[PATCH v2] date.c: allow ISO 8601 reduced precision times

["Đoàn Trần Công Danh" <congdanhqx@gmail.com>]

ISO 8601 permits "reduced precision" time representations to omit the
seconds value or both the minutes and the seconds values.  The
abbreviate times could look like 17:45 or 1745 to omit the seconds,
or simply as 17 to omit both the minutes and the seconds.

parse_date_basic accepts the 17:45 format but it rejects the other two.
Change it to accept 4-digit and 2-digit time values when they follow a
recognized date and a 'T'.

Before this change:

$ TZ=UTC test-tool date approxidate 2022-12-13T23:00 2022-12-13T2300 2022-12-13T23
2022-12-13T23:00 -> 2022-12-13 23:00:00 +0000
2022-12-13T2300 -> 2022-12-13 23:54:13 +0000
2022-12-13T23 -> 2022-12-13 23:54:13 +0000

After this change:

$ TZ=UTC helper/test-tool date approxidate 2022-12-13T23:00 2022-12-13T2300 2022-12-13T23
2022-12-13T23:00 -> 2022-12-13 23:00:00 +0000
2022-12-13T2300 -> 2022-12-13 23:00:00 +0000
2022-12-13T23 -> 2022-12-13 23:00:00 +0000

Note: ISO 8601 also allows reduced precision date strings such as
"2022-12" and "2022". This patch does not attempt to address these.

Reported-by: Pat LaVarre <plavarre@purestorage.com>
Signed-off-by: Phil Hord <phil.hord@gmail.com>
Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
---

Since this is a complete re-implementation from Phil Hord's version.
I'm reassigning the author to me.

This version change the implementation to only treat the string as ISO8601 if
a 'T' existed and date has been parsed. I also added a test for parsing
RFC-822, which Hord accidentally broke.

The commit message has been changed:
* The example has been changed to be independent from local timezone
* Remove the mention of adding test-cases, since it's obviously necessary.

 date.c          | 37 +++++++++++++++++++++++++++++++++++++
 t/t0006-date.sh |  8 ++++++++
 2 files changed, 45 insertions(+)

diff --git a/date.c b/date.c
index 53bd6a7932..6f45eeb356 100644
--- a/date.c
+++ b/date.c
@@ -493,6 +493,12 @@ static int match_alpha(const char *date, struct tm *tm, int *offset)
 		return 2;
 	}
 
+	/* ISO-8601 allows yyyymmDD'T'HHMMSS, with less precision */
+	if (*date == 'T' && isdigit(date[1]) && tm->tm_hour == -1) {
+		tm->tm_min = tm->tm_sec = 0;
+		return 1;
+	}
+
 	/* BAD CRAP */
 	return skip_alpha(date);
 }
@@ -638,6 +644,18 @@ static inline int nodate(struct tm *tm)
 		tm->tm_sec) < 0;
 }
 
+/*
+ * Have we seen an ISO-8601-alike date, i.e. 20220101T0,
+ * In which, hour is still unset,
+ * and minutes and second has been set to 0.
+ */
+static inline int maybeiso8601(struct tm *tm)
+{
+	return tm->tm_hour == -1 &&
+		tm->tm_min == 0 &&
+		tm->tm_sec == 0;
+}
+
 /*
  * We've seen a digit. Time? Year? Date?
  */
@@ -701,6 +719,25 @@ static int match_digit(const char *date, struct tm *tm, int *offset, int *tm_gmt
 		return end - date;
 	}
 
+	/* reduced precision of ISO-8601's time: HHMM or HH */
+	if (maybeiso8601(tm)) {
+		unsigned int num1 = num;
+		unsigned int num2 = 0;
+		if (n == 4) {
+			num1 = num / 100;
+			num2 = num % 100;
+		}
+		if ((n == 4 || n == 2) && !nodate(tm) &&
+		    set_time(num1, num2, 0, tm) == 0)
+			return n;
+		/*
+		 * We thought this is an ISO-8601 time string,
+		 * we set minutes and seconds to 0,
+		 * turn out it isn't, rollback the change.
+		 */
+		tm->tm_min = tm->tm_sec = -1;
+	}
+
 	/* Four-digit year or a timezone? */
 	if (n == 4) {
 		if (num <= 1400 && *offset == -1) {
diff --git a/t/t0006-date.sh b/t/t0006-date.sh
index 2490162071..e18b160286 100755
--- a/t/t0006-date.sh
+++ b/t/t0006-date.sh
@@ -88,6 +88,13 @@ check_parse 2008-02-14 bad
 check_parse '2008-02-14 20:30:45' '2008-02-14 20:30:45 +0000'
 check_parse '2008-02-14 20:30:45 -0500' '2008-02-14 20:30:45 -0500'
 check_parse '2008.02.14 20:30:45 -0500' '2008-02-14 20:30:45 -0500'
+check_parse '20080214T20:30:45' '2008-02-14 20:30:45 +0000'
+check_parse '20080214T20:30' '2008-02-14 20:30:00 +0000'
+check_parse '20080214T20' '2008-02-14 20:00:00 +0000'
+check_parse '20080214T203045' '2008-02-14 20:30:45 +0000'
+check_parse '20080214T2030' '2008-02-14 20:30:00 +0000'
+check_parse '20080214T000000.20' '2008-02-14 00:00:00 +0000'
+check_parse '20080214T00:00:00.20' '2008-02-14 00:00:00 +0000'
 check_parse '20080214T203045-04:00' '2008-02-14 20:30:45 -0400'
 check_parse '20080214T203045 -04:00' '2008-02-14 20:30:45 -0400'
 check_parse '20080214T203045.019-04:00' '2008-02-14 20:30:45 -0400'
@@ -99,6 +106,7 @@ check_parse '2008-02-14 20:30:45 -05' '2008-02-14 20:30:45 -0500'
 check_parse '2008-02-14 20:30:45 -:30' '2008-02-14 20:30:45 +0000'
 check_parse '2008-02-14 20:30:45 -05:00' '2008-02-14 20:30:45 -0500'
 check_parse '2008-02-14 20:30:45' '2008-02-14 20:30:45 -0500' EST5
+check_parse 'Thu, 7 Apr 2005 15:14:13 -0700' '2005-04-07 15:14:13 -0700'
 
 check_approxidate() {
 	echo "$1 -> $2 +0000" >expect
-- 
2.39.0.287.g690a66fa66