dfa: prune_overlong -> clear_deeper_reports

Rather than pruning states, we simply clear reports on states that are
too deep and allow Hopcroft minimisation to reduce the size of the DFA
afterwards.

src/nfa/mcclellancompile_util.cpp

@@ -167,70 +167,8 @@ u32 calc_min_dist_from_bob(raw_dfa &raw, vector<u32> *dist_in) {
     return last_d;
 }
 
-static
-vector<vector<dstate_id_t>> find_in_edges(const raw_dfa &raw) {
-    vector<vector<dstate_id_t>> in_edges(raw.states.size());
-    flat_set<dstate_id_t> seen;
-
-    for (u32 s = 1; s < raw.states.size(); s++) {
-        seen.clear();
-        for (u32 j = 0; j < raw.alpha_size; j++) {
-            dstate_id_t t = raw.states[s].next[j];
-            if (!seen.insert(t).second) {
-                continue;
-            }
-            in_edges[t].push_back(s);
-        }
-    }
-
-    return in_edges;
-}
-
-static
-vector<u32> calc_min_dist_to_accept(const raw_dfa &raw,
-                                const vector<vector<dstate_id_t>> &in_edges) {
-    vector<u32> dist(raw.states.size(), ~0U);
-
-    /* for reporting states to start from */
-    deque<dstate_id_t> to_visit;
-    for (u32 s = 0; s < raw.states.size(); s++) {
-        if (state_has_reports(raw, s)) {
-            to_visit.push_back(s);
-            dist[s] = 0;
-        }
-    }
-
-    /* bfs */
-    UNUSED u32 last_d = 0;
-    while (!to_visit.empty()) {
-        dstate_id_t s = to_visit.front();
-        to_visit.pop_front();
-        assert(s != DEAD_STATE);
-
-        u32 d = dist[s];
-        assert(d >= last_d);
-        assert(d != ~0U);
-
-        for (auto t : in_edges[s]) {
-            if (t == DEAD_STATE) {
-                continue;
-            }
-            if (dist[t] == ~0U) {
-                to_visit.push_back(t);
-                dist[t] = d + 1;
-            } else {
-                assert(dist[t] <= d + 1);
-            }
-        }
-
-        last_d = d;
-    }
-
-    return dist;
-}
-
-bool prune_overlong(raw_dfa &raw, u32 max_offset) {
-    DEBUG_PRINTF("pruning to at most %u\n", max_offset);
+bool clear_deeper_reports(raw_dfa &raw, u32 max_offset) {
+    DEBUG_PRINTF("clearing reports on states deeper than %u\n", max_offset);
     vector<u32> bob_dist;
     u32 max_min_dist_bob = calc_min_dist_from_bob(raw, &bob_dist);
 
@@ -238,47 +176,18 @@ bool prune_overlong(raw_dfa &raw, u32 max_offset) {
         return false;
     }
 
-    vector<u32> accept_dist = calc_min_dist_to_accept(raw, find_in_edges(raw));
-
-    /* look over the states and filter out any which cannot reach a report
-     * states before max_offset */
-    vector<dstate_id_t> new_ids(raw.states.size());
-    vector<dstate> new_states;
-    u32 count = 1;
-    new_states.push_back(raw.states[DEAD_STATE]);
-
+    bool changed = false;
     for (u32 s = DEAD_STATE + 1; s < raw.states.size(); s++) {
-        if (bob_dist[s] + accept_dist[s] > max_offset) {
-            DEBUG_PRINTF("pruned %u: bob %u, report %u\n", s, bob_dist[s],
-                          accept_dist[s]);
-            new_ids[s] = DEAD_STATE;
-        } else {
-            new_ids[s] = count++;
-            new_states.push_back(raw.states[s]);
-            assert(new_states.size() == count);
-            assert(new_ids[s] <= s);
+        if (bob_dist[s] > max_offset && state_has_reports(raw, s)) {
+            DEBUG_PRINTF("clearing reports on %u (depth %u)\n", s, bob_dist[s]);
+            auto &ds = raw.states[s];
+            ds.reports.clear();
+            ds.reports_eod.clear();
+            changed = true;
         }
     }
 
-    /* swap states */
-    DEBUG_PRINTF("pruned %zu -> %u\n", raw.states.size(), count);
-    raw.states = std::move(new_states);
-    new_states.clear();
-
-    /* update edges and daddys to refer to the new ids */
-    for (u32 s = DEAD_STATE + 1; s < raw.states.size(); s++) {
-        for (u32 j = 0; j < raw.alpha_size; j++) {
-            dstate_id_t old_t = raw.states[s].next[j];
-            raw.states[s].next[j] = new_ids[old_t];
-        }
-        raw.states[s].daddy = new_ids[raw.states[s].daddy];
-    }
-
-    /* update specials */
-    raw.start_floating = new_ids[raw.start_floating];
-    raw.start_anchored = new_ids[raw.start_anchored];
-
-    return true;
+    return changed;
 }
 
 set<ReportID> all_reports(const raw_dfa &rdfa) {

src/nfa/mcclellancompile_util.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015-2016, Intel Corporation
+ * Copyright (c) 2015-2017, Intel Corporation
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
@@ -39,10 +39,12 @@ namespace ue2 {
 u32 remove_leading_dots(raw_dfa &raw);
 
 /**
- * Prunes any states which cannot be reached within max_offset from start of
- * stream. Returns false if no changes are made to the rdfa
+ * \brief Clear reports on any states that are deeper than \a max_offset from
+ * start of stream.
+ *
+ * Returns false if no changes are made to the DFA.
  */
-bool prune_overlong(raw_dfa &raw, u32 max_offset);
+bool clear_deeper_reports(raw_dfa &raw, u32 max_offset);
 
 std::set<ReportID> all_reports(const raw_dfa &rdfa);
 bool has_eod_accepts(const raw_dfa &rdfa);

src/smallwrite/smallwrite_build.cpp

@@ -279,7 +279,7 @@ void SmallWriteBuildImpl::add(const NGHolder &g, const ExpressionInfo &expr) {
         return;
     }
 
-    if (prune_overlong(*r, cc.grey.smallWriteLargestBuffer)) {
+    if (clear_deeper_reports(*r, cc.grey.smallWriteLargestBuffer)) {
         minimize_hopcroft(*r, cc.grey);
     }
 
@@ -725,7 +725,7 @@ bytecode_ptr<NFA> prepEngine(raw_dfa &rdfa, u32 roseQuality,
         if (*small_region <= *start_offset) {
             return nullptr;
         }
-        if (prune_overlong(rdfa, *small_region - *start_offset)) {
+        if (clear_deeper_reports(rdfa, *small_region - *start_offset)) {
             minimize_hopcroft(rdfa, cc.grey);
             if (rdfa.start_anchored == DEAD_STATE) {
                 DEBUG_PRINTF("all patterns pruned out\n");