Add c/isogram

c/isogram/src/isogram.c

@@ -0,0 +1,67 @@
+#include <stdbool.h>
+#include <stdio.h>
+
+int length(const char string[])
+{
+  int i = 0;
+  while (string[i] != '\0') {
+    i++;
+  }
+  return i+1; // Account for terminating null character.
+}
+
+void copy_phrase(const char phrase[], char copy[]) {
+  int i;
+  for (i = 0; i < length(phrase); i++) {
+    copy[i] = phrase[i];
+  }
+  copy[i+1]= '\0';
+}
+
+char lowercase(char* c) {
+  // Lowercase only ASCII capital characters.
+  if (*c >= 65 && *c <= 90) {
+    return *c + 32;
+  }
+  return *c;
+}
+
+void lowercase_phrase(char phrase[]) {
+  for (int i = 0; i < length(phrase); i++) {
+    phrase[i] = lowercase(&phrase[i]);
+  }
+}
+
+bool is_symbol_char(char c) {
+  // Ignore symbols that precede alpha chars.
+  if (c < 65 || c > 122) {
+    return true;
+  }
+  // Ignore symbols intervening the upper and lowercase chars.
+  if (c > 90 && c < 97) {
+    return true;
+  }
+  return false;
+}
+
+bool is_isogram(const char phrase[]) {
+  if (phrase == NULL) { return false; }
+  if (length(phrase) == 0) { return true; }
+  // Lowercase the phrase.
+  // We make a copy, since the input is constant.
+  char lower_phrase[(length(phrase))];
+  copy_phrase(phrase, lower_phrase);
+  lowercase_phrase(lower_phrase);
+  // Check if the lowercased phrase is an isogram.
+  for (int i = 0; i < length(lower_phrase); i++) {
+    if (is_symbol_char(lower_phrase[i])) { continue; } // Ignore non-alpha chars.
+    for (int j = 0; j < length(lower_phrase); j++) {
+      if (is_symbol_char(lower_phrase[j])) { continue; }
+      if (j == i) { continue; }
+      if (lower_phrase[i] == lower_phrase[j]) {
+        return false;
+      }
+    }
+  }
+  return true;
+}