repl.it
@renatorib/

parser-v5

Nodejs

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  • index.js
index.js
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const lexer = input => {
  let tokens = []
  let current = 0
  
  while (current < input.length) {
    let char = input[current]
    
    if (char === " ") {
      current++
      continue
    }

    if (char === "+") {
      tokens.push({ type: "Plus" })
      current++
      continue
    }

    if (char === "-") {
      tokens.push({ type: "Minus" })
      current++
      continue
    }

    if (char === "*") {
      tokens.push({ type: "Star" })
      current++
      continue
    }

    if (char === "/") {
      tokens.push({ type: "Slash" })
      current++
      continue
    }
    
    if (char === "(") {
      tokens.push({ type: "LeftParen" })
      current++
      continue
    }

    if (char === ")") {
      tokens.push({ type: "RightParen" })
      current++
      continue
    }

    if (/[0-9]/.test(char)) {
      let value = ''

      while (/[0-9]/.test(char)) {
        value += char
        current++
        char = input[current]
      }
      
      tokens.push({ type: "Number", value: parseInt(value, 10) })
      
      continue
    }

    throw new Error(`Unexpected char ${char}`)
  }

  tokens.push({ type: "End" })
  
  return tokens
} 

const nodes = {
  LiteralNode: ({ token }) => 
    ({ type: "LiteralNode", token }),

  BinaryOpNode: ({ left, right, token }) => 
    ({ type: "BinaryOpNode", left, right, token }),

  UnaryOpNode: ({ expr, token }) => 
    ({ type: "UnaryOpNode", expr, token })
}

const parser = tokens => {
  let current = 0

  const peek = () => tokens[current]
  const prev = () => tokens[current - 1]
  const match = type => {
    if (peek().type === type) {
      current++
      return true
    }
    return false
  }

  const expr = () => {
    return add()
  }

  const add = () => {
    let node = mul()

    while (match('Plus') || match('Minus')) {
      node = nodes.BinaryOpNode({ token: prev(), left: node, right: mul() })
    }

    return node
  }

  const mul = () => {
    let node = una()

    while (match('Star') || match('Slash')) {
      node = nodes.BinaryOpNode({ token: prev(), left: node, right: una() })
    }    

    return node
  }

  const una = () => {
    if (match('Minus') || match('Plus')) {
      return nodes.UnaryOpNode({ token: prev(), expr: una() })
    }

    return prim()
  }

  const prim = () => {
    if (match('Number')) {
      return nodes.LiteralNode({ token: prev() })
    }

    if (match('LeftParen')) {
      const result = expr()
      
      if (match('RightParen')) {
        return result
      }
      
      throw SyntaxError(`Unexpected ${peek().type}, expecting closing paren`)
    }

    throw new Error(`Unexpected ${peek().type} token`)
  }

  return expr()
}

const interpreter = (ast) => {
  const visitors = {
    LiteralNode(node) {
      return node.token.value
    },

    BinaryOpNode(node, visit) {
      const leftVal = visit(node.left)
      const rightVal = visit(node.right)

      if (node.token.type === 'Plus') {
        return leftVal + rightVal
      }
      
      if (node.token.type === 'Minus') {
        return leftVal - rightVal
      }

      if (node.token.type === 'Star') {
        return leftVal * rightVal
      }

      if (node.token.type === 'Slash') {
        return leftVal / rightVal
      }
    },

    UnaryOpNode(node, visit) {
      const exprVal = visit(node.expr)

      if (node.token.type === 'Plus') {
        return + exprVal        
      }

      if (node.token.type === 'Minus') {
        return - exprVal
      }
    } 
  }

  const visit = (node) => {
    if (visitors[node.type]) {
      return visitors[node.type](node, visit)
    }
  }

  return visit(ast)
}

/** DEBUG PURPOSES */

const __debug = (input) => {
  console.log('------------------------------------------------')
  console.log(input)
  console.log('------------------------------------------------')
  console.log('')

  const lexemes = lexer(input)
  console.log('Lexemes:', JSON.stringify(lexemes, null, 2))
  console.log('')

  const ast = parser(lexemes)
  console.log('AST:', JSON.stringify(ast, null, 2))
  console.log('')

  const result = interpreter(ast)
  console.log('Result:', result)

  console.log('')
  console.log('')
  console.log('')
}

__debug('(5 + 4 + 3 - - 2) * + 2')
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