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[wip] more basic info added to README re: requirements, installation, usage

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@ -7,6 +7,73 @@
Nim wrapper for [Leopard-RS](https://github.com/catid/leopard): a fast library for [Reed-Solomon](https://en.wikipedia.org/wiki/Reed%E2%80%93Solomon_error_correction) erasure correction coding.
## Requirements
* Same as Leopard-RS' requirements, e.g. CMake 3.7 or newer.
* Nim 1.2 or newer.
## Installation
With [Nimble](https://github.com/nim-lang/nimble)
```text
$ nimble install leopard
```
In a project's `.nimble` file
```nim
requires "leopard >= 0.0.1 & < 0.0.2"
```
In a [nimbus-build-system](https://github.com/status-im/nimbus-build-system) project
```text
$ git submodule add https://github.com/status-im/nim-leopard.git vendor/nim-leopard
$ make update
```
### Submodule
#### Init
[status-im/leopard](https://github.com/status-im/leopard), a fork of [catid/leopard](https://github.com/catid/leopard) (Leopard-RS), is a submodule of nim-leopard.
When nim-leopard is installed with `nimble install leopard`, or as a dependency in a Nimble project, or vendored in a nimbus-build-system project, submodule init is handled automatically.
If the nim-leopard repo is cloned directly, then before running `nimble develop` or `nimble install` in the root of the clone, it's necessary to init the submodule
```text
$ git submodule update --init
```
#### Build
The submodule is automatically built (in the `nimcache` dir) and statically linked during compilation of any Nim module that has `import leopard` or `import leopard/wrapper`.
If the `nimcache` dir is set to a custom value, it must be an absolute path.
For the build to work on Windows, `nimble` or `nim c` must be run from a Bash shell, e.g. Git Bash or an MSYS2 shell, and all needed tools (e.g. `cmake` and `make`) must be available in and suitable for that environment.
##### OpenMP
Leopard-RS' `CMakeLists.txt` checks for [OpenMP](https://en.wikipedia.org/wiki/OpenMP) support. If it is available then it is enabled in the build of `libleopard.a`.
Build toolchains commonly installed on Linux and Windows come with support for OpenMP.
The clang/++ compiler in Apple's Xcode does not support OpenMP, but the one installed with `brew install llvm` does support it, though it's also necessary to `brew install libomp`.
So, on macOS, when running `nimble test` of nim-leopard or compiling a project that imports nim-leopard:
* If libomp is not installed and Apple's clang is used, no extra flags need to be passed to the Nim compiler. OpenMP support will not be enabled in `libleopard.a`.
* If libomp is installed and Apple's clang is used, this flag should be passed to `nim c`
```text
-d:LeopardCmakeFlags="-DCMAKE_BUILD_TYPE=Release -DENABLE_OPENMP=off"
```
* If the intent is to use brew-installed clang + libomp, the shell environment should be modified
```text
$ export PATH="$(brew --prefix)/opt/llvm/bin:${PATH}"
$ export LDFLAGS="-L$(brew --prefix)/opt/libomp/lib -L$(brew --prefix)/opt/llvm/lib -Wl,-rpath,$(brew --prefix)/opt/llvm/lib"
```
and these flags should be passed to `nim c`
```text
-d:LeopardCmakeFlags="-DCMAKE_BUILD_TYPE=Release -DCMAKE_C_COMPILER=$(brew --prefix)/opt/llvm/bin/clang -DCMAKE_CXX_COMPILER=$(brew --prefix)/opt/llvm/bin/clang++" -d:LeopardExtraCompilerlags="-fopenmp" -d:LeopardExtraLinkerFlags="-fopenmp -L$(brew --prefix)/opt/libomp/lib"
```
## Usage
```nim
@ -20,11 +87,10 @@ var
data: seq[seq[byte]]
# RS(256,239) :: 239 data symbols, 17 parity symbols
assert RS(256,239).code == 239
assert RS(256,239).data == 239
assert RS(256,239).parity == 17
# Choose some N
# Choose some N for symbolBytes
N = 1
# For RS(256,239) fill data such that
assert data.len == 239
@ -62,6 +128,10 @@ else:
assert recoveredData.error.code == LeopardNeedMoreData
```
### OpenMP
When OpenMP is enabled, whether or not parallel processing kicks in depends on the symbol and byte counts. On a local machine with an Intel processor `RS(256,239)` with `symbolBytes == 64` seems to be the lower bound for triggering parallel processing.
## Versioning
nim-leopard generally follows the upstream `master` branch such that changes there will result in a version bump for this package.