#!/usr/bin/env sh

set -e

usage() {
    # shellcheck disable=SC2016
    echo 'usage: nextbin [-v] "$0" "$@"' >&2
}

case "$1" in
    -v) action="echo"; shift;;
    -*|'') usage; exit 1;;
    *) action="exec"
esac

orig_exe=$1
shift

die() {
    fmt=$1
    shift
    printf "nextbin %s failed: $fmt\n" "$orig_exe" "$@" >&2
    exit 1
}

[ / -ef / ] || exec bash "$0" "$orig_exe" "$@" || die "test -ef doesn't work, and bash not found"
[ -n "$PATH" ] || die 'PATH is empty'

exe_name=${orig_exe##*/}
path=$PATH
phase=1
found=
while :; do
    exe=${path%%:*}/$exe_name
    if [ -f "$exe" ] && [ -x "$exe" ]; then
        if [ "$phase" = 1 ]; then
            if [ "$exe" -ef "$orig_exe" ]; then
                phase=2
            fi
        elif [ "$exe" -ef "$orig_exe" ]; then
            [ -z "$found" ] || die 'duplicate PATH entries found'
        elif [ -z "$found" ]; then
            found=$exe
        fi
    fi
    case $path in
        *:*) path=${path#*:} ;;
        *) break ;;
    esac
done

if [ "$phase" = 1 ]; then
    found=$(command -v "$exe_name") || die '%s is not in PATH' "$exe_name"
elif [ -z "$found" ]; then
    die '%s is last instance' "$orig_exe"
fi
"$action" "$found" "$@"