From ebefc041b5e6daf974e252754b949d757cf33f4e Mon Sep 17 00:00:00 2001
From: Sam Hocevar <sam@hocevar.net>
Date: Tue, 15 Nov 2011 20:19:32 +0000
Subject: [PATCH] tutorial: replace log() calls with fast approximations.

---
 test/math/remez.cpp     |  9 ++-------
 test/tutorial/tut03.cpp | 12 ++++++++----
 2 files changed, 10 insertions(+), 11 deletions(-)

diff --git a/test/math/remez.cpp b/test/math/remez.cpp
index c45d2aee..32338fb7 100644
--- a/test/math/remez.cpp
+++ b/test/math/remez.cpp
@@ -30,25 +30,20 @@ using namespace std;
 /* The function we want to approximate */
 real myfun(real const &y)
 {
-real k1024 = 1024;
-real klog32 = log((real)32);
-return (y - k1024) / log2(log(sqrt(y))/klog32);
     real x = sqrt(y);
     return (sin(x) - x) / (x * y);
 }
 
 real myerr(real const &y)
 {
-return myfun(y);
-return real::R_1;
     real x = sqrt(y);
     return sin(x) / (x * y);
 }
 
 int main(int argc, char **argv)
 {
-    RemezSolver<3> solver;
-    solver.Run((real)(1024.001), (real)(1024 * 1024), myfun, myerr, 40);
+    RemezSolver<2> solver;
+    solver.Run(real::R_1 >> 400, real::R_PI_2 * real::R_PI_2, myfun, myerr, 40);
 
     return EXIT_SUCCESS;
 }
diff --git a/test/tutorial/tut03.cpp b/test/tutorial/tut03.cpp
index 5d21eae2..175bc021 100644
--- a/test/tutorial/tut03.cpp
+++ b/test/tutorial/tut03.cpp
@@ -166,7 +166,7 @@ public:
             for (int j = ((m_frame + 1) % 4) / 2; j < m_size.y; j += 2)
             for (int i = m_frame % 2; i < m_size.x; i += 2)
             {
-                double const maxlen = 32;
+                double const maxsqlen = 32;
 
                 f64cmplx z0 = m_center + ScreenToWorldOffset(ivec2(i, j));
                 f64cmplx r0 = z0;
@@ -177,7 +177,7 @@ public:
                 //f64cmplx r0(0.3245046418497685, 0.04855101129280834);
                 f64cmplx z;
                 int iter = MAX_ITERATIONS;
-                for (z = z0; iter && z.sqlen() < maxlen * maxlen; z = z * z + r0)
+                for (z = z0; iter && z.sqlen() < maxsqlen; z = z * z + r0)
                     --iter;
 
                 if (iter)
@@ -185,13 +185,17 @@ public:
                     double f = iter;
                     double n = z.sqlen();
 
-                    double k = log(n) * 0.5f / log(maxlen);
+                    /* Approximate log(sqrt(n))/log(sqrt(maxsqlen)) */
+                    union { double n; uint64_t x; } u = { n };
+                    double k = (u.x >> 42) - (((1 << 10) - 1) << 10);
+                    k *= 1.0 / (1 << 10) / log2(maxsqlen);
+
                     /* Approximate log2(k) in [1,2]. */
                     f += (- 0.344847817623168308695977510213252644185 * k
                           + 2.024664188044341212602376988171727038739) * k
                           - 1.674876738008591047163498125918330313237;
 
-                    *m_pixelstart++ = m_palette[(int)(f * PALETTE_STEP + 0.25 * m_frame)];
+                    *m_pixelstart++ = m_palette[(int)(f * PALETTE_STEP)];
                 }
                 else
                 {