#!/bin/ksh

# Get the GDAS1 ancillary data file closest to a MODIS granule date and time

# Check number of arguments
if [ $# != 3 ]; then
  echo "Usage: get_gdas dir date time"
  echo "  dir  is the directory containing the GDAS1 files"
  echo "  date is the date (yyyyddd)"
  echo "  time is the time (hhmm)"
  exit 1
fi

# Get arguments
dir=$1
date=$2
time=$3

# Set the day and time of the closest GDAS1 file
if   [[ $time -ge 0000 && $time -le 0255 ]]; then
  gdas_time="00"
  gdas_delta=0
elif [[ $time -ge 0300 && $time -le 0855 ]]; then
  gdas_time="06"
  gdas_delta=0
elif [[ $time -ge 0900 && $time -le 1455 ]]; then
  gdas_time="12"
  gdas_delta=0
elif [[ $time -ge 1500 && $time -le 2055 ]]; then
  gdas_time="18"
  gdas_delta=0
elif [[ $time -ge 2100 && $time -le 2355 ]]; then
  gdas_time="00"
  gdas_delta=1
fi

# Compute the Gregorian date (yyyymmdd) of the closest GDAS1 file
modis_date=`dateplus -J $date`
gdas_date=`dateplus $gdas_delta $modis_date | cut -c3-8`

# Find the GDAS1 file
gdas_name="gdas1.PGrbF00."${gdas_date}.${gdas_time}"z"
gdas_file=`find $dir -size +20000000c -name $gdas_name`

# Check for success
if [[ $gdas_file != "" ]]; then
  echo $gdas_file
  exit 0
else
  exit 1
fi