#!/bin/sh

# TODO:
# support "$0 FLAG NEW" when no upstream default

set -e +x

USAGE="Usage: $0 FLAG OLD NEW"

if [ $# -ne 3 ]; then
    echo "$USAGE"
    exit 1
fi

FLAG=$1
OLD=$2
NEW=$3

if [ "$OLD" = "$NEW" ]; then
  echo "Old and new value can't be the same!"
  exit 1
fi

CABALFILE=$(ls *.cabal)

if [ $(echo $CABALFILE | wc -w) -ne 1 ]; then
   echo "There needs to be one .cabal file in the current dir!"
   exit 1
fi

if ! grep -q -i "^flag *$FLAG" $CABALFILE; then
   echo "$CABALFILE does have flag $FLAG"
   exit 1
fi

if ! grep -A3 -i "^flag *$FLAG" $CABALFILE | grep -q -i "default:"; then
   echo "$CABALFILE: $FLAG flag might not have a default"
   exit 1
fi

if ! grep -A3 -i "^flag *$FLAG" $CABALFILE | grep -q -i "default: *$OLD"; then
   echo "$CABALFILE: $FLAG flag already defaults to $NEW"
   exit 1
fi

if [ ! -f $CABALFILE.orig ]; then
    BACKUP=.orig
fi

sed -i$BACKUP -e "/[Ff]lag *$FLAG/,/[Dd]efault: *$OLD/ s/\([Dd]efault: *\)$OLD/\1$NEW/" $CABALFILE