1 files changed, 81 insertions, 0 deletions
diff --git a/luhncheck.c b/luhncheck.c
new file mode 100644
index 0000000..51f04ab
--- /dev/null
+++ b/luhncheck.c
@@ -0,0 +1,81 @@
+/* See LICENSE file for copyright and license details. */
+#include <libsimple-arg.h>
+#include <errno.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+NUSAGE(2, "number ...");
+
+
+static int
+check_luhn(const char *s)
+{
+	unsigned sum = 0;
+
+	switch (strlen(s) & 1U) {
+		for (;;) {
+		case 0:
+			if ('0' <= *s && *s <= '4')
+				sum += 2U * (unsigned)(*s - '0');
+			else if ('5' <= *s && *s <= '9')
+				sum += 2U * (unsigned)(*s - '0') - 9U;
+			else if (*s)
+				return 0;
+			else
+				break;
+			s++;
+
+		case 1:
+			if ('0' <= *s && *s <= '9')
+				sum += (unsigned)(*s - '0');
+			else if (*s)
+				return 0;
+			else
+				break;
+			s++;
+
+			sum %= 10U;
+		}
+	}
+
+	return (sum % 10U) == 0U;
+}
+
+
+int
+main(int argc, char *argv[])
+{
+	int use_colour;
+	int ret = 0;
+	int r, ok;
+
+	ARGBEGIN {
+	default:
+		usage();
+	} ARGEND;
+
+	if (!argc)
+		usage();
+
+	use_colour = isatty(STDOUT_FILENO);
+
+	for (; *argv; argv++) {
+		ok = check_luhn(*argv);
+		ret |= !ok;
+		r = printf("%s%s %s%s\n", use_colour ? ok ? "\033[1;32m" : "\033[1;31m" : "",
+		           *argv, ok ? "is OK" : "is invalid", use_colour ? "\033[m" : "");
+		if (r < 0) {
+			fprintf(stderr, "%s: printf: %s\n", argv0, strerror(errno));
+			exit(2);
+		}
+	}
+
+	if (fflush(stdout) || fclose(stdout)) {
+		fprintf(stderr, "%s: printf: %s\n", argv0, strerror(errno));
+		exit(2);
+	}
+
+	return ret;
+}