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>

1 files changed, 17 insertions, 0 deletions

diff --git a/git-compat-util.h b/git-compat-util.h
index 061e33c774..8e808c01d2 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -464,6 +464,23 @@ static inline int skip_prefix(const char *str, const char *prefix,
 }
 
 /*
+ * 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.
  */