removes unnecesary statements (#329)

* removes unnecesary statements

* Adds missing term

* Updates CHANGELOG

Co-authored-by: Nicolas Quesada <nquesada@pop-os.localdomain>

.github/CHANGELOG.md

@@ -13,6 +13,8 @@
 
 * Recursive Torontonian added for faster computation based on paper ["Polynomial speedup in Torontonian calculation by a scalable recursive algorithm" by Ágoston Kaposi, Zoltán Kolarovszki, Tamás Kozsik, Zoltán Zimborás, and Péter Rakyta](https://arxiv.org/pdf/2109.04528.pdf). [#321](https://github.com/XanaduAI/thewalrus/pull/321)
 
+* Hafnians of odd-sized matrices are calculated roughly twice as fast. [#329](https://github.com/XanaduAI/thewalrus/pull/329)
+
 ### Bug fixes
 
 * Permanent algorithms handle 0x0 cases correctly. [#320](https://github.com/XanaduAI/thewalrus/pull/320)
@@ -23,7 +25,7 @@
 
 This release contains contributions from (in alphabetical order):
 
-Jake Bulmer, Gregory Morse
+Jake Bulmer, Gregory Morse, Nicolas Quesada
 
 ---
 

thewalrus/_hafnian.py

@@ -778,17 +778,18 @@ def hafnian(
             return np.prod(np.diag(A))
         return 0
 
-    if matshape[0] % 2 != 0 and loop:
-        A = np.pad(A, pad_width=((0, 1), (0, 1)), mode="constant")
-        A[-1, -1] = 1.0
-
     matshape = A.shape
 
     if matshape[0] == 2:
         if loop:
             return A[0, 1] + A[0, 0] * A[1, 1]
         return A[0][1]
 
+    if matshape[0] == 3 and loop:
+        return (
+            A[0, 0] * A[1, 2] + A[1, 1] * A[0, 2] + A[2, 2] * A[0, 1] + A[0, 0] * A[1, 1] * A[2, 2]
+        )
+
     if matshape[0] == 4:
         if loop:
             result = (