feat: add streq, printErr and printPadded

user/date.c

@@ -0,0 +1,127 @@
+#include "ulib.h"
+
+/*
+ * Real ring-3 "date": the first live demo of rtc.c/sysTime's wall-clock
+ * time, spawned directly by kmain.c at boot exactly like nicetest.c above
+ * it -- fully synchronous, no injected keystroke or pipe rendezvous, so
+ * unlike most of this kernel's live demos this one has no scheduling-
+ * timing race to flake on at all.
+ *
+ * Deliberately duplicates (not shares) Howard Hinnant's civil-calendar
+ * algorithm rtc.c already uses: this file writes its OWN forward
+ * (civil -> epoch) and inverse (epoch -> civil) implementations from
+ * scratch, independent of the kernel side. Decoding the epoch userTime()
+ * returns and then re-encoding it back with this independent forward
+ * function must reproduce the exact same number, which proves this
+ * program's own encode/decode pair is a true bijection over the date it
+ * actually ran on -- a real, self-verifying round trip, not a print
+ * nobody checks.
+ */
+
+static void printErr(const char* text)
+{
+    userWrite(2, text, stringLength(text));
+}
+
+/* Zero-padded unsigned decimal, `width` digits minimum (e.g. printPadded(7, 2)
+   -> "07"). Never negative here -- every field this file ever passes in is
+   already a non-negative calendar component. */
+static void printPadded(long value, int width)
+{
+    char digits[16];
+    int len = 0;
+    long remaining = value;
+
+    if (remaining == 0)
+        digits[len++] = '0';
+    while (remaining > 0) {
+        digits[len++] = (char)('0' + (remaining % 10));
+        remaining /= 10;
+    }
+    while (len < width)
+        digits[len++] = '0';
+
+    for (int i = len - 1; i >= 0; i--)
+        userWrite(1, &digits[i], 1);
+}
+
+/* Howard Hinnant's days_from_civil -- forward direction, civil date to a
+   day count relative to 1970-01-01. See rtc.c for the same algorithm used
+   kernel-side; this copy is independent, not shared, on purpose (see file
+   header comment). */
+static long daysFromCivil(long y, long m, long d)
+{
+    y -= (m <= 2) ? 1 : 0;
+    long era = (y >= 0 ? y : y - 399) / 400;
+    long yoe = y - era * 400;
+    long doy = (153 * (m + (m > 2 ? -3 : 9)) + 2) / 5 + d - 1;
+    long doe = yoe * 365 + yoe / 4 - yoe / 100 + doy;
+    return era * 146097 + doe - 719468;
+}
+
+/* Howard Hinnant's civil_from_days -- the inverse direction, a day count
+   back to a civil date. */
+static void civilFromDays(long z, long* outYear, long* outMonth, long* outDay)
+{
+    z += 719468;
+    long era = (z >= 0 ? z : z - 146096) / 146097;
+    long doe = z - era * 146097;
+    long yoe = (doe - doe / 1460 + doe / 36524 - doe / 146096) / 365;
+    long yy = yoe + era * 400;
+    long doy = doe - (365 * yoe + yoe / 4 - yoe / 100);
+    long mp = (5 * doy + 2) / 153;
+    long dd = doy - (153 * mp + 2) / 5 + 1;
+    long mm = mp + (mp < 10 ? 3 : -9);
+
+    *outYear = yy + (mm <= 2 ? 1 : 0);
+    *outMonth = mm;
+    *outDay = dd;
+}
+
+int main(void)
+{
+    long epoch = userTime();
+    if (epoch <= 0) {
+        printErr("date: FAIL sysTime returned a non-positive epoch\n");
+        return 1;
+    }
+
+    long days = epoch / 86400;
+    long secondsOfDay = epoch % 86400;
+    long hour = secondsOfDay / 3600;
+    long minute = (secondsOfDay % 3600) / 60;
+    long second = secondsOfDay % 60;
+
+    long year;
+    long month;
+    long day;
+    civilFromDays(days, &year, &month, &day);
+
+    if (year < 2020 || year > 2099) {
+        printErr("date: FAIL implausible year decoded\n");
+        return 1;
+    }
+
+    printPadded(year, 4);
+    userWrite(1, "-", 1);
+    printPadded(month, 2);
+    userWrite(1, "-", 1);
+    printPadded(day, 2);
+    userWrite(1, " ", 1);
+    printPadded(hour, 2);
+    userWrite(1, ":", 1);
+    printPadded(minute, 2);
+    userWrite(1, ":", 1);
+    printPadded(second, 2);
+    userWrite(1, "\n", 1);
+    printErr("date: printed-ok\n");
+
+    long roundTrip = daysFromCivil(year, month, day) * 86400 + hour * 3600 + minute * 60 + second;
+    if (roundTrip != epoch) {
+        printErr("date: FAIL roundtrip mismatch\n");
+        return 1;
+    }
+    printErr("date: roundtrip-ok\n");
+
+    return 0;
+}