/**
 * Moves var statements to the top of functions.
 *
 * When a var is moved, it is initialized to undefined. The
 * transformation introduces assignment statements where the var statement
 * originally occurred.
 */
import { NodePath, Visitor } from 'babel-traverse';
import * as t from 'babel-types';
import { tag } from '@stopify/util';

type Hoisted<T> = T & {
  hoisted?: boolean
};
const hoisted = <T>(t: T) => tag('hoisted', t, true);

function declToAssign(decl: t.VariableDeclarator): t.AssignmentExpression {
  return decl.init === null ?
    t.assignmentExpression('=', decl.id, t.unaryExpression('void', t.numericLiteral(0))) :
    t.assignmentExpression('=', decl.id, decl.init);
}

const lift: Visitor = {
  VariableDeclaration:
  function (path: NodePath<Hoisted<t.VariableDeclaration>>): void {
    if (path.node.hoisted) {
      return;
    }
    const { kind, declarations } = path.node;
    if (kind === 'var') {
      const topScope = path.getFunctionParent();
      switch (topScope.node.type) {
        case 'FunctionDeclaration':
        case 'FunctionExpression': {
          const ids = declarations.map(decl => decl.id);
          // TODO(sbaxter): Update babel typescript declaration to type
          // `unshiftContainer` and remove `any` cast
          (<any>topScope.get('body')).unshiftContainer('body',
            hoisted(t.variableDeclaration('var',
            ids.map(id => t.variableDeclarator(id, undefined)))));
          const exp = declarations.length === 1 ?
            declToAssign(declarations[0]):
            t.sequenceExpression(declarations.map(decl => declToAssign(decl)));
          path.replaceWith(exp);
          break;
        }
        case 'Program':
          const ids = declarations.map(decl => decl.id);
          // TODO(sbaxter): Update babel typescript declaration to type
          // `unshiftContainer` and remove `any` cast
          (<any>topScope).unshiftContainer('body',
            hoisted(t.variableDeclaration('var',
            ids.map(id => t.variableDeclarator(id, undefined)))));
          const exp = declarations.length === 1 ?
            declToAssign(declarations[0]):
            t.sequenceExpression(declarations.map(decl => declToAssign(decl)));
          path.replaceWith(t.expressionStatement(exp));
          break;
        default:
          throw new Error(
            `Expected to find function/program parent, but found ${topScope.type} instead`);
      }
    }
  }
};

module.exports = function() {
  return { visitor: lift };
};