import nonEnterpriseDefaultVersion from '@/versions/lib/non-enterprise-default-version'
import { getPathWithoutVersion } from '@/frame/lib/path-utils'

import type { Permalink } from '@/types'

type Redirects = Record<string, string>

export default function permalinkRedirects(
  permalinks: Permalink[],
  redirectFrom: string[],
): Redirects {
  const redirects: Redirects = {}
  if (!permalinks.length) return redirects

  // The following is handling for versionless redirect fallbacks!
  // We put an entry into `redirects` without any version prefix that goes to the first supported
  // version in the lib/all-versions.ts order. For example, we want this versionless path:
  //   /billing/managing-billing-for-your-github-account/managing-invoices-for-your-enterprise
  // to redirect to its first supported version, which is GHEC:
  //   /enterprise-cloud@latest/billing/managing-billing-for-your-github-account/managing-invoices-for-your-enterprise
  if (permalinks[0].pageVersion !== nonEnterpriseDefaultVersion) {
    redirects[getPathWithoutVersion(permalinks[0].hrefWithoutLanguage)] =
      permalinks[0].hrefWithoutLanguage
  }

  // For every "old" path in a content file's redirect_from frontmatter, also add that path to
  // the redirects object as a key, where the value is the content file's permalink.
  for (let frontmatterOldPath of redirectFrom) {
    if (!frontmatterOldPath.startsWith('/')) {
      throw new Error(
        `'${frontmatterOldPath}' is not a valid redirect_from frontmatter value because it doesn't start with a /`,
      )
    }

    // Exceptions where the `redirect_from` entries are too old
    // Only replace /enterprise/ when it's at the start of the path followed by /admin/
    // This handles legacy patterns like /enterprise/admin/... → /admin/...
    // but preserves paths like /early-access/enterprise/... where enterprise is a directory name
    frontmatterOldPath = frontmatterOldPath
      .replace('/admin/guides/', '/admin/')
      .replace(/^\/enterprise\/admin\//, '/admin/')

    for (let index = 0; index < permalinks.length; index++) {
      const permalink = permalinks[index]
      // For the first supported permalink (the order is determined by lib/all-versions),
      // put an entry into `redirects` without any version prefix.
      if (index === 0) {
        redirects[frontmatterOldPath] = permalink.hrefWithoutLanguage
      }

      // For every permalink, put an entry into `redirects` with the version prefix.
      redirects[`/${permalink.pageVersion}${frontmatterOldPath}`] = permalink.hrefWithoutLanguage
    }
  }

  return redirects
}