lalrpop 0.12.2

convenient LR(1) parser generator
Documentation 
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use lexer::dfa::{DFA, Kind, NFAIndex, START};

pub fn interpret<'text>(dfa: &DFA, input: &'text str) -> Option<(NFAIndex, &'text str)> {
    let mut longest: Option<(NFAIndex, usize)> = None;
    let mut state_index = START;

    for (offset, ch) in input.char_indices() {
        let state = &dfa.states[state_index.0];

        let target =
            dfa.state(state_index).test_edges
                                  .iter()
                                  .filter_map(|&(test, target)| {
                                      if test.contains_char(ch) {
                                          Some(target)
                                      } else {
                                          None
                                      }
                                  })
                                  .next();

        if let Some(target) = target {
            state_index = target;
        } else {
            state_index = state.other_edge;
        }

        match dfa.state(state_index).kind {
            Kind::Accepts(nfa) => { longest = Some((nfa, offset + ch.len_utf8())); }
            Kind::Reject => { break; }
            Kind::Neither => { }
        }
    }

    longest.map(|(index, offset)| (index, &input[..offset]))
}