Make minor improvements

libc/dns/comparednsnames.c

@@ -0,0 +1,77 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2020 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ Permission to use, copy, modify, and/or distribute this software for         │
+│ any purpose with or without fee is hereby granted, provided that the         │
+│ above copyright notice and this permission notice appear in all copies.      │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
+│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
+│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
+│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
+│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
+│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
+│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
+│ PERFORMANCE OF THIS SOFTWARE.                                                │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/dns/dns.h"
+#include "libc/macros.internal.h"
+#include "libc/str/str.h"
+
+forceinline void FindDnsLabel(const char *A, size_t *i, size_t *n) {
+  while (*i) {
+    if (A[*i - 1] == '.') {
+      if (*i == *n) {
+        --(*n);
+      } else {
+        break;
+      }
+    }
+    --(*i);
+  }
+}
+
+/**
+ * Compares DNS hostnames in reverse lexicographical asciibetical order.
+ * @return <0, 0, or >0
+ * @see test/libc/dns/comparednsnames_test.c (the code that matters)
+ */
+int CompareDnsNames(const char *A, const char *B) {
+  int res;
+  bool first;
+  size_t n, m, i, j;
+  if (A == B) return 0;
+  n = strlen(A);
+  m = strlen(B);
+  if (!n || !m || ((A[n - 1] == '.') ^ (B[m - 1] == '.'))) {
+    if (n && m && A[n - 1] == '.' && strchr(B, '.')) {
+      --m;
+    } else if (n && m && B[m - 1] == '.' && strchr(A, '.')) {
+      --n;
+    } else {
+      return A[n ? n - 1 : 0] - B[m ? m - 1 : 0];
+    }
+  }
+  i = n;
+  j = m;
+  first = true;
+  for (;;) {
+    FindDnsLabel(A, &i, &n);
+    FindDnsLabel(B, &j, &m);
+    if (first) {
+      first = false;
+      if (!i && j) return 1;
+      if (!j && i) return -1;
+    }
+    if ((res = strncasecmp(&A[i], &B[j], MIN(n - i + 1, m - j + 1)))) {
+      return res;
+    }
+    if (!i || !j) {
+      return i - j;
+    }
+    n = i;
+    m = j;
+  }
+}