Adding computus in C (#697)

* Editting computus.md

* Adding gauss_easter.c

Author: Gathros

contents/computus/code/c/gauss_easter.c

@@ -0,0 +1,68 @@
+#include <stdio.h>
+
+char *computus(int year, int servois, char *out, size_t out_size) {
+    // Year's position on the 19 year metonic cycle
+    int a = year % 19;
+
+    // Century index
+    int k = year / 100;
+
+    //Shift of metonic cycle, add a day offset every 300 years
+    int p = (13 + 8 * k) / 25;
+
+    // Correction for non-observed leap days
+    int q = k / 4;
+
+    // Correction to starting point of calculation each century
+    int M = (15 - p + k - q) % 30;
+
+    // Number of days from March 21st until the full moon
+    int d = (19 * a + M) % 30;
+
+    // Returning if user wants value for Servois' table
+    if (servois) {
+        snprintf(out, out_size, "%d",(21 + d) % 31);
+        return out;
+    }
+
+    // Finding the next Sunday
+    // Century-based offset in weekly calculation
+    int N = (4 + k - q) % 7;
+
+    // Correction for leap days
+    int b = year % 4;
+    int c = year % 7;
+
+    // Days from d to next Sunday
+    int e = (2 * b + 4 * c + 6 * d + N) % 7;
+
+    // Historical corrections for April 26 and 25
+    if ((d == 29 && e == 6) || (d == 28 && e == 6 && a > 10)) {
+        e = -1;
+    }
+
+    if ((22 + d + e) > 31) {
+        snprintf(out, out_size, "April %d", d + e - 9);
+    } else {
+        snprintf(out, out_size, "March %d", 22 + d + e);
+    }
+
+    return out;
+}
+
+int main() {
+    char tmp1[9], tmp2[9];
+
+    printf("The following are the dates of the Paschal full moon (using "
+           "Servois notation) and the date of Easter for 2020-2030 AD:\n");
+
+    printf("Year\tServois number\tEaster\n");
+
+    for (int year = 2020; year <= 2030; year++) {
+        printf("%d\t\t%s\t%s\n", year, computus(year, 1, tmp1, 9),
+               computus(year, 0, tmp2, 9));
+    }
+
+    return 0;
+}
+

contents/computus/computus.md

@@ -131,7 +131,7 @@ $$
 k = \left\lfloor\frac{\text{year}}{100}\right\rfloor,
 $$
 where $$\lfloor\cdot\rfloor$$ is a flooring operation of rounding the value down to the nearest integer.
-With this, we can calculate the shift in the Metonic cycle to be, 
+With this, we can calculate the shift in the Metonic cycle to be,
 
 $$
 p = \left\lfloor\frac{13+8k}{25}\right\rfloor.
@@ -285,6 +285,8 @@ For now, we have the code outputting a tuple of $$d$$ and $$e$$, so users can us
 [import, lang:"python"](code/python/gauss_easter.py)
 {% sample lang="crystal" %}
 [import, lang:"crystal"](code/crystal/gauss_easter.cr)
+{% sample lang="c" %}
+[import, lang:"c"](code/c/gauss_easter.c)
 {% endmethod %}