import path from 'path'
import process from 'process'
import Module from 'module'

const builtins = Module.builtinModules
const JS_EXTENSIONS = new Set(['.js', '.mjs'])

const baseURL = new URL('file://')
baseURL.pathname = `${process.cwd()}/`

export function resolve (specifier, parentModuleURL = baseURL, defaultResolve) {
  if (builtins.includes(specifier)) {
    return {
      url: specifier,
      format: 'builtin'
    }
  }
  if (/^\.{0,2}[/]/.test(specifier) !== true && !specifier.startsWith('file:')) {
    // For node_modules support:
    // return defaultResolve(specifier, parentModuleURL);
    throw new Error(
      `imports must begin with '/', './', or '../'; '${specifier}' does not`)
  }
  const resolved = new URL(specifier, parentModuleURL)
  const ext = path.extname(resolved.pathname)
  if (!JS_EXTENSIONS.has(ext)) {
    throw new Error(
      `Cannot load file with non-JavaScript file extension ${ext}.`)
  }
  return {
    url: resolved.href,
    format: 'esm'
  }
}