sz3: Add wasm build with emscripten

Much of the works was already done in
https://github.com/otytlandsvik/SZ3-wasm

Thanks @oletytlandsvik :)

by-name/sz3-wasm/default.nix

@@ -0,0 +1,75 @@
+{
+  lib,
+  stdenv,
+  fetchFromGitHub,
+  emscripten,
+  zstd,
+}:
+stdenv.mkDerivation {
+  pname = "sz3-wasm";
+  # NOTE: the SZ3 `rev` below MUST stay in sync with `by-name/sz3` — the SZ3
+  # bitstream format is tied to a specific version (the stream embeds SZ3_DATA_VER
+  # and is checked on decode), so the wasm decoder must be built from the exact
+  # same source as whatever encoder produced the chunks.
+  version = "3.3.2";
+
+  src = fetchFromGitHub {
+    owner = "szcompressor";
+    repo = "SZ3";
+    rev = "34b36f2af0d43aa4f5dea0e39500e91f6d35c9cf";
+    sha256 = "0fckf5939m9d2w3095isnq3qfrz8fxpppn7y3c2hwzdh72vdzic7";
+  };
+
+  # zstd is SZ3's lossless backend. SZ3's tools/zstd uses CMake FetchContent to
+  # download zstd at build time, which does not work offline, so we compile the
+  # zstd C sources from nixpkgs' zstd source directly.
+  zstdSrc = zstd.src;
+
+  nativeBuildInputs = [ emscripten ];
+
+  buildPhase = ''
+    runHook preBuild
+
+    # Emscripten's cache in the nix store is read-only; give it a writable one.
+    export EM_CACHE=$TMPDIR/emcache
+    export HOME=$TMPDIR
+    mkdir -p $EM_CACHE
+
+    mkdir -p gen/SZ3
+    substitute include/SZ3/version.hpp.in gen/SZ3/version.hpp \
+      --subst-var-by PROJECT_NAME SZ3 \
+      --subst-var-by PROJECT_VERSION 3.3.2 \
+      --subst-var-by PROJECT_VERSION_MAJOR 3 \
+      --subst-var-by PROJECT_VERSION_MINOR 3 \
+      --subst-var-by PROJECT_VERSION_PATCH 2 \
+      --subst-var-by PROJECT_VERSION_TWEAK 0 \
+      --subst-var-by SZ3_DATA_VERSION 3.3.2
+
+    ZSTD_SRCS="$zstdSrc/lib/common/*.c $zstdSrc/lib/compress/*.c $zstdSrc/lib/decompress/*.c"
+
+    em++ -std=c++17 -O3 -DNDEBUG -sMEMORY64=1 \
+      -I gen -I include -I tools/sz3c/include -I "$zstdSrc/lib" \
+      tools/sz3c/src/sz3c.cpp $ZSTD_SRCS \
+      -sMODULARIZE=1 -sEXPORT_NAME=createSz3Module -sEXPORT_ES6=1 -sSINGLE_FILE=1 \
+      -sEXPORTED_FUNCTIONS='["_SZ_decompress","_malloc","_free"]' \
+      -sEXPORTED_RUNTIME_METHODS='["HEAPU8","HEAPF32","cwrap","ccall"]' \
+      -sALLOW_MEMORY_GROWTH=1 -sENVIRONMENT=web,worker \
+      -o sz3_decode.js
+
+    runHook postBuild
+  '';
+
+  installPhase = ''
+    runHook preInstall
+    mkdir -p $out
+    install -m644 sz3_decode.js $out/sz3_decode.js
+    runHook postInstall
+  '';
+
+  meta = with lib; {
+    description = "WebAssembly (wasm64) build of the SZ3 lossy decompressor C API for the browser";
+    homepage = "https://github.com/szcompressor/SZ3";
+    license = licenses.bsd3;
+    platforms = platforms.unix;
+  };
+}

flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1783522502,
+        "narHash": "sha256-iffAls3iaNTyJC2faYcUXSI+Gp02cDjYl+MygxKl2GI=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "0bb7ec54c8483066ec9d7720e780a5caa71f8612",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}