Creating a C compiler in JavaScript
Tags: javascript, c
Published: 2024-06-30 03:57:33

Creating a C compiler in JavaScript is a complex and ambitious project that involves several components, including lexical analysis, parsing, semantic analysis, and code generation. Below is a simplified and high-level example of how you might start building such a compiler. This example will focus on the lexical analysis (tokenization) and parsing stages, which are the first steps in compiling C code.

### Step 1: Lexical Analysis (Tokenization)

The lexical analyzer (lexer) converts the input C code into a stream of tokens.

```javascript
class Lexer {
  constructor(input) {
    this.input = input;
    this.tokens = [];
    this.current = 0;
  }

  tokenize() {
    while (this.current < this.input.length) {
      let char = this.input[this.current];

      if (/\s/.test(char)) {
        this.current++;
        continue;
      }

      if (/[a-zA-Z_]/.test(char)) {
        let start = this.current;
        while (/[a-zA-Z0-9_]/.test(this.input[this.current])) {
          this.current++;
        }
        this.tokens.push({ type: 'IDENTIFIER', value: this.input.slice(start, this.current) });
        continue;
      }

      if (/[0-9]/.test(char)) {
        let start = this.current;
        while (/[0-9]/.test(this.input[this.current])) {
          this.current++;
        }
        this.tokens.push({ type: 'NUMBER', value: this.input.slice(start, this.current) });
        continue;
      }

      switch (char) {
        case '+':
          this.tokens.push({ type: 'PLUS', value: '+' });
          this.current++;
          break;
        case '-':
          this.tokens.push({ type: 'MINUS', value: '-' });
          this.current++;
          break;
        case '*':
          this.tokens.push({ type: 'STAR', value: '*' });
          this.current++;
          break;
        case '/':
          this.tokens.push({ type: 'SLASH', value: '/' });
          this.current++;
          break;
        case '=':
          this.tokens.push({ type: 'EQUAL', value: '=' });
          this.current++;
          break;
        case ';':
          this.tokens.push({ type: 'SEMICOLON', value: ';' });
          this.current++;
          break;
        case '(':
          this.tokens.push({ type: 'LPAREN', value: '(' });
          this.current++;
          break;
        case ')':
          this.tokens.push({ type: 'RPAREN', value: ')' });
          this.current++;
          break;
        default:
          throw new TypeError('Unexpected character: ' + char);
      }
    }
    return this.tokens;
  }
}
```

### Step 2: Parsing

The parser converts the stream of tokens into an abstract syntax tree (AST).

```javascript
class Parser {
  constructor(tokens) {
    this.tokens = tokens;
    this.current = 0;
  }

  parse() {
    let ast = {
      type: 'Program',
      body: []
    };

    while (this.current < this.tokens.length) {
      ast.body.push(this.parseStatement());
    }

    return ast;
  }

  parseStatement() {
    let token = this.tokens[this.current];
    
    if (token.type === 'IDENTIFIER' && this.tokens[this.current + 1].type === 'EQUAL') {
      return this.parseAssignment();
    }

    throw new TypeError('Unknown statement: ' + token.type);
  }

  parseAssignment() {
    let identifier = this.tokens[this.current];
    this.current++; // skip identifier
    this.current++; // skip equal sign

    let value = this.parseExpression();

    this.expect('SEMICOLON');

    return {
      type: 'Assignment',
      identifier: identifier.value,
      value: value
    };
  }

  parseExpression() {
    let token = this.tokens[this.current];

    if (token.type === 'NUMBER') {
      this.current++;
      return {
        type: 'Literal',
        value: Number(token.value)
      };
    }

    throw new TypeError('Unknown expression: ' + token.type);
  }

  expect(type) {
    let token = this.tokens[this.current];
    if (token.type !== type) {
      throw new TypeError('Expected ' + type + ' but found ' + token.type);
    }
    this.current++;
  }
}
```

### Step 3: Code Generation

Finally, the code generator converts the AST into the target language, which could be JavaScript or any other language.

```javascript
class CodeGenerator {
  generate(node) {
    switch (node.type) {
      case 'Program':
        return node.body.map(statement => this.generate(statement)).join('\n');
      case 'Assignment':
        return `let ${node.identifier} = ${this.generate(node.value)};`;
      case 'Literal':
        return node.value;
      default:
        throw new TypeError('Unknown node type: ' + node.type);
    }
  }
}
```

### Putting It All Together

Here is how you might use the lexer, parser, and code generator:

```javascript
const input = `x = 42;`;
const lexer = new Lexer(input);
const tokens = lexer.tokenize();
console.log('Tokens:', tokens);

const parser = new Parser(tokens);
const ast = parser.parse();
console.log('AST:', JSON.stringify(ast, null, 2));

const generator = new CodeGenerator();
const output = generator.generate(ast);
console.log('Output:', output);
```

This will tokenize the input, parse it into an AST, and generate JavaScript code from the AST.

### Note

This example is highly simplified and only handles a tiny subset of the C language. A full-fledged C compiler would require handling a much larger set of tokens, parsing complex expressions, statements, declarations, types, and generating more sophisticated code.