[bibtex] Add bib-due script filter options

Added options to bib-due script to pass input file (-i filename.bib), to
display simple help (-h), to specify which field to filter by (-l
fieldname), to specify which values to retain (-r
'values|in|regex|groups'), until which date to show (-u 2020-05-13).

Parsing remains brittle, which makes -l and -r not very useful as of
now; the parser would just get confused with different fields being
present.

bibtex/.local/bin/bib-due

@@ -4,28 +4,121 @@
 # FIXME: reimplementation with real library needed
 #
 
+OPTIND=1 # Reset in case getopts has been used previously in the shell.
 fields="due|priority|\bauthor\b|\btitle\b"
 filterby="due"
+file="${BIBFILE}"
+until=""
+
+show_help() {
+  printf "%s\n" \
+    "" \
+    " bib-due       Show due readings from bibtex file." \
+    "" \
+    " Usage: bib-due [-hv] -i input.bib -r 'due|priority|\bauthor|\btitle' -l 'due' -u '2020-05-12'" \
+    "" \
+    " Options:" \
+    "" \
+    "     -i [bibtex-file]                    Input bibtex file to scrape and get items from." \
+    "" \
+    "     -r [fields]                         Field values to read in file." \
+    "" \
+    "     -l [filter]                         Field to use as filter entity." \
+    "                                         This field is required for the scraper to pick entries up." \
+    "" \
+    "     help    | -h | --help               Print out this help." \
+    "" \
+    " Invoked without arguments, bib-due will scrape the file defined in BIBFILE environment variable, " \
+    " filtering entries with the 'due' field, and getting the values for 'due', 'priority', 'author', " \
+    " and 'title' fields. It will then print the entries to stdout." \
+    "" \
+    " Example output line:" \
+    "" \
+    ' 2020-06-25 (1): Sergei Gerasymchuk -- “Ze” time in Ukraine (Gerasymchuk2019) ' \
+    ""
+}
+
+filter_until() {
+  # filter for dates, with line numbers
+  filtered=$(echo "$entries" | grep -noE '[[:digit:]]{4}-[[:digit:]]{2}-[[:digit:]]{2}')
+
+  # redirect entries to fifo pipe
+  mkfifo filteredentries
+  finish() {
+    rm filteredentries
+  }
+  trap finish EXIT
+  echo "$filtered" >filteredentries &
+
+  # find first date past until filter
+  lastline=""
+  while IFS= read -r line; do
+    cond=$(printf '%s' "$line" | cut -d: -f2)
+    cond=$(date -d "$cond" +%s)
+    if [ "$cond" -gt "$until" ]; then
+      lastline=$(printf '%s' "$line" | cut -d: -f1)
+      break
+    fi
+  done <filteredentries
+
+  # special cases all in filter, or none in filter
+  if [ -z "$lastline" ]; then
+    return
+  elif [ "$lastline" -eq 1 ]; then
+    entries=""
+  # filter
+  else
+    # remove lines below found
+    lastprinted="$((lastline - 1))"
+    entries=$(echo "$entries" | sed -n "1,${lastprinted}p;${lastline}q")
+  fi
+
+}
 
 main() {
-  # use system default bib file or passed argument
+  if [ -z "$file" ]; then
-  if [ -n "$1" ]; then
-    file="$1"
-  elif [ -n "$BIBFILE" ]; then
-    file="$BIBFILE"
-  else
     echo "Requires a bibtex file as argument."
     exit 1
   fi
 
   # filter all entries for those containing filterby field (default: due)
   # retain values of all fields mentioned in fields variable
-  entries=$(grep -E "^@|$fields" "$file" | awk 1 ORS=' ' | sed -E 's/@\w+\{/\n/g' | grep $filterby | tail -n +2 | sed -E 's/(,\s+(\w+)\s+=\s+|,\s*$)/\t/g' | awk -F'\t' '{ print $4 "\t" $5 "\t" $2 "\t" $3 "\t" $1 }')
+  entries=$(grep -E "^@|$fields" "$file" | awk 1 ORS=' ' | sed -E 's/@\w+\{/\n/g' | grep "$filterby" | tail -n +2 | sed -E 's/(,\s+(\w+)\s+=\s+|,\s*$)/\t/g' | awk -F'\t' '{ print $4 "\t" $5 "\t" $2 "\t" $3 "\t" $1 }')
 
   # prettify and sort the entries for display (remove {}, order by date,prio,author,title)
   entries=$(echo "$entries" | awk -F'\t' '{ gsub(/{/,""); gsub(/}/,""); gsub(/prio/,"",$2) } { print $1 " (" $2 "): " $3 " -- " $4 " (" $5 ")"}' | sort)
 
+  if [ -n "$until" ]; then
+    filter_until
+  fi
+
   echo "$entries"
 }
 
+while getopts "h?i:u:r:l:" opt; do
+  case "$opt" in
+  h | \?)
+    show_help
+    exit 0
+    ;;
+    # v)  verbose=1
+    # ;;
+  r)
+    fields="$OPTARG"
+    ;;
+  l)
+    filterby="$OPTARG"
+    ;;
+  i)
+    file="$OPTARG"
+    ;;
+  u)
+    until="$(date -d "$OPTARG" +%s)"
+    ;;
+  esac
+done
+shift $((OPTIND - 1))
+
+[ "${1:-}" = "--" ] && shift
+
 main "$@"