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.
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).
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.
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:
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.
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