From 46a9f55050cb852410da9c80c7e248d8df0328a8 Mon Sep 17 00:00:00 2001
From: Abigail <abigail@abigail.freedom.nl>
Date: Tue, 11 Jan 2022 19:04:33 +0100
Subject: Week 147: C solutions

---
 challenge-147/abigail/c/ch-1.c | 70 ++++++++++++++++++++++++++++++++++++++++++
 challenge-147/abigail/c/ch-2.c | 66 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 136 insertions(+)
 create mode 100644 challenge-147/abigail/c/ch-1.c
 create mode 100644 challenge-147/abigail/c/ch-2.c

diff --git a/challenge-147/abigail/c/ch-1.c b/challenge-147/abigail/c/ch-1.c
new file mode 100644
index 0000000000..d02433f475
--- /dev/null
+++ b/challenge-147/abigail/c/ch-1.c
@@ -0,0 +1,70 @@
+# include <stdlib.h>
+# include <stdio.h>
+# include <string.h>
+# include <stdbool.h>
+
+/*
+ * See https://theweeklychallenge.org/blog/perl-weekly-challenge-147
+ */
+
+/*
+ * Run as: cc -o ch-1.o ch-1.c; ./ch-1.o
+ */
+# define COUNT 20
+
+bool is_prime (int n) {
+    if (n % 2 == 0) {return n == 2;}
+    for (int i = 3; i * i <= n; i += 2) {
+        if (n % i == 0) {return false;}
+    }
+    return true;
+}
+
+int main (void) {
+    int * todo;
+    if ((todo = (int *) malloc (4 * sizeof (int))) == NULL) {
+        perror ("Malloc failed");
+        return (1);
+    }
+    todo [0]    = 2;
+    todo [1]    = 3;
+    todo [2]    = 5;
+    todo [3]    = 7;
+    size_t high = 4;
+
+    for (size_t i = 0; i < high; i ++) {
+        printf ("%d ", todo [i]);
+    }
+
+    int count = COUNT - high;
+
+    int pow = 10;
+    while (count > 0) {
+        int * next;
+        int next_high = 0;
+        if ((next = (int *) malloc (9 * high * sizeof (int))) == NULL) {
+            perror ("Malloc failed");
+            return (1);
+        }
+        for (int d = 1; d <= 9 && count > 0; d ++) {
+            for (size_t i = 0; i < high && count > 0; i ++) {
+                int candidate = d * pow + todo [i];
+                if (is_prime (candidate)) {
+                    next [next_high ++] = candidate;
+                    printf ("%d ", candidate);
+                    count --;
+                }
+            }
+        }
+        if (!next_high) {
+            break;
+        }
+        free (todo);
+        todo = next;
+        high = next_high;
+        pow *= 10;
+    }
+    free (todo);
+
+    printf ("\n");
+}
diff --git a/challenge-147/abigail/c/ch-2.c b/challenge-147/abigail/c/ch-2.c
new file mode 100644
index 0000000000..02ff39bd1e
--- /dev/null
+++ b/challenge-147/abigail/c/ch-2.c
@@ -0,0 +1,66 @@
+# include <stdlib.h>
+# include <stdio.h>
+# include <string.h>
+# include <stdbool.h>
+
+/*
+ * See https://theweeklychallenge.org/blog/perl-weekly-challenge-147
+ */
+
+/*
+ * Run as: cc -o ch-2.o ch-2.c; ./ch-2.o
+ */
+
+bool is_pentagonal (int candidate, int * pentagon, size_t max) {
+    size_t low  = 0;
+    size_t high = max;
+    while (low < high) {
+        size_t mid = (low + high) / 2;
+        if (pentagon [mid] == candidate) {return true;}
+        if (pentagon [mid]  < candidate) {low  = mid + 1;}
+        if (pentagon [mid]  > candidate) {high = mid;}
+    }
+    return false;
+}
+
+
+int main (void) {
+    int *  pentagon =  NULL;
+    int    n        =     0;
+    int    p        =     0;
+    size_t cap      =   100;
+    size_t high     =     0;
+    bool   done     = false;
+
+    if ((pentagon = (int *) malloc (cap * sizeof (int))) == NULL) {
+        perror ("Malloc failed");
+        return (1);
+    }
+
+    while (!done) {
+        p += n + n + n + 1;
+        n ++;
+        if (high >= cap) {
+            cap *= 2;
+            if ((pentagon =
+                     (int *) realloc (pentagon, cap * sizeof (int))) == NULL) {
+                perror ("Realloc failed");
+                return (1);
+            }
+        }
+        pentagon [high ++] = p;
+
+        for (size_t i = 0; i < high &&
+                           pentagon [i] + pentagon [i] < p &&
+                          !done; i ++) {
+            int seen = pentagon [i];
+            if (is_pentagonal (p - seen,        pentagon, high) &&
+                is_pentagonal (p - seen - seen, pentagon, high)) {
+                printf ("%d %d\n", seen, p - seen);
+                done = true;
+            }
+        }
+    }
+
+    return (0);
+}
-- 
cgit