#!/bin/bash
#
# Run commands in parallel from a bash script, e.g.
#
#   source "${path_to}/libparallel.sh"
#   for user in $( cat $user_file ); do parallel 5
#     user_migrate $user &
#   done
# 
# runs 5 user_migrate processes at a time, when one completes, 
# a new one is started
#
#
# FOR MORE COMPLEX THINGS, SCROLL DOWN
#
parallel_debug() { [[ "${VERBOSE}" != "" ]] && echo "$@"; }
parallel_jobs() {
	local i=0
	while read job; do 
		(( i++ ))
	done <<< "$( jobs )"
	echo $i
}
parallel_exit() {
	wait
	[[ "${PARALLEL_JOBS}" != "" ]] && {
		echo "parallelized ${PARALLEL_JOBS} jobs with ${PARALLEL_SLEEPS} sleeps"
	}
	PARALLEL_JOBS=""
	PARALLEL_SLEEPS=""
}
parallel_sleep() {
	while [[ $( parallel_jobs ) -ge $max ]] ; do  
		(( PARALLEL_SLEEPS++ ))
		parallel_debug "  sleeping until queue not full"
		sleep 1
	done
}
parallel() {
	local max=10
	[[ "${1}" =~ ^[0-9]+$ ]] && {
		max=$1
	}
	[[ "${PARALLEL_EXIT_HANDLER_IS_DISABLED}" == "" ]] && {
		trap parallel_exit EXIT
	}
	parallel_debug $( printf "queue depth %3d/%d\n" $( parallel_jobs) $max )
	parallel_sleep
}
#
# To maintain serialization, use a subshell, e.g.
#
#   for user in $( cat $user_file ); do parallel 5
#     ( user_migrate $user 
#       fw_update $user ) & 
#   done 
#
# To get more reporting per job, use parallel_exec, e.g.
#
#   for user in $( cat $user_file ); do 
#     N=5 parallel_exec user_migrate $user 
#   done
#
# Default parallelism = 10, change with N
#
# To get progress reporting, set VERBOSE=yes, e.g.
#
#   VERBOSE=yes
#   for user in $( cat $user_file ); do parallel 5
#     user_migrate $user &
#   done
#
# If you trap EXIT already, set 
#   PARALLEL_EXIT_HANDLER_IS_DISABLED=yes
#
# and call parallel_exit when your loop is done
# see parallel_test() below
#
parallel_exec() {
	local max=10
	[[ "${N}" =~ ^[0-9]+$ ]] && {
		max=$N
	}
	[[ "$PARALLEL_JOBS" == "" ]] && {
		PARALLEL_JOBS=0
		PARALLEL_SLEEPS=0
		trap parallel_exit EXIT
	}

	( "$@" ) &
	parallel_debug $( printf "queue depth %3d/%d, running %s\n" $( parallel_jobs) $max "$*" )
	(( PARALLEL_JOBS++ ))
	parallel_sleep
}
parallel_test() {
	for user in $( cat /etc/passwd ); do parallel 10
		echo $user >/dev/null
		sleep 5 &
	done 
	parallel_exit

	for user in $( cat /etc/passwd ); do 
		echo $user >/dev/null
		N=5 parallel_exec sleep 1 
	done 
}