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README.md

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+## Problem statement
+
+This solves the following problem:
+```
+Given a set of rectilinear rectangles in the plane, each by coordinates of opposing corners,
+calculate the area of their union.
+```
+It was a task to show that this is solvable in `O(n^2)` on one of my exercise sheets.
+Because I had some spare time, I decided to implement this.
+
+Actually, this can be done in `O(nlog n)` time using a sweepline algorithm
+that makes use of [segment trees][st].
+
+The implementation tries to be reasonably fast and respects `nlog n` running time,
+but is certainly not too optimized and more of a showcase.
+
+
+## Build
+A standard `C++` installation along with `CMake` should to the job.
+Remove the benchmark targets if you do not need them.
+
+
+## Benchmark
+
+Just for fun, I played around with the [Google Benchmark Library][gl-benchmark].
+You will have to install the library and statically link to it to run the
+benchmarks.
+See the github page of the benchmark library for details.
+
+
+[gl-benchmark]: https://github.com/google/benchmark
+[st]: https://en.wikipedia.org/wiki/Segment_tree