def _mdbook_html_impl(ctx):
    # We will be generating our own book.toml based on this rule's configuration.
    #
    # We do this because:
    #  - The book.toml must contain a reference to the source files of the
    #    generated book. This only works as long as the source files are not
    #    generated by Bazel.
    #  - The book.toml file effectively describes a build, so it makes sense to
    #    port that over to be fully managed by the Bazel rule for mdbook. This
    #    makes things more consistent with the rest of our Bazel usage, at the
    #    expense of slightly deviating from how upstream does things.
    #
    # We emit the toml into `book.toml` because that's what mdbook needs.
    # However, instead of emitting it in a subdirectory named after this
    # target, we do that in `${target}_`. This is so that we can use the target
    # name as a directory containing the actual generated HTML files, making
    # the life of developers using this rule a bit easier.
    out_book_toml = ctx.actions.declare_file(ctx.attr.name + "_/book.toml")

    # Find root of given handbook from srcs - there must be exactly one
    # SUMMARY.md and the parent directory of that is the root.
    summary = None
    for f in ctx.files.srcs:
        if not f.path.endswith("/SUMMARY.md"):
            continue
        if summary != None:
            fail("More then one SUMMARY.md provided.")
        summary = f
    if summary == None:
        fail("No SUMMARY.md provided in srcs.")

    # We now have the SUMMARY.md file from which we can figure out the source
    # directory of the book. However, mdbook takes a source root path relative
    # to the book.toml file, not one relative to the current working
    # directory... Thus, we need to prepend a list of '../' elements that bring
    # mdbook down from the book.toml location back into the workspace execution
    # root, which is where our SUMMARY.md path is itself rooted.
    #
    # For example, if book.toml lives in:
    #   execroot/_main/bazel-out/k8-fastbuild/bin/metropolis/handbook/handbook_/book.toml
    # Then we will need to prepend:
    #   ../../../../../../../
    # To get back to execroot/.
    prepend = len(out_book_toml.path.split("/")) - 1
    src_dir_path = ("../" * prepend) + summary.dirname

    # Generate book.toml.
    # Bazel does not have a toml library. We abuse JSON encoding to get
    # serialized list/string data as an acceptable substitute to building a
    # fully self-standing toml serializer for Bazel.
    book_toml_contents = [
        "[book]",
        "title = {}".format(json.encode(ctx.attr.title)),
        "authors = {}".format(json.encode(ctx.attr.authors)),
        "language = {}".format(json.encode(ctx.attr.language)),
        "multilingual = false",
        "src = {}".format(json.encode(src_dir_path)),
    ]
    ctx.actions.write(
        output = out_book_toml,
        content = "\n".join(book_toml_contents),
    )

    out_dir = ctx.actions.declare_directory(ctx.attr.name)

    # We also have to prepend the out dir path, for the same reasons for which
    # we prepend src_dir_path above.
    out_dir_path = ("../" * prepend) + out_dir.path
    ctx.actions.run(
        executable = ctx.executable._mdbook,
        arguments = [
            "build",
            "-d",
            out_dir_path,
            out_book_toml.dirname,
        ],
        inputs = ctx.files.srcs + [out_book_toml],
        outputs = [out_dir],
    )
    return [
        DefaultInfo(
            files = depset([out_dir]),
        ),
    ]

mdbook_html = rule(
    doc = "Build an mdbook source root into HTML files.",
    implementation = _mdbook_html_impl,
    attrs = {
        "title": attr.string(
            doc = "The title of the generated book.",
        ),
        "authors": attr.string_list(
            default = ["Monogon Project Authors"],
            doc = "The authors of the generated book.",
        ),
        "language": attr.string(
            default = "en",
            doc = "The language of the generated book.",
        ),
        "srcs": attr.label_list(
            allow_files = True,
            doc = "The sources of the generated book. Exaclty one file must be named SUMMARY.md, and that file's location will be used to determine the root of the book sources.",
        ),
        "_mdbook": attr.label(
            doc = "The mdbook tool.",
            executable = True,
            cfg = "exec",
            default = "@crate_index//:mdbook__mdbook",
        ),
    },
)