Add parseSchema (#72)

* Add parseSchema

* fmt

Author: kylebarron

README.md

@@ -24,6 +24,18 @@ const WASM_MEMORY: WebAssembly.Memory = ...
 const field = parseField(WASM_MEMORY.buffer, fieldPtr);
 ```
 
+### `parseSchema`
+
+Parse an [`ArrowSchema`](https://arrow.apache.org/docs/format/CDataInterface.html#the-arrowschema-structure) C FFI struct into an `arrow.Schema` instance. Note that the underlying field **must** be a `Struct` type. In essence a `Struct` field is used to mimic a `Schema` while only being one field.
+
+- `buffer` (`ArrayBuffer`): The [`WebAssembly.Memory`](https://developer.mozilla.org/en-US/docs/WebAssembly/JavaScript_interface/Memory) instance to read from.
+- `ptr` (`number`): The numeric pointer in `buffer` where the C struct is located.
+
+```js
+const WASM_MEMORY: WebAssembly.Memory = ...
+const schema = parseSchema(WASM_MEMORY.buffer, fieldPtr);
+```
+
 ### `parseData`
 
 Parse an [`ArrowArray`](https://arrow.apache.org/docs/format/CDataInterface.html#the-arrowarray-structure) C FFI struct into an [`arrow.Data`](https://arrow.apache.org/docs/js/classes/Arrow_dom.Data.html) instance. Multiple `Data` instances can be joined to make an [`arrow.Vector`](https://arrow.apache.org/docs/js/classes/Arrow_dom.Vector.html).

src/index.ts

@@ -1,3 +1,4 @@
-export { parseVector, parseData } from "./vector";
 export { parseField } from "./field";
 export { parseRecordBatch } from "./record-batch";
+export { parseSchema } from "./schema";
+export { parseVector, parseData } from "./vector";

src/schema.ts

@@ -0,0 +1,33 @@
+import * as arrow from "apache-arrow";
+import { parseField } from "./field";
+
+/**
+Parse an [`ArrowSchema`](https://arrow.apache.org/docs/format/CDataInterface.html#the-arrowschema-structure) C FFI struct into an `arrow.Schema` instance. Note that the underlying field **must** be a `Struct` type. In essence a `Struct` field is used to mimic a `Schema` while only being one field.
+
+- `buffer` (`ArrayBuffer`): The [`WebAssembly.Memory`](https://developer.mozilla.org/en-US/docs/WebAssembly/JavaScript_interface/Memory) instance to read from.
+- `ptr` (`number`): The numeric pointer in `buffer` where the C struct is located.
+
+```js
+const WASM_MEMORY: WebAssembly.Memory = ...
+const schema = parseSchema(WASM_MEMORY.buffer, fieldPtr);
+```
+ */
+export function parseSchema(buffer: ArrayBuffer, ptr: number): arrow.Schema {
+  const field = parseField(buffer, ptr);
+  if (!isStructField(field)) {
+    throw new Error("Expected struct");
+  }
+
+  return unpackStructField(field);
+}
+
+function isStructField(field: arrow.Field): field is arrow.Field<arrow.Struct> {
+  return field.typeId == arrow.Type.Struct;
+}
+
+function unpackStructField(field: arrow.Field<arrow.Struct>): arrow.Schema {
+  const fields = field.type.children;
+  const metadata = field.metadata;
+  // TODO: support dictionaries parameter for dictionary-encoded arrays
+  return new arrow.Schema(fields, metadata);
+}