Fix problem with setting the month of certain leap years.

src/dex/time/libtime.c

@@ -1132,35 +1132,55 @@ DeeTime_SetRepr(DeeTimeObject *__restrict self,
 	}	break;
 
 	case TIME_REPR_MONTH: { /* Month in Year */
-		Dee_int128_t old_months, mday, extra_nano;
+		Dee_int128_t old_months, nano_since_start_of_month, start_of_month;
 		Dee_int128_t new_months, month_delta, new_year;
+		Dee_int128_t new_month_length;
+		uint16_t new_month_length_in_days;
 		uint8_t old_month, new_month;
 		struct month const *new_year_months;
-		uint16_t new_month_length;
-		_DeeTime_GetRepr(&old_months, self, TIME_REPR_MONTHS);
-		_DeeTime_GetRepr(&mday, self, TIME_REPR_MDAY);
+
+		/* Figure out the accurate month number since "0" */
+		old_months = self->t_nanos;
+		time_inplace_nanosecond2month(&old_months);
+
+		/* Figure out the # of days since the start of the relevant month. */
+		start_of_month = old_months;
+		time_inplace_month2nanosecond(&start_of_month);
+		nano_since_start_of_month = self->t_nanos;
+		__hybrid_int128_sub128(nano_since_start_of_month, start_of_month);
+
+		/* Figure out the month within the relevant year, and determine delta. */
 		__hybrid_int128_floormod8_r(old_months, MONTHS_PER_YEAR, old_month);
 		++old_month; /* 1-based */
 		month_delta = *p_value;
 		__hybrid_int128_sub8(month_delta, old_month);
+
+		/* Apply delta to months */
 		new_months = old_months;
 		__hybrid_int128_add128(new_months, month_delta);
-		__hybrid_int128_floordivmod8(new_months, MONTHS_PER_YEAR, new_year, new_month);
-		new_year_months  = month_info_for_year(&new_year);
-		new_month_length = month_getlen(&new_year_months[new_month]);
 
-		/* Clamp day-of-month to the max valid value in the context of the new month */
-		__hybrid_int128_dec(mday); /* We need a 0-based day-of-month */
-		if (__hybrid_int128_ge16(mday, new_month_length))
-			__hybrid_int128_set16(mday, new_month_length - 1);
+		/* Figure out the new, relevant year+month */
+		__hybrid_int128_floordivmod8(new_months, MONTHS_PER_YEAR, new_year, new_month);
+		new_year_months = month_info_for_year(&new_year);
+		new_month_length_in_days = month_getlen(&new_year_months[new_month]);
+		__hybrid_uint128_set16(*(Dee_uint128_t *)&new_month_length, new_month_length_in_days);
+		time_inplace_days2nanoseconds(&new_month_length);
+
+		/* Clamp day-of-month to the max valid value in the context of the new month.
+		 * This is needed for:
+		 * >> local t = Time(year: 1976, month: 1, day: 31);
+		 * >> t.month = 2;
+		 * >> print repr t;
+		 * >> assert t == Time(year: 1976, month: 2, day: 29, hour: 23, minute: 59, second: 59, nanosecond: 999999999);
+		 * Without this, the date's month would unexpectedly be in March instead. */
+		if (__hybrid_int128_ge128(nano_since_start_of_month, new_month_length)) {
+			nano_since_start_of_month = new_month_length;
+			__hybrid_int128_dec(nano_since_start_of_month);
+		}
 
-		/* Combine `new_months' and `mday' to set the total number of days */
+		/* Combine `new_months' and `nano_since_start_of_month' to form a new timestamp. */
 		time_inplace_month2nanosecond(&new_months);
-		time_inplace_days2nanoseconds(&mday);
-		__hybrid_int128_add128(new_months, mday);
-		extra_nano = self->t_nanos; /* Keep hour+minute+second+nanosecond */
-		__hybrid_int128_floormod64(extra_nano, NANOSECONDS_PER_DAY);
-		__hybrid_int128_add128(new_months, extra_nano);
+		__hybrid_int128_add128(new_months, nano_since_start_of_month);
 		self->t_nanos = new_months;
 	}	break;
 

util/test/deemon-string-format-extended.dee

@@ -125,6 +125,7 @@ assert "x = 11" == "x = {_$1 + $1}".format({ "_$1": 10 });
 
 /* Do some more tests on function calls done from within template strings. */
 function func(a = 30, b = 40) -> a + b;
+assert "x = 70" == "x = {func()}".format({ "func": func });
 assert "x = 50" == "x = {func($10)}".format({ "func": func });
 assert "x = 30" == "x = {func($10, $20)}".format({ "func": func });
 assert "x = 30" == "x = {func($10, b: $20)}".format({ "func": func });

util/test/time-month.dee

@@ -0,0 +1,51 @@
+#!/usr/bin/deemon
+/* Copyright (c) 2018-2024 Griefer@Work                                       *
+ *                                                                            *
+ * This software is provided 'as-is', without any express or implied          *
+ * warranty. In no event will the authors be held liable for any damages      *
+ * arising from the use of this software.                                     *
+ *                                                                            *
+ * Permission is granted to anyone to use this software for any purpose,      *
+ * including commercial applications, and to alter it and redistribute it     *
+ * freely, subject to the following restrictions:                             *
+ *                                                                            *
+ * 1. The origin of this software must not be misrepresented; you must not    *
+ *    claim that you wrote the original software. If you use this software    *
+ *    in a product, an acknowledgement (see the following) in the product     *
+ *    documentation is required:                                              *
+ *    Portions Copyright (c) 2018-2024 Griefer@Work                           *
+ * 2. Altered source versions must be plainly marked as such, and must not be *
+ *    misrepresented as being the original software.                          *
+ * 3. This notice may not be removed or altered from any source distribution. *
+ */
+
+import * from deemon;
+import * from time;
+
+function getYMD(t: Time): (int, int, int) {
+	return (t.year, t.month, t.mday);
+}
+
+
+/* Make sure that months in leap-years are handled correctly.
+ *
+ * Before, there was an off-by-one error, where setting the
+ * month of a leap-year would subtract 1 year. */
+assert getYMD(Time(year: 103, month: 2, day: 29)) == (103, 3, 1);
+assert getYMD(Time(year: 104, month: 2, day: 29)) == (104, 2, 29);
+assert getYMD(Time(year: 105, month: 2, day: 29)) == (105, 3, 1);
+assert getYMD(Time(year: 1975, month: 2, day: 29)) == (1975, 3, 1);
+assert getYMD(Time(year: 1976, month: 2, day: 29)) == (1976, 2, 29);
+assert getYMD(Time(year: 1977, month: 2, day: 29)) == (1977, 3, 1);
+
+/* Assert that sub-month times are properly clamped when setting a
+ * month while the sub-month time would not fit into the new month. */
+local t = Time(year: 1977, month: 1, day: 31);
+t.month = 2;
+assert t == Time(year: 1977, month: 2, day: 28, hour: 23, minute: 59, second: 59, nanosecond: 999999999);
+
+/* Also make sure that month-day clamping correctly makes use of leap days. */
+local t = Time(year: 1976, month: 1, day: 31);
+t.month = 2;
+assert t == Time(year: 1976, month: 2, day: 29, hour: 23, minute: 59, second: 59, nanosecond: 999999999);
+