docs.sheetjs.com/09_hermes.md at 686b5c55af08d2ef9ef3394dabc17d0c13ac7cd1

sheetjs/docs.sheetjs.com

2023-05-30 03:27:28 -04:00

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import current from '/version.js'; import CodeBlock from '@theme/CodeBlock';

Hermes is an embeddable JS engine written in C++. With some light shims, it can run the standalone browser scripts.

The Standalone scripts can be parsed and evaluated in a Hermes context.

:::caution Here be Dragons

The main target for Hermes is React Native. At the time of writing, there was no official documentation for embedding the Hermes engine in C++ programs.

:::

Integration Details

Initialize Hermes

The runtime can be initialized in one line:

std::unique_ptr<facebook::jsi::Runtime> rt(facebook::hermes::makeHermesRuntime());

Hermes does not expose a console or global variable, but those can be synthesized from JS code in the runtime:

auto src = std::make_shared<facebook::jsi::StringBuffer>(
  /* create global object */
  "var global = (function(){ return this; }).call(null);"
  /* create a fake `console` from the hermes `print` builtin */
  "var console = { log: function(x) { print(x); } };"
);
auto js = rt->prepareJavaScript(src, std::string("<eval>"));
rt->evaluatePreparedJavaScript(js);

Load SheetJS Scripts

The main library can be loaded by reading the script from the file system and evaluating in the Hermes context:

static char *read_file(const char *filename, size_t *sz) {
  FILE *f = fopen(filename, "rb");
  if(!f) return NULL;
  long fsize; { fseek(f, 0, SEEK_END); fsize = ftell(f); fseek(f, 0, SEEK_SET); }
  char *buf = (char *)malloc(fsize * sizeof(char));
  *sz = fread((void *) buf, 1, fsize, f);
  fclose(f);
  return buf;
}

/* Unfortunately the library provides no C-friendly Buffer classes */
class CBuffer : public facebook::jsi::Buffer {
  public:
    CBuffer(const uint8_t *data, size_t size) : buf(data), sz(size) {}
    size_t size() const override { return sz; }
    const uint8_t *data() const override { return buf; }

  private:
    const uint8_t *buf;
    size_t sz;
};

// ...
  /* load SheetJS library */
  size_t sz; char *xlsx_full_min_js = read_file("xlsx.full.min.js", &sz);
  auto src = std::make_shared<CBuffer>(CBuffer((uint8_t *)xlsx_full_min_js, sz));
  auto js = rt->prepareJavaScript(src, std::string("xlsx.full.min.js"));
  rt->evaluatePreparedJavaScript(js);

To confirm the library is loaded, XLSX.version can be printed to the console:

auto src = std::make_shared<facebook::jsi::StringBuffer>(
  "console.log('SheetJS Library Version: ' + XLSX.version)"
);
auto js = rt->prepareJavaScript(src, std::string("<eval>"));
rt->evaluatePreparedJavaScript(js);

Reading Files

Hermes supports ArrayBuffer but has no simple helper to read raw memory. Libraries are expected to implement MutableBuffer:

/* ArrayBuffer constructor expects MutableBuffer*/
class CMutableBuffer : public facebook::jsi::MutableBuffer {
  public:
    CMutableBuffer(uint8_t *data, size_t size) : buf(data), sz(size) {}
    size_t size() const override { return sz; }
    uint8_t *data() override { return buf; }

  private:
    uint8_t *buf;
    size_t sz;
};
// ...
  /* load payload as ArrayBuffer */
  size_t sz; char *data = read_file(argv[1], &sz);
  auto payload = std::make_shared<CMutableBuffer>(CMutableBuffer((uint8_t *)data, sz));
  auto ab = facebook::jsi::ArrayBuffer(*rt, payload);

It is strongly recommended to create a stub function to perform the entire workflow in JS code and pass the final result back to C++.

JS Stub function

function(buf) {
  /* `buf` will be an ArrayBuffer */
  var wb = XLSX.read(buf);
  return XLSX.utils.sheet_to_csv(wb.Sheets[wb.SheetNames[0]]);
}

C++ integration code

  /* define stub function to read and convert first sheet to CSV */
  auto src = std::make_shared<facebook::jsi::StringBuffer>(
    "(function(buf) {"
      "var wb = XLSX.read(buf);"
      "return XLSX.utils.sheet_to_csv(wb.Sheets[wb.SheetNames[0]]);"
    "})"
  );
  auto js = rt->prepareJavaScript(src, std::string("<eval>"));
  auto func = rt->evaluatePreparedJavaScript(js);

  /* call stub function and capture result */
  auto csv = func.asObject(*rt).asFunction(*rt).call(*rt, ab);

  /* interpret as utf8 and print to stdout */
  std::string str = csv.getString(*rt).utf8(*rt);

Complete Example

The "Integration Example" covers a traditional integration in a C++ application, while the "CLI Test" demonstrates other concepts using the hermes CLI tool.

Integration Example

:::note

This demo was last tested on 2023 May 30 against Hermes commit 869312f on a Intel Mac. llvm-g++ -v printed:

Apple clang version 14.0.0 (clang-1400.0.29.202)
Target: x86_64-apple-darwin21.6.0

:::

Make a project directory:

mkdir sheetjs-hermes
cd sheetjs-hermes

Download the Makefile:

curl -LO https://docs.sheetjs.com/hermes/Makefile

Download sheetjs-hermes.cpp:

curl -LO https://docs.sheetjs.com/hermes/sheetjs-hermes.cpp

Build the library (this is the init target):

make init

Build the application:

make sheetjs-hermes

Download the standalone script and test file:

xlsx.full.min.js
pres.numbers

{\ curl -LO https://cdn.sheetjs.com/xlsx-${current}/package/dist/xlsx.full.min.js curl -LO https://sheetjs.com/pres.numbers}

Run the application:

./sheetjs-hermes pres.numbers

If successful, the program will print the library version number and the contents of the first sheet as CSV rows.

CLI Test

:::note

This demo was last tested on 2023 May 30 against Hermes version 0.11.0.

:::

Due to limitations of the standalone binary, this demo will encode a test file as a Base64 string and directly add it to an amalgamated script.

Install the hermes command line tool
Download the standalone script and test file:

xlsx.full.min.js
pres.numbers