Fix icon and logo to point to nodes

[Beorn] / build.js

#!/usr/bin/env -S deno run --allow-read --allow-write
// 🧸📔 Bjørn ∷ build.js
// ====================================================================
//
// Copyright © 2022‐2023 Lady [@ Lady’s Computer].
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at <https://mozilla.org/MPL/2.0/>.

// As the shebang at the top of this file indicates, it must be run via
// Deno with read and write permissions. You can get Deno from
// <https://deno.land/>.
//
// This file generates a minimal, Atom‐supporting blog from a handful
// of R·D·F∕X·M·L files and a couple of templates. It will silently
// overwrite the files it generates. This is usually what you want.
//
// ※ The actual run script is at the very end of this file. It is
// preceded by a number of helper function definitions, of which the
// most important is `applyMetadata` (which is used to generate the
// HTML and Atom elements from the processed metadata).
//
// ※ The list of supported metadata properties and their R·D·F
// representations is provided by `context`. You can add support for
// new metadata fields simply by adding them to the `context` and then
// handling them appropriately in `applyMetadata`.
//
// This script is a bit of a mess and you shouldn’t bother trying to
// understand the whole thing before you start hacking on it. Just find
// the parts relevant to whatever you want to change, and assume that
// things will work in sensible (if cumbersome) browser‐like ways.

// This import polyfills a (mediocre but sufficient) D·O·M environment
// onto the global object.
import "https://git.ladys.computer/Lemon/blob_plain/0.2.2:/window/mod.js";

// Much of the H·T·M·L generation in this script uses the 🍋🏷 Lemon
// library for convenience (<https://git.ladys.computer/Lemon>).
//
// Frequently this will be bound to a different document than the
// global one and called as `LMN`.
import Lemon from "https://git.ladys.computer/Lemon/blob_plain/0.2.2:/mod.js";

// Markdown processing uses rusty_markdown, which uses Rust’s
// pulldown-cmark behind the scenes via WebAssembly.
import {
  html as markdownTokensToHTML,
  tokens as markdownTokens,
} from "https://deno.land/x/rusty_markdown@v0.4.1/mod.ts";

// Various namespaces.
const AWOL = "http://bblfish.net/work/atom-owl/2006-06-06/";
const DC11 = "http://purl.org/dc/elements/1.1/";
const FOAF = "http://xmlns.com/foaf/0.1/";
const RDF = "http://www.w3.org/1999/02/22-rdf-syntax-ns#";
const RDFS = "http://www.w3.org/2000/01/rdf-schema#";
const SIOC = "http://rdfs.org/sioc/ns#";
const XML = "http://www.w3.org/XML/1998/namespace";
const XHTML = "http://www.w3.org/1999/xhtml";

/**
 * Adds the provided content to the provided element.
 *
 * Content may be either nullish (in which case nothing is added), a
 * string (object or literal), a NodeList, or an array of zero or more
 * of these values. Nodes need not belong to the same document as the
 * provided element; they will be imported. During this process,
 * special care is taken to ensure that the resulting content is
 * correctly language‐tagged for its new context.
 *
 * ☡ If the provided element is not attached to anything, then it won’t
 * be possible to walk its parent nodes in search of language
 * information. Generally, it is best to attach elements to a document
 * BEFORE calling this function.
 */
const addContent = (element, content) => {
  const { ownerDocument: document } = element;
  const LMN = Lemon.bind({ document });
  if (content == null) {
    // The provided content is nullish.
    /* do nothing */
  } else if (Array.isArray(content)) {
    // The provided content is an array.
    content.forEach(addContent.bind(null, element));
  } else if (Object(content) instanceof String) {
    // The provided content is a string (object or literal).
    const { lang } = content;
    if (lang && lang != getLanguage(element)) {
      const newChild = element.appendChild(LMN.span`${content}`);
      setLanguage(newChild, lang);
    } else {
      element.appendChild(document.createTextNode(`${content}`));
    }
  } else {
    // Assume the provided content is a NodeList.
    if (element.hasAttribute("property")) {
      // The provided element has an R·D·F∕A property; note that its
      // datatype is `XMLLiteral`.
      element.setAttribute("datatype", "XMLLiteral");
    } else {
      // The provided element does not have an R·D·F∕A property.
      /* do nothing */
    }
    for (const child of Array.from(content)) {
      // Iterate over the nodes in the provided NodeList and handle
      // them appropriately.
      const lang = getLanguage(child);
      const newChild = (() => {
        const imported = document.importNode(child, true);
        if (lang && lang != getLanguage(element)) {
          // The imported node’s language would change if it were just
          // imported directly into the provided element.
          if (imported.nodeType == 1) {
            // The imported node is an element node.
            setLanguage(imported, lang);
            return imported;
          } else if (imported.nodeType <= 4) {
            // The imported node is a text node.
            const result = LMN.span`${imported}`;
            setLanguage(result, lang);
            return result;
          } else {
            // The imported node is not an element or text.
            return imported;
          }
        } else {
          // The imported node’s language will not change if imported
          // directly into the provided element.
          return imported;
        }
      })();
      element.appendChild(newChild);
    }
  }
  return element;
};

/**
 * Adds HTML for the provided people to the provided element, tagging
 * them with the provided property.
 *
 * ☡ As with `addContent`, it is best to attach elements to a document
 * PRIOR to providing them to this function, for language‐detection
 * reasons.
 */
const addPeople = (element, people, property) => {
  const { ownerDocument: document } = element;
  const LMN = Lemon.bind({ document });
  const { length } = people;
  for (const [index, { uri, name }] of people.entries()) {
    const personElement = element.appendChild(
      uri
        ? LMN.a.rel(`${property}`).href(`${uri}`)``
        : LMN.span.rel(`${property}`)``,
    );
    if (name == null) {
      // The current person has no name; provide its `uri`.
      personElement.appendChild(LMN.code`${uri}`);
    } else {
      // The current person has a name.
      addContent(
        personElement.appendChild(
          LMN.span.property(`${FOAF}name`)``,
        ),
        name,
      );
    }
    if (index < length - 2) {
      // The current person is two or greater from the end.
      addContent(element, ", ");
    } else if (index < length - 1) {
      // The current person is one from the end.
      addContent(element, " & ");
    } else {
      // The current person is the last.
      /* do nothing */
    }
  }
  return element;
};

/**
 * Applies the provided metadata to the provided node by creating and
 * inserting the appropriate elements, then returns the provided node.
 *
 * If the provided node is a document, it is assumed to be an entry
 * template, and full entry H·T·M·L is generated. If it is a document
 * fragment, it is assumed to be a document fragment collecting entries
 * for the H·T·M·L feed index page, and entry H·T·M·L links are
 * generated. Otherwise, the provided node is assumed to be an Atom
 * element, and Atom metadata elements are generated.
 */
const applyMetadata = (node, metadata) => {
  if (node.nodeType == 9) {
    // The provided node is a document.
    //
    // Assume it is an entry template document and insert the full
    // entry H·T·M·L accordingly.
    const document = node;
    const { documentElement } = document;
    if (hasExpandedName(documentElement, XHTML, "html")) {
      // This is an XHTML template.
      const LMN = Lemon.bind({ document });
      const {
        id,
        title,
        author,
        contributor,
        published,
        content,
        rights,
        updated,
      } = metadata;
      fillOutHead(document, metadata, "entry");
      const contentPlaceholder = document.getElementsByTagNameNS(
        XHTML,
        "bjørn-content",
      ).item(0);
      if (contentPlaceholder != null) {
        // The content placeholder exists; replace it with the content
        // nodes.
        const { parentNode: contentParent } = contentPlaceholder;
        const contentElement = contentParent.insertBefore(
          LMN.article.about(`${id}`)`${"\n"}`,
          contentPlaceholder,
        );

        // Handle the entry content header.
        contentElement.appendChild(
          document.createComment(" BEGIN ENTRY HEADER "),
        );
        addContent(contentElement, "\n");
        const contentHeader = contentElement.appendChild(
          LMN.header.id("entry.header")`${"\n\t"}`,
        );
        addContent(
          contentHeader.appendChild(
            LMN.h1.id("entry.title").property(`${DC11}title`)``,
          ),
          title,
        );
        if (author.length > 0) {
          // This entry has authors.
          addContent(contentHeader, "\n\t");
          addPeople(
            contentHeader.appendChild(
              LMN.p.id("entry.author")``,
            ),
            author,
            `${DC11}creator`,
          );
        } else {
          // This entry does not have authors.
          /* do nothing */
        }
        if (contributor.length > 0) {
          // This entry has contributors.
          addContent(contentHeader, "\n\t");
          addContent(
            addPeople(
              contentHeader.appendChild(
                LMN.p.id(
                  "entry.contributor",
                )`With contributions from `,
              ),
              contributor,
              `${DC11}contributor`,
            ),
            ".",
          );
        } else {
          // This entry does not have contributors.
          /* do nothing */
        }
        if (published) {
          // This entry has a publication date.
          addContent(contentHeader, "\n\t");
          contentHeader.appendChild(
            LMN.p.id("entry.published")`Published: ${LMN.time.property(
              `${DC11}date`,
            )`${published}`}.`,
          );
        } else {
          // This entry does not have a publication date.
          /* do nothing */
        }
        addContent(contentHeader, "\n");
        addContent(contentElement, "\n");
        contentElement.appendChild(
          document.createComment(" END ENTRY HEADER "),
        );
        addContent(contentElement, "\n");

        // Handle the entry content.
        contentElement.appendChild(
          document.createComment(" BEGIN ENTRY CONTENT "),
        );
        addContent(contentElement, "\n");
        addContent(
          contentElement.appendChild(
            LMN.div.id("entry.content").property(`${SIOC}content`)``,
          ),
          content,
        );
        addContent(contentElement, "\n");
        contentElement.appendChild(
          document.createComment(" END ENTRY CONTENT "),
        );
        addContent(contentElement, "\n");

        // Handle the entry content footer.
        contentElement.appendChild(
          document.createComment(" BEGIN ENTRY FOOTER "),
        );
        addContent(contentElement, "\n");
        const contentFooter = contentElement.appendChild(
          LMN.footer.id("entry.footer")`${"\n\t"}`,
        );
        if (rights) {
          addContent(
            contentFooter.appendChild(
              LMN.div.id("entry.rights").property(`${DC11}rights`)``,
            ),
            rights,
          );
          addContent(contentFooter, "\n\t");
        }
        contentFooter.appendChild(
          LMN.p.id("entry.updated")`Last updated: ${LMN.time.property(
            `${AWOL}updated`,
          )`${updated}`}.`,
        );
        addContent(contentFooter, "\n");
        addContent(contentElement, "\n");
        contentElement.appendChild(
          document.createComment(" END ENTRY FOOTER "),
        );
        addContent(contentElement, "\n");

        // Remove the placeholder.
        contentParent.removeChild(contentPlaceholder);
      } else {
        // There is no content placeholder.
        /* do nothing */
      }
      globalThis.bjørnTransformEntryHTML?.(document, metadata);
    } else {
      // This is not an XHTML template.
      /* do nothing */
    }
  } else if (node.nodeType == 11) {
    // The provided node is a document fragment.
    //
    // Assume it is collecting H·T·M·L feed entry links and insert a
    // new one for the provided metadata.
    const { ownerDocument: document } = node;
    const LMN = Lemon.bind({ document });
    const {
      id,
      title,
      author,
      published,
      summary,
    } = metadata;
    // The content placeholder exists; replace it with the content
    // nodes.
    node.appendChild(
      document.createComment(` <${id}> `),
    );
    addContent(node, "\n");
    const contentElement = node.appendChild(
      LMN.li.resource(`${id}`)`${"\n"}`,
    );
    addContent(
      contentElement.appendChild(
        LMN.a.href(`${id}`)``,
      ).appendChild(
        LMN.h3.property(`${DC11}title`)``,
      ),
      title,
    );
    if (author.length > 0) {
      // This entry has authors.
      addContent(contentElement, "\n");
      addPeople(
        contentElement.appendChild(
          LMN.p``,
        ),
        author,
        `${DC11}creator`,
      );
    } else {
      // This entry does not have authors.
      /* do nothing */
    }
    if (published) {
      // This entry has a publication date.
      addContent(contentElement, "\n");
      contentElement.appendChild(
        LMN.time.property(`${DC11}date`)`${published}`,
      );
    } else {
      // This entry does not have a publication date.
      /* do nothing */
    }
    if (summary) {
      // This entry has a summary.
      addContent(contentElement, "\n");
      addContent(
        contentElement.appendChild(
          LMN.div.property(`${DC11}abstract`)``,
        ),
        summary,
      );
    } else {
      // This entry does not have a summary.
      /* do nothing */
    }
    addContent(contentElement, "\n");
    addContent(node, "\n");
  } else {
    // The provided node is not a document or document fragment.
    //
    // Assume it is an Atom element of some sort and add the
    // the appropriate metadata as child elements.
    const { ownerDocument: document } = node;
    const alternateLink = node.appendChild(
      document.createElement("link"),
    );
    alternateLink.setAttribute("rel", "alternate");
    alternateLink.setAttribute("type", "application/xhtml+xml");
    alternateLink.setAttribute("href", metadata.id);
    for (const [property, values] of Object.entries(metadata)) {
      for (const value of Array.isArray(values) ? values : [values]) {
        const propertyNode = document.createElement(property);
        switch (context[property]?.type) {
          case "person": {
            // The property describes a person.
            const { name, uri } = value;
            if (uri) {
              // The person has a U·R·I.
              const subnode = document.createElement("uri");
              subnode.textContent = uri;
              propertyNode.appendChild(subnode);
            } else {
              // The person does not have a U·R·I.
              /* do nothing */
            }
            if (name != null) {
              // The person has a name.
              const subnode = document.createElement("name");
              subnode.textContent = name;
              propertyNode.appendChild(subnode);
            } else {
              // The person does not have a name.
              /* do nothing */
            }
            if (propertyNode.childNodes.length == 0) {
              // Neither a U·R·I nor a name was added; skip adding this
              // property.
              continue;
            } else {
              break;
            }
          }
          default: {
            // The property describes (potentially rich) text.
            if (value == null) {
              // The property has no value; skip appending it to the
              // node.
              continue;
            } else if (Object(value) instanceof String) {
              // The property has a string value.
              propertyNode.textContent = value;
              break;
            } else {
              // The property value is a list of nodes.
              propertyNode.setAttribute("type", "xhtml");
              const div = document.createElementNS(XHTML, "div");
              for (const child of Array.from(value)) {
                div.appendChild(document.importNode(child, true));
              }
              propertyNode.appendChild(div);
              break;
            }
          }
        }
        node.appendChild(propertyNode);
      }
    }
  }
  return node;
};

/**
 * The base path from which to pull files and generate resulting
 * documents.
 */
const basePath = `./${Deno.args[0] ?? ""}`;

/**
 * Mappings from Atom concepts to R·D·F∕X·M·L ones.
 *
 * `namespace` and `localName` give the R·D·F representation for the
 * concept. Three `type`s are supported :—
 *
 * - "person": Has a `name` (`foaf:name`) and an `iri` (`rdf:about`).
 *   Emails are NOT supported.
 *
 * - "text": Can be string, markdown, or X·M·L content.
 *
 * - "literal": This is a plaintext field.
 */
const context = Object.freeze({
  author: Object.freeze({
    namespace: DC11,
    localName: "creator",
    type: "person",
  }),
  // category is not supported at this time
  content: Object.freeze({
    namespace: SIOC,
    localName: "content",
    type: "text",
  }),
  contributor: Object.freeze({
    namespace: DC11,
    localName: "contributor",
    type: "person",
  }),
  // generator is provided by the build script
  icon: Object.freeze({
    namespace: AWOL,
    localName: "icon",
    type: "node",
  }),
  // link is provided by the build script
  logo: Object.freeze({
    namespace: AWOL,
    localName: "logo",
    type: "node",
  }),
  published: Object.freeze({
    namespace: DC11,
    localName: "date",
    type: "literal",
  }),
  rights: Object.freeze({
    namespace: DC11,
    localName: "rights",
    type: "text",
  }),
  // source is not supported at this time
  subtitle: Object.freeze({
    namespace: RDFS,
    localName: "label",
    type: "text",
  }),
  summary: Object.freeze({
    namespace: DC11,
    localName: "abstract",
    type: "text",
  }),
  title: Object.freeze({
    namespace: DC11,
    localName: "title",
    type: "text",
  }),
  // updated is provided by the build script
});

const {
  /**
   * Returns a new document created from the source code of the named
   * template.
   */
  documentFromTemplate,
} = (() => {
  const cache = Object.create(null);
  return {
    documentFromTemplate: async (name) =>
      parser.parseFromString(
        name in cache ? cache[name] : (
          cache[name] = await Deno.readTextFile(
            `${basePath}/index#${name}.xhtml`,
          )
        ),
        "application/xml",
      ),
  };
})();

/**
 * Fills out the `head` of the provided H·T·M·L document with the
 * appropriate metadata.
 */
const fillOutHead = (document, metadata, type) => {
  const { documentElement } = document;
  const LMN = Lemon.bind({ document });
  const head =
    Array.from(documentElement.childNodes).find(($) =>
      hasExpandedName($, XHTML, "head")
    ) ?? documentElement.insertBefore(
      LMN.head``,
      documentElement.childNodes.item(0),
    );
  const titleElement =
    Array.from(head.childNodes).find(($) =>
      hasExpandedName($, XHTML, "title")
    ) ?? head.appendChild(LMN.title``);
  const {
    title,
    author,
    subtitle,
    summary,
  } = metadata;
  titleElement.textContent = Object(title) instanceof String
    ? title
    : Array.from(title ?? []).map(($) => $.textContent).join("");
  for (const person of author) {
    // Iterate over authors and add appropriate meta tags.
    head.appendChild(
      LMN.meta({
        name: "author",
        content: person.name ?? person.uri,
      })``,
    );
  }
  head.appendChild(
    LMN.meta({ name: "generator", content: "🧸📔 Bjørn" })``,
  );
  if (type == "entry") {
    // The provided document is an entry document.
    if (summary) {
      // The entry has a summary.
      head.appendChild(
        LMN.meta({
          name: "description",
          content: Object(summary) instanceof String
            ? summary
            : Array.from(summary).map(($) => $.textContent).join(""),
        })``,
      );
    } else {
      /* do nothing */
    }
    head.appendChild(
      LMN.link
        .rel("alternate")
        .type("application/atom+xml")
        .href("../../feed.atom")``,
    );
  } else {
    // The provided document is not an entry document.
    if (subtitle) {
      // The entry has a subtitle.
      head.appendChild(
        LMN.meta({
          name: "description",
          content: Object(subtitle) instanceof String
            ? summary
            : Array.from(subtitle).map(($) => $.textContent).join(""),
        })``,
      );
    } else {
      /* do nothing */
    }
    head.appendChild(
      LMN.link
        .rel("alternate")
        .type("application/atom+xml")
        .href("./feed.atom")``,
    );
  }
  globalThis.bjørnTransformHead?.(head, metadata, type);
};

/**
 * Returns the language of the provided node, or null if it has no
 * language.
 *
 * ※ This function returns null regardless of whether a node has no
 * language implicitly (because no parent element had a language set)
 * or explicitly (because the value of the @xml:lang attribute was "").
 *
 * ※ If no language is set, the language of the node is ambiguous.
 * Because this typically occurs when a node is expected to inherit its
 * language from some parent context, this lack of language information
 * SHOULD NOT be preserved when inserting the node into a document
 * where language information is present (instead allowing the node to
 * inherit language information from the node it is being inserted
 * into). There are explicit language codes which may be used if this
 * behaviour is undesirable: If you want to signal that a node does not
 * contain linguistic content, use the language code `zxx`. If you want
 * to signal that a node’s language is undetermined in a way which will
 * never inherit from a parent node, use the language code `und`.
 */
const getLanguage = (node) => {
  const { nodeType } = node;
  if (nodeType == 1) {
    // The provided node is an element.
    const ownLanguage =
      node.namespaceURI == XHTML && node.hasAttribute("lang") &&
        node.getAttribute("lang") ||
      node.hasAttributeNS(XML, "lang") &&
        node.getAttributeNS(XML, "lang");
    if (ownLanguage) {
      // The element has a recognized language attribute set to a
      // nonempty value.
      return ownLanguage;
    } else if (ownLanguage === "") {
      // The element explicitly has no language.
      return null;
    } else {
      // The element has no language attribute, but may inherit a
      // language from its parent.
      const { parentNode } = node;
      if (parentNode != null && parentNode.nodeType != 9) {
        // The provided node has a nondocument parent; get the language
        // from there.
        return getLanguage(parentNode);
      } else {
        // The provided node has no parent and consequently no language.
        return null;
      }
    }
  } else if (nodeType == 9) {
    // The provided node is a document.
    return getLanguage(node.documentElement);
  } else if (nodeType == 11) {
    // The provided node is a document fragment.
    return getLanguage(node.ownerDocument.documentElement);
  } else {
    // The provided node may inherit a language from a parent node.
    const { parentNode } = node;
    if (parentNode != null) {
      // The provided node has a parent; get the language from there.
      return getLanguage(parentNode);
    } else {
      // The provided node has no parent and consequently no language.
      return null;
    }
  }
};

/**
 * Returns whether the provided node has the provided namespace and
 * local name.
 */
const hasExpandedName = (node, namespace, localName) =>
  node.namespaceURI == namespace && node.localName == localName;

/**
 * Processes an RDF document and returns an object of Atom metadata.
 *
 * See `context` for the metadata properties supported.
 */
const metadataFromDocument = (
  { documentElement: root, lastModified },
) => {
  const contextEntries = [...Object.entries(context)];
  const documentType = hasExpandedName(root, AWOL, "Feed")
    ? "feed"
    : "entry";
  const result = Object.assign(Object.create(null), {
    id: root.getAttributeNS(RDF, "about"),
    updated: documentType == "feed" || lastModified == null
      ? new Date().toISOString()
      : lastModified.toISOString(),
    title: null,
    author: [],
    contributor: [],
    rights: null,
    ...(documentType == "feed"
      ? { // additional feed properties
        subtitle: null,
        logo: null,
        icon: null,
      }
      : { // additional entry properties
        published: null,
        summary: null,
        content: null,
      }),
  });
  for (
    const node of [
      ...Array.from(root.attributes),
      ...Array.from(root.childNodes),
    ]
  ) {
    // Iterate over all child nodes and attributes, finding ones which
    // correspond to Atom properties and assigning the appropriate
    // metadata.
    const [name, { type }] = contextEntries.find(
      ([_, value]) =>
        hasExpandedName(node, value.namespace, value.localName),
    ) ?? [, {}];
    if (name != null && name in result) {
      // The current node corresponds with an Atom property.
      const { [name]: existing } = result;
      const content = (() => {
        switch (type) {
          case "node": {
            // The node points to another node.
            return node.getAttributeNS(RDF, "resource");
          }
          case "person": {
            // The node describes a person.
            const base =
              node.getAttributeNS?.(RDF, "parseType") == "Resource"
                ? node
                : Array.from(node.childNodes).find(($) =>
                  $.nodeType == 1
                );
            const person = {
              name: null,
              uri: base.getAttributeNS?.(RDF, "about") || null,
            };
            for (
              const subnode of [
                ...Array.from(base.attributes),
                ...Array.from(base.childNodes),
              ]
            ) {
              // Process child nodes and attributes for the current
              // person, looking for name metadata.
              if (hasExpandedName(subnode, FOAF, "name")) {
                // This is a name node.
                if (person.name == null) {
                  // No name has been set yet.
                  person.name = subnode.textContent;
                } else {
                  // A name has already been set.
                  throw new TypeError(
                    `Duplicate name found for person${
                      person.id != null ? ` <${person.id}>` : ""
                    } while processing <${result.id}>.`,
                  );
                }
              } else {
                // This is not a name node
                /* do nothing */
              }
            }
            return person;
          }
          case "text": {
            // The node describes (potentially rich) textual content.
            //
            // ☡ Don’t return an Array here or it will look like a list
            // of multiple values. Return the NodeList of child nodes
            // directly.
            const parseType = node.getAttributeNS?.(RDF, "parseType");
            if (parseType == "Markdown") {
              // This is an element with Markdown content (which
              // hopefully can be converted into X·M·L).
              return parser.parseFromString(
                `<福 xmlns="${XHTML}" lang="${
                  getLanguage(node) ?? ""
                }">${
                  markdownTokensToHTML(
                    markdownTokens(node.textContent),
                  )
                }</福>`,
                "application/xml",
              ).documentElement.childNodes;
            } else if (parseType == "Literal") {
              // This is an element with literal X·M·L contents.
              return node.childNodes;
            } else {
              // This is an element without literal X·M·L contents.
              /* do nothing */
            }
          } // falls through
          default: {
            // The node describes something in plaintext.
            const text = new String(node.textContent);
            const lang = getLanguage(node);
            if (lang) {
              text.lang = lang;
            } else {
              /* do nothing */
            }
            return text;
          }
        }
      })();
      if (existing == null) {
        // The property takes at most one value, but none has been set.
        result[name] = content;
      } else if (Array.isArray(existing)) {
        // The property takes multiple values.
        existing.push(content);
      } else {
        // The property takes at most one value, and one has already
        // been set.
        throw new TypeError(
          `Duplicate content found for ${name} while processing <${result.id}>.`,
        );
      }
    } else {
      // The current node does not correspond with an Atom property.
      /* do nothing */
    }
  }
  globalThis.bjørnTransformMetadata?.(result, documentType);
  return validateMetadata(result, documentType);
};

/** The DOMParser used by this script. */
const parser = new DOMParser();

/** The XMLSerializer used by this script. */
const serializer = new XMLSerializer();

/**
 * Sets the @xml:lang attribute of the provided element, and if it is
 * an H·T·M·L element also sets the @lang.
 */
const setLanguage = (element, lang) => {
  element.setAttributeNS(XML, "xml:lang", lang ?? "");
  if (element.namespaceURI == XHTML) {
    element.setAttribute("lang", lang ?? "");
  } else {
    /* do nothing */
  }
};

/**
 * Throws if the provided metadata does not conform to expectations for
 * the provided type, and otherwise returns it.
 */
const validateMetadata = (metadata, type) => {
  if (metadata.id == null) {
    throw new TypeError("Missing id.");
  } else if (metadata.title == null) {
    throw new TypeError(`Missing title for item <${metadata.id}>.`);
  } else if (type == "feed" && metadata.author == null) {
    throw new TypeError(`Missing author for feed <${metadata.id}>.`);
  } else if (type == "entry" && metadata.content == null) {
    throw new TypeError(`Missing content for entry <${metadata.id}>.`);
  } else {
    return metadata;
  }
};

{ // Set up global variables for use in hooks.
  //
  // Bjørn is principally built to be run from the command line (as a
  // shell script) rather than conforming to typical Ecmascript module
  // patterns. However, it recognizes hooks through various
  // specially‐named properties on `globalThis`. After defining these
  // hooks, a script can use a *dynamic* `import("./path/to/build.js")`
  // to run the Bjørn build steps.
  //
  // To make writing scripts which make use of these hooks easier,
  // infrastructural dependencies and useful functions are provided on
  // `globalThis` so that wrapping scripts don’t have to attempt to
  // manage the dependencies themselves.
  //
  // Note that the `Lemon/window` polyfill will already have
  // established some D·O·M‐related global properties by the time this
  // runs, so they don’t need to be redeclared here.
  globalThis.Lemon = Lemon;
  globalThis.Bjørn = {
    addContent,
    context,
    getLanguage,
    setLanguage,
  };
}

await (async () => { // this is the run script
  const writes = [];

  // Set up the feed metadata and Atom feed document.
  const feedDocument = parser.parseFromString(
    await Deno.readTextFile(`${basePath}/#feed.rdf`),
    "application/xml",
  );
  const feedMetadata = metadataFromDocument(feedDocument);
  const feedURI = new URL(feedMetadata.id);
  const document = parser.parseFromString(
    `<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom"><generator>🧸📔 Bjørn</generator><link rel="self" type="application/atom+xml" href="${new URL(
      "./feed.atom",
      feedURI,
    )}"/></feed>`,
    "application/xml",
  );
  const { documentElement: feed } = document;
  const feedLanguage = getLanguage(feedDocument);
  if (feedLanguage) {
    // The feed element has a language.
    setLanguage(feed, feedLanguage);
  } else {
    // There is no language for the feed.
    /* do nothing */
  }
  applyMetadata(feed, feedMetadata);

  // Set up the index page.
  const feedTemplate = await documentFromTemplate("feed");
  const { documentElement: feedTemplateRoot } = feedTemplate;
  if (feedLanguage && !getLanguage(feedTemplateRoot)) {
    // The root element of the template does not have an
    // assigned language, but the feed does.
    setLanguage(feedTemplateRoot, feedLanguage);
  } else {
    // Either the template root already has a language, or the
    // entry doesn’t either.
    /* do nothing */
  }
  const feedEntries = feedTemplate.createDocumentFragment();

  // Process entries and save the resulting index files.
  for (
    const { name: date } of Array.from(
      Deno.readDirSync(`${basePath}/`),
    ).filter(({ name: date, isDirectory }) =>
      // Exclude non‐dated directories.
      isDirectory && /^[0-9]{4}-[0-9]{2}-[0-9]{2}$/u.test(date)
    ).sort(({ name: a }, { name: b }) =>
      // Sort the directories.
      a < b ? 1 : a > b ? -1 : 0
    )
  ) {
    // Iterate over each dated directory and process its entries.
    for (
      const { name: entryName } of Array.from(
        Deno.readDirSync(`${basePath}/${date}/`),
      ).filter(({ name: entryName, isDirectory }) =>
        // Exclude non‐entry directories.
        isDirectory &&
        //deno-lint-ignore no-control-regex
        !/[\x00-\x20\x22#%/<>?\\^\x60{|}\x7F]/u.test(entryName)
      ).sort(({ name: a }, { name: b }) =>
        // Sort the directories.
        a < b ? 1 : a > b ? -1 : 0
      )
    ) {
      // Iterate over each entry directory and process its contents.
      const entry = document.createElement("entry");
      const entryPath = `${basePath}/${date}/${entryName}/#entry.rdf`;
      const entryDocument = parser.parseFromString(
        await Deno.readTextFile(entryPath),
        "application/xml",
      );
      const { documentElement: entryRoot } = entryDocument;
      entryDocument.lastModified = (await Deno.lstat(entryPath)).mtime;
      if (!entryRoot.hasAttributeNS(RDF, "about")) {
        // The entry doesn’t have an identifier; let’s give it one.
        entryRoot.setAttributeNS(
          RDF,
          "about",
          new URL(`./${date}/${entryName}/`, feedURI),
        );
      } else {
        // The entry already has an identifier.
        /* do nothing */
      }
      const entryMetadata = metadataFromDocument(entryDocument);
      if (entryMetadata.author.length == 0) {
        // The entry metadata did not supply an author.
        entryMetadata.author = feedMetadata.author;
      } else {
        // The entry metadata supplied its own author.
        /* do nothing */
      }
      const entryTemplate = await documentFromTemplate("entry");
      const { documentElement: templateRoot } = entryTemplate;
      const lang = getLanguage(entryRoot);
      if (lang && !getLanguage(templateRoot)) {
        // The root element of the template does not have an
        // assigned language, but the entry does.
        setLanguage(templateRoot, lang);
      } else {
        // Either the template root already has a language, or the
        // entry doesn’t either.
        /* do nothing */
      }
      writes.push(
        Deno.writeTextFile(
          `${basePath}/${date}/${entryName}/index.xhtml`,
          serializer.serializeToString(
            applyMetadata(entryTemplate, entryMetadata),
          ) + "\n",
        ),
      );
      applyMetadata(entry, entryMetadata);
      applyMetadata(feedEntries, entryMetadata);
      feed.appendChild(entry);
    }
  }

  // Apply the feed metadata to the feed template and save the
  // resulting index file.
  if (hasExpandedName(feedTemplateRoot, XHTML, "html")) {
    // This is an XHTML template.
    const LMN = Lemon.bind({ document: feedTemplate });
    const {
      id,
      title,
      subtitle,
      rights,
      updated,
    } = feedMetadata;
    fillOutHead(feedTemplate, feedMetadata, "feed");
    const contentPlaceholder = feedTemplate.getElementsByTagNameNS(
      XHTML,
      "bjørn-content",
    ).item(0);
    if (contentPlaceholder != null) {
      // There is a content placeholder.
      const { parentNode: contentParent } = contentPlaceholder;
      const contentElement = contentParent.insertBefore(
        LMN.nav.about(`${id}`)`${"\n"}`,
        contentPlaceholder,
      );
      const contentHeader = contentElement.appendChild(
        LMN.header`${"\n\t"}`,
      );
      addContent(
        contentHeader.appendChild(LMN.h1.property(`${DC11}title`)``),
        title,
      );
      addContent(contentHeader, "\n");
      if (subtitle) {
        // The feed has a subtitle.
        addContent(
          contentHeader.appendChild(LMN.p.property(`${RDFS}label`)``),
          subtitle,
        );
        addContent(contentHeader, "\n");
      } else {
        // The feed has no subtitle.
        /* do nothing */
      }
      addContent(contentElement, "\n");
      const entriesElement = contentElement.appendChild(
        LMN.ul.rel(`${AWOL}entry`)`${"\n"}`,
      );
      entriesElement.appendChild(feedEntries);
      addContent(contentElement, "\n");
      const contentFooter = contentElement.appendChild(
        LMN.footer`${"\n\t"}`,
      );
      if (rights) {
        // The feed has a rights statement.
        addContent(
          contentFooter.appendChild(
            LMN.div.property(`${DC11}rights`)``,
          ),
          rights,
        );
        addContent(contentFooter, "\n\t");
      } else {
        // The feed has no rights statement.
        /* do nothing */
      }
      contentFooter.appendChild(
        LMN.p.id("entry.updated")`Last updated: ${LMN.time.property(
          `${AWOL}updated`,
        )`${updated}`}.`,
      );
      addContent(contentFooter, "\n");
      addContent(contentElement, "\n");
      contentParent.removeChild(contentPlaceholder);
    } else {
      /* do nothing */
    }
  }
  globalThis.bjørnTransformFeedHTML?.(feedTemplate, feedMetadata);
  writes.push(
    Deno.writeTextFile(
      "index.xhtml",
      serializer.serializeToString(feedTemplate) + "\n",
    ),
  );

  // Save the feed Atom file.
  globalThis.bjørnTransformFeedAtom?.(document, feedMetadata);
  writes.push(
    Deno.writeTextFile(
      "feed.atom",
      serializer.serializeToString(document) + "\n",
    ),
  );

  // Await all writes.
  await Promise.all(writes);
})();