allow purging orderbook data that is no longer needed

this definiately has bugs :)
tried to get a grasp of the orderbook code here but not fully there yet. Must be revised.

mm2src/mm2_main/src/lp_ordermatch.rs

@@ -39,6 +39,7 @@ use futures::{compat::Future01CompatExt, lock::Mutex as AsyncMutex, TryFutureExt
 use hash256_std_hasher::Hash256StdHasher;
 use hash_db::Hasher;
 use http::Response;
+use itertools::Itertools;
 use keys::{AddressFormat, KeyPair};
 use mm2_core::mm_ctx::{from_ctx, MmArc, MmWeak};
 use mm2_err_handle::prelude::*;
@@ -2554,8 +2555,44 @@ impl Orderbook {
 
     fn find_order_by_uuid(&self, uuid: &Uuid) -> Option<OrderbookItem> { self.order_set.get(uuid).cloned() }
 
-    fn purge_pair(&mut self, _base: &str, _rel: &str) {
-        // FIXME: To be filled! (ordered & unordred maps must have hashable keys)
+    fn purge_pair(&mut self, base: &str, rel: &str, delete_sub: bool) {
+        let base_rel = (base.to_string(), rel.to_string());
+        let rel_base = (rel.to_string(), base.to_string());
+        let topic = orderbook_topic_from_base_rel(base, rel);
+        let alb_pair = alb_ordered_pair(base, rel);
+        // Only delete the subscription if instructed to.
+        if delete_sub {
+            self.topics_subscribed_to.remove(&topic);
+        }
+        // Remove the pair from the ordered map.
+        self.ordered.remove(&base_rel);
+        self.ordered.remove(&rel_base);
+        // Remove the pair from the unordred map and get their orders' uuids.
+        let uuids = {
+            let base_rel_uuids = self.unordered.remove(&base_rel).unwrap_or_default();
+            let rel_base_uuids = self.unordered.remove(&rel_base).unwrap_or_default();
+            base_rel_uuids.into_iter().chain(rel_base_uuids.into_iter())
+        };
+        let pubkeys_for_pair = uuids
+            // Remove these uuids from the orders set, and collect the pubkeys to update the pubkeys state.
+            .filter_map(|uuid| self.order_set.remove(&uuid).map(|order| (order.pubkey, uuid)))
+            .into_group_map();
+        for (pubkey, uuids) in pubkeys_for_pair {
+            let pubkey_state = match self.pubkeys_state.get_mut(&pubkey) {
+                Some(state) => state,
+                None => continue,
+            };
+
+            if pubkey_state.trie_roots.get(&alb_pair).is_none() {
+                continue;
+            }
+            for uuid in uuids {
+                pubkey_state.orders_uuids.remove(&(uuid, alb_pair.clone()));
+            }
+
+            pubkey_state.order_pairs_trie_state_history.remove(alb_pair.clone());
+            pubkey_state.trie_roots.remove(&alb_pair);
+        }
     }
 
     fn insert_or_update_order_update_trie(&mut self, order: OrderbookItem) {
@@ -5712,6 +5749,29 @@ async fn subscribe_actively_to_orderbook_topic(
     Ok(())
 }
 
+#[allow(dead_code)]
+fn unsubscribe_actively_from_orderbook_topic(ctx: &MmArc, base: &str, rel: &str) -> Result<(), String> {
+    let topic = orderbook_topic_from_base_rel(base, rel);
+    let ordermatch_ctx = try_s!(OrdermatchContext::from_ctx(ctx));
+    let mut orderbook = ordermatch_ctx.orderbook.lock();
+
+    let no_one_is_listening = match orderbook.topics_subscribed_to.get_mut(&topic) {
+        Some(sub_state) => {
+            sub_state.is_listening_actively = false;
+            !sub_state.is_listening_passively
+        },
+        None => false,
+    };
+
+    // If we don't have a passive listener to this topic we want to delete the subscription.
+    orderbook.purge_pair(base, rel, no_one_is_listening);
+    if no_one_is_listening {
+        unsubscribe_from_topic(ctx, topic)
+    }
+
+    Ok(())
+}
+
 fn subscribe_passively_to_orderbook_topic(ctx: &MmArc, base: &str, rel: &str) -> Result<(), String> {
     let topic = orderbook_topic_from_base_rel(base, rel);
     let ordermatch_ctx = try_s!(OrdermatchContext::from_ctx(ctx));
@@ -5741,7 +5801,7 @@ fn unsubscribe_passively_from_orderbook_topic(ctx: &MmArc, base: &str, rel: &str
     };
 
     if remove_pair_from_orderbook {
-        orderbook.purge_pair(base, rel);
+        orderbook.purge_pair(base, rel, true);
         unsubscribe_from_topic(ctx, topic)
     }