Preallocate pmap according to PRecord fields count

[kkpattern]

Since the fields in a PRecord is fixed, the total size of the PMap in the record is already known before allocating the map. We can use the length of _precord_fields to pre-allocate the map to avoid reallocating later.

For example, if we add a log in _reallocate and give the following test code:

from pyrsistent import field
from pyrsistent import PRecord


class DRecord(PRecord):
    a = field()
    b = field()
    c = field()
    d = field()
    e = field()
    f = field()
    g = field()
    h = field()
    i = field()
    j = field()
    k = field()
    m = field()
    n = field()
    o = field()
    p = field()
    q = field()


r = DRecord()
for k in ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "m", "n", "o", "p", "q"]:
    r = r.set(k, 1)
    print(r)

Without the pre-allocating, the output will be:

DRecord(a=1)
DRecord(b=1, a=1)
DRecord(c=1, b=1, a=1)
DRecord(c=1, b=1, d=1, a=1)
DRecord(c=1, b=1, e=1, d=1, a=1)
DRecord(f=1, c=1, b=1, e=1, d=1, a=1)
DRecord(f=1, c=1, g=1, b=1, e=1, d=1, a=1)
DRecord(f=1, c=1, g=1, b=1, h=1, e=1, d=1, a=1)
DRecord(f=1, c=1, g=1, i=1, b=1, h=1, e=1, d=1, a=1)
DRecord(f=1, c=1, g=1, i=1, b=1, j=1, h=1, e=1, d=1, a=1)
DRecord(f=1, c=1, g=1, i=1, b=1, k=1, j=1, h=1, e=1, d=1, a=1)
DRecord(f=1, c=1, g=1, i=1, b=1, k=1, j=1, h=1, e=1, d=1, a=1, m=1)
_reallocate
DRecord(g=1, i=1, b=1, h=1, d=1, a=1, n=1, f=1, c=1, k=1, j=1, e=1, m=1)
DRecord(g=1, i=1, b=1, h=1, d=1, a=1, n=1, f=1, c=1, k=1, j=1, e=1, o=1, m=1)
DRecord(g=1, i=1, b=1, p=1, h=1, d=1, a=1, n=1, f=1, c=1, k=1, j=1, e=1, o=1, m=1)
DRecord(g=1, i=1, b=1, p=1, h=1, d=1, a=1, q=1, n=1, f=1, c=1, k=1, j=1, e=1, o=1, m=1)

And with the pre-allocating, the reallocating is avoided:

DRecord(a=1)
DRecord(a=1, b=1)
DRecord(c=1, a=1, b=1)
DRecord(c=1, a=1, b=1, d=1)
DRecord(c=1, a=1, b=1, d=1, e=1)
DRecord(c=1, a=1, b=1, d=1, f=1, e=1)
DRecord(c=1, a=1, b=1, d=1, f=1, e=1, g=1)
DRecord(c=1, a=1, b=1, d=1, f=1, e=1, h=1, g=1)
DRecord(i=1, c=1, a=1, b=1, d=1, f=1, e=1, h=1, g=1)
DRecord(i=1, c=1, a=1, b=1, d=1, f=1, e=1, h=1, j=1, g=1)
DRecord(i=1, c=1, a=1, b=1, d=1, k=1, f=1, e=1, h=1, j=1, g=1)
DRecord(i=1, c=1, a=1, b=1, d=1, k=1, m=1, f=1, e=1, h=1, j=1, g=1)
DRecord(i=1, c=1, a=1, b=1, d=1, k=1, m=1, f=1, n=1, e=1, h=1, j=1, g=1)
DRecord(i=1, c=1, a=1, b=1, d=1, k=1, m=1, f=1, n=1, e=1, h=1, j=1, g=1, o=1)
DRecord(i=1, c=1, a=1, b=1, d=1, k=1, m=1, f=1, n=1, e=1, p=1, h=1, j=1, g=1, o=1)
DRecord(i=1, c=1, q=1, a=1, b=1, d=1, k=1, m=1, f=1, n=1, e=1, p=1, h=1, j=1, g=1, o=1)

[tobgu]

Thanks, nice little improvement!