feat: add 'select' handlebars helper

include/mrdocs/Support/Handlebars.hpp

@@ -1044,6 +1044,17 @@ MRDOCS_DECL
 void
 registerAntoraHelpers(Handlebars& hbs);
 
+/** Register logical helpers into a Handlebars instance
+
+    This function registers a number of common helpers that perform
+    logical operations.
+
+    @param hbs The Handlebars instance to register the helpers into
+ */
+MRDOCS_DECL
+void
+registerLogicalHelpers(Handlebars& hbs);
+
 /** Register string helpers into a Handlebars instance
 
     This function registers a number of common helpers that operate on
@@ -1130,6 +1141,18 @@ MRDOCS_DECL
 bool
 not_fn(dom::Array const& arg);
 
+/** "select" helper function
+ *
+ *  The "select" helper returns the second argument if the first argument is
+ *  truthy, and the third argument otherwise.
+ */
+MRDOCS_DECL
+dom::Value
+select_fn(
+    dom::Value condition,
+    dom::Value result_true,
+    dom::Value result_false);
+
 /** "increment" helper function
  *
  *  The "increment" helper adds 1 to the value if it's an integer and converts

src/lib/Support/Handlebars.cpp

@@ -3674,6 +3674,17 @@ registerAntoraHelpers(Handlebars& hbs)
     hbs.registerHelper("year", dom::makeInvocable(year_fn));
 }
 
+void
+registerLogicalHelpers(Handlebars& hbs)
+{
+    hbs.registerHelper("and", dom::makeVariadicInvocable(and_fn));
+    hbs.registerHelper("eq", dom::makeVariadicInvocable(eq_fn));
+    hbs.registerHelper("ne", dom::makeVariadicInvocable(ne_fn));
+    hbs.registerHelper("not", dom::makeVariadicInvocable(not_fn));
+    hbs.registerHelper("or", dom::makeVariadicInvocable(or_fn));
+    hbs.registerHelper("select", dom::makeInvocable(select_fn));
+}
+
 bool
 and_fn(dom::Array const& args)
 {
@@ -3749,6 +3760,16 @@ increment_fn(dom::Value const& value)
     return 1;
 }
 
+dom::Value
+select_fn(
+    dom::Value condition,
+    dom::Value result_true,
+    dom::Value result_false)
+{
+    return isEmpty(condition) ?
+        result_false : result_true;
+}
+
 dom::Value
 detag_fn(dom::Value html)
 {