mdbook/crates/mdbook-html/src/html_handlebars/helpers/theme.rs

use handlebars::{
    Context, Handlebars, Helper, Output, RenderContext, RenderError, RenderErrorReason,
};
use log::trace;

pub(crate) fn theme_option(
    h: &Helper<'_>,
    _r: &Handlebars<'_>,
    ctx: &Context,
    rc: &mut RenderContext<'_, '_>,
    out: &mut dyn Output,
) -> Result<(), RenderError> {
    trace!("theme_option (handlebars helper)");

    let param = h.param(0).and_then(|v| v.value().as_str()).ok_or_else(|| {
        RenderErrorReason::ParamTypeMismatchForName(
            "theme_option",
            "0".to_owned(),
            "string".to_owned(),
        )
    })?;

    let default_theme = rc.evaluate(ctx, "@root/default_theme")?;
    let default_theme_name = default_theme.as_json().as_str().ok_or_else(|| {
        RenderErrorReason::ParamTypeMismatchForName(
            "theme_option",
            "default_theme".to_owned(),
            "string".to_owned(),
        )
    })?;

    out.write(param)?;
    if param.to_lowercase() == default_theme_name.to_lowercase() {
        out.write(" (default)")?;
    }

    Ok(())
}