From fe4829a0fa9647e94011df58621a5982df2cd7e8 Mon Sep 17 00:00:00 2001
From: Ben Millwood <thebenmachine@gmail.com>
Date: Fri, 6 Apr 2012 21:33:24 +0000
Subject: * Calculate some exponentiation more efficiently/sensibly in bg_lib.c

---
 src/game/bg_lib.c | 106 +++++++++++++++++++++++-------------------------------
 1 file changed, 44 insertions(+), 62 deletions(-)

(limited to 'src/game')

diff --git a/src/game/bg_lib.c b/src/game/bg_lib.c
index dd31e946..94af5f18 100644
--- a/src/game/bg_lib.c
+++ b/src/game/bg_lib.c
@@ -818,6 +818,43 @@ double rint( double v )
     return floor( v );
 }
 
+/*
+===============
+powN
+
+Raise a double to a integer power
+===============
+*/
+static double powN( double base, int exp )
+{
+  if( exp >= 0 )
+  {
+    double result = 1.0;
+
+    // calculate x, x^2, x^4, ... by repeated squaring
+    // and multiply together the ones corresponding to the
+    // binary digits of the exponent
+    // e.g. x^73 = x^(1 + 8 + 64) = x * x^8 * x^64
+    while( exp > 0 )
+    {
+      if( exp % 2 == 1 )
+        result *= base;
+
+      base *= base;
+      exp /= 2;
+    }
+
+    return result;
+  }
+  // if exp is INT_MIN, the next clause will be upset,
+  // because -exp isn't representable
+  else if( exp == INT_MIN )
+    return powN( base, exp + 1 ) / base;
+  // x < 0
+  else
+    return 1.0 / powN( base, -exp );
+}
+
 double tan( double x )
 {
   return sin( x ) / cos( x );
@@ -1540,28 +1577,8 @@ double strtod( const char *nptr, char **endptr )
           *endptr = (char *)nptr;
         return res;
       }
-      if( exp > 0 )
-      {
-        while( exp-- > 0 )
-        {
-          res2 = res * 2;
-          // check for infinity
-          if( res2 <= res )
-            break;
-          res = res2;
-        }
-      }
-      else
-      {
-        while( exp++ < 0 )
-        {
-          res2 = res / 2;
-          // check for underflow
-          if( res2 >= res )
-            break;
-          res = res2;
-        }
-      }
+
+      res *= powN( 2, exp );
     }
     if( endptr )
       *endptr = (char *)end;
@@ -1604,30 +1621,8 @@ double strtod( const char *nptr, char **endptr )
           *endptr = (char *)nptr;
         return res;
       }
-      if( exp > 0 )
-      {
-        while( exp-- > 0 )
-        {
-          res10 = res * 10;
-          // check for infinity to save us time
-          if( res10 <= res )
-            break;
-          res = res10;
-        }
-      }
-      else if( exp < 0 )
-      {
-        while( exp++ < 0 )
-        {
-          res10 = res / 10;
-          // check for underflow
-          // (test for 0 would probably be just
-          // as good)
-          if( res10 >= res )
-            break;
-          res = res10;
-        }
-      }
+
+      res *= powN( 10, exp );
     }
     if( endptr )
       *endptr = (char *)end;
@@ -2540,19 +2535,6 @@ static LDOUBLE abs_val (LDOUBLE value)
   return result;
 }
 
-static LDOUBLE pow10 (int exp)
-{
-  LDOUBLE result = 1;
-
-  while (exp)
-  {
-    result *= 10;
-    exp--;
-  }
-
-  return result;
-}
-
 static long round (LDOUBLE value)
 {
   long intpart;
@@ -2615,12 +2597,12 @@ static int fmtfp (char *buffer, size_t *currlen, size_t maxlen,
   /* We "cheat" by converting the fractional part to integer by
    * multiplying by a factor of 10
    */
-  fracpart = round ((pow10 (max)) * (ufvalue - intpart));
+  fracpart = round ((powN (10, max)) * (ufvalue - intpart));
 
-  if (fracpart >= pow10 (max))
+  if (fracpart >= powN (10, max))
   {
     intpart++;
-    fracpart -= pow10 (max);
+    fracpart -= powN (10, max);
   }
 
 #ifdef DEBUG_SNPRINTF
-- 
cgit