From 4630e2d37ba0a104b1bbebfebe1bedcbebd382ac Mon Sep 17 00:00:00 2001
From: Tomasz Kramkowski <tk@the-tk.com>
Date: Fri, 1 Apr 2016 19:30:35 +0200
Subject: rot.c: Fix this terribly broken rot

---
 rot.c | 130 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 111 insertions(+), 19 deletions(-)

diff --git a/rot.c b/rot.c
index c7c6771..985cc69 100644
--- a/rot.c
+++ b/rot.c
@@ -8,32 +8,124 @@
  * along with this program.  If not, see [http://www.gnu.org/licenses/].
  */
 
-#include <stdio.h>
-#include <string.h>
 #include <ctype.h>
+#include <errno.h>
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdio.h>
 #include <stdlib.h>
+#include <stdnoreturn.h>
+#include <string.h>
 
-int main(int argc, char **argv)
+#define ACHAR_CNT 26
+
+static noreturn void die(char *fmt, ...)
 {
-	if (argc != 3) {
-		fprintf(stderr, "Usage: <text> <rotation>\a\n");
-		exit(1);
-	}
+	va_list ap;
+
+	va_start(ap, fmt);
+	vfprintf(stderr, fmt, ap);
+	va_end(ap);
+
+	exit(EXIT_FAILURE);
+}
+
+static int aclamp(long val)
+{
+	while (val < 0)
+		val += ACHAR_CNT;
+
+	return val % ACHAR_CNT;
+}
+
+static int getrot(const char *str)
+{
+	long val;
+	char *endptr;
+
+	if (!*str)
+		die("Invalid argument '%s'\n", str);
+
+	errno = 0;
+	val = strtol(str, &endptr, 10);
+	if (errno)
+		die("Invalid argument '%s': %s", str, strerror(errno));
+	if (*endptr)
+		die("Invalid argument '%s'\n", str);
+
+	return aclamp(val);
+}
 
-	int rot = strtol(argv[2], NULL, 10);
-	unsigned length = strlen(argv[1]);
-
-	for (unsigned i = 0; i < length; i++) {
-		if (isalpha(argv[1][i])) {
-			if (argv[1][i] - 'A' < 26)
-				putchar((argv[1][i] - 'A' + rot) % 26 + 'A');
-			else
-				putchar((argv[1][i] - 'a' + rot) % 26 + 'a');
-		} else
-			putchar(argv[1][i]);
+static int chval(int c)
+{
+	switch (tolower(c)) {
+	case 'a': return  0;
+	case 'b': return  1;
+	case 'c': return  2;
+	case 'd': return  3;
+	case 'e': return  4;
+	case 'f': return  5;
+	case 'g': return  6;
+	case 'h': return  7;
+	case 'i': return  8;
+	case 'j': return  9;
+	case 'k': return 10;
+	case 'l': return 11;
+	case 'm': return 12;
+	case 'n': return 13;
+	case 'o': return 14;
+	case 'p': return 15;
+	case 'q': return 16;
+	case 'r': return 17;
+	case 's': return 18;
+	case 't': return 19;
+	case 'u': return 20;
+	case 'v': return 21;
+	case 'w': return 22;
+	case 'x': return 23;
+	case 'y': return 24;
+	case 'z': return 25;
 	}
 
-	putchar('\n');
+	return -1;
+}
+
+static int valch(int c, bool upper)
+{
+	static const char chars[] = "abcdefghijklmnopqrstuvwxyz";
+	int ret;
+
+	if (c < 0 || c > 25)
+		return 0;
+
+	ret = chars[aclamp(c)];
+
+	if (upper)
+		ret = toupper(ret);
+
+	return ret;
+}
+
+static int rot(int c, int r)
+{
+	return valch(aclamp(chval(c) + r), isupper(c));
+}
+
+int main(int argc, char **argv)
+{
+	int r, c;
+
+	if (argc != 2)
+		die("Usage: %s <rotation>\n", argv[0]);
+
+	r = getrot(argv[1]);
+
+	while (c = getchar(), c != EOF) {
+		if (isalpha(c))
+			putchar(rot(c, r));
+		else
+			putchar(c);
+	}
 
 	return 0;
 }
-- 
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