feat(inventory): add upc.c (Phase 1: scan + UPC validation)

code/inventory/upc.c

@@ -0,0 +1,54 @@
+/*
+ * upc.c - UPC-A validation.
+ *
+ * A UPC-A barcode is 12 digits. The last digit is a mod-10 checksum over the
+ * first 11: digits in odd positions (1,3,5,7,9,11) are weighted x3 and digits
+ * in even positions (2,4,6,8,10) x1; the check digit is whatever makes the
+ * weighted total a multiple of 10.
+ */
+#include "upc.h"
+
+static int allDigits(const char *s, int n)
+{
+    int i;
+    for (i = 0; i < n; i++) {
+        if (s[i] < '0' || s[i] > '9')
+            return 0;
+    }
+    return 1;
+}
+
+int upcCheckDigit(const char *s)
+{
+    int i, sum = 0;
+
+    if (!allDigits(s, 11))
+        return -1;
+
+    for (i = 0; i < 11; i++) {
+        int d = s[i] - '0';
+        if ((i % 2) == 0)   /* 0-based even index = 1st,3rd,... digit: weight 3 */
+            sum += 3 * d;
+        else
+            sum += d;
+    }
+    return (10 - (sum % 10)) % 10;
+}
+
+int upcIsValid(const char *s)
+{
+    int n = 0;
+    int check;
+
+    while (s[n] != '\0')
+        n++;
+    if (n != UPC_A_LEN)
+        return 0;
+    if (!allDigits(s, UPC_A_LEN))
+        return 0;
+
+    check = upcCheckDigit(s);
+    if (check < 0)
+        return 0;
+    return (s[11] - '0') == check;
+}