mpeg-sdl-parser

ISO/IEC 14496-34 Syntactic Description Language (MPEG SDL) parser implemented in TypeScript

Web Editor

A browser based web editor using this module is available at https://github.com/MPEGGroup/mpeg-sdl-editor

Bun Module Usage

Add the module:

bun add @mpeggroup/mpeg-sdl-parser

import {
  SdlStringInput,
  createLenientSdlParser,
  collateParseErrors,
  buildAst,
  dispatchNodeHandler,
  prettyPrint
} from "@mpeggroup/mpeg-sdl-parser";

// Create a Lezer based SDL parser
// This will create a lenient parser which recovers from parse errors and places error nodes in the parse tree.
// A strict parser which will throw SyntacticParseError can be created with createStrictSdlParser().
const parser = await createLenientSdlParser();

// Prepare the SDL input
const sdlStringInput = new SdlStringInput("computed int i;");

// Parse SDL input and produce a parse tree
const sdlParseTree = sdlParser.parse(sdlStringInput);

// Traverse and print the parse tree
const cursor = sdlParseTree.cursor();
do {
  console.log(`Node ${cursor.name} from ${cursor.from} to ${cursor.to}`)
} while (cursor.next());

// Print any parsing errors by collating any error nodes in the parse tree
const parseErrors = collateParseErrors(sdlParseTree, sdlStringInput);

console.log(JSON.stringify(parseErrors);

// Build an Abstract Syntax Tree (AST) of the SDL specification from the parse tree
// Note that this will throw a SyntacticParseError if the parse tree contains parsing errors.
const specification = buildAst(sdlParseTree, sdlStringInput);

// Define a simple AST Node handler
class MyNodeHandler implements NodeHandler {
  beforeVisit(node: AbstractCompositeNode) {
    console.log("About to visit child nodes");
  }

  visit(node: AbstractLeafNode) {
    console.log("Visiting leaf node");
  }

  afterVisit(node: AbstractCompositeNode) {
    console.log("Finished visiting child nodes");
  }
}

// Dispatch the handler to visit all nodes in the AST
dispatchNodeHandler(specification, new MyNodeHandler());

// Pretty print the specification (retaining comments)
let prettifiedSpecification = await prettyPrint(specification, sdlStringInput)

console.log(prettifiedSpecification);

// A Prettier (prettier.io) plugin for SDL is also available:
import * as prettier from "prettier/standalone.js";
import { prettierPluginSdl } from "@mpeggroup/mpeg-sdl-parser"; 

prettifiedSpecification = await prettier.format("computed int i;", { 
  parser: "sdl",
  plugins: [prettierPluginSdl],
});

console.log(prettifiedSpecification);

Development

Install dependencies:

bun install

Generate parser from grammar:

bun run generate

Test:

bun test

NOTE: The following tasks use Deno as it excels at these and Bun does not currently provide such functionality:

Format:

deno fmt

Lint:

deno lint index.ts src/ tests/

Generate HTML API Documentation:

deno doc --html --name=mpeg-sdl-parser index.ts

Documentation

Overview

The parser is implemented using Lezer using the Lezer grammar defined in sdl.lezer.grammar.

For reference purposes an SDL EBNF grammar is also provided in sdl.ebnf.grammar

Abstract Syntax Tree Model

classDiagram

  class Location {
    row: number
    column: number
    position: number
  }

  class AbstractNode {
    nodeKind: NodeKind
    text: string
    accept(visitor: NodeVisitor) boolean
  }

  class AbstractLeafNode {
    isCompositeNode = false
  }

  class AbstractCompositeNode {
    isCompositeNode = true
    getChildNodeIterable() IterableIterator
  }

  class NodeHandler {
    beforeVisit(node: AbstractCompositeNode)
    visit(node: AbstractLeafNode)
    afterVisit(node: AbstractCompositeNode)
  }

  AbstractNode --> Location : location
  AbstractLeafNode --|> AbstractNode

  AbstractCompositeNode --|> AbstractNode
  AbstractCompositeNode --> "*" AbstractNode : childNodes

  LeafNodeXXX "*" --|> AbstractLeafNode
  CompositeNodeYYY "*" --|> AbstractCompositeNode

  Specification --|> AbstractCompositeNode

  NodeHandler ..> AbstractCompositeNode : beforeVisit
  NodeHandler ..> AbstractLeafNode : visit
  NodeHandler ..> AbstractCompositeNode : afterVisit

Loading

API

Link to auto-generated API docs:

API Documentation

Debug Logging

Internal framework logging can be enabled by setting the MPEG_SDL_PARSER_DEBUG environment variable. If running in a browser open the developer console and run:

localStorage.setItem("MPEG_SDL_PARSER_DEBUG", "true")

The logging implementation will look for an object conforming to the Logger interface and use it if found. If not found, a simple logging implementation using the console object will be used.

License

MIT © Flowscripter