#!/bin/bash
#
# Synopsis:
#
#   #!/bin/bash
#   . export-json.bash
#   export_JSON <variables>
#
# Example:
#
#   $ (
#           source export-json.bash &&
#           export_JSON SSH_TTY \
#                       SSH_CONNECTION \
#                       SSH_CLIENT \
#                       SSH_AUTH_SOCK
#     )
#
#   {
#     "SSH_TTY": "/dev/pts/0",
#     "SSH_CONNECTION": "192.0.2.140 41564 198.51.100.25 22",
#     "SSH_CLIENT": "192.0.2.140 41564 22",
#     "SSH_AUTH_SOCK": "/tmp/ssh-XXXXcePKJl/agent.36326"
#   }
#
# This bash function exports shell variables as
# JSON ("javascript object notation") strings.
#
# The string is printed to stdout.
#
# JSON is a format that represents data as
# Javascript source code so modern programmers
# can read it unlike bash source code.
#
# Variable names are given to the function as its
# argument list:
#
#       $ source export-json.bash &&
#         export_JSON PATH
#
# The output is a single JSON object containing
# key-value mappings between JSON strings:
#
#       {
#         "PATH": "/usr/local/sbin:[...]"
#       }
#
# It is possible to use a different name for the
# variable in the JSON object key field than in
# the shell environment, like this:
#
#       $ export_JSON DBUS_SESSION_BUS_ADDRESS=p
#       {
#         "p": "unix:path=/run/user/1000/bus"
#       }
#
# It uses the external tool "jq" to parse string
# values placed in jq's argument list by bash and
# then encode them as JSON string values. This
# is no accidental dependency. The jq program is
# the foundation of the trustworthiness of this
# code. If we were encoding JSON strings in bash
# we would have to be a lot more careful.

arg1_to_env0()
{
	case "$1" in
		*=*=* )
			set -- "${1#*=}"
			echo "Error: invalid variable: ${1@Q}" >&2
			return 30
			;;
		*[^a-zA-Z0-9_=]* | [^a-zA-Z_]* | '')
			echo "Error: invalid variable: ${1@Q}" >&2
			return 10
			;;
	esac
	set -- "${1##*=}" "${1%%=*}"
	if [ -v "$2" ]
	then
		printf '%s=%s\0' "$1" "${!2}"
	else
		echo "Warning: ignoring unset variable: ${2@Q}" >&2
	fi
}

# Hygienic loop.  Doesn't touch the shell
# environment.
for_each()
{
	while [ $# -ge 2 ]
	do
		$1 "$2" || return
		set -- "$1" "${@:3}"
	done
}

to_JSON_all()
{
	(
		if [ "$1" = '-a' ]
		then
			jq_env
		fi
		for_each to_JSON1 $(compgen -A arrayvar)
	) |
	jq -s 'add'
}

jq_zip2()
{
	cat <<'END'
$ARGS.positional |
[
  .[ length/2 : ],
  .[ : length/2 ]
] |
transpose |
map ({ (.[0]): .[1] }) |
add
END
}

json_encode_associative_array()
{
	eval \
		'set -- "$1" \
			"${'"$1"'[@]}" \
			"${!'"$1"'[@]}";
		' &&
	if [ $# -ge 3 ]
	then
		jq -n "{ (\$k): $(jq_zip2) }" \
			--arg k "$1" \
			--args "${@:2}"
	else
		jq -n '{ ($k): {} }' --arg k "$1"
	fi
}

json_encode_indexed_array()
{
	eval \
		'set -- "$1" \
			"${'"$1"'[@]}" \
		' &&
	jq -n '{ ($k): $ARGS.positional }' \
		--arg k "$1" \
		--args "${@:2}"
}

# This uses the more complex implementation that,
# when used with multiple arguments, does not
# require extra calls to jq. Since this is only
# using a single argument, a more straightforward
# implementation could be used just as well.
json_encode_string()
{
	[ -v "$1" ] &&
	export_JSON_unsafe "$1"
}

# The straightforward implementation:
json_encode_string()
{
	[ -v "$1" ] &&
	jq -n '{ ($k): $v }' --arg k "$1" --arg v "${!1}"
}
# But is having two implementations really
# straightforward?  And what about this
# parenthetical commentary? Moreso wobbly, if not
# winding, error-prone complexity finding itself
# grinding up on the accidents of time passing it
# might be anyway but the takeaway is don't touch
# it till it croak, the tests will run and the
# result will speak, may truth spread through the
# dendrites of Samizdat!

to_JSON()
{
	case "$#$1" in
		0 )
			printf \
				'%s\n' \
				'usage: to_JSON -a    # print all' \
				'       to_JSON -A    # print arrays' \
				'       to_JSON <variable name> [...]' \
				>&2
			return -1
			;;
		1-a | 1-A )
			to_JSON_all $1
			;;
		* )
			for_each to_JSON1 "$@"
			;;
	esac
}

to_JSON1()
{
	case "${!1@a}" in
		*a* )
			json_encode_indexed_array "$1"
			;;
		*A* )
			json_encode_associative_array "$1"
			;;
		* )
			json_encode_string "$1"
			;;
	esac
}

env0_to_JSON()
{
	set --
	while read -d ''
	do
		set -- "$@" --arg "${REPLY%%=*}" "${REPLY#*=}"
	done
	jq -n -r '$ARGS.named' "$@"
}

export_JSON_unsafe()
{
	(
		set -o pipefail
		for_each arg1_to_env0 "$@" | env0_to_JSON
	)
}

safety_pipe()
{
	[ $# -ge 2 ] || return
	set -- "$(mktemp)" "$@" || return
	(
		trap "rm ${1@Q}" EXIT
		"${@:3}" > "$1" &&
		$2 < "$1"
	)
}

export_JSON()
{
	safety_pipe env0_to_JSON for_each arg1_to_env0 "$@"
}

filter_vars()
{
	while read
	do
		if [ -v "$REPLY" ]
		then
			printf '%s\n' "$REPLY"
		fi
	done
}

jq_env()
{
	export_JSON $(compgen -v | filter_vars) | jq "$@"
}

jq_exports()
{
	export_JSON $(compgen -e | filter_vars) | jq "$@"
}

try()
{
	"$@"
	printf '(Exit %s) <- [%s]\n' "$?" "${*@Q}" >&2
}

runtest()
{
	set -- SSH_CLIENT SSH_TTY SSH_AUTH_SOCK SSH_CONNECTION
	try export_JSON "$@"
	unset unsetvar
	try export_JSON SSH_TTY unsetvar
	try export_JSON unsetvar SSH_TTY
	try export_JSON
	try export_JSON ''
	try export_JSON 'invalid!' SSH_TTY
	try export_JSON SSH_TTY 'invalid!'
	try jq_env .unsetvar
	emptyvar= try jq_env .emptyvar
	try jq_exports '.|{TERM,LANG,HOSTTYPE,EDITOR,SHELL}'
	try jq_env '.|{TERM,LANG,HOSTTYPE,EDITOR,SHELL}'
	try to_JSON PATH BASH_ARGV BASH_VERSINFO BASH_ALIASES BASH_CMDS
}

# code poetry flowetry toiletry coiled spring load
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# any accepting returns of types unknown or known
#
# defensively typed values for anomic millennial
# hackers
#
# atomized castaways of aging social bodies
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# or falter to a byzantine emperors structures
# span relativities messages