#!/bin/bash
Help(){
	echo "
NAME
    git_update.sh
SYNOPSIS
    git_update.sh [-h] [-r ref] [-d dest] [-H] [-a] repository
OPTIONS
	-h prints the help. 
	-r specifies the reference to the commit to be synchronized. It can be a tag or a branch. By default, it is the last commit of branch main.
	-d specifies the destination of the clone or update. Directory must be empty if a new clone is to be made.
	If the repository to be cloned is local, and its path is passed as a relative path, the path should start from the destination. 
	To avoid mistakes, absolute paths are advised. 
	-H allows the $HOME directory to be used by git_update.sh. By default, git_update.sh cannot access $HOME to prevent default behavior. 
	If you need the global .gitconfig located in your $HOME to be used, you should supply the -H option. 	
	-a specifies that the aggressive option of the git garbage collection must be used. Only advised when changes happen in many different objects. Will slow down the execution.   
DESCRIPTION
	This script will replace the destination with the wanted commit of a git repository. The history is not preserved but tags are. Untracked files remain.
	The git commands have been chosen so as to minimize the memory and bandwidth usages."
}

#variables
summary="$0 [options] <repo>"
ref=main
dst='.'
use_home=false
be_aggressive="false"

while getopts ":hr:d:H" option; do
   case $option in
      h) # display Help
         Help
         exit;;
      r) # desired branch or tag 
            ref="$OPTARG";;
      d) # destination of clone
            dst="$OPTARG";; 
	  H) # use real home dir
	  		use_home="true";;
	  a) #use -a in git gc call 
	  		be_aggressive="true";;
     \?) # invalid option
         echo "Error: Invalid option here"
         exit;;
   esac
done
shift $((OPTIND-1))

repo="$1"
if [ -z "$repo" ] ; then
	exit "$0: Empty repo given\n$summary"
fi

if [ ! $use_home ] ; then
	set -a
	export HOME=/dev/null
	set +a
fi

mkdir -p "$dst"
cd "$dst"

if [ -d .git ] ; then
	echo "updating..."
	git fetch --tags --depth=1 --prune --prune-tags --force origin $ref 
	git reset --hard --recurse-submodules FETCH_HEAD 
	git submodule update --init --recursive --force --depth=1 --remote 
	#garbage collection of anything unreachable at the moment
	git reflog expire --expire=now --all 
	if "$be_aggressive" ; then
		git gc --prune=now --aggressive
	elsels
		git gc --prune=now 
	fi
else
echo "cloning..."
	clone_dst='.'
    git clone -b "$ref" --recurse-submodules --shallow-submodules --depth 1 "$repo" "$clone_dst" 
fi