Replace loops with O(1) computation.

.gitignore

@@ -134,6 +134,7 @@ Makefile
 /tests/test_cdf_read
 /tests/test_cdf_write
 /tests/test_grib
+/tests/test_month_adjust
 /tests/test_resource_copy
 /tests/test_resource_copy_mpi
 /tests/test_table

src/calendar.h

@@ -6,6 +6,8 @@
 
 // clang-format off
 
+#include <stdlib.h>
+
 #include "cdi.h"
 
 #ifdef __cplusplus
@@ -30,6 +32,20 @@ calendar_dpy(int calendar)
 int days_per_year(int calendar, int year);
 int days_per_month(int calendar, int year, int month);
 
+struct yearMonth
+{
+  int year, month;
+};
+
+/* normalizes month to range [1,12] and adjusts year accordingly */
+static inline struct yearMonth
+normalize_month(int year, int month)
+{
+  div_t modres = div(month - 1, 12);
+  year += modres.quot - ((month < 1) & (modres.rem != 0));
+  return (struct yearMonth){ .year = year, .month = (modres.rem + 12) % 12 + 1 };
+}
+
 #ifdef __cplusplus
 }
 #endif

src/taxis.c

@@ -1035,10 +1035,9 @@ rtimeval2datetime(double timevalue, taxis_t *taxis)
 
       month += nmon;
 
-      // clang-format off
-      while (month > 12) { month -= 12; year++; }
-      while (month <  1) { month += 12; year--; }
-      // clang-format on
+      struct yearMonth ym = normalize_month(year, month);
+      year = ym.year;
+      month = ym.month;
 
       timeunits = TUNIT_DAY;
       timevalue = fmon * days_per_month(calendar, year, month);
@@ -1084,10 +1083,9 @@ datetime2rtimeval(CdiDateTime vDateTime, taxis_t *taxis)
       int nmonth = (int) value;
       month -= nmonth;
 
-      // clang-format off
-      while (month > 12) { month -= 12; year++; }
-      while (month <  1) { month += 12; year--; }
-      // clang-format on
+      struct yearMonth ym = normalize_month(year, month);
+      year = ym.year;
+      month = ym.month;
 
       const int dpm = days_per_month(calendar, year, month);
 
@@ -1267,10 +1265,9 @@ cdiSetForecastPeriod(double timevalue, taxis_t *taxis)
 
       month -= nmon;
 
-      // clang-format off
-      while (month > 12) { month -= 12; year++; }
-      while (month <  1) { month += 12; year--; }
-      // clang-format on
+      struct yearMonth ym = normalize_month(year, month);
+      year = ym.year;
+      month = ym.month;
 
       timeunits = TUNIT_DAY;
       timevalue = fmon * days_per_month(calendar, year, month);

tests/Makefile.am

@@ -8,6 +8,7 @@ TESTS = cksum_verify \
 	test_cksum_service test_cksum_nc2 test_cksum_nc4 test_cksum_ieg \
 	test_chunk_cksum \
 	calendar_test1 \
+	test_month_adjust \
 	pio_write_run pio_cksum_mpinonb pio_cksum_fpguard \
 	pio_cksum_asynch pio_cksum_writer \
 	pio_cksum_cdf \
@@ -23,6 +24,7 @@ TESTS = cksum_verify \
 
 check_PROGRAMS = cksum_verify test_grib cksum_write cksum_read \
 	calendar_test1 \
+	test_month_adjust \
 	pio_write \
 	test_resource_copy cksum_write_chunk pio_write_deco2d test_table \
 	test_byteswap
@@ -41,6 +43,7 @@ cksum_read_SOURCES = cksum_read.c \
 	stream_cksum.c stream_cksum.h \
 	ensure_array_size.h ensure_array_size.c
 calendar_test1_SOURCES = calendar_test1.c
+test_month_adjust_SOURCES = test_month_adjust.c
 pio_write_SOURCES = pio_write.c pio_write.h simple_model.c \
 	simple_model_helper.h simple_model_helper.c \
 	pio_write_setup_grid.h pio_write_setup_grid.c

tests/test_month_adjust.c

+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/time.h>
+
+#include "calendar.h"
+
+int
+main(void)
+{
+  static const int vals[] = { -12, -17, 17, 11, -769652651 };
+  enum
+  {
+    numVals = sizeof(vals) / sizeof(vals[0])
+  };
+  for (size_t i = 0; i < numVals; ++i)
+    {
+      enum
+      {
+        startYear = 1900
+      };
+      int month = vals[i];
+      struct yearMonth ym = normalize_month(startYear, month);
+      if ((long) ym.year * 12 + ym.month != (long) startYear * 12 + month)
+        {
+          fprintf(stderr,
+                  "problem: month=%d, ym.month=%d, ym.year=%d\n"
+                  "(long)ym.year * 12 + ym.month = %ld\n"
+                  "(long)startYear * 12 + month = %ld\n",
+                  month, ym.month, ym.year, (long) ym.year * 12 + ym.month, (long) startYear * 12 + month);
+          abort();
+        }
+    }
+  {
+    struct timeval tv;
+    if (gettimeofday(&tv, NULL))
+      {
+        perror("failed to get time for random number generator initialization");
+        exit(EXIT_FAILURE);
+      }
+    srandom((unsigned) (tv.tv_sec ^ tv.tv_usec));
+  }
+  for (size_t j = 0; j < 1000000; ++j)
+    {
+      int year = (int) (random() - RAND_MAX / 2), month = (int) (random() - RAND_MAX / 2);
+      struct yearMonth ym = normalize_month(year, month);
+      if ((long) ym.year * 12 + ym.month != (long) year * 12 + month)
+        {
+          fprintf(stderr,
+                  "problem: month=%d, ym.month=%d, ym.year=%d\n"
+                  "(long)ym.year * 12 + ym.month = %ld\n"
+                  "(long)year * 12 + month = %ld\n",
+                  month, ym.month, ym.year, (long) ym.year * 12 + ym.month, (long) year * 12 + month);
+          abort();
+        }
+    }
+  return EXIT_SUCCESS;
+}