# Source code for netket.hilbert.fock

```# Copyright 2021 The NetKet Authors - All rights reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

from typing import Optional, Union

import numpy as np

from netket.utils import StaticRange

from .homogeneous import HomogeneousHilbert
from .index.constraints import SumConstraint

FOCK_MAX = np.iinfo(np.intp).max - 1
"""
Maximum number of particles in the fock space.
It is `maxvalue(np.int64)-1` because we use N+1 in several formulas
and it would overflow.
"""

[docs]
class Fock(HomogeneousHilbert):
r"""Hilbert space obtained as tensor product of local fock basis."""

[docs]
def __init__(
self,
n_max: Optional[int] = None,
N: int = 1,
n_particles: Optional[int] = None,
):
r"""
Constructs a new ``Boson`` given a maximum occupation number, number of sites
and total number of bosons.

Args:
n_max: Maximum occupation for a site (inclusive). If None, the local
occupation number is unbounded.
N: number of bosonic modes (default = 1)
n_particles: Constraint for the number of particles. If None, no constraint
is imposed.

Examples:
Simple boson hilbert space.

>>> from netket.hilbert import Fock
>>> hi = Fock(n_max=5, n_particles=11, N=3)
>>> print(hi.size)
3
>>> print(hi.n_states)
15
"""
self._n_max = n_max

if n_particles is not None:
if not isinstance(n_particles, int):
raise TypeError(
f"n_particles must be an integer. Got {n_particles} ({type(n_particles)})"
)

n_particles = int(n_particles)
if n_particles < 0:
raise ValueError(
f"Number of particles must be >= 0, but received {n_particles}."
)
self._n_particles = n_particles

if self._n_max is None:
self._n_max = n_particles
else:
if self._n_max * N < n_particles:
raise Exception(
"""The required total number of bosons is not compatible
with the given n_max."""
)
constraints = SumConstraint(n_particles)
else:
constraints = None
self._n_particles = None

if self._n_max is not None:
# assert self._n_max > 0
local_states = StaticRange(
0, 1, self._n_max + 1, dtype=np.int8 if self._n_max < 2**6 else int
)
else:
self._n_max = FOCK_MAX
local_states = None

super().__init__(local_states, N, constraints)

@property
def n_max(self) -> Optional[int]:
r"""The maximum number of bosons per site, or None
if the number is unconstrained."""
return self._n_max

@property
def n_particles(self) -> Optional[int]:
r"""The total number of particles, or None
if the number is unconstrained."""
return self._n_particles

def __pow__(self, n) -> "Fock":
if self.n_particles is None:
return Fock(self.n_max, self.size * n)

return NotImplemented

def _mul_sametype_(self, other: "Fock") -> "Fock":
assert type(self) == type(other)
if self.n_max == other.n_max:
if self._n_particles is None and other._n_particles is None:
return Fock(self.n_max, N=self.size + other.size)

return NotImplemented

[docs]
def ptrace(self, sites: Union[int, list]) -> Optional["Fock"]:
if isinstance(sites, int):
sites = [sites]

for site in sites:
if site < 0 or site >= self.size:
raise ValueError(
f"Site {site} not in this hilbert space of site {self.size}"
)

if self.n_particles is not None:
raise TypeError(
"Cannot take the partial trace with a total particles constraint."
)

Nsites = len(sites)

if self.size - Nsites == 0:
return None
else:
return Fock(self.n_max, N=self.size - Nsites)

def __repr__(self):
n_particles = (
f", n_particles={self._n_particles}"
if self._n_particles is not None
else ""
)
nmax = self._n_max if self._n_max < FOCK_MAX else "FOCK_MAX"
return f"Fock(n_max={nmax}{n_particles}, N={self.size})"

@property
def _attrs(self):
return (self.size, self._n_max, self._n_particles)

```