Add C solution

1 files changed, 45 insertions, 0 deletions

diff --git a/challenge-030/paulo-custodio/c/ch-1.c b/challenge-030/paulo-custodio/c/ch-1.c
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+/*
+Challenge 030
+
+Task #1
+Write a script to list dates for Sunday Christmas between 2019 and 2100. For
+example, 25 Dec 2022 is Sunday.
+*/
+
+#include <assert.h>
+#include <stdio.h>
+#include <stdbool.h>
+
+enum { Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday };
+
+int day_of_week(int y, int m, int d) {
+    return (d += m < 3 ? y-- : y - 2, 23*m/9 + d + 4 + y/4- y/100 + y/400)%7;
+}
+
+bool is_leap_year(int y) {
+    return y%400==0 ? true : y%100==0 ? false : y%4==0 ? true : false;
+}
+
+int days_of_month(int y, int m) {
+    switch (m) {
+        case 1: case 3: case 5: case 7: case 8: case 10: case 12: return 31;
+        case 4: case 6: case 9: case 11: return 30;
+        case 2: return is_leap_year(y) ? 29 : 28;
+        default: assert(0);
+    }
+}
+
+void print_sunday_christmas(int start_year, int end_year) {
+    const char* sep = "";
+    for (int y = start_year; y <= end_year; y++) {
+        if (day_of_week(y, 12, 25) == Sunday) {
+            printf("%s%04d", sep, y);
+            sep = ", ";
+        }
+    }
+    printf("\n");
+}
+
+int main() {
+    print_sunday_christmas(2019, 2100);
+}