Optimize nearest structure border iteration

Getting the nearest generated structure contains a nested set of loops that
iterates over all chunks at a specific chessboard distance. It does this by
iterating over the entire square of chunks within that distance, and checking
if the coordinates are at exactly the right distance to be on the border.

This patch optimizes the iteration by only iterating over the border chunks.
This evaluated chunks are the same, and in the same order, as before, to
ensure that the returned found structure (which may for example be a buried
treasure that will be marked on a treasure map) is the same as in vanilla.

paper-server/patches/sources/net/minecraft/world/level/chunk/ChunkGenerator.java.patch

@@ -44,7 +44,27 @@
  
                  blockposition_mutableblockposition.set(SectionPos.sectionToBlockCoord(chunkcoordintpair.x, 8), 32, SectionPos.sectionToBlockCoord(chunkcoordintpair.z, 8));
                  double d1 = blockposition_mutableblockposition.distSqr(center);
-@@ -312,29 +331,29 @@
+@@ -247,12 +266,15 @@
+         int i1 = placement.spacing();
+ 
+         for (int j1 = -radius; j1 <= radius; ++j1) {
+-            boolean flag1 = j1 == -radius || j1 == radius;
++            // Paper start - Perf: iterate over border chunks instead of entire square chunk area
++            boolean flag1 = j1 == -radius || j1 == radius; final boolean onBorderAlongZAxis = flag1; // Paper - OBFHELPER
+ 
+-            for (int k1 = -radius; k1 <= radius; ++k1) {
+-                boolean flag2 = k1 == -radius || k1 == radius;
++            for (int k1 = -radius; k1 <= radius; k1 += onBorderAlongZAxis ? 1 : radius * 2) {
++                // boolean flag2 = k1 == -radius || k1 == radius;
+ 
+-                if (flag1 || flag2) {
++                // if (flag1 || flag2) {
++                if (true) {
++                    // Paper end - Perf: iterate over border chunks instead of entire square chunk area
+                     int l1 = centerChunkX + i1 * j1;
+                     int i2 = centerChunkZ + i1 * k1;
+                     ChunkPos chunkcoordintpair = placement.getPotentialStructureChunk(seed, l1, i2);
+@@ -312,29 +334,29 @@
          }
      }
  
@@ -81,7 +101,7 @@
  
                      Objects.requireNonNull(set);
                      palettedcontainerro.getAll(set::add);
-@@ -345,7 +364,7 @@
+@@ -345,7 +367,7 @@
              int j = list.size();
  
              try {
@@ -90,7 +110,7 @@
                  int k = Math.max(GenerationStep.Decoration.values().length, j);
  
                  for (int l = 0; l < k; ++l) {
-@@ -353,7 +372,7 @@
+@@ -353,7 +375,7 @@
                      Iterator iterator;
                      CrashReportCategory crashreportsystemdetails;
  
@@ -99,7 +119,7 @@
                          List<Structure> list1 = (List) map.getOrDefault(l, Collections.emptyList());
  
                          for (iterator = list1.iterator(); iterator.hasNext(); ++i1) {
-@@ -368,9 +387,9 @@
+@@ -368,9 +390,9 @@
                              };
  
                              try {
@@ -112,7 +132,7 @@
                                  });
                              } catch (Exception exception) {
                                  CrashReport crashreport = CrashReport.forThrowable(exception, "Feature placement");
-@@ -418,11 +437,18 @@
+@@ -418,11 +440,18 @@
                                  return (String) optional.orElseGet(placedfeature::toString);
                              };
  
@@ -134,7 +154,7 @@
                              } catch (Exception exception1) {
                                  CrashReport crashreport1 = CrashReport.forThrowable(exception1, "Feature placement");
  
-@@ -435,7 +461,7 @@
+@@ -435,15 +464,42 @@
                      }
                  }
  
@@ -143,10 +163,12 @@
              } catch (Exception exception2) {
                  CrashReport crashreport2 = CrashReport.forThrowable(exception2, "Biome decoration");
  
-@@ -445,6 +471,33 @@
+                 crashreport2.addCategory("Generation").setDetail("CenterX", (Object) chunkcoordintpair.x).setDetail("CenterZ", (Object) chunkcoordintpair.z).setDetail("Decoration Seed", (Object) i);
-         }
+                 throw new ReportedException(crashreport2);
-     }
++            }
- 
++        }
++    }
++
 +   // CraftBukkit start
 +    public void applyBiomeDecoration(WorldGenLevel world, ChunkAccess chunk, StructureManager structureAccessor) {
 +        this.applyBiomeDecoration(world, chunk, structureAccessor, true);
@@ -167,17 +189,16 @@
 +                WorldgenRandom seededrandom = new WorldgenRandom(new net.minecraft.world.level.levelgen.LegacyRandomSource(generatoraccessseed.getSeed()));
 +                seededrandom.setDecorationSeed(generatoraccessseed.getSeed(), x, z);
 +                populator.populate(world, new org.bukkit.craftbukkit.util.RandomSourceWrapper.RandomWrapper(seededrandom), x, z, limitedRegion);
-+            }
+             }
 +            limitedRegion.saveEntities();
 +            limitedRegion.breakLink();
-+        }
+         }
-+    }
+     }
 +    // CraftBukkit end
-+
+ 
      private static BoundingBox getWritableArea(ChunkAccess chunk) {
          ChunkPos chunkcoordintpair = chunk.getPos();
-         int i = chunkcoordintpair.getMinBlockX();
+@@ -521,7 +577,7 @@
-@@ -521,7 +574,7 @@
                  }
              }
  
@@ -186,7 +207,7 @@
                  if (list.size() == 1) {
                      this.tryGenerateStructure((StructureSet.StructureSelectionEntry) list.get(0), structureAccessor, registryManager, randomstate, structureTemplateManager, placementCalculator.getLevelSeed(), chunk, chunkcoordintpair, sectionposition, dimension);
                  } else {
-@@ -582,6 +635,14 @@
+@@ -582,6 +638,14 @@
          StructureStart structurestart = structure.generate(weightedEntry.structure(), dimension, dynamicRegistryManager, this, this.biomeSource, noiseConfig, structureManager, seed, pos, j, chunk, predicate);
  
          if (structurestart.isValid()) {