random: remove generator choice from easy API

it's supposed to be an easy replacement for std::rand, we don't need to
let the user choose their generator.

memory/buffer/circular.cpp

@@ -48,7 +48,7 @@ tmpname (std::string &str, size_t length)
 
     str.resize (length);
     std::generate_n (str.begin (), length, [&] (void) {
-        return util::rand::choose (alphanum);
+        return util::random::choose (alphanum);
     });
 }
 

random.hpp

@@ -14,36 +14,52 @@
  * Copyright 2016 Danny Robson <danny@nerdcruft.net>
  */
 
-#ifndef __UTIL_RANDOM_HPP
-#define __UTIL_RANDOM_HPP
+#ifndef CRUFT_UTIL_RANDOM_HPP
+#define CRUFT_UTIL_RANDOM_HPP
 
 #include <random>
 
 
-///////////////////////////////////////////////////////////////////////////////
-namespace util::rand {
-    //-------------------------------------------------------------------------
-    using default_generator = std::minstd_rand;
-
-
-    //-------------------------------------------------------------------------
-    template <typename Generator = default_generator>
-    Generator&
-    thread_engine (void)
+namespace util::random {
+    ///////////////////////////////////////////////////////////////////////////
+    /// return correctly initialised thread-local generator of an unspecified,
+    /// but not entirely useless, type. ie, not LCG.
+    auto&
+    generator (void)
     {
-        std::random_device rd;
-        thread_local Generator gen (rd ());
+        static thread_local std::mt19937_64 gen { std::random_device {}() };
         return gen;
     }
 
 
-    //-------------------------------------------------------------------------
-    template <typename Generator = default_generator, typename T, size_t N>
-    T&
-    choose (T (&t)[N], Generator gen = thread_engine<Generator> ())
+    ///////////////////////////////////////////////////////////////////////////
+    /// select a value uniformly from the range [lo, hi)
+    template <typename T>
+    std::enable_if_t<std::is_floating_point_v<T>,T>
+    uniform (T lo, T hi)
     {
-        std::uniform_int_distribution<size_t> dist (0, N-1);
-        return t[dist (gen)];
+        return std::uniform_real_distribution { lo, hi } (generator ());
+    }
+
+
+    //-------------------------------------------------------------------------
+    /// select a value uniformly from the range [lo, hi)
+    template <typename T>
+    std::enable_if_t<std::is_integral_v<T>,T>
+    uniform (T lo, T hi)
+    {
+        return std::uniform_int_distribution { lo, hi } (generator ());
+    }
+
+
+    ///////////////////////////////////////////////////////////////////////////
+    /// choose a value at random from an array
+    template <typename T, size_t N>
+    T&
+    choose (T (&t)[N])
+    {
+        std::uniform_int_distribution<size_t> dist (0, N - 1);
+        return t[dist (generator ())];
     }
 }