Make *c* and *d* rounds less intrusively configurable

README.md

@@ -42,13 +42,20 @@ verifies 64 test vectors, and
 does the same and prints intermediate values.
 
 The code can be adapted to implement SipHash-*c*-*d*, the version of SipHash
-with *c* compression rounds and *d* finalization rounds, by tweaking the
-lines
-```C
-#define cROUNDS 2
-#define dROUNDS 4
+with *c* compression rounds and *d* finalization rounds, by defining `cROUNDS`
+or `dROUNDS` when compiling.  This can be done with `-D` command line arguments
+to many compilers such as below.
+
+```sh
+gcc -Wall --std=c99 -DcROUNDS=2 -DdROUNDS=4 siphash.c halfsiphash.c test.c -o test
 ```
 
+The `makefile` also takes *c* and *d* rounds values as parameters.
+
+```sh
+make cROUNDS=2 dROUNDS=4
+``` 
+
 Obviously, if the number of rounds is modified then the test vectors
 won't verify.
 

halfsiphash.c

@@ -19,8 +19,12 @@
 #include <string.h>
 
 /* default: SipHash-2-4 */
-#define cROUNDS 2
-#define dROUNDS 4
+#ifndef cROUNDS
+    #define cROUNDS 2
+#endif
+#ifndef dROUNDS
+    #define dROUNDS 4
+#endif
 
 #define ROTL(x, b) (uint32_t)(((x) << (b)) | ((x) >> (32 - (b))))
 

makefile

@@ -3,6 +3,14 @@ CFLAGS=-Wall --std=c99
 SRC=siphash.c halfsiphash.c test.c
 BIN=test debug vectors
 
+ifneq ($(cROUNDS),)
+CFLAGS:=$(CFLAGS) -DcROUNDS=$(cROUNDS)
+endif
+
+ifneq ($(dROUNDS),)
+CFLAGS:=$(CFLAGS) -DdROUNDS=$(dROUNDS)
+endif
+
 all:                    $(BIN)
 
 test:                   $(SRC)

siphash.c

@@ -20,8 +20,12 @@
 #include <string.h>
 
 /* default: SipHash-2-4 */
-#define cROUNDS 2
-#define dROUNDS 4
+#ifndef cROUNDS
+    #define cROUNDS 2
+#endif
+#ifndef dROUNDS
+    #define dROUNDS 4
+#endif
 
 #define ROTL(x, b) (uint64_t)(((x) << (b)) | ((x) >> (64 - (b))))