fix: support input_offset() (#48)

Add support for fetching the Extism address of the input block. Notably,
the input block _may_ be null, in which case the function returns `0n`.

Fixes #47

src/call-context.ts

@@ -199,6 +199,11 @@ export class CallContext {
       block?.view.setBigUint64(Number(offset), n, true);
     },
 
+    input_offset: () => {
+      const blockIdx = this.#stack[this.#stack.length - 1][0];
+      return Block.indexToAddress(blockIdx || 0);
+    },
+
     input_length: () => {
       return BigInt(this.#input?.byteLength ?? 0);
     },

src/mod.test.ts

@@ -505,6 +505,17 @@ if (typeof WebAssembly === 'undefined') {
     }
   });
 
+  test('plugin can call input_offset', async () => {
+    const plugin = await createPlugin('http://localhost:8124/wasm/input_offset.wasm');
+    try {
+      const input = 'hello world';
+      const hw = await plugin.call('input_offset_length', input);
+      assert.equal(hw?.getBigUint64(0, true), input.length);
+    } finally {
+      await plugin.close();
+    }
+  });
+
   test('plugin can fail gracefully', async () => {
     const plugin = await createPlugin('http://localhost:8124/wasm/fail.wasm');
     try {