maths: add renormalise function

maths.hpp

@@ -23,10 +23,11 @@
 #include "debug.hpp"
 #include "types/traits.hpp"
 
+#include <cmath>
 #include <cstdint>
+#include <limits>
 #include <type_traits>
 #include <utility>
-#include <cmath>
 
 template <typename T>
 T
@@ -307,6 +308,34 @@ smoothstep [[gnu::pure]] (T a, T b, T x)
     return x * x * (3 - 2 * x);
 }
 
+#include "types/string.hpp"
+
+//-----------------------------------------------------------------------------
+template <typename T, typename U>
+U
+renormalise [[gnu::pure]] (T t)
+{
+    static const T T_max = std::numeric_limits<T>::max ();
+    static const U U_max = std::numeric_limits<U>::max ();
+    static const bool shrinking = sizeof (U) < sizeof (T);
+    static const bool T_float = std::is_floating_point<T>::value;
+    static const bool U_float = std::is_floating_point<U>::value;
+
+    if (T_float && U_float)
+        return U (t);
+
+    if (T_float) {
+        return U (limit (t, 0, 1) * U_max);
+    }
+
+    if (U_float)
+        return U (t) / T_max;
+
+    if (shrinking)
+        return U (t / (sizeof (T) / sizeof (U)));
+    else
+        return U (t) * (sizeof (U) / sizeof (T));
+}
 
 #include "maths.ipp"
 

test/maths.cpp

@@ -51,6 +51,39 @@ test_comparisons (void)
 }
 
 
+void
+test_normalisations (void)
+{
+    // u8 to float
+    {
+        auto a = renormalise<uint8_t,float> (255);
+        CHECK_EQ (a, 1.f);
+
+        auto b = renormalise<uint8_t,float> (0);
+        CHECK_EQ (b, 0.f);
+    }
+
+    // float to u8
+    {
+        struct {
+            float   a;
+            uint8_t b;
+        } TESTS[] = {
+            {  1.f, 255 },
+            {  0.f,   0 },
+            {  2.f, 255 },
+            { -1.f,   0 }
+        };
+
+        for (auto i: TESTS) {
+            std::cerr << "x";
+            auto v = renormalise<decltype(i.a),decltype(i.b)> (i.a);
+            CHECK_EQ ((unsigned)v, (unsigned)i.b);
+        }
+    }
+}
+
+
 int
 main (int, char **) {
     // Max out the precision in case we trigger debug output
@@ -59,6 +92,8 @@ main (int, char **) {
 
     test_comparisons ();
 
+    test_normalisations ();
+
     CHECK_EQ (util::min (-2, 0, 2), -2);
     CHECK_EQ (util::max (-2, 0, 2),  2);