Split up Wasm binaries so Cloudflare deployments stay below 25 MB per file

frontend/vite.config.ts

@@ -1,6 +1,6 @@
 import { execSync } from "child_process";
 import { createHash } from "crypto";
-import { copyFileSync, cpSync, existsSync, readdirSync, readFileSync, statSync, writeFileSync } from "fs";
+import { copyFileSync, cpSync, existsSync, readdirSync, readFileSync, rmSync, statSync, writeFileSync } from "fs";
 import path from "path";
 import { svelte } from "@sveltejs/vite-plugin-svelte";
 import { defineConfig } from "vite";
@@ -11,7 +11,17 @@ const projectRootDir = path.resolve(__dirname);
 // https://vitejs.dev/config/
 export default defineConfig(({ mode }) => {
 	return {
-		plugins: [svelteGlobalStyles(), webkitUserSelectPrefix(), svelte(), staticAssets(), mode !== "native" && thirdPartyLicenses(), mode !== "native" && serviceWorker()],
+		plugins: [
+			svelteGlobalStyles(),
+			webkitUserSelectPrefix(),
+			svelte(),
+			staticAssets(),
+			mode !== "native" && thirdPartyLicenses(),
+			mode !== "native" && wasmSplitting(),
+			mode !== "native" && serviceWorker(),
+		],
+		// Default for builds that exclude the `wasmSplitting` plugin, which overrides this when active
+		define: { __WASM_PART_COUNT__: "1" },
 		resolve: {
 			alias: [{ find: /\/..\/branding\/(.*\.svg)/, replacement: path.resolve(projectRootDir, "../branding", "$1?raw") }],
 		},
@@ -136,6 +146,48 @@ function thirdPartyLicenses(): PluginOption {
 	};
 }
 
+// Splits the Wasm binary into parts small enough for Cloudflare Pages' 25 MiB single-file limit, rejoined at runtime by `initWasm()` in `src/utility-functions/wasm-loader.ts`.
+// Only active when the `SPLIT_WASM` environment variable is set, which CI does for deployments; local builds keep the single file.
+function wasmSplitting(): PluginOption {
+	const PART_SIZE = 24 * 1024 * 1024;
+
+	let partCount = 1;
+
+	return {
+		name: "wasm-splitting",
+		config(_, { command }) {
+			// Measure the Wasm binary (already built by `cargo run build web` before Vite runs) to decide how many parts are needed
+			if (command === "build" && process.env.SPLIT_WASM) {
+				const wasmPath = path.resolve(projectRootDir, "wrapper/pkg/graphite_wasm_wrapper_bg.wasm");
+				if (!existsSync(wasmPath)) throw new Error(`SPLIT_WASM is set but the Wasm binary is missing at ${wasmPath}`);
+				partCount = Math.ceil(statSync(wasmPath).size / PART_SIZE);
+			}
+
+			// Bake the part count into the bundle so `initWasm()` knows how many parts to fetch and rejoin
+			return { define: { __WASM_PART_COUNT__: String(partCount) } };
+		},
+		// Synchronous so it completes before the `serviceWorker` plugin's `writeBundle` collects the precache manifest
+		writeBundle(options) {
+			if (partCount <= 1) return;
+
+			const assetsDir = path.join(options.dir || "dist", "assets");
+			const wasmFileName = readdirSync(assetsDir).find((name) => name.startsWith("graphite_wasm_wrapper_bg-") && name.endsWith(".wasm"));
+			if (!wasmFileName) throw new Error("Could not find the emitted Wasm asset to split");
+			const wasmPath = path.join(assetsDir, wasmFileName);
+
+			const contents = readFileSync(wasmPath);
+			if (Math.ceil(contents.length / PART_SIZE) !== partCount) throw new Error("Wasm binary size changed during the build, invalidating the baked-in part count");
+
+			// Replace the single Wasm file with its parts so only they get deployed and precached
+			for (let index = 0; index < partCount; index += 1) {
+				const partName = wasmFileName.replace(/\.wasm$/, `-part${index}.wasm`);
+				writeFileSync(path.join(assetsDir, partName), contents.subarray(index * PART_SIZE, (index + 1) * PART_SIZE));
+			}
+			rmSync(wasmPath);
+		},
+	};
+}
+
 function serviceWorker(): PluginOption {
 	// Files that should never be precached
 	const EXCLUDED_FILES = new Set(["service-worker.js"]);