CSMA/CD 协议的模拟实现

// Random.cpp: implementation of the Random class. // ////////////////////////////////////////////////////////////////////// //#include "stdafx.h" #include "time.h" #include <iostream> #include "Random.h" using namespace std; #define TYPE_0 0 /* linear congruential */ #define BREAK_0 8 #define DEG_0 0 #define SEP_0 0 #define TYPE_1 1 /* x**7 + x**3 + 1 */ #define BREAK_1 32 #define DEG_1 7 #define SEP_1 3 #define TYPE_2 2 /* x**15 + x + 1 */ #define BREAK_2 64 #define DEG_2 15 #define SEP_2 1 #define TYPE_3 3 /* x**31 + x**3 + 1 */ #define BREAK_3 128 #define DEG_3 31 #define SEP_3 3 #define TYPE_4 4 /* x**63 + x + 1 */ #define BREAK_4 256 #define DEG_4 63 #define SEP_4 1 ////////////////////////////////////////////////////////////////////// // Construction/Destruction ////////////////////////////////////////////////////////////////////// unsigned int Random::next(unsigned int n) { return next() % n; } char Random::nextChar() { return char((next() >> 3) & 0xFF); } bool Random::nextBool() { return (next() & 0x1000) != 0; } float Random::nextFloat() { return float(next()) / 0x7FFFFFFF; } double Random::nextDouble() { return double(next()) / 0x7FFFFFFF; } Random::Random(int stateSize) { _pBuffer = new char[stateSize]; initState((unsigned int) time(NULL), _pBuffer, stateSize); } Random::~Random() { delete [] _pBuffer; } /* * Compute x = (7^5 * x) mod (2^31 - 1) * wihout overflowing 31 bits: * (2^31 - 1) = 127773 * (7^5) + 2836 * From "Random number generators: good ones are hard to find", * Park and Miller, Communications of the ACM, vol. 31, no. 10, * October 1988, p. 1195. */ unsigned int Random::goodRand(int x) { int hi, lo; if (x == 0) x = 123459876; hi = x / 127773; lo = x % 127773; x = 16807 * lo - 2836 * hi; if (x < 0) x += 0x7FFFFFFF; return x; } /* * Initialize the random number generator based on the given seed. If the * type is the trivial no-state-information type, just remember the seed. * Otherwise, initializes state[] based on the given "seed" via a linear * congruential generator. Then, the pointers are set to known locations * that are exactly rand_sep places apart. Lastly, it cycles the state * information a given number of times to get rid of any initial dependencies * introduced by the L.C.R.N.G. Note that the initialization of randtbl[] * for default usage relies on values produced by this routine. */ void Random::seed(unsigned int x) { int i, lim; _state[0] = x; if (_randType == TYPE_0) lim = NSHUFF; else { for (i = 1; i < _randDeg; i++) _state[i] = goodRand(_state[i - 1]); _fptr = &_state[_randSep]; _rptr = &_state[0]; lim = 10 * _randDeg; } for (i = 0; i < lim; i++) next(); } /* * Many programs choose the seed value in a totally predictable manner. * This often causes problems. We seed the generator using the much more * secure random(4) interface. Note that this particular seeding * procedure can generate states which are impossible to reproduce by * calling srandom() with any value, since the succeeding terms in the * state buffer are no longer derived from the LC algorithm applied to * a fixed seed. */ void Random::seed() { streamsize len; if (_randType == TYPE_0) len = sizeof _state[0]; else len = _randDeg * sizeof _state[0]; //RandomInputStream rstr; //rstr.read((char*) _state, len); } /* * Initialize the state information in the given array of n bytes for future * random number generation. Based on the number of bytes we are given, and * the break values for the different R.N.G.'s, we choose the best (largest) * one we can and set things up for it. srandom() is then called to * initialize the state information. * * Note that on return from srandom(), we set state[-1] to be the type * multiplexed with the current value of the rear pointer; this is so * successive calls to initstate() won't lose this information and will be * able to restart with setstate(). * * Note: the first thing we do is save the current state, if any, just like * setstate() so that it doesn't matter when initstate is called. * * Returns a pointer to the old state. * * Note: The Sparc platform requires that arg_state begin on an int * word boundary; otherwise a bus error will occur. Even so, lint will * complain about mis-alignment, but you should disregard these messages. */ void Random::initState(unsigned int s, char* argState, int n) { unsigned int* intArgState = (unsigned int*) argState; if (n < BREAK_0) { printf("not enough state"); return; } if (n < BREAK_1) { _randType = TYPE_0; _randDeg = DEG_0; _randSep = SEP_0; } else if (n < BREAK_2) { _randType = TYPE_1; _randDeg = DEG_1; _randSep = SEP_1; } else if (n < BREAK_3) { _randType = TYPE_2; _randDeg = DEG_2; _randSep = SEP_2; } else if (n < BREAK_4) { _randType = TYPE_3; _randDeg = DEG_3; _randSep = SEP_3; } else { _randType = TYPE_4; _randDeg = DEG_4; _randSep = SEP_4; } _state = intArgState + 1; /* first location */ _endPtr = &_state[_randDeg]; /* must set end_ptr before seed */ seed(s); if (_randType == TYPE_0) intArgState[0] = _randType; else intArgState[0] = MAX_TYPES * (int) (_rptr - _state) + _randType; } /* * Next: * * If we are using the trivial TYPE_0 R.N.G., just do the old linear * congruential bit. Otherwise, we do our fancy trinomial stuff, which is * the same in all the other cases due to all the global variables that have * been set up. The basic operation is to add the number at the rear pointer * into the one at the front pointer. Then both pointers are advanced to * the next location cyclically in the table. The value returned is the sum * generated, reduced to 31 bits by throwing away the "least random" low bit. * * Note: the code takes advantage of the fact that both the front and * rear pointers can't wrap on the same call by not testing the rear * pointer if the front one has wrapped. * * Returns a 31-bit random number. */ unsigned int Random::next() { unsigned int i; unsigned int *f, *r; if (_randType == TYPE_0) { i = _state[0]; _state[0] = i = goodRand(i) & 0x7FFFFFFF; } else { /* * Use local variables rather than static variables for speed. */ f = _fptr; r = _rptr; *f += *r; i = (*f >> 1) & 0x7FFFFFFF; /* chucking least random bit */ if (++f >= _endPtr) { f = _state; ++r; } else if (++r >= _endPtr) { r = _state; } _fptr = f; _rptr = r; } return i; }