add new readme

README.md

@@ -1,6 +1,206 @@
 # Extism Zig Host SDK
 
-Zig SDK for the extism WebAssembly Plugin-System.
+This repo contains the Zig code for integrating with the [Extism](https://extism.org/) runtime. Install this library into your host Zig application to run Extism plug-ins.
+
+> **Note**: If you're unsure what Extism is or what an SDK is see our homepage: [https://extism.org](https://extism.org).
 
 > **Note**: This is an early 1.0 release and is unstable until we hit 1.0. If you are looking to integrate now consider looking at the 0.x version in the [extism/extism](https://github.com/extism/extism/tree/main/zig) repo.
 
+## Installation
+
+### Install the Extism Runtime Dependency
+
+For this library, you first need to install the Extism Runtime. You can [download the shared object directly from a release](https://github.com/extism/extism/releases) or use the [Extism CLI](https://github.com/extism/cli) to install it:
+
+```bash
+sudo extism lib install latest
+
+#=> Fetching https://github.com/extism/extism/releases/download/v0.5.2/libextism-aarch64-apple-darwin-v0.5.2.tar.gz
+#=> Copying libextism.dylib to /usr/local/lib/libextism.dylib
+#=> Copying extism.h to /usr/local/include/extism.h
+```
+
+> **Note**: This library has breaking changes and targets 1.0 of the runtime. For the time being, install the runtime from our nightly development builds on git: `sudo extism lib install --version git`.
+
+# within your Zig project directory:
+mkdir -p libs
+cd libs
+git clone https://github.com/extism/extism.git
+
+## Getting Started
+
+This guide should walk you through some of the concepts in Extism and this Zig library.
+
+### Creating A Plug-in
+
+The primary concept in Extism is the [plug-in](https://extism.org/docs/concepts/plug-in). You can think of a plug-in as a code module stored in a `.wasm` file.
+
+Since you may not have an Extism plug-in on hand to test, let's load a demo plug-in from the web:
+
+```zig
+// First require the library
+const extism = @import("extism");
+
+const wasm_url = extism.manifest.WasmUrl{ .url = "https://github.com/extism/plugins/releases/latest/download/count_vowels.wasm" };
+const manifest = .{ .wasm = &[_]extism.manifest.Wasm{.{ .wasm_url= wasm_url }} };
+
+var gpa = std.heap.GeneralPurposeAllocator(.{}){};
+const allocator = gpa.allocator();
+
+var plugin = try extism.Plugin.initFromManifest(
+    allocator,
+    manifest,
+    &[_]extism.Function{},
+    false,  
+);
+
+defer plugin.deinit();
+```
+
+> **Note**: See [the Manifest docs](https://github.com/extism/zig-sdk/blob/main/src/manifest.zig#L32) as it has a rich schema and a lot of options.
+
+### Calling A Plug-in's Exports
+
+This plug-in was written in Rust and it does one thing, it counts vowels in a string. As such, it exposes one "export" function: `count_vowels`. We can call exports using [Extism::Plugin#call](https://github.com/extism/zig-sdk/blob/main/src/plugin.zig#L61):
+
+```zig
+try plugin.call("count_vowels", "Hello, World!");
+# => {"count": 3, "total": 3, "vowels": "aeiouAEIOU"}
+```
+
+All exports have a simple interface of bytes-in and bytes-out. This plug-in happens to take a string and return a JSON encoded string with a report of results.
+
+### Plug-in State
+
+Plug-ins may be stateful or stateless. Plug-ins can maintain state b/w calls by the use of variables. Our count vowels plug-in remembers the total number of vowels it's ever counted in the "total" key in the result. You can see this by making subsequent calls to the export:
+
+```zig
+try plugin.call("count_vowels", "Hello, World!");
+# => {"count": 3, "total": 6, "vowels": "aeiouAEIOU"}
+try plugin.call("count_vowels", "Hello, World!");
+# => {"count": 3, "total": 9, "vowels": "aeiouAEIOU"}
+```
+
+These variables will persist until this plug-in is freed or you initialize a new one.
+
+### Configuration
+
+Plug-ins may optionally take a configuration object. This is a static way to configure the plug-in. Our count-vowels plugin takes an optional configuration to change out which characters are considered vowels. Example:
+
+```zig
+try plugin.call("count_vowels", "Yellow, World!");
+# => {"count": 3, "total": 3, "vowels": "aeiouAEIOU"}
+
+var config = std.json.ArrayHashMap([]const u8){};
+defer config.deinit(allocator);
+
+try config.map.put(allocator, "vowels", "aeiouyAEIOUY");
+try plugin.setConfig(allocator, config);
+
+try plugin.call("count_vowels", "Yellow, World!");
+# => {"count": 4, "total": 4, "vowels": "aeiouAEIOUY"}
+```
+
+### Host Functions
+
+Let's extend our count-vowels example a little bit: Instead of storing the `total` in an ephemeral plug-in var, let's store it in a persistent key-value store!
+
+Wasm can't use our KV store on it's own. This is where [Host Functions](https://extism.org/docs/concepts/host-functions) come in.
+
+[Host functions](https://extism.org/docs/concepts/host-functions) allow us to grant new capabilities to our plug-ins from our application. They are simply some zig methods you write which can be passed down and invoked from any language inside the plug-in.
+
+Let's load the manifest like usual but load up this `count_vowels_kvstore` plug-in:
+
+```zig
+const wasm_url = extism.manifest.WasmUrl{ .url = "https://github.com/extism/plugins/releases/latest/download/count_vowels_kvstore.wasm" };
+const manifest = .{ .wasm = &[_]extism.manifest.Wasm{.{ .wasm_url= wasm_url }} };
+```
+
+> *Note*: The source code for this is [here](https://github.com/extism/plugins/blob/main/count_vowels_kvstore/src/lib.rs) and is written in rust, but it could be written in any of our PDK languages.
+
+Unlike our previous plug-in, this plug-in expects you to provide host functions that satisfy our its import interface for a KV store.
+
+We want to expose two functions to our plugin, `kv_write(key: String, value: Bytes)` which writes a bytes value to a key and `kv_read(key: String) -> Bytes` which reads the bytes at the given `key`.
+
+```zig
+// pretend this is Redis or something
+const HashMap = std.StringHashMap([]const u8);
+var KV_STORE: HashMap = undefined;
+
+export fn kv_read(plugin_ptr: ?*extism.c.ExtismCurrentPlugin, inputs: [*c]const extism.c.ExtismVal, n_inputs: u64, outputs: [*c]extism.c.ExtismVal, n_outputs: u64, user_data: ?*anyopaque) callconv(.C) void {
+
+    var curr_plugin = extism.CurrentPlugin.getCurrentPlugin(plugin_ptr orelse unreachable);
+
+    // retrieve the key from the plugin
+    var input_slice = inputs[0..n_inputs];
+    const key = curr_plugin.inputBytes(&input_slice[0]);
+
+    // Try to get the value from KV_STORE
+    const val = KV_STORE.get(key) orelse null;
+
+    // return the value to the plugin
+    var output_slice = outputs[0..n_outputs];
+    curr_plugin.returnBytes(&output_slice[0], val);
+}
+
+export fn kv_write(plugin_ptr: ?*extism.c.ExtismCurrentPlugin, inputs: [*c]const extism.c.ExtismVal, n_inputs: u64, outputs: [*c]extism.c.ExtismVal, n_outputs: u64, user_data: ?*anyopaque) callconv(.C) void {
+
+    var curr_plugin = extism.CurrentPlugin.getCurrentPlugin(plugin_ptr orelse unreachable);
+
+    // retrieve key and value from the plugin
+    var input_slice = inputs[0..n_inputs];
+    const key = curr_plugin.inputBytes(&input_slice[0]);
+    const val = curr_plugin.inputBytes(&input_slice[1]);
+
+    // write to the KV
+    _ = KV_STORE.put(key, val) catch unreachable;
+}
+
+```
+
+Now we just need to create a new host environment and pass it in when loading the plug-in. Here our environment initializer takes no arguments, but you could imagine putting some customer specific instance variables in there:
+
+```zig
+ var gpa = std.heap.GeneralPurposeAllocator(.{}){};
+const allocator = gpa.allocator();
+
+KV_STORE = std.StringHashMap([]const u8).init(allocator);
+defer KV_STORE.deinit();
+
+var f_read = extism.Function.init(
+    "kv_read",
+    &[_]extism.c.ExtismValType{extism.c.I64},
+    &[_]extism.c.ExtismValType{extism.c.I64},
+    &kv_read,
+    @constCast(@as(*const anyopaque, @ptrCast("user data"))),
+);
+defer f_read.deinit();
+
+var f_write = extism.Function.init(
+    "kv_write",
+    &[_]extism.c.ExtismValType{extism.c.I64, extism.c.I64},
+    &[_]extism.c.ExtismValType{},
+    &kv_write,
+    @constCast(@as(*const anyopaque, @ptrCast("user data"))),
+);
+defer f_write.deinit();
+
+var plugin = try extism.Plugin.initFromManifest(
+    allocator,
+    manifest,
+    &[_]extism.Function{f_read, f_write},
+    false,  
+);
+defer plugin.deinit();
+```
+
+Now we can invoke the event:
+
+```zig
+try plugin.call("count_vowels", "Hello, World!");
+# => {"count": 3, "total": 3, "vowels": "aeiouAEIOU"}
+
+try plugin.call("count_vowels", "Hello, World!");
+# => {"count": 3, "total": 6, "vowels": "aeiouAEIOU"}
+```
+