date-week

#!/bin/bash
about()
{
cat <<'EOF'
---
summary: Wrapper for GNU date which parses ISO Week dates (e.g. `2025W022` from `date-week -d 2025-01-07 '+%YW%V%u'`).
version: 0.0.5
status: unstable
dateCreated: 2025-01-07T21:33:51Z
dateModified: 2025-10-23T13:47:53Z
uuid: 785d13d6-3195-4c4d-a0e9-c4696412ae74
...
EOF
}

DATEFORMAT_WEEKDATE='%YW%V%u';

main()
{
  local dateArgs=();
  local inputDate='';
  local dateFormat=;
  while [[ $# -gt 0 ]]; do
    case "$1" in
      --format)
        if [[ -n $dateFormat ]]; then
          echo "$dateFormat";
        else
          echo "$DATEFORMAT_WEEKDATE";
        fi
        return;;
      -d) inputDate="$2"; shift;;
      -W|--format=weekdate) dateFormat="$DATEFORMAT_WEEKDATE";;
      +*) dateFormat="${1#+}";;
      --format=*) dateFormat="${1#--format=}";; # for discoverability beside =weekdate
      -v) ;; # ignore, handled outside main
      *) dateArgs+=("$1");;
    esac
    shift;
  done
  dateArgs+=("+${dateFormat}");
  local PAT_ISO_WEEK_NAMED='(?<year>[+-]?[0-9]+)-?W(?<week>[0-5][0-9])-?(?<day>[0-7])?(?<after>$|[^0-9][[:alnum:]:.-]*)';
  local PAT_ISO_WEEK_TEST='W[0-5][0-9][0-7]?([^0-9]|$)';
  if [[ -n $inputDate ]]; then
    if [[ $inputDate =~ $PAT_ISO_WEEK_TEST ]]; then
      whisper 2 "[date-week] input matches PAT_ISO_WEEK_TEST /$PAT_ISO_WEEK_TEST/";
      if [[ ${inputDate:0:1} == W ]]; then
        local thisYear="$(date '+%Y')";
        inputDate="${thisYear}${inputDate}";
      fi
      whisper 1 "[date-week] inputDate(${#inputDate}): ${inputDate}";
      local oIFS="$IFS";
      # may require perl 5.10+ for '$+{name}' references (relatedLink: https://stackoverflow.com/a/288989)
      local substitution='s/'"$PAT_ISO_WEEK_NAMED"'/$+{year}\t$+{week}\t$+{day}\t$+{after}/';
      local IFS=$'\t';
      local inputDateParts=($(echo -n "$inputDate" | perl -pe "$substitution"));
      IFS="$oIFS";
      whisper 1 "[date-week] inputDateParts(${#inputDateParts[@]}): ${inputDateParts[*]}";
      if [[ "${inputDateParts[0]}" != "$inputDate" ]]; then
        local year=${inputDateParts[0]};
        local weeks=${inputDateParts[1]#0};
        local days=${inputDateParts[2]};
        local after=${inputDateParts[3]};
        if [[ -z $days ]]; then
          days=1; # Starting week with Monday
        fi
        local firstDayOfYearDate="${year}-01-01";
        local firstDayOfYearWeekday="$(date -d "${firstDayOfYearDate}" '+%u')";
        let 'days=days + (weeks-1)*7 - firstDayOfYearWeekday';
        dateArgs+=(-d "${firstDayOfYearDate}${after} +${days}days");
      else
        >&2 echo "[date-week] WARNING: no change in input, despite matching week pattern in input.";
      fi
    else
      dateArgs+=(-d "$inputDate");
    fi
  fi
  whisper 1 "[date-week] (${#dateArgs[@]} args) \$ date ${dateArgs[*]}";
  date "${dateArgs[@]}";
}

showHelp()
{
  echo "USAGE: $(basename "$0") [-d DATE] [+FORMAT] […]";
  echo "SUMMARY: Wrapper for GNU date which parses ISO Week dates (e.g. '2025W023').";
  echo "OPTIONS:";
  echo " -d DATE, +FORMAT, …";
  echo "    GNU date options are passed through.";
  echo;
  echo " -W, --format=weekdate";
  echo "    Use FORMAT '+${DATEFORMAT_WEEKDATE}'.";
  echo;
  echo " --format";
  echo "    Show date-week strftime format string and exit.";
  echo;
  echo " --test";
  echo "    Run tests and exit.";
  echo;
  echo " -V, --about";
  echo "    Show script metadata and exit.";
  echo;
  echo " -h, --help";
  echo "    Show this help and exit.";
  echo;
  echo " -v, --verbose";
  echo "    Show more info. Can be used multiple times.";
}

runTests()
{
  debug()
  {
    local name="$1";
    shift;
    local logPrefix="[date-week:test(${name})]";
    >&2 echo "${logPrefix} \$ $*";
    local result="$("$@")";
    local status=$?;
    >&2 echo "${logPrefix} status $status, output: ${result}";
    >&2 echo;
    return $status;
  }

  local debugOutputFormat='%Y-%m-%dT%H:%M:%S%z';
  debug "after" main -d '2025W022T08:00-08' "+${debugOutputFormat}";
  local now="$(date "+${debugOutputFormat}")";
  local nowWeek="$(date -d "$now" '+%YW%V%uT%H:%M:%S%z')";
  whisper 1 "[date-week:test] now: ${now}, inputting '${nowWeek}'";
  debug "this week" main -d "$nowWeek" "+${debugOutputFormat}";
  debug "week only" main -d 'W05' "+${debugOutputFormat}";
  debug "dashes" main -d '2025-W15-3' "+${debugOutputFormat}";
  debug "08" main -d '2025W08' "+${debugOutputFormat}";
}

g_verbosity=0;
knock() { [[ $g_verbosity -ge $1 ]]; }
whisper() { knock $1 || return; shift; >&2 echo "$@"; }

# Handle args which switch/affect outside of 'main' function.
for arg in "$@"; do
  case "$arg" in
    -v|--verbose) let g_verbosity++;;
    -V|--about) about; exit;; # "V" for "version"
    -h|--help) showHelp; exit;;
    --test) runTests; exit;;
  esac
done

main "$@";

exit;