datalab （原發布 csdn 2018年09月21日 20:42:54）

首先聲明datalab本人未完成，有4道題目沒有作出來。本文博客記錄下本身的解析，以便之後回憶。若是能幫助到你就更好了，若是以爲本文沒啥技術含量，也望多多包涵。

/* 
 * bitAnd - x&y using only ~ and | 
 *   Example: bitAnd(6, 5) = 4
 *   Legal ops: ~ |
 *   Max ops: 8
 *   Rating: 1
 */
int bitAnd(int x, int y) {
  return ~(~x | ~y);
}
/* 
 * getByte - Extract byte n from word x
 *   Bytes numbered from 0 (LSB) to 3 (MSB)
 *   Examples: getByte(0x12345678,1) = 0x56
 *   Legal ops: ! ~ & ^ | + << >>
 *   Max ops: 6
 *   Rating: 2
 */
int getByte(int x, int n) {
  int offsetValue = 0xff;
  int offsetIndex = n << 3;
  int value = (x & (offsetValue << offsetIndex)) >> offsetIndex;
  return value & offsetValue;
}
/* 
 * logicalShift - shift x to the right by n, using a logical shift
 *   Can assume that 0 <= n <= 31
 *   Examples: logicalShift(0x87654321,4) = 0x08765432
 *   Legal ops: ! ~ & ^ | + << >>
 *   Max ops: 20
 *   Rating: 3 
 */
int logicalShift(int x, int n) {
    int offset = 0x1 << 31;
    int offsetValue = ~(offset >> n << 1);  
    return (x >> n) & offsetValue;
}
/*
 * bitCount - returns count of number of 1's in word
 *   Examples: bitCount(5) = 2, bitCount(7) = 3
 *   Legal ops: ! ~ & ^ | + << >>
 *   Max ops: 40
 *   Rating: 4
 */
int bitCount(int x) {
    return 2;
}
/* 
 * bang - Compute !x without using !
 *   Examples: bang(3) = 0, bang(0) = 1
 *   Legal ops: ~ & ^ | + << >>
 *   Max ops: 12
 *   Rating: 4 
 */
int bang(int x) {   
    return 2;
}
/* 
 * tmin - return minimum two's complement integer 
 *   Legal ops: ! ~ & ^ | + << >>
 *   Max ops: 4
 *   Rating: 1
 */
int tmin(void) {
  return (0x1 << 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 offsetValue = 0x1 << n;
  int addValue = (offsetValue >> 1) & (~offsetValue);//2^(n-1)
  int value1 = x + addValue;//x - {-[2^(n-1)]}
  int value2 = addValue + (~x);//[2^(n-1)-1] - x
  int maxValue = 0x1 << 31;
  return (n >> 5) | ((!(value1 & maxValue)) & (!(value2 & maxValue)));
}
/* 
 * 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) {
  int maxValue = 0x1 << 31;
  int offsetValue = ~(0x1 << 31 >> (32 + ~n));
  int andValue = offsetValue & x;
  return (x >> n) + ((!!(x & maxValue)) & (!!(andValue)));
}
/* 
 * 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 offsetValue = 0x1;
    int offsetIndex = 31;
    int offsetSign = offsetValue << offsetIndex;
    int signX = !(x & offsetSign);
    int signY = !(y & offsetSign);
    int value1 = ((!signX) & signY )^ 0x0;
    int value2 = (signX & (!signY)) ^ 0x1;
    int value3 = (!((y + ~x + 1) & offsetSign)) ^ 0x0;
    return value1 | (value2 & value3);
}
/*
 * 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) {
    return 2;
}
/* 
 * 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
 *   they are to be interpreted as the bit-level representations of
 *   single-precision floating point values.
 *   When argument is NaN, return argument.
 *   Legal ops: Any integer/unsigned operations incl. ||, &&. also if, while
 *   Max ops: 10
 *   Rating: 2
 */
unsigned float_neg(unsigned uf) {
    int offsetValue = 0x1;
    int offsetIndex = 0;
    int andValue = 0;
    int signValue;
    while (offsetIndex < 31)
    {
        signValue = (uf & offsetValue) >> offsetIndex;
        if (offsetIndex < 23)
        {
            andValue = andValue | signValue;
        }
        else
        {
            andValue = andValue & signValue;
        }
        offsetIndex += 1;
        offsetValue <<= 1;
    }
    if (andValue)
    {
        return uf;//NaN
    }
    return uf ^ offsetValue;
}
/* 
 * float_i2f - Return bit-level equivalent of expression (float) x
 *   Result is returned as unsigned int, but
 *   it is to be interpreted as the bit-level representation of a
 *   single-precision floating point values.
 *   Legal ops: Any integer/unsigned operations incl. ||, &&. also if, while
 *   Max ops: 30
 *   Rating: 4
 */
unsigned float_i2f(int x) {
    return 2;
}
/* 
 * float_twice - Return bit-level equivalent of expression 2*f for
 *   floating point argument f.
 *   Both the argument and result are passed as unsigned int's, but
 *   they are to be interpreted as the bit-level representation of
 *   single-precision floating point values.
 *   When argument is NaN, return argument
 *   Legal ops: Any integer/unsigned operations incl. ||, &&. also if, while
 *   Max ops: 30
 *   Rating: 4
 */
unsigned float_twice(unsigned uf) {
    int signIndex = 31;
    int expIndex = 23;
    int offsetValue = 0x1;
    int offsetSign = offsetValue << signIndex;
    int andValue = 1;
    int orValue = 0;
    int signValue;
    int offsetIndex = expIndex;
    while (offsetIndex < signIndex)
    {
        signValue = (uf & (offsetValue << offsetIndex)) >> offsetIndex;
        andValue = andValue & signValue;
        orValue = orValue | signValue;
        offsetIndex += 1;
    }
    if (andValue == 1)//exp==255
    {
        return uf;
    }
    else if (orValue == 0)//非規格化
    {
        signValue = !!(uf & offsetSign);
        uf <<= 1;
        if (signValue == 0)
        {
            return uf & (~offsetSign);
        }
        return uf | offsetSign;
    }
    else
    {
        signValue = ((uf >> expIndex) + 1) << expIndex;
        offsetIndex = expIndex;
        while (offsetIndex < signIndex)
        {
            uf &= ~(offsetValue << offsetIndex);
            offsetIndex += 1;
        }
        return uf | signValue;
    }
}