Add a FastRandomContext::randrange and use it

src/crypto/common.h

@@ -79,4 +79,25 @@ void static inline WriteBE64(unsigned char* ptr, uint64_t x)
     memcpy(ptr, (char*)&v, 8);
 }
 
+/** Return the smallest number n such that (x >> n) == 0 (or 64 if the highest bit in x is set. */
+uint64_t static inline CountBits(uint64_t x)
+{
+#ifdef HAVE_DECL___BUILTIN_CLZL
+    if (sizeof(unsigned long) >= sizeof(uint64_t)) {
+        return x ? 8 * sizeof(unsigned long) - __builtin_clzl(x) : 0;
+    }
+#endif
+#ifdef HAVE_DECL___BUILTIN_CLZLL
+    if (sizeof(unsigned long long) >= sizeof(uint64_t)) {
+        return x ? 8 * sizeof(unsigned long long) - __builtin_clzll(x) : 0;
+    }
+#endif
+    int ret = 0;
+    while (x) {
+        x >>= 1;
+        ++ret;
+    }
+    return ret;
+}
+
 #endif // BITCOIN_CRYPTO_COMMON_H