[Annotation] Improve the performance of the code for getting glyphs which belongs to annotations bounding boxes (bug 1987914)

Instead of looking at every bbox, we use a grid (64x64) where each cell of the grid is associated with the bboxes
touching it.
In order to get the potential bboxes containing a point, we just have to compute the number of the cell containing
it and in using the associated described above, we can quickly know if the point is contained.
With the pdf in the mentioned bug, it's ~20 times faster.

src/core/intersector.js

@@ -16,13 +16,13 @@
 class SingleIntersector {
   #annotation;
 
-  #minX = Infinity;
+  minX = Infinity;
 
-  #minY = Infinity;
+  minY = Infinity;
 
-  #maxX = -Infinity;
+  maxX = -Infinity;
 
-  #maxY = -Infinity;
+  maxY = -Infinity;
 
   #quadPoints = null;
 
@@ -40,30 +40,21 @@ class SingleIntersector {
     if (!quadPoints) {
       // If there are no quad points, we use the rectangle to determine the
       // bounds of the annotation.
-      [this.#minX, this.#minY, this.#maxX, this.#maxY] = annotation.data.rect;
+      [this.minX, this.minY, this.maxX, this.maxY] = annotation.data.rect;
       return;
     }
 
     for (let i = 0, ii = quadPoints.length; i < ii; i += 8) {
-      this.#minX = Math.min(this.#minX, quadPoints[i]);
-      this.#maxX = Math.max(this.#maxX, quadPoints[i + 2]);
-      this.#minY = Math.min(this.#minY, quadPoints[i + 5]);
-      this.#maxY = Math.max(this.#maxY, quadPoints[i + 1]);
+      this.minX = Math.min(this.minX, quadPoints[i]);
+      this.maxX = Math.max(this.maxX, quadPoints[i + 2]);
+      this.minY = Math.min(this.minY, quadPoints[i + 5]);
+      this.maxY = Math.max(this.maxY, quadPoints[i + 1]);
     }
     if (quadPoints.length > 8) {
       this.#quadPoints = quadPoints;
     }
   }
 
-  overlaps(other) {
-    return !(
-      this.#minX >= other.#maxX ||
-      this.#maxX <= other.#minX ||
-      this.#minY >= other.#maxY ||
-      this.#maxY <= other.#minY
-    );
-  }
-
   /**
    * Check if the given point intersects with the annotation's quad points.
    * The point (x, y) is supposed to be the center of the glyph.
@@ -72,12 +63,7 @@ class SingleIntersector {
    * @returns {boolean}
    */
   #intersects(x, y) {
-    if (
-      this.#minX >= x ||
-      this.#maxX <= x ||
-      this.#minY >= y ||
-      this.#maxY <= y
-    ) {
+    if (this.minX >= x || this.maxX <= x || this.minY >= y || this.maxY <= y) {
       return false;
     }
 
@@ -154,56 +140,91 @@ class SingleIntersector {
   }
 }
 
+// The grid is STEPS x STEPS.
+const STEPS = 64;
+
 class Intersector {
-  #intersectors = new Map();
+  #intersectors = [];
+
+  #grid = [];
+
+  #minX;
+
+  #minY;
+
+  #invXRatio;
+
+  #invYRatio;
 
   constructor(annotations) {
+    let minX = Infinity;
+    let minY = Infinity;
+    let maxX = -Infinity;
+    let maxY = -Infinity;
+    const intersectors = this.#intersectors;
     for (const annotation of annotations) {
       if (!annotation.data.quadPoints && !annotation.data.rect) {
         continue;
       }
       const intersector = new SingleIntersector(annotation);
-      for (const [otherIntersector, overlapping] of this.#intersectors) {
-        if (otherIntersector.overlaps(intersector)) {
-          if (!overlapping) {
-            this.#intersectors.set(otherIntersector, new Set([intersector]));
-          } else {
-            overlapping.add(intersector);
+      intersectors.push(intersector);
+      minX = Math.min(minX, intersector.minX);
+      minY = Math.min(minY, intersector.minY);
+      maxX = Math.max(maxX, intersector.maxX);
+      maxY = Math.max(maxY, intersector.maxY);
+    }
+    this.#minX = minX;
+    this.#minY = minY;
+    this.#invXRatio = (STEPS - 1) / (maxX - minX);
+    this.#invYRatio = (STEPS - 1) / (maxY - minY);
+    for (const intersector of intersectors) {
+      const iMin = this.#getGridIndex(intersector.minX, intersector.minY);
+      const iMax = this.#getGridIndex(intersector.maxX, intersector.maxY);
+      const w = (iMax - iMin) % STEPS;
+      const h = Math.floor((iMax - iMin) / STEPS);
+      for (let i = iMin; i <= iMin + h * STEPS; i += STEPS) {
+        for (let j = 0; j <= w; j++) {
+          let existing = this.#grid[i + j];
+          if (!existing) {
+            this.#grid[i + j] = existing = [];
           }
+          existing.push(intersector);
         }
       }
-      this.#intersectors.set(intersector, null);
     }
   }
 
+  #getGridIndex(x, y) {
+    const i = Math.floor((x - this.#minX) * this.#invXRatio);
+    const j = Math.floor((y - this.#minY) * this.#invYRatio);
+    return i >= 0 && i < STEPS && j >= 0 && j < STEPS ? i + j * STEPS : -1;
+  }
+
   addGlyph(transform, width, height, glyph) {
     const x = transform[4] + width / 2;
     const y = transform[5] + height / 2;
-    let overlappingIntersectors;
-    for (const [intersector, overlapping] of this.#intersectors) {
-      if (overlappingIntersectors) {
-        if (overlappingIntersectors.has(intersector)) {
-          intersector.addGlyph(x, y, glyph);
-        } else {
-          intersector.disableExtraChars();
-        }
-        continue;
-      }
-      if (!intersector.addGlyph(x, y, glyph)) {
-        continue;
-      }
-      overlappingIntersectors = overlapping;
+    const index = this.#getGridIndex(x, y);
+    if (index < 0) {
+      return;
+    }
+    const intersectors = this.#grid[index];
+    if (!intersectors) {
+      return;
+    }
+
+    for (const intersector of intersectors) {
+      intersector.addGlyph(x, y, glyph);
     }
   }
 
   addExtraChar(char) {
-    for (const intersector of this.#intersectors.keys()) {
+    for (const intersector of this.#intersectors) {
       intersector.addExtraChar(char);
     }
   }
 
   setText() {
-    for (const intersector of this.#intersectors.keys()) {
+    for (const intersector of this.#intersectors) {
       intersector.setText();
     }
   }