#!/usr/bin/ksh93

#
# diff watch - interactively watch evolution of file
#
# Example usage:
# $ ./watchdiff.ksh /proc/fs/nfsd/clients/2497/states
#
# Written by Roland Mainz <roland.mainz@nrubsig.org>
#

function diffvars
{
	# use nameref here for efficiency
	nameref o=$1
	nameref c=$2
	
	diff -u \
		<( cat <<<"$o" ) \
		<( cat <<<"$c" ) || true
}

function main
{
	set -o errexit
	set -o nounset

	typeset	basefile="$1"
	typeset	orig_basefilecontent="$( < "$basefile" )"
	typeset	curr_basefilecontent="$orig_basefilecontent"

	integer	term_lines
	float	ts # timestamp
	typeset	usrcmd
	typeset	headtailcmd='head'

	for (( ;; )) ; do
		term_lines="$(tput lines)"
		tput clear

		printf $"# %s: Watching file $'%q' in %s mode:\n" \
			"watchdiff.ksh" \
			"${basefile}" \
			"${headtailcmd}"

		curr_basefilecontent="$( < "$basefile" )"
		diffvars orig_basefilecontent curr_basefilecontent 2>&1 | \
			${headtailcmd} -n $(( term_lines - 3 ))

		for (( ts=SECONDS ; (SECONDS-ts) < 10. ; )) ; do
			while read -n1 -t1 usrcmd ; do
				case "${usrcmd}" in
					$'r') # reload original file
						orig_basefilecontent="$curr_basefilecontent"
						break 2
						;;
					$' ') # refresh
						break 2
						;;
					$'h') # use head
						headtailcmd='head'
						break 2
						;;
					$'t') # use tail
						headtailcmd='tail'
						break 2
						;;
					$'q' | $'\E') # quit
						break 3
						;;
					*)
						printf $"Unknown cmd %q\n" "$usrcmd"
						;;
				esac
			done
		done
	done
	
	return 0
}

# main
export PATH='/bin:/usr/bin'

builtin cat
builtin head

main "$@"
exit $?