#include <stdio.h>

typedef struct { char a,b,c; } Triple;

unsigned long div3(Triple *v, char *r) {
  if ((long)v <= 2)  
    return (unsigned long)r;
  return div3(&v[-1], &r[1]);
}

int main() {
  unsigned long v = 21; 
  int r = div3((Triple*)v, 0); 
  printf("%ld / 3 = %d\n", v, r); 
  return 0;
}

要将一个32位数字除以3，可以将其乘以0x55555556，然后取64位结果的前32位。

现在剩下要做的就是使用位运算和移位来实现乘法…

使用cblas，作为OS X加速框架的一部分。

[02:31:59] [william@relativity ~]$ cat div3.c
#import <stdio.h>
#import <Accelerate/Accelerate.h>

int main() {
    float multiplicand = 123456.0;
    float multiplier = 0.333333;
    printf("%f * %f == ", multiplicand, multiplier);
    cblas_sscal(1, multiplier, &multiplicand, 1);
    printf("%f\n", multiplicand);
}

[02:32:07] [william@relativity ~]$ clang div3.c -framework Accelerate -o div3 && ./div3
123456.000000 * 0.333333 == 41151.957031

如果我们认为__div__不是正字法上的/

def divBy3(n):
    return n.__div__(3)

print divBy3(9), 'or', 9//3

好吧，我想我们都同意这不是一个现实世界的问题。为了好玩，这里是如何用Ada和多线程来做这件事:

with Ada.Text_IO;

procedure Divide_By_3 is

   protected type Divisor_Type is
      entry Poke;
      entry Finish;
   private
      entry Release;
      entry Stop_Emptying;
      Emptying : Boolean := False;
   end Divisor_Type;

   protected type Collector_Type is
      entry Poke;
      entry Finish;
   private
      Emptying : Boolean := False;
   end Collector_Type;

   task type Input is
   end Input;
   task type Output is
   end Output;

   protected body Divisor_Type is
      entry Poke when not Emptying and Stop_Emptying'Count = 0 is
      begin
         requeue Release;
      end Poke;
      entry Release when Release'Count >= 3 or Emptying is
         New_Output : access Output;
      begin
         if not Emptying then
            New_Output := new Output;
            Emptying := True;
            requeue Stop_Emptying;
         end if;
      end Release;
      entry Stop_Emptying when Release'Count = 0 is
      begin
         Emptying := False;
      end Stop_Emptying;
      entry Finish when Poke'Count = 0 and Release'Count < 3 is
      begin
         Emptying := True;
         requeue Stop_Emptying;
      end Finish;
   end Divisor_Type;

   protected body Collector_Type is
      entry Poke when Emptying is
      begin
         null;
      end Poke;
      entry Finish when True is
      begin
         Ada.Text_IO.Put_Line (Poke'Count'Img);
         Emptying := True;
      end Finish;
   end Collector_Type;

   Collector : Collector_Type;
   Divisor : Divisor_Type;

   task body Input is
   begin
      Divisor.Poke;
   end Input;

   task body Output is
   begin
      Collector.Poke;
   end Output;

   Cur_Input : access Input;

   -- Input value:
   Number : Integer := 18;
begin
   for I in 1 .. Number loop
      Cur_Input := new Input;
   end loop;
   Divisor.Finish;
   Collector.Finish;
end Divide_By_3;

#include <stdio.h>

typedef struct { char a,b,c; } Triple;

unsigned long div3(Triple *v, char *r) {
  if ((long)v <= 2)  
    return (unsigned long)r;
  return div3(&v[-1], &r[1]);
}

int main() {
  unsigned long v = 21; 
  int r = div3((Triple*)v, 0); 
  printf("%ld / 3 = %d\n", v, r); 
  return 0;
}