesterday)
    BASE=$yesterday
  ;;
  tomorrow)
    BASE=$tomorrow
  ;;
  *)
    BASE=`date -d $1 +%s 2> /dev/null`
    RC=$?
    if [ $RC -ne 0 ]; then
	echo "Error: \"$1\" is not a valid date"
	exit $RC
    fi
  ;;
  esac
  math $BASE $2
;;
*)
  echo "Usage: `basename $0` [yesterday|today|tomorrow|<DATE>] [+|-]<N>[D|W|M|Y]"
  echo ""
  echo "Examples:"
  echo "	`basename $0` +1M"
  echo "	`basename $0` MM/DD/YYYY -1W"
  echo "	`basename $0` yesterday +2D"
  echo ""
  exit $E_BADARGS
;;
esac