lox/src/dup_filter.rs

/*! Filter duplicate shows of credentials and open invitations by id
 * (which will typically be a Scalar).  This implementation just keeps
 * the table of seen ids in memory, but a production one would of course
 * use a disk-backed database. */

use std::cmp::Eq;
use std::collections::HashMap;
use std::hash::Hash;

/// Each instance of DupFilter maintains its own independent table of
/// seen ids.  IdType will typically be Scalar.
#[derive(Default, Debug)]
pub struct DupFilter<IdType> {
    seen_table: HashMap<IdType, ()>,
}

impl<IdType: Hash + Eq + Copy> DupFilter<IdType> {
    /// Check to see if the id is in the seen table, but do not add it
    /// to the seen table.  Return true if it is already in the table,
    /// false if not.
    pub fn check(&self, id: &IdType) -> bool {
        self.seen_table.contains_key(id)
    }

    /// As atomically as possible, check to see if the id is in the seen
    /// table, and add it if not.  Return Ok(()) if it was not already
    /// in the table, and Err(()) if it was.
    pub fn filter(&mut self, id: &IdType) -> Result<(), ()> {
        match self.seen_table.insert(*id, ()) {
            None => Ok(()),
            Some(()) => Err(()),
        }
    }
}