hash: Implement hash function for Boolean values.

This will be used by the configuration database, and it's generally useful
to have around.

diff --git a/lib/hash.h b/lib/hash.h
index 2015a63..33c5c42 100644 (file)
--- a/lib/hash.h
+++ b/lib/hash.h
@@ -16,6 +16,7 @@
 #ifndef HASH_H
 #define HASH_H 1
 
+#include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>
 #include <string.h>
@@ -70,6 +71,15 @@ static inline uint32_t hash_int(uint32_t x, uint32_t basis)
     return x + basis;
 }
 
+/* An attempt at a useful 1-bit hash function.  Has not been analyzed for
+ * quality. */
+static inline uint32_t hash_boolean(bool x, uint32_t basis)
+{
+    enum { P0 = 0xc2b73583 };   /* This is hash_int(1, 0). */
+    enum { P1 = 0xe90f1258 };   /* This is hash_int(2, 0). */
+    return (x ? P0 : P1) ^ HASH_ROT(basis, 1);
+}
+
 static inline uint32_t hash_double(double x, uint32_t basis)
 {
     BUILD_ASSERT_DECL(sizeof x == 8);