Determine if a string has any duplicate characters

c/unique.c

@@ -0,0 +1,43 @@
+// not uniq(1)
+
+#include <stdint.h>
+#include <stdio.h>
+#include <ctype.h>
+
+const char *unique(const char *str, const char **dup) {
+  const char *alphabet[26] = { NULL };
+
+  for(; *str; str += 1) {
+    uint8_t pos = tolower(*str) - 'a';
+
+    if(pos >= 0 && pos <= 25) {
+      if(alphabet[pos]) {
+        *dup = str;
+        return alphabet[pos];
+      } else {
+        alphabet[pos] = str;
+      }
+    }
+  }
+
+  return NULL;
+}
+
+int main(int argc, char **argv) {
+  const char *str = "";
+
+  if(argc > 1) {
+    str = argv[1];
+  }
+
+  const char *dup = NULL;
+  const char *idx = unique(str, &dup);
+
+  if(dup) {
+    uint8_t idx_spacing = (int) (idx - str) + 1;
+    uint8_t dup_spacing = (int) (dup - str) - idx_spacing + 1;
+    printf("%s\n%*s%*s\n", str, idx_spacing, "^", dup_spacing, "^");
+  }
+
+  return 0;
+}