// 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())
}