From 46fb24e9fd20baa6c21623a895f01c53cf6ac107 Mon Sep 17 00:00:00 2001
From: Matthew Honnibal <honnibal@gmail.com>
Date: Tue, 23 Jun 2015 00:02:22 +0200
Subject: [PATCH] * Add cycle-checking code in gold.pyx

---
 spacy/gold.pyx | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/spacy/gold.pyx b/spacy/gold.pyx
index f3ed33d10..489b8b124 100644
--- a/spacy/gold.pyx
+++ b/spacy/gold.pyx
@@ -242,6 +242,16 @@ cdef class GoldParse:
                                 self.heads[w2] = None
                                 self.labels[w2] = ''
 
+        # Check there are no cycles in the dependencies, i.e. we are a tree
+        for w in range(self.length):
+            seen = set([w])
+            head = w
+            while self.heads[head] != head and self.heads[head] != None:
+                head = self.heads[head]
+                if head in seen:
+                    raise Exception("Cycle found: %s" % seen)
+                seen.add(head)
+
         self.brackets = {}
         for (gold_start, gold_end, label_str) in brackets:
             start = self.gold_to_cand[gold_start]