fix(util): fix decaying function output (#3495)

* fix(util): fix decaying function output

* fix(util): better test and adhere to code standards

* fix(util): correct variable name, pytestify test, update website text

.github/contributors/SamuelLKane.md

@@ -0,0 +1,104 @@
+# spaCy contributor agreement
+
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+## Contributor Details
+
+| Field                          | Entry                |
+|------------------------------- | -------------------- |
+| Name                           | Samuel Kane          |
+| Date                           | 3/20/19              |
+| GitHub username                | SamuelLKane          |
+| Website (optional)             | samuel-kane.com      |

spacy/tests/regression/test_issue3447.py

@@ -0,0 +1,10 @@
+from spacy.util import decaying
+
+def test_decaying():
+    sizes = decaying(10., 1., .5)
+    size = next(sizes)
+    assert size == 10.
+    size = next(sizes)
+    assert size == 10. - 0.5
+    size = next(sizes)
+    assert size == 10. - 0.5 - 0.5

spacy/util.py

@@ -507,13 +507,10 @@ def stepping(start, stop, steps):
 def decaying(start, stop, decay):
     """Yield an infinite series of linearly decaying values."""
 
-    def clip(value):
-        return max(value, stop) if (start > stop) else min(value, stop)
-
-    nr_upd = 1.0
+    curr = float(start)
     while True:
-        yield clip(start * 1.0 / (1.0 + decay * nr_upd))
-        nr_upd += 1
+        yield max(curr, stop)
+        curr -= (decay)
 
 
 def minibatch_by_words(items, size, tuples=True, count_words=len):

website/docs/api/top-level.md

@@ -622,10 +622,10 @@ Yield an infinite series of linearly decaying values.
 > #### Example
 >
 > ```python
-> sizes = decaying(1., 10., 0.001)
-> assert next(sizes) == 1.
-> assert next(sizes) == 1. - 0.001
-> assert next(sizes) == 0.999 - 0.001
+> sizes = decaying(10., 1., 0.001)
+> assert next(sizes) == 10.
+> assert next(sizes) == 10. - 0.001
+> assert next(sizes) == 9.999 - 0.001
 > ```
 
 | Name       | Type        | Description          |