From ade625b24f12025b278ef2cfaaf857ac81df4319 Mon Sep 17 00:00:00 2001
From: Abigail <abigail@abigail.be>
Date: Wed, 17 Feb 2021 18:43:33 +0100
Subject: C solution for week 100, part 1

---
 challenge-100/abigail/README.md |  1 +
 challenge-100/abigail/c/ch-1.c  | 75 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 76 insertions(+)
 create mode 100644 challenge-100/abigail/c/ch-1.c

diff --git a/challenge-100/abigail/README.md b/challenge-100/abigail/README.md
index ddce3fd34a..ff62f9e3ba 100644
--- a/challenge-100/abigail/README.md
+++ b/challenge-100/abigail/README.md
@@ -21,6 +21,7 @@ Output: 07:15 pm or 07:15pm
 * [GNU AWK](awk/ch-1.gawk)
 * [Bash](bash/ch-1.sh)
 * [Befunge-93](befunge-93/ch-1.bf93)
+* [C](c/ch-1.c)
 * [Perl](perl/ch-1.pl)
 
 ### Blog
diff --git a/challenge-100/abigail/c/ch-1.c b/challenge-100/abigail/c/ch-1.c
new file mode 100644
index 0000000000..8ea41facc9
--- /dev/null
+++ b/challenge-100/abigail/c/ch-1.c
@@ -0,0 +1,75 @@
+# include <stdlib.h>
+# include <stdio.h>
+# include <string.h>
+# include <ctype.h>
+
+/*
+ * See ../README.md
+ */
+
+/*
+ * Run as: cc -o ch-1.o ch-1.c; ./ch-1.o < input-file
+ */
+
+int main (void) {
+    char *  line    = NULL;
+    size_t  len     = 0;
+    size_t  str_len;
+
+    while ((str_len = getline (&line, &len, stdin)) != -1) {
+        char * ptr = line;
+
+        /*
+         * Parse hour and minute from line
+         */
+        int  hour = atoi (ptr);
+        while (isdigit (* ptr)) {
+            ptr ++;
+        }
+        ptr ++;   /* The ':' separating hour and minute */
+        int  minute = atoi (ptr);
+
+        /*
+         * Advance the pointer to the first character of the
+         * AM/PM marker; if there is none, it will point 
+         * to the NUL byte. (And * ptr is then false).
+         */
+        while (* ptr && (isdigit (* ptr) || isspace (* ptr))) {
+            ptr ++;
+        }
+
+        /*
+         * Calculate the new AM/PM marker:
+         *   - If there was one, there new one will be empty.
+         *   - Else, if the hour is 12 or larger, it's PM
+         *   - Else, it's AM
+         */
+        char * new_ampm = * ptr ? "" : hour >= 12 ? "pm" : "am";
+
+        /*
+         * Calculate the new hour:
+         *   - we always mod the hour by 12;
+         *        this has the effect of making 12 AM (and 12 PM)
+         *        equal to 0, and subtracts 12 from time in 24-hour
+         *        format, for times after noon.
+         *   - if we started from a 24 hour time, and the hour is
+         *     now 0, make it 12.
+         *   - if we started from PM time, add 12 to the hour.
+         */
+        hour %= 12;
+        if (!* ptr && hour == 0) {
+            hour  = 12;
+        }
+        if (tolower (* ptr) == 'p') {
+            hour += 12;
+        }
+        
+        /*
+         * Print the result.
+         */
+        printf ("%02d:%02d%s\n", hour, minute, new_ampm);
+    }
+    free (line);
+
+    return (0);
+}
-- 
cgit