added readme

LICENSE

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+Copyright (c) 2014-2019, Radovan Bast
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+* Redistributions of source code must retain the above copyright notice, this
+  list of conditions and the following disclaimer.
+
+* Redistributions in binary form must reproduce the above copyright notice,
+  this list of conditions and the following disclaimer in the documentation
+  and/or other materials provided with the distribution.
+
+* Neither the name of git-bisect-exercise nor the names of its
+  contributors may be used to endorse or promote products derived from
+  this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

README.md

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+[![License](https://img.shields.io/badge/license-%20BSD--3-blue.svg)](../master/LICENSE)
+
+
+## Git bisect exercise
+
+### Motivation
+
+The motivation for this exercise is to be able to do archaeology with Git on a
+source code where the bug is difficult to see visually. **Finding the offending
+commit is often more than half the debugging**.
+
+
+### Background
+
+The script `get_pi.py` approximates pi using terms of the Nilakantha series. It
+should produce 3.14 but it does not. The script broke at some point and
+produces 3.57 using the last commit:
+
+```
+$ python get_pi.py
+
+3.57
+```
+
+At some point within the 500 first commits, an error was introduced. The only
+thing we know is that the first commit worked correctly.
+
+
+### Your task
+
+Clone or fork this repository and use `git bisect` to find the commit which
+broke the computation.

get_pi.py

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-# commit 492
+# commit 500
 t0 = 3.0 + 0.16666666666666666
 t1 = t0 + -0.03333333333333333
 t2 = t1 + 0.011904761904761904
 t3 = t2 + -0.005555555555555556
-t4 = t3 + 0.0030303030303030303
+t4 = t3 + 0.0030303030303030303  # this comment is added to obfuscate git blame
 t5 = t4 + -0.0018315018315018315
 t6 = t5 + 0.0011904761904761906
 t7 = t6 + 0.4312830065359477
 t8 = t7 + 0.0005847953216374269
 t9 = t8 + -0.0004329004329004329
 t10 = t9 + 0.00032938076416337287
-t11 = t10 + -0.0002564102564102564  # this comment is added to obfuscate git blame
+t11 = t10 + -0.0002564102564102564
 t12 = t11 + 0.0002035002035002035
 t13 = t12 + -0.00016420361247947455
 print('%.2f' % (t13))