#!/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.
	It can be different from the actual branch. CAREFUL, the command git branch will always show the original branch name even though a branch switch has happened.
	-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.   
	-o ssh options for ssh clone
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"
ssh_opts="ssh"

while getopts ":ho:r:d:H" option; do
   case $option in
      h) # display Help
         Help
         exit;;
      r) # desired branch or tag 
            ref="$OPTARG";;
      d) # destination of clone
            dst="$OPTARG";; 
	  H) # use real home dir
	  		use_home="true";;
	  a) #use -a in git gc call 
	  		be_aggressive="true";;
	  o) # ssh options
		  	ssh_opts="$ssh_opts $OPTARG";;
     \?) # invalid option
         echo "Error: Invalid option '$option'"
         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 "$ref" --recurse-submodules --shallow-submodules --depth 1 --config core.sshCommand="$ssh_opts" "$repo" "$clone_dst" 
fi