Source

pypy / pypy / rlib / rrandom.py

# this code is a version of the mersenne twister random number generator which
# is supposed to be used from RPython without the Python interpreter wrapping
# machinery etc.

# this is stolen from CPython's _randommodule.c

from pypy.rlib.rarithmetic import r_uint

N = 624
M = 397
MATRIX_A = r_uint(0x9908b0df) # constant vector a
UPPER_MASK  = r_uint(0x80000000) # most significant w-r bits
LOWER_MASK = r_uint(0x7fffffff) # least significant r bits
MASK_32 = r_uint(0xffffffff)
TEMPERING_MASK_A = r_uint(0x9d2c5680)
TEMPERING_MASK_B = r_uint(0xefc60000)
MAGIC_CONSTANT_A = r_uint(1812433253)
MAGIC_CONSTANT_B = r_uint(19650218)
MAGIC_CONSTANT_C = r_uint(1664525)
MAGIC_CONSTANT_D = r_uint(1566083941)


class Random(object):
    def __init__(self, seed=r_uint(0)):
        self.state = [r_uint(0)] * N
        self.index = 0
        self.init_genrand(seed)

    def init_genrand(self, s):
        mt = self.state
        mt[0]= s & MASK_32
        for mti in range(1, N):
            mt[mti] = (MAGIC_CONSTANT_A *
                           (mt[mti - 1] ^ (mt[mti - 1] >> 30)) + r_uint(mti))
            # See Knuth TAOCP Vol2. 3rd Ed. P.106 for multiplier.
            # In the previous versions, MSBs of the seed affect
            # only MSBs of the array mt[].
            # for >32 bit machines 
            mt[mti] &= MASK_32
        self.index = N

    def init_by_array(self, init_key):
        key_length = len(init_key)
        mt = self.state
        self.init_genrand(MAGIC_CONSTANT_B)
        i = 1
        j = 0
        if N > key_length:
            max_k = N
        else:
            max_k = key_length
        for k in range(max_k, 0, -1):
            mt[i] = ((mt[i] ^
                         ((mt[i - 1] ^ (mt[i - 1] >> 30)) * MAGIC_CONSTANT_C))
                     + init_key[j] + r_uint(j)) # non linear
            mt[i] &= MASK_32 # for WORDSIZE > 32 machines
            i += 1
            j += 1
            if i >= N:
                mt[0] = mt[N - 1]
                i = 1
            if j >= key_length:
                j = 0
        for k in range(N - 1, 0, -1):
            mt[i] = ((mt[i] ^
                        ((mt[i - 1] ^ (mt[i - 1] >> 30)) * MAGIC_CONSTANT_D))
                     - i) # non linear
            mt[i] &= MASK_32 # for WORDSIZE > 32 machines
            i += 1
            if (i>=N):
                mt[0] = mt[N - 1]
                i = 1
        mt[0] = UPPER_MASK

    def genrand32(self):
        mag01 = [0, MATRIX_A]
        mt = self.state
        if self.index >= N:
            for kk in range(N - M):
                y = (mt[kk] & UPPER_MASK) | (mt[kk + 1] & LOWER_MASK)
                mt[kk] = mt[kk+M] ^ (y >> 1) ^ mag01[y & r_uint(1)]
            for kk in range(N - M, N - 1):
                y = (mt[kk] & UPPER_MASK) | (mt[kk + 1] & LOWER_MASK)
                mt[kk] = mt[kk + (M - N)] ^ (y >> 1) ^ mag01[y & r_uint(1)]
            y = (mt[N - 1] & UPPER_MASK) | (mt[0] & LOWER_MASK)
            mt[N - 1] = mt[M - 1] ^ (y >> 1) ^ mag01[y & r_uint(1)]
            self.index = 0
        y = mt[self.index]
        self.index += 1
        y ^= y >> 11
        y ^= (y << 7) & TEMPERING_MASK_A
        y ^= (y << 15) & TEMPERING_MASK_B
        y ^= (y >> 18)
        return y

    def random(self):
        a = self.genrand32() >> 5
        b = self.genrand32() >> 6
        return (a * 67108864.0 + b) * (1.0 / 9007199254740992.0)

    def jumpahead(self, n):
        mt = self.state
        for i in range(N - 1, 0, -1):
            j = n % i
            mt[i], mt[j] = mt[j], mt[i]
        for i in range(N):
            mt[i] += r_uint(i + 1)
        self.index = N
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