From e29f63136bb41aa5c5c5ef33e67a4c7fa1a8e36e Mon Sep 17 00:00:00 2001
From: Alon Zakai <alonzakai@gmail.com>
Date: Sun, 29 Jan 2012 09:59:40 -0800
Subject: bisection bugfinding tool

---
 tools/bisect_pair.py | 84 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 84 insertions(+)
 create mode 100644 tools/bisect_pair.py

diff --git a/tools/bisect_pair.py b/tools/bisect_pair.py
new file mode 100644
index 00000000..3b880b28
--- /dev/null
+++ b/tools/bisect_pair.py
@@ -0,0 +1,84 @@
+'''
+Given two similar files, for example one with an additional optimization pass,
+and with different results, will bisect between them to find the smallest
+diff that makes the outputs different.
+'''
+
+import os, sys, shutil
+from subprocess import Popen, PIPE, STDOUT
+
+__rootpath__ = os.path.abspath(os.path.dirname(os.path.dirname(__file__)))
+def path_from_root(*pathelems):
+  return os.path.join(__rootpath__, *pathelems)
+exec(open(path_from_root('tools', 'shared.py'), 'r').read())
+
+file1 = open(sys.argv[1]).read()
+file2 = open(sys.argv[2]).read()
+
+leftf = open('left', 'w')
+leftf.write(file1)
+leftf.close()
+
+rightf = open('right', 'w')
+rightf.write(file2)
+rightf.close()
+
+print 'running files'
+left_result = run_js('left', stderr=PIPE)
+right_result = run_js('right', stderr=PIPE) # right as in left-right, not as in correct
+assert left_result != right_result
+
+# Calculate diff chunks
+print 'diffing'
+diff = Popen(['diff', '-U', '5', 'left', 'right'], stdout=PIPE).communicate()[0].split('\n')
+pre_diff = diff[:2]
+diff = diff[2:]
+
+chunks = []
+curr = []
+for i in range(len(diff)):
+  if diff[i].startswith('@'):
+    if len(curr) > 0:
+      chunks.append(curr)
+    curr = [diff[i]]
+  else:
+    curr.append(diff[i])
+if len(curr) > 0:
+  chunks.append(curr)
+
+# Bisect both sides of the span, until we have a single chunk
+low = 0
+high = len(chunks)
+
+print 'beginning bisection, %d chunks' % high
+
+while high-low > 2:
+  mid = (low + high)/2
+  print '  current status: %d - %d - %d' % (low, mid, high)
+  # Take chunks from the middle and on. This is important because the eliminator removes variables, so starting from the beginning will add errors
+  curr_diff = '\n'.join(map(lambda parts: '\n'.join(parts), chunks[mid:])) + '\n'
+  difff = open('diff.diff', 'w')
+  difff.write(curr_diff)
+  difff.close()
+  shutil.copy('left', 'middle')
+  Popen(['patch', 'middle', 'diff.diff'], stdout=PIPE, stderr=PIPE).communicate()
+  result = run_js('middle', stderr=PIPE)
+  if result == left_result:
+    high = mid+1
+  else:
+    low = mid
+
+critical = '\n'.join(chunks[low]) + '\n'
+
+c = open('critical.diff', 'w')
+c.write(critical)
+c.close()
+print 'sanity check'
+shutil.copy('middle', 'middle2')
+Popen(['patch', 'middle2', 'critical.diff'], stdout=PIPE, stderr=PIPE).communicate()
+assert run_js('middle', stderr=PIPE) == left_result
+assert run_js('middle2', stderr=PIPE) != left_result
+
+print 'middle is like left, middle2 is like right, critical.diff is the difference that matters,'
+print critical
+
-- 
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