Method evaluate

src/braille.rs:1659–1685

* Returns a string with the correct number of nesting chars (could be an empty string) * @param(node) -- current node * @param(char) -- char (string) that should be repeated * Note: as a side effect, an attribute with the value so repeated calls to this or a child will be fast */

(&self,
                        _context: &context::Evaluation<'_, 'd>,
                        args: Vec<Value<'d>>)

 * Note: as a side effect, an attribute with the value so repeated calls to this or a child will be fast
 */
 fn evaluate<'d>(&self,
                        _context: &context::Evaluation<'_, 'd>,
                        args: Vec<Value<'d>>)
                        -> StdResult<Value<'d>, XPathError>
    {
        let mut args = Args(args);
        args.exactly(2)?;
        let repeat_char = args.pop_string()?;
        let node = crate::xpath_functions::validate_one_node(args.pop_nodeset()?, "NestingChars")?;
        if let Node::Element(el) = node {
            let name = name(&el);
            // it is likely a bug to call this one a non mfrac
            if name == "mfrac" {
                // because it is called on itself, the fraction is counted one too many times -- chop one off
                // this is slightly messy because we are chopping off a char, not a byte
                const BRAILLE_BYTE_LEN: usize = "⠹".len();      // all Unicode braille symbols have the same number of bytes
                return Ok( Value::String( NemethNestingChars::nemeth_frac_value(&el, &repeat_char)[BRAILLE_BYTE_LEN..].to_string() ) );
            } else if name == "msqrt" || name == "mroot" {
                return Ok( Value::String( NemethNestingChars::nemeth_root_value(&el, &repeat_char)? ) );
            } else {
                panic!("NestingChars chars should be used only on 'mfrac'. '{}' was passed in", name);
            }
        } else {
            // not an element, so nothing to do
            return Ok( Value::String("".to_string()) );
        }
    }
}

pub struct BrailleChars;