Add architecture.md

docs/architecture.md

@@ -0,0 +1,57 @@
+# Architecture
+
+This is a document going over my various design decisions I made
+with physis, to onboard future contributors and also point a light
+at possible mistakes and what we can do to solve them!
+
+## Goals
+
+I think it's important to go over what problems physis is set up to solve. It's meant to:
+
+* Make patching easier for custom launchers
+* Users who want to write one-off tools that interact with the game data (like scrapers)
+* Modding tools that want to read and write game data in a safe way
+
+Physis is all about game data, reading, writing and modifying it. However, it's keen to
+keep it _safe_ and prevent invalid data from being written, and if there is invalid data
+being read - to make it obvious to the developer. So there is a high-level representation
+of a file format, which is built on top of our custom parsers:
+
+```rust
+#[derive(Debug)]
+pub struct IndexEntry {
+    pub hash: u64,
+    pub data_file_id: u8,
+    pub offset: u32,
+}
+
+#[binrw]
+#[br(little)]
+pub struct IndexFile {
+    sqpack_header: SqPackHeader,
+
+    #[br(seek_before = SeekFrom::Start(sqpack_header.size.into()))]
+    index_header: SqPackIndexHeader,
+
+    #[br(seek_before = SeekFrom::Start(index_header.index_data_offset.into()))]
+    #[br(count = index_header.index_data_size / 16)]
+    pub entries: Vec<IndexHashTableEntry>,
+}
+```
+
+Here's a section of `src/index.rs` that showcases an example of this methodology, headers and other auto-generated data
+is hidden by the developer (as there is very little to worry about there anyway) and they only need access to modifying and
+reading entries. There is still work to be done to improve upon this, but this is the general idea when writing for
+game format parsing.
+
+## Top-level Architecture
+
+There is a bunch of methods to crack open your dat files, but the best way is to `GameData` and `BootData`. These two
+structures help parse and ensure boot and game data is valid.
+
+There is helper methods in `GameData` such as `extract(path)` and `exists(path)` which are wrappers around other public
+APIs, but make it easier for developers to get started.
+
+When parsing game data, you'll notice many formats only accept a memory buffer, which is just a `Vec<u8>`. These are
+passed as references, and are purely non-owning. The purpose of this is because some files are not backed by a file on disk,
+but instead are extracted and processed entirely in memory.