[jak2] Fix collision renderer extract (#3081)

Fixes missing collision geometry reported in
#3011

The issue happens when there are 256 polygons. In this case `num-polys`
is 0 (it's a u8). There are actual cases where there are 0 polygons, so
we have to do a more complicated check to get the real count. I should
have done this in the first place, but it seemed to work...

decompiler/level_extractor/extract_collide_frags.cpp

@@ -296,16 +296,69 @@ void handle_collide_fragment(const TypedRef& collide_fragment,
 
   level_tools::Vector bbox_min;
   bbox_min.read_from_file(get_field_ref(collide_fragment, "bbox", dts));
+
+  // There's a lot of indirection:
+  // grid -> buckets -> index -> poly -> vertex.
+
+  // First step: 3D coordinates are converted to a cell in a 3D grid.
+  // The dimensions of this grid are stored as u8's in the dimension array.
+  u32 dim_array = deref_u32(get_field_ref(collide_fragment, "dimension-array", dts), 0);
+  u8 counts[4];
+  memcpy(counts, &dim_array, 4);
+  ASSERT(counts[3] == 0);  // unused
+
+  // Each grid cell maps to a bucket (they go in order, no fancy hash function)
+  u32 num_buckets = read_plain_data_field<uint16_t>(collide_fragment, "num-buckets", dts);
+  // this should match the grid dims
+  ASSERT(counts[0] * counts[1] * counts[2] == num_buckets);
+
+  // read the buckets
+  struct BucketEntry {
+    s16 index, count;
+  };
+  std::vector<BucketEntry> buckets(num_buckets);
+  memcpy_from_plain_data((u8*)buckets.data(),
+                         deref_label(get_field_ref(collide_fragment, "bucket-array", dts)),
+                         sizeof(BucketEntry) * num_buckets);
+
+  // Each bucket references a series of entries in the index list. Check all buckets to see the
+  // length of the index list.
+  int max_from_buckets = 0;
+  for (const auto& bucket : buckets) {
+    int end = bucket.count + bucket.index;
+    if (end > max_from_buckets) {
+      max_from_buckets = end;
+    }
+  }
+  // confirm the index list length matches
+  u32 num_indices = read_plain_data_field<uint16_t>(collide_fragment, "num-indices", dts);
+  ASSERT(max_from_buckets == (int)num_indices);
+
+  // read the index list
+  std::vector<u8> index_list(num_indices);
+  memcpy_from_plain_data((u8*)index_list.data(),
+                         deref_label(get_field_ref(collide_fragment, "index-array", dts)),
+                         num_indices);
+  u8 max_in_index_list = 0;
+  for (auto x : index_list) {
+    max_in_index_list = std::max(x, max_in_index_list);
+  }
+
   u8 poly_count = read_plain_data_field<uint8_t>(collide_fragment, "poly-count", dts);
   u8 num_polys = read_plain_data_field<uint8_t>(collide_fragment, "num-polys", dts);
   ASSERT(poly_count == num_polys);
+  if (poly_count == 0) {
+    ASSERT(max_in_index_list == 255);
+  } else {
+    ASSERT(num_polys == max_in_index_list + 1);
+  }
 
   // this value seems to be bogus
   // u16 vert_count = read_plain_data_field<uint16_t>(collide_fragment, "num-verts", dts);
-  std::vector<Poly> polys(poly_count);
+  std::vector<Poly> polys(max_in_index_list + 1);
   memcpy_from_plain_data((u8*)polys.data(),
                          deref_label(get_field_ref(collide_fragment, "poly-array", dts)),
-                         4 * poly_count);
+                         4 * (max_in_index_list + 1));
   int max_vi = 0;
   int max_pat = 0;
   for (const auto& p : polys) {
@@ -402,7 +455,7 @@ void extract_collide_frags(const level_tools::CollideHash& chash,
     data_ref.byte_offset += 4;
   }
 
-  // now, ties
+  // first, all ties. Store collide-hash-fragments that are associated by ties.
   for (auto tt : ties) {
     auto last_array = tt->arrays.back().get();
     auto as_instance_array = dynamic_cast<level_tools::DrawableInlineArrayInstanceTie*>(last_array);