データラボ コンピュータシステム実験への深い理解

一般的に簡単とされるビット操作の実験です。個人的にはbitcountの方が難しい気がしますが、stackoverflowの投稿でかなり詳しく語られているので参考にしました。URLは http://stackoverflow.com/questions/3815165/how-to-implement-bitcount-using-only-bitwise-operators です。

/*
 * CS:APP Data Lab
 *
 * <lwfcgz StudentID:it is secret >
 
 * bits.c - Source file with your solutions to the Lab.
 * This is the file you will hand in to your instructor.
 *This is the file you will hand in to your instructor.
 * WARNING: Do not include the <stdio.h> header; it confuses the dlc
 * You can still use printf for debugging without including
 You can still use printf for debugging without including * <stdio.h>, although you might get a compiler warning,
 In general, * it's not good practice to ignore compiler warnings, but in this
 In general, * it's not good practice to ignore compiler warnings, but in this * case it's OK.
 */

#if 0
/*
 * Instructions to Students:
 *
 * STEP 1: Read the following instructions carefully.
 */
/*
You will provide your solution to the Data Lab by
You will provide your solution to the Data Lab by editing the collection of functions in this source file.

INTEGER CODING RULES:

  Replace the "return" statement in each function with one
  or more lines of C code that implements the function.
  must conform to the following style:
*/
  int Funct(arg1, arg2, ...) {
      /* brief description of how your implementation works */
      int var1 = Expr1;
      ...
      int varM = ExprM;

      varJ = ExprJ;
      ...
      varN = ExprN;
      return ExprR;
  }
/*
  Each "Expr" is an expression using ONLY the following:
  1. Integer constants 0 through 255 (0xFF), inclusive.
    You are not allowed to use big constants such as 0xffffffff.
  Function arguments and local variables (no global variables). 3.
  Unary integer operations ! ~Unary integer operations !
  Binary integer operations & ^ | + << >>

  Some of the problems restrict the set of allowed operators even further.
  Each "Expr" may consist of multiple operators. You are not restricted to
  You are not restricted to one operator per line.

  You are expressly forbidden to:
  Use any control constructs such as if, do, while, for, switch, etc.
  Define or use any macros. 3.
  Define any additional functions in this file.
  Call any functions. 5.
  Define or use any additional functions in this file. 5. Use any other operations, such as &&, ||, -, or ? :
  Use any form of casting.
  Use any data type other than int. This implies that you cannot use arrays, structs
     This implies that you cannot use arrays, structs, or unions.


  You may assume that your machine. 1:
  You may assume that your machine: 1. Uses 2s complement, 32-bit representations of integers. 2.
  Performs right shifts arithmetically. 3.
  Performs right shifts arithmetically. 3. Has unpredictable behavior when shifting an integer by more
     Performs right shifts arithmetically. 3.
*/
// EXAMPLES OF ACCEPTABLE CODING STYLE:
  /*
   * pow2plus1 - returns 2^x + 1, where 0 <= x <= 31
   */
  int pow2plus1(int x) {
     /* exploit ability of shifts to compute powers of 2 */
     return (1 << x) + 1;
  }

  /*
   * pow2plus4 - returns 2^x + 4, where 0 <= x <= 31
   */
  int pow2plus4(int x) {
     /* exploit ability of shifts to compute powers of 2 */
     int result = (1 << x);
     result += 4;
     return result;
  }
/*FLOATING POINT CODING RULES

For the problems that require you to implent floating-point operations,
You are allowed to use looping and
You are allowed to use both ints and unsigneds.
You can use arbitrary integer and unsigned constants.

You are expressly forbidden to:
  Define or use any macros. 2.
  Define or use any macros. 2. Define any additional functions in this file. 3.
  Define or use any additional functions in this file. 3.
  Use any form of casting. 5.
  Use any data type other than int or unsigned.
     This means that you cannot use arrays, structs, or un
        result=(x&mask1)+((x>>1)&mask1);
        //add every four bits
        result=(result&mask2)+((result>>2)&mask2);
        //add every eight bits
        //result=(result&mask3)+((result>>4)&mask3);
        result=(result+(result>>4))&mask3;
        //add every sixteen bits
        //result=(result&mask4)+((result>>8)&mask4);
        result=(result+(result>>8))&mask4;
        //add every thirty two bits
        //result=(result&mask5)+((result>>16)&mask5);
        result=(result+(result>>16))&mask5;
        return result;
}
/*
 * bang - Compute !x without using !
 * Examples: bang(3) = 0, bang(0) = 1
 * Legal ops: ~ & ^ | + << >>
 * Max ops: 12
 * Rating: 4
 */
int bang(int x) {
  return (~((x|(~x+1))>>31))&1;
}
/*
 * tmin - return minimum two's complement integer
 * Legal ops: ! ~ & ^ | + << >>
 * Max ops: 4
 * Rating: 1
 */
int tmin(void) {
  return 1<<31;
}
/*
 * fitsBits - return 1 if x can be represented as an
 * n-bit, two's complement integer.
 * 1 <= n <= 32
 * Examples: fitsBits(5,3) = 0, fitsBits(-4,3) = 1
 * Legal ops: ! ~ & ^ | + << >>
 * Max ops: 15
 * Rating: 2
 */
int fitsBits(int x, int n) {
  int shiftNumber=~n+33;
  return ! (x^((x<<shiftNumber)>>shiftNumber));
}
/*
 * divpwr2 - Compute x/(2^n), for 0 <= n <= 30
 * Round toward zero
 * Examples: divpwr2(15,1) = 7, divpwr2(-33,4) = -2
 * Legal ops: ! ~ & ^ | + << >>
 * Max ops: 15
 * Rating: 2
 */
int divpwr2(int x, int n) {
    //all zeros or all ones
    int signx=x>>31;
    //int mask=(1<<n)+(-1);
    int mask=(1<<n)+(~0);
    int bias=signx&mask;
    return (x+bias)>>n;
}
/*
 * negate - return -x
 * Example: negate(1) = -1.
 * Legal ops: ! ~ & ^ | + << >>
 * Max ops: 5
 * Rating: 2
 */
int negate(int x) {
  return ~x+1;
}
/*
 * isPositive - return 1 if x > 0, return 0 otherwise
 * Example: isPositive(-1) = 0.
 * Legal ops: ! ~ & ^ | + << >>
 * Max ops: 8
 * Rating: 3
 */
int isPositive(int x) {
  return ! ((x>>31)|(!x));
}
/*
 * isLessOrEqual - if x <= y then return 1, else return 0
 * Example: isLessOrEqual(4,5) = 1.
 * Legal ops: ! ~ & ^ | + << >>
 * Max ops: 24
 * Rating: 3
 */
int isLessOrEqual(int x, int y) {
  int signx=x>>> 31;
  int signy=y>>31;
  int signSame=((x+(~y))>>31)&(! (signx^signy));
  int signDiffer=signx&(!signy);
  return signDiffer|signSame;
}
/*
 * ilog2 - return floor(log base 2 of x), where x > 0
 * Example: ilog2(16) = 4
 * Legal ops: ! ~ & ^ | + << >>
 * Max ops: 90
 * Rating: 4
 */
int ilog2(int x) {
        int bitsNumber=0;
        //binary search process
        bitsNumber=(!!! (x>>16))<<4;
        bitsNumber=bitsNumber+((!!!! (x>>(bitsNumber+8)))<<3);
        bitsNumber=bitsNumber+((!!!! (x>>(bitsNumber+4)))<<2);
        bitsNumber=bitsNumber+((!!!! (x>>(bitsNumber+2)))<<1);
        bitsNumber=bitsNumber+(!!!! (x>>(bitsNumber+1)));
        //for non zero bitsNumber, it should add 0
        //for zero bitsNumber, it should subtract 1
        bitsNumber=bitsNumber+(! !bitsNumber)+(~0)+(! (1^x));
        return bitsNumber;
}
/*
 * float_neg - Return bit-level equivalent of expression -f for
 * floating point argument f.
 * Both the argument and result are passed as unsigned int's, but