New tzselect menu option ‘now’

* Makefile (ZONETABLES, VERSION_DEPS): Add zonenow.tab.
(CHECK_NOW_TIMESTAMP, CHECK_NOW_FUTURE_YEARS, CHECK_NOW_FUTURE_SECS):
New macros.
(check_now): New target rule.
(check_mild): Depend on it.
* NEWS: Mention this new feature.
* checknow.awk, zonenow.tab: New files.
* checktab.awk: Adjust (i.e., loosen) checks for zonenow.tab.
* tzselect.ksh (TZ_ZONETABTYPE_TABLE, TZ_ZONENOW_TABLE):
New vars, so that the code can switch back and for easily between
zone1970.tab and zonenow.tab.
(TZ_ZONE_TABLE): Set to one of those two vars, depending on user.
Let users choose ‘now’ if they care only about timestamps from now on.
Narrow ‘regions’ based on ‘now’ selection, if possible (which it
always is, with the current data).

Makefile

@@ -580,7 +580,7 @@ YDATA=		$(PRIMARY_YDATA) etcetera
 NDATA=		factory
 TDATA_TO_CHECK=	$(YDATA) $(NDATA) backward
 TDATA=		$(YDATA) $(NDATA) $(BACKWARD)
-ZONETABLES=	zone1970.tab zone.tab
+ZONETABLES=	zone.tab zone1970.tab zonenow.tab
 TABDATA=	iso3166.tab $(TZDATA_TEXT) $(ZONETABLES)
 LEAP_DEPS=	leapseconds.awk leap-seconds.list
 TZDATA_ZI_DEPS=	ziguard.awk zishrink.awk version $(TDATA) \
@@ -619,7 +619,7 @@ VERSION_DEPS= \
 		tzfile.5 tzfile.h tzselect.8 tzselect.ksh \
 		workman.sh zdump.8 zdump.c zic.8 zic.c \
 		ziguard.awk zishrink.awk \
-		zone.tab zone1970.tab
+		zone.tab zone1970.tab zonenow.tab
 
 all:		tzselect zic zdump libtz.a $(TABDATA) \
 		  vanguard.zi main.zi rearguard.zi
@@ -825,7 +825,8 @@ tzselect:	tzselect.ksh version
 
 check: check_back check_mild
 check_mild:	check_character_set check_white_space check_links \
-		  check_name_lengths check_slashed_abbrs check_sorted \
+		  check_name_lengths check_now \
+		  check_slashed_abbrs check_sorted \
 		  check_tables check_web check_ziguard check_zishrink check_tzs
 
 check_character_set: $(ENCHILADA)
@@ -893,7 +894,29 @@ check_links:	checklinks.awk tzdata.zi
 		  -f checklinks.awk tzdata.zi
 		touch $@
 
-check_tables:	checktab.awk $(YDATA) backward $(ZONETABLES)
+# Check timestamps from now through 28 years from now, to make sure
+# that zonenow.tab contains all sequences of planned timestamps,
+# without any duplicate sequences.  In theory this might require
+# 2800 years but that would take a long time to check.
+CHECK_NOW_TIMESTAMP = `./date +%s`
+CHECK_NOW_FUTURE_YEARS = 28
+CHECK_NOW_FUTURE_SECS = $(CHECK_NOW_FUTURE_YEARS) '*' 366 '*' 24 '*' 60 '*' 60
+check_now:	checknow.awk date tzdata.zi zdump zic zone1970.tab zonenow.tab
+		rm -fr $@.dir
+		mkdir $@.dir
+		./zic -d $@.dir tzdata.zi
+		now=$(CHECK_NOW_TIMESTAMP) && \
+		  future=`expr $(CHECK_NOW_FUTURE_SECS) + $$now` && \
+		  ./zdump -i -t $$now,$$future \
+		     $$(find $$PWD/$@.dir/????*/ -type f) \
+		     >$@.dir/zdump.tab
+		$(AWK) \
+		  -v zdump_table=$@.dir/zdump.tab \
+		  -f checknow.awk zonenow.tab
+		rm -fr $@.dir
+		touch $@
+
+check_tables:	checktab.awk $(YDATA) backward zone.tab zone1970.tab
 		for tab in $(ZONETABLES); do \
 		  test "$$tab" = zone.tab && links='$(BACKWARD)' || links=''; \
 		  $(AWK) -f checktab.awk -v zone_table=$$tab $(YDATA) $$links \

NEWS

@@ -5,6 +5,7 @@ Unreleased, experimental changes
   Briefly:
     Ittoqqortoormiit, Greenland changes time zones on 2024-03-31.
     Code and data fixes for Palestine timestamps starting in 2072.
+    A new data file zonenow.tab for timestamps starting now.
 
   Changes to future timestamps
 
@@ -25,13 +26,23 @@ Unreleased, experimental changes
     This does not affect UTC offsets, only the tm_isdst flag.
     (Thanks to Thomas M. Steenholdt.)
 
+  New data file
+
+    A new data file zonenow.tab helps configure applications that use
+    timestamps dated from now on.  This simplifies configuration,
+    since users choose from a smaller Zone set.  The file's format is
+    experimental and subject to change.
+
   Changes to code
 
     localtime.c no longer mishandles TZif files that contain a single
     transition into a DST regime.  Previously, it incorrectly assumed
     DST was in effect before the transition too.  (Thanks to Alois
     Treindl for debugging help.)
 
+    tzselect now optionally reads zonenow.tab, to simplify when
+    configuring only for timestamps dated from now on.
+
     tzselect no longer creates temporary files.
 
     tzselect no longer mishandles the following:

checknow.awk

@@ -0,0 +1,53 @@
+# Check zonenow.tab for consistency with primary data.
+
+# Contributed by Paul Eggert.  This file is in the public domain.
+
+function record_zone(zone, data) {
+  if (zone) {
+    zone_data[zone] = data
+    zones[data] = zones[data] " " zone
+  }
+}
+
+BEGIN {
+  while (getline <zdump_table) {
+    if ($0 ~ /^TZ/) {
+      record_zone(zone, data)
+      zone = $0
+      sub(/.*\.dir\//, "", zone)
+      sub(/"/, "", zone)
+      data = ""
+    } else if ($0 ~ /./)
+      data = data $0 "\n"
+  }
+  record_zone(zone, data)
+  FS = "\t"
+}
+
+/^[^#]/ {
+  zone = $3
+  data = zone_data[zone]
+  if (!data) {
+    printf "%s: no data\n"
+    status = 1
+  } else {
+    zone2 = zonenow[data]
+    if (zone2) {
+      printf "zones %s and %s identical from now on\n", zone, zone2
+      status = 1
+    } else
+      zonenow[data] = zone
+  }
+}
+
+END {
+ for (zone in zone_data) {
+    data = zone_data[zone]
+    if (!zonenow[data]) {
+      printf "checknow.tab should have one of:%s\n", zones[data]
+      zonenow[data] = zone # This suppresses duplicate diagnostics.
+      status = 1
+    }
+ }
+ exit status
+}

checktab.awk

@@ -83,7 +83,7 @@ BEGIN {
 		    cc = cca[i]
 		    if (cc2name[cc]) {
 			cc_used[cc]++
-		    } else {
+		    } else if (! (cc == "XX" && zone_table == "zonenow.tab")) {
 			printf "%s:%d: %s: unknown country code\n", \
 				zone_table, zone_NR, cc >>"/dev/stderr"
 			status = 1
@@ -110,7 +110,7 @@ BEGIN {
 	      used_max_cc = cc
 	    }
 	  }
-	  if (used_max <= 1 && comments) {
+	  if (used_max <= 1 && comments && zone_table != "zonenow.tab") {
 	    printf "%s:%d: unnecessary comment '%s'\n", \
 	      zone_table, i, comments \
 	      >>"/dev/stderr"
@@ -149,7 +149,8 @@ $1 ~ /^#/ { next }
 		if ($3 ~ /%/) rulePercentUsed[$2] = 1
 	}
 	if (tz && tz ~ /\// && tz !~ /^Etc\//) {
-		if (!tztab[tz] && FILENAME != "backward") {
+		if (!tztab[tz] && FILENAME != "backward" \
+		    && zone_table != "zonenow.tab") {
 			printf "%s: no data for '%s'\n", zone_table, tz \
 				>>"/dev/stderr"
 			status = 1

tzselect.ksh

@@ -196,7 +196,8 @@ read_file() {
   }
 }
 read_file TZ_COUNTRY_TABLE "$TZDIR/iso3166.tab"
-read_file TZ_ZONE_TABLE "$TZDIR/$zonetabtype.tab"
+read_file TZ_ZONETABTYPE_TABLE "$TZDIR/$zonetabtype.tab"
+TZ_ZONENOW_TABLE=
 
 newline='
 '
@@ -385,6 +386,7 @@ while
   country_result=
   region=
   time=
+  TZ_ZONE_TABLE=$TZ_ZONETABTYPE_TABLE
 
   case $coord in
   ?*)
@@ -393,7 +395,8 @@ while
 
     # Ask the user for continent or ocean.
 
-    echo >&2 'Please select a continent, ocean, "coord", "TZ", or "time".'
+    echo >&2 \
+      'Please select a continent, ocean, "coord", "TZ", "time", or "now".'
 
     quoted_continents=`
       $AWK '
@@ -406,10 +409,10 @@ while
 	  printf "'\''%s'\''\n", entry
 	}
 	BEGIN {
-	  TZ_ZONE_TABLE = substr(ARGV[1], 2)
+	  TZ_ZONETABTYPE_TABLE = substr(ARGV[1], 2)
 	  ARGV[1] = ""
 	  FS = "\t"
-	  nlines = split(TZ_ZONE_TABLE, line, /\n/)
+	  nlines = split(TZ_ZONETABTYPE_TABLE, line, /\n/)
 	  for (i = 1; i <= nlines; i++) {
 	    $0 = line[i]
 	    if ($0 ~ /^[^#]/)
@@ -421,7 +424,7 @@ while
 	    }
 	  }
 	}
-      ' ="$TZ_ZONE_TABLE" |
+      ' ="$TZ_ZONETABTYPE_TABLE" |
       sort -u |
       tr '\n' ' '
       echo ''
@@ -431,12 +434,19 @@ while
       doselect '"$quoted_continents"' \
 	"coord - I want to use geographical coordinates." \
 	"TZ - I want to specify the timezone using the POSIX TZ format." \
-	"time - I know local time already."
+	"time - I know local time already." \
+	"now - Like \"time\", but configure only for timestamps from now on."
       continent=$select_result
       case $continent in
       Americas) continent=America;;
       *" "*)    continent=`expr "$continent" : '\''\([^ ]*\)'\''`
       esac
+      case $zonetabtype,$continent in
+      zonenow,*) ;;
+      *,now)
+	${TZ_ZONENOW_TABLE:+:} read_file TZ_ZONENOW_TABLE "$TZDIR/zonenow.tab"
+	TZ_ZONE_TABLE=$TZ_ZONENOW_TABLE
+      esac
     '
   esac
 
@@ -529,7 +539,7 @@ while
       `;;
     *)
       case $continent in
-      time)
+      now|time)
 	minute_format='%a %b %d %H:%M'
 	old_minute=`TZ=UTC0 date +"$minute_format"`
 	for i in 1 2 3
@@ -609,6 +619,31 @@ while
 	  ="$continent_re" ="$TZ_COUNTRY_TABLE" ="$zone_table" |
 	sort -f
       `
+      # If all zone table entries have comments, and there are
+      # at most 22 entries, asked based on those comments.
+      # This fits the prompt onto old-fashioned 24-line screens.
+      regions=`
+	$AWK '
+	  BEGIN {
+	    TZ_ZONE_TABLE = substr(ARGV[1], 2)
+	    ARGV[1] = ""
+	    FS = "\t"
+	    nlines = split(TZ_ZONE_TABLE, line, /\n/)
+	    for (i = 1; i <= nlines; i++) {
+	      $0 = line[i]
+	      if ($0 ~ /^[^#]/ && !missing_comment) {
+		if ($4)
+		  comment[++inlines] = $4
+		else
+		  missing_comment = 1
+	      }
+	    }
+	    if (!missing_comment && inlines <= 22)
+	      for (i = 1; i <= inlines; i++)
+		print comment[i]
+	  }
+	' ="$zone_table"
+      `
 
       # If there's more than one country, ask the user which one.
       case $countries in