LUCENE-9125: Optimize Automaton.step() with binary search and introduce Automaton.next().

Closes #1160

lucene/CHANGES.txt

@@ -118,6 +118,8 @@ Optimizations
 
 * LUCENE-9113: Faster merging of SORTED/SORTED_SET doc values. (Adrien Grand)
 
+* LUCENE-9125: Optimize Automaton.step() with binary search and introduce Automaton.next(). (Bruno Roustant)
+
 Bug Fixes
 ---------------------
 

lucene/core/src/java/org/apache/lucene/util/automaton/Automaton.java

@@ -656,22 +656,77 @@ public class Automaton implements Accountable {
    * @return destination state, -1 if no matching outgoing transition
    */
   public int step(int state, int label) {
+    return next(state, 0, label, null);
+  }
+
+  /**
+   * Looks for the next transition that matches the provided label, assuming determinism.
+   * <p>
+   * This method is similar to {@link #step(int, int)} but is used more efficiently
+   * when iterating over multiple transitions from the same source state. It keeps
+   * the latest reached transition index in {@code transition.transitionUpto} so
+   * the next call to this method can continue from there instead of restarting
+   * from the first transition.
+   *
+   * @param transition The transition to start the lookup from (inclusive, using its
+   *                   {@link Transition#source} and {@link Transition#transitionUpto}).
+   *                   It is updated with the matched transition;
+   *                   or with {@link Transition#dest} = -1 if no match.
+   * @param label      The codepoint to look up.
+   * @return The destination state; or -1 if no matching outgoing transition.
+   */
+  public int next(Transition transition, int label) {
+    return next(transition.source, transition.transitionUpto, label, transition);
+  }
+
+  /**
+   * Looks for the next transition that matches the provided label, assuming determinism.
+   *
+   * @param state               The source state.
+   * @param fromTransitionIndex The transition index to start the lookup from (inclusive); negative interpreted as 0.
+   * @param label               The codepoint to look up.
+   * @param transition          The output transition to update with the matching transition; or null for no update.
+   * @return The destination state; or -1 if no matching outgoing transition.
+   */
+  private int next(int state, int fromTransitionIndex, int label, Transition transition) {
     assert state >= 0;
     assert label >= 0;
-    int trans = states[2*state];
+    int stateIndex = 2 * state;
-    int limit = trans + 3*states[2*state+1];
+    int firstTransitionIndex = states[stateIndex];
-    // TODO: we could do bin search; transitions are sorted
+    int numTransitions = states[stateIndex + 1];
-    while (trans < limit) {
-      int dest = transitions[trans];
-      int min = transitions[trans+1];
-      int max = transitions[trans+2];
-      if (min <= label && label <= max) {
-        return dest;
-      }
-      trans += 3;
-    }
 
-    return -1;
+    // Since transitions are sorted,
+    // binary search the transition for which label is within [minLabel, maxLabel].
+    int low = Math.max(fromTransitionIndex, 0);
+    int high = numTransitions - 1;
+    while (low <= high) {
+      int mid = (low + high) >>> 1;
+      int transitionIndex = firstTransitionIndex + 3 * mid;
+      int minLabel = transitions[transitionIndex + 1];
+      if (minLabel > label) {
+        high = mid - 1;
+      } else {
+        int maxLabel = transitions[transitionIndex + 2];
+        if (maxLabel < label){
+          low = mid + 1;
+        } else {
+          int destState = transitions[transitionIndex];
+          if (transition != null) {
+            transition.dest = destState;
+            transition.min = minLabel;
+            transition.max = maxLabel;
+            transition.transitionUpto = mid;
+          }
+          return destState;
+        }
+      }
+    }
+    int destState = -1;
+    if (transition != null) {
+      transition.dest = destState;
+      transition.transitionUpto = low;
+    }
+    return destState;
   }
 
   /** Records new states and transitions and then {@link

lucene/core/src/java/org/apache/lucene/util/automaton/MinimizationOperations.java

@@ -94,12 +94,15 @@ final public class MinimizationOperations {
       }
     }
     // find initial partition and reverse edges
+    Transition transition = new Transition();
     for (int q = 0; q < statesLen; q++) {
       final int j = a.isAccept(q) ? 0 : 1;
       partition[j].add(q);
       block[q] = j;
+      transition.source = q;
+      transition.transitionUpto = -1;
       for (int x = 0; x < sigmaLen; x++) {
-        final ArrayList<Integer>[] r = reverse[a.step(q, sigma[x])];
+        final ArrayList<Integer>[] r = reverse[a.next(transition, sigma[x])];
         if (r[x] == null) {
           r[x] = new ArrayList<>();
         }

lucene/core/src/java/org/apache/lucene/util/automaton/RunAutomaton.java

@@ -78,10 +78,13 @@ public abstract class RunAutomaton implements Accountable {
     accept = new boolean[size];
     transitions = new int[size * points.length];
     Arrays.fill(transitions, -1);
+    Transition transition = new Transition();
     for (int n=0;n<size;n++) {
       accept[n] = a.isAccept(n);
+      transition.source = n;
+      transition.transitionUpto = -1;
       for (int c = 0; c < points.length; c++) {
-        int dest = a.step(n, points[c]);
+        int dest = a.next(transition, points[c]);
         assert dest == -1 || dest < size;
         transitions[n * points.length + c] = dest;
       }