add skip_prefix_mem helper

The skip_prefix function has been very useful for
simplifying pointer arithmetic and avoiding repeated magic
numbers, but we have no equivalent for length-limited
buffers. So we're stuck with:

if (3 <= len && skip_prefix(buf, "foo", &buf))
len -= 3;

That's not that complicated, but it needs to use magic
numbers for the length of the prefix (or else write out
strlen("foo"), repeating the string). By using a helper, we
can get the string length behind the scenes (and often at
compile time for string literals).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/git-compat-util.h b/git-compat-util.h
index 061e33c7749dc496e1f5e78aabdfd4c6f1e82736..8e808c01d2508edd4d59994d4714486a13afeb42 100644 (file)
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -463,6 +463,23 @@ static inline int skip_prefix(const char *str, const char *prefix,
        return 0;
 }
 
+/*
+ * Like skip_prefix, but promises never to read past "len" bytes of the input
+ * buffer, and returns the remaining number of bytes in "out" via "outlen".
+ */
+static inline int skip_prefix_mem(const char *buf, size_t len,
+                                 const char *prefix,
+                                 const char **out, size_t *outlen)
+{
+       size_t prefix_len = strlen(prefix);
+       if (prefix_len <= len && !memcmp(buf, prefix, prefix_len)) {
+               *out = buf + prefix_len;
+               *outlen = len - prefix_len;
+               return 1;
+       }
+       return 0;
+}
+
 /*
  * If buf ends with suffix, return 1 and subtract the length of the suffix
  * from *len. Otherwise, return 0 and leave *len untouched.