Update 736-parse-lisp-expression.js

Author: everthis

736-parse-lisp-expression.js

@@ -31,3 +31,89 @@ const e = (x, v = []) =>
           ),
       }[x[0]]()),
   }[typeof x]())
+
+// another
+
+/**
+ * @param {string} expression
+ * @return {number}
+ */
+const evaluate = function (expression) {
+  const tokens = tokenizer(expression)
+  let i = 0
+  function exec(scope) {
+    let value = null
+    const next = tokens[i++]
+    if (next === '(') {
+      scope = enter(scope)
+      switch (tokens[i++]) {
+        case 'add':
+          const a = exec(scope)
+          const b = exec(scope)
+          value = a + b
+          break
+        case 'mult':
+          const x = exec(scope)
+          const y = exec(scope)
+          value = x * y
+          break
+        case 'let':
+          while (tokens[i] !== '(' && tokens[i + 1] !== ')') {
+            scope.variables[tokens[i++]] = exec(scope)
+          }
+          value = exec(scope)
+          break
+      }
+      scope = exit(scope)
+      i++
+    } else if (isNumber(next)) {
+      value = Number(next)
+    } else {
+      // Find variable in current scope otherwise go to parent
+      let t = scope
+      while (t) {
+        if (next in t.variables) {
+          value = t.variables[next]
+          break
+        }
+        t = t.parent
+      }
+    }
+    return value
+  }
+  return exec(newScope())
+}
+function tokenizer(expression) {
+  const tokens = []
+  let token = ''
+  for (const c of expression) {
+    if (c === '(' || c === ')') {
+      if (token) tokens.push(token)
+      tokens.push(c)
+      token = ''
+    } else if (c === ' ') {
+      if (token) tokens.push(token)
+      token = ''
+    } else {
+      token += c
+    }
+  }
+  if (token) {
+    tokens.push(token)
+  }
+  return tokens
+}
+function isNumber(n) {
+  return !isNaN(n)
+}
+function newScope() {
+  return { parent: null, variables: {} }
+}
+function enter(scope) {
+  const next = newScope()
+  next.parent = scope
+  return next
+}
+function exit(scope) {
+  return scope.parent
+}