#!/bin/sh
#
# git-backdate-tag: Change the date of an existing tag, replacing the
# tag reference with the newly-generated tag object.
#
# Usage: git-backdate-tag TAG DATE

usage () {
    echo "Usage: git-backdate-tag TAG DATE"
}

if [ -n "$3" ]; then
    usage
    exit 1
elif [ -z "$2" ]; then
    usage
    exit 1
fi

# Set parameters
tag="$1"
date=$(date --date="$2" +%s)

if [ $? -ne 0 ]; then
    echo Could not parse date
    exit 1
fi

# Replace old date with new date
newtagobj=$(git cat-file tag "$tag" | \
    sed -r -e "s/^(tagger .+) ([^ \\n]+) ([^ \\n]+)\$/\1 $date \3/1" | \
    git mktag)

if [ $? -ne 0 ]; then
    echo Could not create replacement tag object
    exit 1
fi

# Set tag to new tag object
git update-ref refs/tags/"$tag" $newtagobj