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lib/aura/Backends/readme.md

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+# Aura Backends
+
+The `/Backends` directory contains target-specific code that may be used on some
+targets to improve performance.
+
+The backends are enabled by default, but if you want to only use the generic
+Haxe sources (for performance comparisons e.g.), compile your Kha project with
+the command line flag `--aura-no-backend`.
+
+## Folder Structure
+
+- `/common_c`:
+  Pure C code that can be used by multiple backends.
+
+- `/hl`:
+  Sources/headers for the Hashlink/C backend. The header files are mostly
+  Hashlink API wrappers around the code in `/common_c`.
+
+Most of the backend sources mirror the respective Haxe code, so please don't
+expect much documentation for the individual functions.
+
+The Haxe implementation/glue code for the backends is in Aura's source files in
+`/Sources`. There are no Haxe files in the `/backend` folder that shadow
+original sources to reduce redundancy and ensure completeness of the API.
+
+Instead, there is usually a static class per backend implementation at the
+bottom of a Haxe source module, whose methods are then called and inlined from
+the original class if the [backend specific define is set](#defines). This way
+all the Haxe functionality for a module stays inside a module and is not
+distributed in many per-backend Haxe files, which also keeps the API consistent
+for each target.
+
+## Defines
+
+If the backends are enabled, Aura sets some defines before compilation which are
+based on the Haxe target to which the project is compiled. They should only be
+used internally, but for the sake of completeness they are documented here:
+
+- `AURA_NO_BACKEND`: Defined if backends are disabled.
+- `AURA_BACKEND_HL`: Defined if backends are enabled and the project is compiled
+  to a Hashlink target.