[PATCH 14/16] compat: helper for detecting unsigned overflow

[Jonathan Nieder <jrnieder@gmail.com>]

The idiom (a + b < a) works fine for detecting that an unsigned
integer has overflowed, but the more explicit

	unsigned_add_overflows(a, b)

might be easier to read.

Define such a macro, expanding roughly to ((a) < UINT_MAX - (b)).
Because the expansion uses each argument only once outside of sizeof()
expressions, it is safe to use this macro with arguments that have
side-effects.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
---
 git-compat-util.h |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)

diff --git a/git-compat-util.h b/git-compat-util.h
index 2af8d3e..817f045 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -31,6 +31,9 @@
 #define maximum_signed_value_of_type(a) \
     (INTMAX_MAX >> (bitsizeof(intmax_t) - bitsizeof(a)))
 
+#define maximum_unsigned_value_of_type(a) \
+    (UINTMAX_MAX >> (bitsizeof(uintmax_t) - bitsizeof(a)))
+
 /*
  * Signed integer overflow is undefined in C, so here's a helper macro
  * to detect if the sum of two integers will overflow.
@@ -40,6 +43,9 @@
 #define signed_add_overflows(a, b) \
     ((b) > maximum_signed_value_of_type(a) - (a))
 
+#define unsigned_add_overflows(a, b) \
+    ((b) > maximum_unsigned_value_of_type(a) - (a))
+
 #ifdef __GNUC__
 #define TYPEOF(x) (__typeof__(x))
 #else
-- 
1.7.2.3