implemented save file compression in browser

src/lib/saveArchive/zstdStream.ts

@@ -0,0 +1,58 @@
+// Zstandard compression and decompression as stream transforms.
+//
+// Saves are re-compressed in the browser because the parser backend runs on a
+// cheap two core server with little disk space. zstd was picked over lzma2 and
+// brotli because its decompression speed does not depend on the level the data
+// was compressed with, so the level can be raised without costing the server
+// anything, and because its wasm build is by far the smallest of the three.
+//
+// @tootallnate/zstd-wasm does the work: it is upstream zstd 1.5.7 built with
+// wasi-sdk and driven through ZSTD_compressStream2, so it compresses a stream
+// as it arrives rather than a buffer at a time. A save of any size comes out
+// as one zstd frame, and memory stays at the library's own input and output
+// buffers.
+
+import {
+    ZstdCompressStream,
+    ZstdDecompressStream,
+} from '@tootallnate/zstd-wasm'
+
+// The wasm file ships inside the package. Turbopack rewrites this into the URL
+// it emits the asset at, so the 534 kB binary is fetched once at runtime and
+// cached by the browser instead of being inlined into the worker bundle.
+const wasmUrl = new URL(
+    '../../../node_modules/@tootallnate/zstd-wasm/dist/zstd.wasm',
+    import.meta.url
+)
+
+// Compiling takes a moment, so it happens once and both directions share it.
+let wasmModule: Promise<WebAssembly.Module> | null = null
+
+function getWasm(): Promise<WebAssembly.Module> {
+    // compileStreaming would need the server to send Content-Type:
+    // application/wasm, which is not worth depending on.
+    wasmModule ??= fetch(wasmUrl)
+        .then((res) => res.arrayBuffer())
+        .then((bytes) => WebAssembly.compile(bytes))
+    return wasmModule
+}
+
+/**
+ * Compresses a stream into a single zstd frame. The level goes from 1 to 22.
+ */
+export function createZstdCompressStream(
+    compressionLevel: number
+): TransformStream<Uint8Array, Uint8Array> {
+    return new ZstdCompressStream(getWasm(), compressionLevel)
+}
+
+/**
+ * Decompresses a zstd stream. Frames that follow one another are read straight
+ * through, so an archive packed as several frames still opens.
+ */
+export function createZstdDecompressStream(): TransformStream<
+    Uint8Array,
+    Uint8Array
+> {
+    return new ZstdDecompressStream(getWasm())
+}