const { readdirSync } = require('fs');
const { parse, resolve, dirname, format } = require('path');

function interpolate(template, context) {
  return template.replace(/\[(\w+)\]/g, (_, key) => context[key] || key);
}

function isNodeModule(filename) {
  return !filename.startsWith('.');
}

/*
 * babel-plugin-alt-import-require
 *
 * Example
 * plugins: [['babel-plugin-alt-import-require', { alt: '[name].mobile[ext]' }], ...otherPlugins]
 *
 * File structure
 * code
 *  \
 *   |_ components
 *   |    \
 *   |    |_ App.js
 *   |    |_ App.mobile.js
 *   |
 *   |_ containers
 *         \
 *         |_ Main.js
 *
 * Main.js
 * ```
 * import App from '../components/App';
 * // or
 * const App = require('../components/App');
 * ```
 *
 * If you're using the plugin after babel has run Main.js would look like this
 *
 * Main.js
 * ```
 * import App from '/code/components/App.mobile.js';
 * // or
 * const App = require('/code/components/App.mobile.js'');
 * ```
 */

// make cache 'global' to all instances of the plugin
const cache = new Map();

module.exports = function(babel) {
  const { types: t } = babel;

  function getAltFile(value, state) {
    const { alt } = state.opts;
    const filename = state.file.opts.filename;

    // if the required/imported file is a node module or
    // there is no alt option bail out
    if (isNodeModule(value) || !alt) return null;

    // get the absolute path the file being processed using the
    // node resolution algo to handle index.js files
    const resolved = require.resolve(resolve(dirname(filename), value));

    const path = parse(resolved);
    const altFile = interpolate(alt, path);

    // if the file being processed is the alt file bail out
    if (path.base === altFile) {
      return null;
    }

    let files;

    // check the cache to see if the dir has been processed before
    if (!cache.has(path.dir)) {
      files = readdirSync(path.dir);
      cache.set(path.dir, files);
    } else {
      files = cache.get(path.dir);
    }

    const hasAltFile = files.includes(altFile);

    return hasAltFile ? format({ ...path, base: altFile }) : null;
  }

  return {
    name: 'alt-import-require',
    visitor: {
      ImportDeclaration(path, state) {
        const altFile = getAltFile(path.node.source.value, state);

        if (altFile) {
          path.node.source.value = altFile;
        }
      },
      CallExpression(path, state) {
        if (t.isIdentifier(path.node.callee, { name: 'require' })) {
          const altFile = getAltFile(path.node.arguments[0].value, state);

          if (altFile) {
            path.node.arguments[0].value = altFile;
          }
        }
      },
    },
  };
};