From 2e16473009176bc4f5a91cb58dbba179ef622a44 Mon Sep 17 00:00:00 2001
From: Neil Traft <ntraft@gmail.com>
Date: Sat, 19 Nov 2022 22:32:48 -0500
Subject: [PATCH] Fix bug #255 and add corresponding unit tests.

---
 neat/graphs.py       |  6 ++++++
 tests/test_graphs.py | 28 +++++++++++++++++++++++-----
 2 files changed, 29 insertions(+), 5 deletions(-)

diff --git a/neat/graphs.py b/neat/graphs.py
index 0f3c6fc2..1b321fae 100644
--- a/neat/graphs.py
+++ b/neat/graphs.py
@@ -73,6 +73,12 @@ def feed_forward_layers(inputs, outputs, connections):
 
     layers = []
     s = set(inputs)
+    # First, find any nodes which do not have inputs.
+    has_input = set(b for (a, b) in connections)  # Get the set of all nodes with an incoming edge.
+    does_not_have_input = required - has_input  # Get required nodes that do NOT have an incoming edge.
+    if does_not_have_input:
+        layers.append(does_not_have_input)
+        s = s.union(does_not_have_input)
     while 1:
         # Find candidate nodes c for the next layer.  These nodes should connect
         # a node in s to a node not in s.
diff --git a/tests/test_graphs.py b/tests/test_graphs.py
index b8d1c9b2..94f10c2f 100644
--- a/tests/test_graphs.py
+++ b/tests/test_graphs.py
@@ -93,19 +93,26 @@ def test_feed_forward_layers():
     outputs = [2]
     connections = [(0, 2), (1, 2)]
     layers = feed_forward_layers(inputs, outputs, connections)
-    assert [{2}] == layers
+    assert [{2}] == layers, f"actually got: {layers}"
 
     inputs = [0, 1]
     outputs = [3]
     connections = [(0, 2), (1, 2), (2, 3)]
     layers = feed_forward_layers(inputs, outputs, connections)
-    assert [{2}, {3}] == layers
+    assert [{2}, {3}] == layers, f"actually got: {layers}"
 
     inputs = [0, 1]
     outputs = [4]
     connections = [(0, 2), (1, 2), (1, 3), (2, 3), (2, 4), (3, 4)]
     layers = feed_forward_layers(inputs, outputs, connections)
-    assert [{2}, {3}, {4}] == layers
+    assert [{2}, {3}, {4}] == layers, f"actually got: {layers}"
+
+    # A graph with dangling input nodes: node 2 has no dependencies, yet some other nodes depend upon it.
+    inputs = [0, 1]
+    outputs = [4]
+    connections = [(1, 3), (2, 3), (2, 4), (3, 4)]
+    layers = feed_forward_layers(inputs, outputs, connections)
+    assert [{2}, {3}, {4}] == layers, f"actually got: {layers}"
 
     inputs = [0, 1, 2, 3]
     outputs = [11, 12, 13]
@@ -114,8 +121,19 @@ def test_feed_forward_layers():
                    (8, 11), (8, 12), (8, 9), (9, 10), (7, 10),
                    (10, 12), (10, 13)]
     layers = feed_forward_layers(inputs, outputs, connections)
-    assert [{4, 5, 6}, {8, 7}, {9, 11}, {10}, {12, 13}] == layers
+    assert [{4, 5, 6}, {8, 7}, {9, 11}, {10}, {12, 13}] == layers, f"actually got: {layers}"
+
+    # Another graph with dangling input nodes (node 5).
+    inputs = [0, 1, 2, 3]
+    outputs = [11, 12, 13]
+    connections = [(0, 4), (1, 4), (2, 6), (3, 6), (3, 7),
+                   (4, 8), (5, 8), (5, 9), (5, 10), (6, 10), (6, 7),
+                   (8, 11), (8, 12), (8, 9), (9, 10), (7, 10),
+                   (10, 12), (10, 13)]
+    layers = feed_forward_layers(inputs, outputs, connections)
+    assert [{5}, {4, 6}, {8, 7}, {9, 11}, {10}, {12, 13}] == layers, f"actually got: {layers}"
 
+    # A graph with dangling output nodes.
     inputs = [0, 1, 2, 3]
     outputs = [11, 12, 13]
     connections = [(0, 4), (1, 4), (1, 5), (2, 5), (2, 6), (3, 6), (3, 7),
@@ -124,7 +142,7 @@ def test_feed_forward_layers():
                    (10, 12), (10, 13),
                    (3, 14), (14, 15), (5, 16), (10, 16)]
     layers = feed_forward_layers(inputs, outputs, connections)
-    assert [{4, 5, 6}, {8, 7}, {9, 11}, {10}, {12, 13}] == layers
+    assert [{4, 5, 6}, {8, 7}, {9, 11}, {10}, {12, 13}] == layers, f"actually got: {layers}"
 
 
 def test_fuzz_feed_forward_layers():