2 files changed, 116 insertions, 0 deletions
diff --git a/challenge-013/paulo-custodio/c/ch-1.c b/challenge-013/paulo-custodio/c/ch-1.c
new file mode 100644
index 0000000000..d15a9dfd1c
--- /dev/null
+++ b/challenge-013/paulo-custodio/c/ch-1.c
@@ -0,0 +1,64 @@
+/*
+Challenge 013
+
+Challenge #1
+Write a script to print the date of last Friday of every month of a given year.
+For example, if the given year is 2019 then it should print the following:
+
+2019/01/25
+2019/02/22
+2019/03/29
+2019/04/26
+2019/05/31
+2019/06/28
+2019/07/26
+2019/08/30
+2019/09/27
+2019/10/25
+2019/11/29
+2019/12/27
+*/
+
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.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 last_friday_month(int y, int m, int* d) {
+    *d = days_of_month(y, m);
+    while (day_of_week(y, m, *d) != Friday)
+        (*d)--;
+}
+
+int main(int argc, char* argv[]) {
+    if (argc != 2) {
+        fputs("usage: ch-1 year", stderr);
+        return EXIT_FAILURE;
+    }
+
+    int y = atoi(argv[1]);
+    for (int m = 1; m <= 12; m++) {
+        int d;
+        last_friday_month(y, m, &d);
+        printf("%04d/%02d/%02d\n", y, m, d);
+    }
+}
diff --git a/challenge-013/paulo-custodio/c/ch-2.c b/challenge-013/paulo-custodio/c/ch-2.c
new file mode 100644
index 0000000000..1cf93500af
--- /dev/null
+++ b/challenge-013/paulo-custodio/c/ch-2.c
@@ -0,0 +1,52 @@
+/*
+Challenge 013
+
+Challenge #2
+Write a script to demonstrate Mutually Recursive methods. Two methods are
+mutually recursive if the first method calls the second and the second calls
+first in turn. Using the mutually recursive methods, generate Hofstadter
+Female and Male sequences.
+
+ F ( 0 ) = 1   ;   M ( 0 ) = 0
+ F ( n ) = n − M ( F ( n − 1 ) ) , n > 0
+ M ( n ) = n − F ( M ( n − 1 ) ) , n > 0.
+*/
+
+#include <stdio.h>
+#include <stdlib.h>
+
+int M(int n);
+
+int F(int n) {
+    if (n == 0) return 1;
+    return n - M( F(n - 1) );
+}
+
+int M(int n) {
+    if (n == 0) return 0;
+    return n - F( M(n - 1) );
+}
+
+int main(int argc, char* argv[]) {
+    if (argc != 2) {
+        fputs("usage: ch-1 N", stderr);
+        return EXIT_FAILURE;
+    }
+
+    int N = atoi(argv[1]);
+    const char* sep = "";
+    printf("F: ");
+    for (int n = 0; n < N; n++) {
+        printf("%s%d", sep, F(n));
+        sep = ", ";
+    }
+    printf(", ...\n");
+
+    sep = "";
+    printf("M: ");
+    for (int n = 0; n < N; n++) {
+        printf("%s%d", sep, M(n));
+        sep = ", ";
+    }
+    printf(", ...\n");
+}