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// before: (add 20 (subtract 4 2))
// after: add(2, subtract(4, 2))
// 分词，处理每个字符内容
function generateToken(str) {
let current = 0;
let tokens = [];
while(current < str.length) {
let char = str[current];
if (char === "(") {
tokens.push({
type: "paren",
value: "("
});
current++;
continue;
}
if (char === ")") {
tokens.push({
type: "paren",
value: ")"
});
current++;
continue;
}
if (/\s/.test(char)) {
current++;
continue;
}
if (/[0-9]/.test(char)) {
let numberValue = '';
while(/[0-9]/.test(char)) {
numberValue += char;
char = str[++current];
}
tokens.push({
type: 'number',
value: numberValue
});
continue;
}
if (/[a-z]/.test(char)) {
let stringValue = '';
while(/[a-z]/.test(char)) {
stringValue += char;
char = str[++current];
}
tokens.push({
type: 'name',
value: stringValue
});
continue;
}
throw new TypeError("未能识别的字符");
}
return tokens;
}
// AST 生成
function generateAST(tokens) {
let current = 0;
let ast = {
type: "Program",
body: [],
};
function walk() {
let token = tokens[current];
if (token.type === 'number') {
current++;
return {
type: "NumberLiteral",
value: token.value,
}
}
if (token.type === 'paren' && token.value === "(") {
token = tokens[++current];
let node = {
type: "CallExpression",
name: token.value,
params: [],
};
token = tokens[++current];
while(
(token.type !== "paren") || (token.type === 'paren' && token.value !== ")")
) {
node.params.push(walk());
token = tokens[current];
}
current++;
return node;
}
throw new TypeError(token.type);
}
while(current < tokens.length) {
ast.body.push(walk())
}
return ast;
}
// AST 转化： babel 插件需要我们处理的部分
function transformer(ast) {
let newAst = {
type: "Program",
body: [],
};
ast._context = newAst.body;
DFS(ast, {
NumberLiteral: {
enter(node, parent) {
parent._context.push({
type: "NumberLiteral",
value: node.value
});
}
},
CallExpression: {
enter(node, parent) {
let expression = {
type: "CallExpression",
callee: {
type: "Identifier",
name: node.name
},
arguments: [],
};
node._context = expression.arguments;
if (parent.type !== "CallExpression") {
expression = {
type: "ExpressionStatement",
expression: expression,
}
}
parent._context.push(expression);
}
}
})
return newAst;
}
// AST 遍历
function DFS(ast, visitor) {
function traverseArray(children, parent) {
children.forEach(child => traverseNode(child, parent));
}
function traverseNode(node, parent) {
let methods = visitor[node.type];
if (methods && methods.enter) {
methods.enter(node, parent);
}
switch(node.type) {
case "Program":
traverseArray(node.body, node);
break;
case "CallExpression":
traverseArray(node.params, node);
break;
case "NumberLiteral":
break;
}
if (methods && methods.exit) {
methods.exit(node, parent);
}
}
return traverseNode(ast, null);
}
// ast -> code 生成代码过程
function generate(ast) {
switch(ast.type) {
case "Program": return ast.body.map(subAst => generate(subAst)).join('\n');
case "ExpressionStatement": return generate(ast.expression) + ";";
case "CallExpression": return generate(ast.callee) + "(" +ast.arguments.map(arg => generate(arg)).join(', ') + ")";
case "Identifier": return ast.name;
case "NumberLiteral": return ast.value;
}
}
function parser(input) {
const tokens = generateToken(input);
console.log(tokens);
const ast = generateAST(tokens);
console.log(JSON.stringify(ast, null, 2));
const newAst = transformer(ast);
console.log(JSON.stringify(newAst, null, 2));
const code = generate(newAst);
return code;
}
module.exports = parser;
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