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 # FileCheck.py
+
 Attempt to reimplement LLVM's FileCheck using Python.
+
+## Background
+
+We know at least two software projects that would benefit from a suite of 
+LLVM LIT integration tests:
+
+1. [Mull project](https://github.com/mull-project/mull)
+
+2. [Doorstop](https://github.com/doorstop-dev/doorstop/pull/431)
+
+The problem is that you need to have to build `FileCheck` from LLVM sources
+which is not a trivial task for 1) people who are not aware with LLVM 
+infrastructure and 2) Python-based projects that would prefer to not have 
+to build anything from LLVM sources in their CI process. 
+
+The option of having pre-compiled binaries is a workaround, but we don't like to 
+keep third-party binary artifacts in source code, 
+(see https://github.com/doorstop-dev/doorstop/pull/431#issuecomment-549237579).
+
+## Goals
+
+The first goal is to make FileCheck.py pass on Mull's current integration 
+tests suite which uses only a very limited subset of FileCheck's features which
+are as follows:
+
+- Commands (both substring and regex matching): 
+  - `CHECK` 
+  - `CHECK-NEXT` 
+  - `CHECK-NOT` 
+  - `CHECK-EMPTY`
+- Options: 
+  - `--strict-whitespace`
+  - `--match-full-lines` 
+  - `--check-prefix`
+
+When this is done, it feels like a good idea to implement full FileCheck's 
+contract and be 100% compatible with the C++ version.
+
+There is a file in this repository: [FileCheck.pdf](FileCheck.pdf) which has 
+some rough implementation coverage: green color means something is implemented, 
+yellow color means that the text is not relevant to implementation, no 
+highlighting means "still to be implemented".
+
+## Notes on implementation
+
+The implementation is made against the latest 
+[FileCheck - Flexible pattern matching file verifier](https://llvm.org/docs/CommandGuide/FileCheck.html)
+document. The plan is to skip looking into the LLVM's implementation of 
+FileCheck, unless some very advanced implementation details are encountered. 
+
+It a nice programming exercise of implementing something from a spec but we also
+want to make FileCheck.py to conform to the same contract this is why 
+FileCheck.py is tested using LIT and the LLVM's FileCheck:
+
+First, real FileCheck is run against a LIT/FileCheck test suite, then
+FileCheck.py is run on the same tests to make sure that it has the same behavior
+as its LLVM FileCheck parent.