Drop Boltons dependency for small custom LRU dict

setup.py

@@ -40,7 +40,6 @@
     url="https://github.com/google-research/torchsde",
     packages=setuptools.find_packages(exclude=['benchmarks', 'diagnostics', 'examples', 'tests']),
     install_requires=[
-        "boltons>=20.2.1",
         "numpy>=1.19",
         "scipy>=1.5",
         "torch>=1.6.0",

torchsde/_brownian/brownian_interval.py

@@ -16,7 +16,6 @@
 import trampoline
 import warnings
 
-import boltons.cacheutils
 import numpy as np
 import torch
 
@@ -112,6 +111,21 @@ def __getitem__(self, item):
         raise KeyError
 
 
+class _LRUDict(dict):
+    def __init__(self, max_size):
+        super().__init__()
+        self._max_size = max_size
+        self._keys = []
+
+    def __setitem__(self, key, value):
+        if key in self:
+            self._keys.remove(key)
+        elif len(self) >= self._max_size:
+            del self[self._keys.pop(0)]
+        super().__setitem__(key, value)
+        self._keys.append(key)
+
+
 class _Interval:
     # Intervals correspond to some subinterval of the overall interval [t0, t1].
     # They are arranged as a binary tree: each node corresponds to an interval. If a node has children, they are left
@@ -505,7 +519,7 @@ def __init__(self,
         elif cache_size == 0:
             self._increment_and_space_time_levy_area_cache = _EmptyDict()
         else:
-            self._increment_and_space_time_levy_area_cache = boltons.cacheutils.LRU(max_size=cache_size)
+            self._increment_and_space_time_levy_area_cache = _LRUDict(max_size=cache_size)
 
         # We keep track of the most recently queried interval, and start searching for the next interval from that
         # element of the binary tree. This is because subsequent queries are likely to be near the most recent query.