Add exponential decay (#189)

* added exponential decay
* updated docstring
* fixed flake8

backoff/__init__.py

@@ -14,13 +14,14 @@
 """
 from backoff._decorator import on_exception, on_predicate
 from backoff._jitter import full_jitter, random_jitter
-from backoff._wait_gen import constant, expo, fibo, runtime
+from backoff._wait_gen import constant, expo, fibo, runtime, decay
 
 __all__ = [
     'on_predicate',
     'on_exception',
     'constant',
     'expo',
+    'decay',
     'fibo',
     'runtime',
     'full_jitter',

backoff/_wait_gen.py

@@ -1,6 +1,7 @@
 # coding:utf-8
 
 import itertools
+import math
 from typing import Any, Callable, Generator, Iterable, Optional, Union
 
 
@@ -31,6 +32,35 @@ def expo(
             yield max_value
 
 
+def decay(
+    initial_value: float = 1,
+    decay_factor: float = 1,
+    min_value: Optional[float] = None
+) -> Generator[float, Any, None]:
+
+    """Generator for exponential decay[1]:
+
+    Args:
+        initial_value: initial quantity
+        decay_factor: exponential decay constant.
+        min_value: The minimum value to yield. Once the value in the
+             true exponential sequence is lower than this, the value
+             of min_value will forever after be yielded.
+
+    [1] https://en.wikipedia.org/wiki/Exponential_decay
+    """
+    # Advance past initial .send() call
+    yield  # type: ignore[misc]
+    t = 0
+    while True:
+        a = initial_value * math.e ** (-t * decay_factor)
+        if min_value is None or a > min_value:
+            yield a
+            t += 1
+        else:
+            yield min_value
+
+
 def fibo(max_value: Optional[int] = None) -> Generator[int, None, None]:
     """Generator for fibonaccial decay.
 

tests/test_wait_gen.py

@@ -1,5 +1,34 @@
 # coding:utf-8
 import backoff
+import math
+
+
+def test_decay():
+    gen = backoff.decay()
+    gen.send(None)
+    for i in range(10):
+        assert math.e ** -i == next(gen)
+
+
+def test_decay_init100():
+    gen = backoff.decay(initial_value=100)
+    gen.send(None)
+    for i in range(10):
+        assert 100 * math.e ** -i == next(gen)
+
+
+def test_decay_init100_decay3():
+    gen = backoff.decay(initial_value=100, decay_factor=3)
+    gen.send(None)
+    for i in range(10):
+        assert 100 * math.e ** (-i * 3) == next(gen)
+
+
+def test_decay_init100_decay3_min5():
+    gen = backoff.decay(initial_value=100, decay_factor=3, min_value=5)
+    gen.send(None)
+    for i in range(10):
+        assert max(100 * math.e ** (-i * 3), 5) == next(gen)
 
 
 def test_expo():