From ec1a74a37c43763e9d24f6dbb578811a7947d5c5 Mon Sep 17 00:00:00 2001
From: Franklin Wei <git@fwei.tk>
Date: Thu, 17 Aug 2017 17:14:21 -0400
Subject: puzzles: faster, smaller sqrt()

Change-Id: I18e170ee49bff131fe76fa4bb1b9e0f120818b82
---
 apps/plugins/puzzles/rbwrappers.c | 70 +++++----------------------------------
 1 file changed, 8 insertions(+), 62 deletions(-)

(limited to 'apps/plugins')

diff --git a/apps/plugins/puzzles/rbwrappers.c b/apps/plugins/puzzles/rbwrappers.c
index 2d857c1cc9..feb1bf79de 100644
--- a/apps/plugins/puzzles/rbwrappers.c
+++ b/apps/plugins/puzzles/rbwrappers.c
@@ -984,70 +984,16 @@ float atan2_wrapper(float y, float x)
         }
 }
 
-/* Square root function, original. */
 float sqrt_wrapper(float x)
 {
-        float z;
-        int32_t sign = (int)0x80000000;
-        int32_t ix,s,q,m,t,i;
-        uint32_t r;
-
-        GET_FLOAT_WORD(ix,x);
-
-    /* take care of Inf and NaN */
-        if((ix&0x7f800000)==0x7f800000) {
-            return x*x+x;               /* sqrt(NaN)=NaN, sqrt(+inf)=+inf
-                                           sqrt(-inf)=sNaN */
-        }
-    /* take care of zero */
-        if(ix<=0) {
-            if((ix&(~sign))==0) return x;/* sqrt(+-0) = +-0 */
-            else if(ix<0)
-                return (x-x)/(x-x);             /* sqrt(-ve) = sNaN */
-        }
-    /* normalize x */
-        m = (ix>>23);
-        if(m==0) {                              /* subnormal x */
-            for(i=0;(ix&0x00800000)==0;i++) ix<<=1;
-            m -= i-1;
-        }
-        m -= 127;       /* unbias exponent */
-        ix = (ix&0x007fffff)|0x00800000;
-        if(m&1) /* odd m, double x to make it even */
-            ix += ix;
-        m >>= 1;        /* m = [m/2] */
-
-    /* generate sqrt(x) bit by bit */
-        ix += ix;
-        q = s = 0;              /* q = sqrt(x) */
-        r = 0x01000000;         /* r = moving bit from right to left */
-
-        while(r!=0) {
-            t = s+r;
-            if(t<=ix) {
-                s    = t+r;
-                ix  -= t;
-                q   += r;
-            }
-            ix += ix;
-            r>>=1;
-        }
-
-    /* use floating add to find out rounding direction */
-        if(ix!=0) {
-            z = one-tiny; /* trigger inexact flag */
-            if (z>=one) {
-                z = one+tiny;
-                if (z>one)
-                    q += 2;
-                else
-                    q += (q&1);
-            }
-        }
-        ix = (q>>1)+0x3f000000;
-        ix += (m <<23);
-        SET_FLOAT_WORD(z,ix);
-        return z;
+    /* find inverse, Quake-style */
+    float xhalf = .5f * x;
+    int i = *(int*)&x;
+    i = 0x5f3759df - (i >> 1);
+    x = *(float*)&i;
+    x = x * (1.5f - (xhalf * x * x));
+
+    return 1.0f / x;
 }
 
 /* hack, simple trig */
-- 
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