write a little readme

Author: ProkopRandacek

lib/fuzzer/README.md

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+There are two things the fuzzer is storing: explored features and the corpus.
+
+Explored features is a sorted buffer of u32 values. Explored feature is
+anything "interesting" that happened in the code when we run it with some input
+and *it did not crash*. Different "interesting things" should correspond to
+different u32 values but collisions never 100% avoidable. Explored features are
+not shared among different workers.
+
+Currently tracked "interesting things" are:
+
+* taken edges in the CFG
+* address of cmp instructions executed
+* address of switch statements executed
+* indirect calls
+
+Fuzzer is trying to maximize the number of unique explored features over all
+inputs.
+
+The corpus is a set of inputs where input is some array of bytes. The initial
+corpus is either provided by the user or generated randomly. The corpus is
+stored as two arrays that are shared among all workers. One of the arrays
+stores the inputs, densely packed one after another. The other is storing some
+metadata and indexes of string ends. Whenever some input explores a new
+feature, it is added to the corpus. The corpus is never shrunk, only appended.
+
+All that the fuzzer does is pick random input, mutate it, see if it hits any
+new features and if so, add the mutated input to the corpus and the new
+features to explored features.
+
+Every file starts with a more detailed documentation on the part of the fuzzer
+that is implemented in that file:
+
+* `feature_capture.zig` - storing and deduplication of features that the user
+  code is emitting
+* `InputPool*.zig` - corpus implementations
+* `main.zig` - the main loop
+* `memory_mapped_list*.zig` - shared growable memory mapped files
+* `mutate.zig` - mutations
+
+Possible improvements:
+
+* Prioritize mutating inputs that hit rare features
+* Table of recently compared values used in mutations
+* In-place mutation to avoid copying?
+* Implement more mutations
+* Multithreading
+* Maybe use hash table for explored features instead of sorted array
+