Diff: fast_math.cpp

Revision:

13:791e20f1af43

Parent:

12:a4b17bb682eb

--- a/fast_math.cpp	Fri Dec 21 22:06:56 2012 +0000
+++ b/fast_math.cpp	Sun Mar 03 09:09:34 2013 +0000
@@ -1,31 +1,37 @@
 #include "mbed.h"
 #include "fast_math.h"
 
-unsigned short  sin60[DEG60+1];  // sin table from 0 to 60 deg. (max precision error is 0.003%)
+unsigned short  sin60[DEG60+1];  // 0～60度, 振幅65535のsinテーブル(最大誤差0.003%) from 0 to 60 deg. (max precision error is 0.003%)
 
 long _sin(unsigned short th){    // return( 65535*sin(th) ), th=rad*DEG60/(PI/3)=rad*(512*3)/PI (0<=rad<2*PI)
-//    init_sin60();
+// 入力 : th = rad*DEG60/(PI/3)=rad*(512*3)/PI, (0<=rad<2*PI)
+// 出力 : 65535*sin(th)
+//    init_fast_math();
 //    if( th>2.*PI ){ th -= 2*PI*(float)((int)(th/(2.*PI)));}
 //    th_int = (unsigned short)(th/(PI/3.)*(float)DEG60+0.5);  // rad to deg
 //    sin = (float)_sin(th)/65535.;
     unsigned short f_minus;
     long    x;
 
+    // sinがマイナスのとき、thから180度引いて、f_minus=1にする
     if( th>=DEG60*3){   f_minus = 1;    th -= DEG60*3;} // if th>=180deg, th = th - 180deg;
     else{               f_minus = 0;}                   // else         , f_minus = on. 
 
-    if( th<DEG60 ){         // th<60deg?
+    if( th<DEG60 ){         // th<60度のとき
         x = sin60[th];                              // sin(th)
-    }else if( th<DEG60*2 ){ // 60<=th<120deg?
+    }else if( th<DEG60*2 ){ // 60≦th<120度のとき
         x = sin60[DEG60*2-th] + sin60[th-DEG60];    // sin(th)=sin(th+60)+sin(th-60)=sin(180-(th+60))+sin(th-60) because sin(th+60)=s/2+c*root(3)/2, sin(th-60)=s/2-c*root(3)/2.
-    }else{// if( th<180 )   // 120<=th<180deg?
+    }else{                  // 120≦th<180度のとき
         x = sin60[DEG60*3-th];                      // sin(60-(th-120))=sin(180-th)
     }
-    if( f_minus==1 ){   x = -x;}
+    if( f_minus==1 ){   x = -x;}  // sinがマイナスのときマイナスにする
     return(x);
 }
 
 long _cos(unsigned short th){    // return( 65535*sin(th) ), th=rad*DEG60/(PI/3)=rad*(512*3)/PI (0<=rad<2*PI)
+// 入力 : th = rad*DEG60/(PI/3)=rad*(512*3)/PI, (0<=rad<2*PI)
+// 出力 : 65535*cos(th)
+    th += DEG60*3/2;
     th += DEG60*3/2;
     if( th>=DEG60*6 ){  th -= DEG60*6;}
     return( _sin(th) );
@@ -34,7 +40,7 @@
 void init_fast_math(){  // sin0-sin60deg; 0deg=0, 60deg=512
     int i;
     
-    for( i=0;i<=DEG60;i++ ){    // set sin table from 0 to 60 deg..
+    for( i=0;i<=DEG60;i++ ){    // 0～60度までのsinテーブルをつくるset sin table from 0 to 60 deg..
 //        sin60[i] = (unsigned short)(sin((float)i/512.*PI/3.));
         sin60[i] = (unsigned short)(65535.*sinf((float)i/(float)DEG60*PI/3.));
     }