From 247d72b1aa1d7d68e4c775f98f63e64e024b7bab Mon Sep 17 00:00:00 2001
From: rwjuk <rwj@serenity.uk.net>
Date: Wed, 5 Jul 2017 19:36:35 +0100
Subject: [PATCH 01/13] Implement sprite cycle checking

---
 src/openrct2/rct2/S6Exporter.cpp |   2 +-
 src/openrct2/rct2/S6Importer.cpp |   4 ++
 src/openrct2/world/sprite.c      | 106 +++++++++++++++++++++++++++++++
 src/openrct2/world/sprite.h      |   3 +-
 4 files changed, 113 insertions(+), 2 deletions(-)

diff --git a/src/openrct2/rct2/S6Exporter.cpp b/src/openrct2/rct2/S6Exporter.cpp
index 7a62e5626b..086456d20f 100644
--- a/src/openrct2/rct2/S6Exporter.cpp
+++ b/src/openrct2/rct2/S6Exporter.cpp
@@ -152,6 +152,7 @@ void S6Exporter::Save(IStream * stream, bool isScenario)
 
 void S6Exporter::Export()
 {
+    openrct2_assert(!(check_for_spatial_index_cycles(false) || check_for_sprite_list_cycles(false)), "A sprite cycle exists.");
     _s6.info = gS6Info;
     uint32 researchedTrackPiecesA[128];
     uint32 researchedTrackPiecesB[128];
@@ -449,7 +450,6 @@ extern "C"
 
         map_reorganise_elements();
         sprite_clear_all_unused();
-
         viewport_set_saved_view();
 
         bool result     = false;
diff --git a/src/openrct2/rct2/S6Importer.cpp b/src/openrct2/rct2/S6Importer.cpp
index 53ac9a2971..d12ea1071e 100644
--- a/src/openrct2/rct2/S6Importer.cpp
+++ b/src/openrct2/rct2/S6Importer.cpp
@@ -413,6 +413,10 @@ public:
         map_update_tile_pointers();
         game_convert_strings_to_utf8();
         map_count_remaining_land_rights();
+
+        // We try to fix the cycles on import, hence the 'true' parameter
+        check_for_sprite_list_cycles(true);
+        check_for_spatial_index_cycles(true);
     }
 
     void Initialise()
diff --git a/src/openrct2/world/sprite.c b/src/openrct2/world/sprite.c
index e313a71106..7f4cf210a4 100644
--- a/src/openrct2/world/sprite.c
+++ b/src/openrct2/world/sprite.c
@@ -267,6 +267,7 @@ void sprite_clear_all_unused()
         sprite->previous = previousSpriteIndex;
         sprite->linked_list_type_offset = SPRITE_LIST_NULL * 2;
         sprite->sprite_index = spriteIndex;
+        sprite->next_in_quadrant = (sprite->next_in_quadrant == 0) ? SPRITE_INDEX_NULL : sprite->next_in_quadrant;
         _spriteFlashingList[spriteIndex] = false;
         spriteIndex = nextSpriteIndex;
     }
@@ -277,6 +278,7 @@ static void sprite_reset(rct_unk_sprite *sprite)
     // Need to retain how the sprite is linked in lists
     uint8 llto = sprite->linked_list_type_offset;
     uint16 next = sprite->next;
+    uint16 next_in_quadrant = sprite->next_in_quadrant;
     uint16 prev = sprite->previous;
     uint16 sprite_index = sprite->sprite_index;
     _spriteFlashingList[sprite_index] = false;
@@ -285,6 +287,7 @@ static void sprite_reset(rct_unk_sprite *sprite)
 
     sprite->linked_list_type_offset = llto;
     sprite->next = next;
+    sprite->next_in_quadrant = next_in_quadrant;
     sprite->previous = prev;
     sprite->sprite_index = sprite_index;
 }
@@ -823,3 +826,106 @@ bool sprite_get_flashing(rct_sprite *sprite)
     assert(sprite->unknown.sprite_index < MAX_SPRITES);
     return _spriteFlashingList[sprite->unknown.sprite_index];
 }
+
+static bool sprite_is_in_cycle(uint16 sprite_idx, bool fix)
+{
+    if (sprite_idx == SPRITE_INDEX_NULL)
+    {
+        return false;
+    }
+    const rct_sprite * fast = get_sprite(sprite_idx);
+    const rct_sprite * slow = fast;
+    bool increment_slow = false;
+    bool cycled = false;
+    while (fast->unknown.sprite_index != SPRITE_INDEX_NULL)
+    {
+        // increment fast every time, unless reached the end
+        if (fast->unknown.next == SPRITE_INDEX_NULL)
+        {
+            break;
+        }
+        else {
+            fast = get_sprite(fast->unknown.next);
+        }
+        // increment slow only every second iteration
+        if (increment_slow)
+        {
+            slow = get_sprite(slow->unknown.next);
+        }
+        increment_slow = !increment_slow;
+        if (fast == slow)
+        {
+            cycled = true;
+            if (fix)
+            {
+                rct_sprite * spr = get_sprite(sprite_idx);
+                // There may be a better solution than simply setting this to 0xFFFF
+                spr->unknown.next = SPRITE_INDEX_NULL;
+            }
+            break;
+        }
+    }
+    return cycled;
+}
+
+static bool sprite_is_in_cycle_quadrant(uint16 sprite_idx, bool fix)
+{
+    if (sprite_idx == SPRITE_INDEX_NULL)
+    {
+        return false;
+    }
+    const rct_sprite * fast = get_sprite(sprite_idx);
+    const rct_sprite * slow = fast;
+    bool increment_slow = false;
+    bool cycled = false;
+    while (fast->unknown.sprite_index != SPRITE_INDEX_NULL)
+    {
+        // increment fast every time, unless reached the end
+        if (fast->unknown.next_in_quadrant == SPRITE_INDEX_NULL)
+        {
+            break;
+        }
+        else {
+            fast = get_sprite(fast->unknown.next_in_quadrant);
+        }
+        // increment slow only every second iteration
+        if (increment_slow)
+        {
+            slow = get_sprite(slow->unknown.next_in_quadrant);
+        }
+        increment_slow = !increment_slow;
+        if (fast == slow)
+        {
+            cycled = true;
+            if (fix)
+            {
+                rct_sprite * spr = get_sprite(sprite_idx);
+
+                // There may be a better solution than simply setting this to 0xFFFF
+                spr->unknown.next_in_quadrant = SPRITE_INDEX_NULL;
+            }
+            break;
+        }
+    }
+    return cycled;
+}
+
+bool check_for_sprite_list_cycles(bool fix)
+{
+    for (sint32 i = 0; i < 6; i++) {
+        if (sprite_is_in_cycle(gSpriteListHead[i], fix)) {
+            return true;
+        }
+    }
+    return false;
+}
+
+bool check_for_spatial_index_cycles(bool fix)
+{
+    for (size_t i = 0; i < 0x10000; i++) {
+        if (sprite_is_in_cycle_quadrant(gSpriteSpatialIndex[i], fix)) {
+            return true;
+        }
+    }
+    return false;
+}
\ No newline at end of file
diff --git a/src/openrct2/world/sprite.h b/src/openrct2/world/sprite.h
index 55dde661bb..1cbe3415d0 100644
--- a/src/openrct2/world/sprite.h
+++ b/src/openrct2/world/sprite.h
@@ -478,6 +478,7 @@ const char *sprite_checksum();
 
 void sprite_set_flashing(rct_sprite *sprite, bool flashing);
 bool sprite_get_flashing(rct_sprite *sprite);
-
+bool check_for_sprite_list_cycles(bool fix);
+bool check_for_spatial_index_cycles(bool fix);
 #endif
 

From 372781cc74d5bd78a6ed22e1b0c0d22a3016ac9b Mon Sep 17 00:00:00 2001
From: rwjuk <rwj@serenity.uk.net>
Date: Wed, 5 Jul 2017 22:33:59 +0100
Subject: [PATCH 02/13] Code quality improvements

---
 src/openrct2/world/sprite.c | 54 ++++++++++++++++++-------------------
 1 file changed, 27 insertions(+), 27 deletions(-)

diff --git a/src/openrct2/world/sprite.c b/src/openrct2/world/sprite.c
index 7f4cf210a4..086d8b8306 100644
--- a/src/openrct2/world/sprite.c
+++ b/src/openrct2/world/sprite.c
@@ -259,15 +259,13 @@ void sprite_clear_all_unused()
     spriteIndex = gSpriteListHead[SPRITE_LIST_NULL];
     while (spriteIndex != SPRITE_INDEX_NULL) {
         sprite = &get_sprite(spriteIndex)->unknown;
-        nextSpriteIndex = sprite->next;
-        previousSpriteIndex = sprite->previous;
-        memset(sprite, 0, sizeof(rct_sprite));
-        sprite->sprite_identifier = SPRITE_IDENTIFIER_NULL;
-        sprite->next = nextSpriteIndex;
-        sprite->previous = previousSpriteIndex;
+        sprite_reset(sprite);
         sprite->linked_list_type_offset = SPRITE_LIST_NULL * 2;
-        sprite->sprite_index = spriteIndex;
-        sprite->next_in_quadrant = (sprite->next_in_quadrant == 0) ? SPRITE_INDEX_NULL : sprite->next_in_quadrant;
+
+        // sprite->next_in_quadrant will only end up as zero owing to corruption
+        // most likely due to previous builds not preserving it when resetting sprites
+        // We reset it to SPRITE_INDEX_NULL to prevent cycles in the sprite lists
+        if (sprite->next_in_quadrant == 0) { sprite->next_in_quadrant = SPRITE_INDEX_NULL; }
         _spriteFlashingList[spriteIndex] = false;
         spriteIndex = nextSpriteIndex;
     }
@@ -290,6 +288,7 @@ static void sprite_reset(rct_unk_sprite *sprite)
     sprite->next_in_quadrant = next_in_quadrant;
     sprite->previous = prev;
     sprite->sprite_index = sprite_index;
+    sprite->sprite_identifier = SPRITE_IDENTIFIER_NULL;
 }
 
 // Resets all sprites in SPRITE_LIST_NULL list
@@ -827,7 +826,7 @@ bool sprite_get_flashing(rct_sprite *sprite)
     return _spriteFlashingList[sprite->unknown.sprite_index];
 }
 
-static bool sprite_is_in_cycle(uint16 sprite_idx, bool fix)
+static bool sprite_is_in_cycle(uint16 sprite_idx)
 {
     if (sprite_idx == SPRITE_INDEX_NULL)
     {
@@ -856,19 +855,13 @@ static bool sprite_is_in_cycle(uint16 sprite_idx, bool fix)
         if (fast == slow)
         {
             cycled = true;
-            if (fix)
-            {
-                rct_sprite * spr = get_sprite(sprite_idx);
-                // There may be a better solution than simply setting this to 0xFFFF
-                spr->unknown.next = SPRITE_INDEX_NULL;
-            }
             break;
         }
     }
     return cycled;
 }
 
-static bool sprite_is_in_cycle_quadrant(uint16 sprite_idx, bool fix)
+static bool sprite_is_in_quadrant_cycle(uint16 sprite_idx)
 {
     if (sprite_idx == SPRITE_INDEX_NULL)
     {
@@ -897,13 +890,6 @@ static bool sprite_is_in_cycle_quadrant(uint16 sprite_idx, bool fix)
         if (fast == slow)
         {
             cycled = true;
-            if (fix)
-            {
-                rct_sprite * spr = get_sprite(sprite_idx);
-
-                // There may be a better solution than simply setting this to 0xFFFF
-                spr->unknown.next_in_quadrant = SPRITE_INDEX_NULL;
-            }
             break;
         }
     }
@@ -912,8 +898,15 @@ static bool sprite_is_in_cycle_quadrant(uint16 sprite_idx, bool fix)
 
 bool check_for_sprite_list_cycles(bool fix)
 {
-    for (sint32 i = 0; i < 6; i++) {
+    for (sint32 i = 0; i < NUM_SPRITE_LISTS; i++) {
         if (sprite_is_in_cycle(gSpriteListHead[i], fix)) {
+            if (fix)
+            {
+                rct_sprite * spr = get_sprite(gSpriteListHead[i]);
+
+                // There may be a better solution than simply setting this to 0xFFFF
+                spr->unknown.next = SPRITE_INDEX_NULL;
+            }
             return true;
         }
     }
@@ -922,10 +915,17 @@ bool check_for_sprite_list_cycles(bool fix)
 
 bool check_for_spatial_index_cycles(bool fix)
 {
-    for (size_t i = 0; i < 0x10000; i++) {
-        if (sprite_is_in_cycle_quadrant(gSpriteSpatialIndex[i], fix)) {
+    for (sint32 i = 0; i < SPATIAL_INDEX_LOCATION_NULL; i++) {
+        if (sprite_is_in_quadrant_cycle(gSpriteSpatialIndex[i])) {
+            if (fix)
+            {
+                rct_sprite * spr = get_sprite(gSpriteSpatialIndex[i]);
+
+                // There may be a better solution than simply setting this to 0xFFFF
+                spr->unknown.next_in_quadrant = SPRITE_INDEX_NULL;
+            }
             return true;
         }
     }
     return false;
-}
\ No newline at end of file
+}

From 28789c37673a9d3a531b27203ac94d0389074b13 Mon Sep 17 00:00:00 2001
From: rwjuk <rwj@serenity.uk.net>
Date: Wed, 5 Jul 2017 22:42:45 +0100
Subject: [PATCH 03/13] Restore resetting of sprite index

---
 src/openrct2/world/sprite.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/openrct2/world/sprite.c b/src/openrct2/world/sprite.c
index 086d8b8306..2da130a022 100644
--- a/src/openrct2/world/sprite.c
+++ b/src/openrct2/world/sprite.c
@@ -262,6 +262,10 @@ void sprite_clear_all_unused()
         sprite_reset(sprite);
         sprite->linked_list_type_offset = SPRITE_LIST_NULL * 2;
 
+        // This shouldn't be necessary, as sprite_reset() preserves the index
+        // but it has been left in as a safety net in case the index isn't set correctly
+        sprite->sprite_index = spriteIndex;
+
         // sprite->next_in_quadrant will only end up as zero owing to corruption
         // most likely due to previous builds not preserving it when resetting sprites
         // We reset it to SPRITE_INDEX_NULL to prevent cycles in the sprite lists

From 72a1ad89a33edd10731d30d5caf8937e575e68ff Mon Sep 17 00:00:00 2001
From: rwjuk <rwj@serenity.uk.net>
Date: Wed, 5 Jul 2017 22:44:05 +0100
Subject: [PATCH 04/13] Remove superfluous parameter

---
 src/openrct2/world/sprite.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/openrct2/world/sprite.c b/src/openrct2/world/sprite.c
index 2da130a022..fb81396841 100644
--- a/src/openrct2/world/sprite.c
+++ b/src/openrct2/world/sprite.c
@@ -903,7 +903,7 @@ static bool sprite_is_in_quadrant_cycle(uint16 sprite_idx)
 bool check_for_sprite_list_cycles(bool fix)
 {
     for (sint32 i = 0; i < NUM_SPRITE_LISTS; i++) {
-        if (sprite_is_in_cycle(gSpriteListHead[i], fix)) {
+        if (sprite_is_in_cycle(gSpriteListHead[i])) {
             if (fix)
             {
                 rct_sprite * spr = get_sprite(gSpriteListHead[i]);

From 67cbe2a2a18c26f1dcfab266afaf06e47b19984a Mon Sep 17 00:00:00 2001
From: rwjuk <rwj@serenity.uk.net>
Date: Wed, 5 Jul 2017 22:55:46 +0100
Subject: [PATCH 05/13] Fix compilation issues and restore line that went
 astray

---
 src/openrct2/world/sprite.c | 57 +++++++++++++++++++------------------
 1 file changed, 29 insertions(+), 28 deletions(-)

diff --git a/src/openrct2/world/sprite.c b/src/openrct2/world/sprite.c
index fb81396841..0efa585d52 100644
--- a/src/openrct2/world/sprite.c
+++ b/src/openrct2/world/sprite.c
@@ -247,34 +247,6 @@ const char * sprite_checksum()
 
 #endif // DISABLE_NETWORK
 
-/**
- * Clears all the unused sprite memory to zero. Probably so that it can be compressed better when saving.
- *  rct2: 0x0069EBA4
- */
-void sprite_clear_all_unused()
-{
-    rct_unk_sprite *sprite;
-    uint16 spriteIndex, nextSpriteIndex, previousSpriteIndex;
-
-    spriteIndex = gSpriteListHead[SPRITE_LIST_NULL];
-    while (spriteIndex != SPRITE_INDEX_NULL) {
-        sprite = &get_sprite(spriteIndex)->unknown;
-        sprite_reset(sprite);
-        sprite->linked_list_type_offset = SPRITE_LIST_NULL * 2;
-
-        // This shouldn't be necessary, as sprite_reset() preserves the index
-        // but it has been left in as a safety net in case the index isn't set correctly
-        sprite->sprite_index = spriteIndex;
-
-        // sprite->next_in_quadrant will only end up as zero owing to corruption
-        // most likely due to previous builds not preserving it when resetting sprites
-        // We reset it to SPRITE_INDEX_NULL to prevent cycles in the sprite lists
-        if (sprite->next_in_quadrant == 0) { sprite->next_in_quadrant = SPRITE_INDEX_NULL; }
-        _spriteFlashingList[spriteIndex] = false;
-        spriteIndex = nextSpriteIndex;
-    }
-}
-
 static void sprite_reset(rct_unk_sprite *sprite)
 {
     // Need to retain how the sprite is linked in lists
@@ -295,6 +267,35 @@ static void sprite_reset(rct_unk_sprite *sprite)
     sprite->sprite_identifier = SPRITE_IDENTIFIER_NULL;
 }
 
+/**
+* Clears all the unused sprite memory to zero. Probably so that it can be compressed better when saving.
+*  rct2: 0x0069EBA4
+*/
+void sprite_clear_all_unused()
+{
+    rct_unk_sprite *sprite;
+    uint16 spriteIndex, nextSpriteIndex;
+
+    spriteIndex = gSpriteListHead[SPRITE_LIST_NULL];
+    while (spriteIndex != SPRITE_INDEX_NULL) {
+        sprite = &get_sprite(spriteIndex)->unknown;
+        nextSpriteIndex = sprite->next;
+        sprite_reset(sprite);
+        sprite->linked_list_type_offset = SPRITE_LIST_NULL * 2;
+
+        // This shouldn't be necessary, as sprite_reset() preserves the index
+        // but it has been left in as a safety net in case the index isn't set correctly
+        sprite->sprite_index = spriteIndex;
+
+        // sprite->next_in_quadrant will only end up as zero owing to corruption
+        // most likely due to previous builds not preserving it when resetting sprites
+        // We reset it to SPRITE_INDEX_NULL to prevent cycles in the sprite lists
+        if (sprite->next_in_quadrant == 0) { sprite->next_in_quadrant = SPRITE_INDEX_NULL; }
+        _spriteFlashingList[spriteIndex] = false;
+        spriteIndex = nextSpriteIndex;
+    }
+}
+
 // Resets all sprites in SPRITE_LIST_NULL list
 void reset_empty_sprites()
 {

From 00848ca629fde254cf793cba87c5b7013aa0081f Mon Sep 17 00:00:00 2001
From: rwjuk <rwj@serenity.uk.net>
Date: Wed, 5 Jul 2017 23:28:14 +0100
Subject: [PATCH 06/13] Correct sprite cycle fixing algorithm

---
 src/openrct2/world/sprite.c | 32 ++++++++++++++++----------------
 1 file changed, 16 insertions(+), 16 deletions(-)

diff --git a/src/openrct2/world/sprite.c b/src/openrct2/world/sprite.c
index 0efa585d52..8f3b835a95 100644
--- a/src/openrct2/world/sprite.c
+++ b/src/openrct2/world/sprite.c
@@ -831,7 +831,7 @@ bool sprite_get_flashing(rct_sprite *sprite)
     return _spriteFlashingList[sprite->unknown.sprite_index];
 }
 
-static bool sprite_is_in_cycle(uint16 sprite_idx)
+static rct_sprite * find_sprite_list_cycle(uint16 sprite_idx)
 {
     if (sprite_idx == SPRITE_INDEX_NULL)
     {
@@ -840,7 +840,7 @@ static bool sprite_is_in_cycle(uint16 sprite_idx)
     const rct_sprite * fast = get_sprite(sprite_idx);
     const rct_sprite * slow = fast;
     bool increment_slow = false;
-    bool cycled = false;
+    rct_sprite * cycle_start = NULL;
     while (fast->unknown.sprite_index != SPRITE_INDEX_NULL)
     {
         // increment fast every time, unless reached the end
@@ -859,14 +859,14 @@ static bool sprite_is_in_cycle(uint16 sprite_idx)
         increment_slow = !increment_slow;
         if (fast == slow)
         {
-            cycled = true;
+            cycle_start = get_sprite(slow->unknown.sprite_index);
             break;
         }
     }
-    return cycled;
+    return cycle_start;
 }
 
-static bool sprite_is_in_quadrant_cycle(uint16 sprite_idx)
+static rct_sprite * find_sprite_quadrant_cycle(uint16 sprite_idx)
 {
     if (sprite_idx == SPRITE_INDEX_NULL)
     {
@@ -875,7 +875,7 @@ static bool sprite_is_in_quadrant_cycle(uint16 sprite_idx)
     const rct_sprite * fast = get_sprite(sprite_idx);
     const rct_sprite * slow = fast;
     bool increment_slow = false;
-    bool cycled = false;
+    rct_sprite * cycle_start = NULL;
     while (fast->unknown.sprite_index != SPRITE_INDEX_NULL)
     {
         // increment fast every time, unless reached the end
@@ -894,23 +894,23 @@ static bool sprite_is_in_quadrant_cycle(uint16 sprite_idx)
         increment_slow = !increment_slow;
         if (fast == slow)
         {
-            cycled = true;
+            cycle_start = get_sprite(slow->unknown.sprite_index);
             break;
         }
     }
-    return cycled;
+    return cycle_start;
 }
 
 bool check_for_sprite_list_cycles(bool fix)
 {
     for (sint32 i = 0; i < NUM_SPRITE_LISTS; i++) {
-        if (sprite_is_in_cycle(gSpriteListHead[i])) {
+        rct_sprite * cycle_start = find_sprite_list_cycle(gSpriteListHead[i]);
+        if (cycle_start != NULL) 
+        {
             if (fix)
             {
-                rct_sprite * spr = get_sprite(gSpriteListHead[i]);
-
                 // There may be a better solution than simply setting this to 0xFFFF
-                spr->unknown.next = SPRITE_INDEX_NULL;
+                cycle_start->unknown.next = SPRITE_INDEX_NULL;
             }
             return true;
         }
@@ -921,13 +921,13 @@ bool check_for_sprite_list_cycles(bool fix)
 bool check_for_spatial_index_cycles(bool fix)
 {
     for (sint32 i = 0; i < SPATIAL_INDEX_LOCATION_NULL; i++) {
-        if (sprite_is_in_quadrant_cycle(gSpriteSpatialIndex[i])) {
+        rct_sprite * cycle_start = find_sprite_quadrant_cycle(gSpriteSpatialIndex[i]);
+        if (cycle_start != NULL)
+        {
             if (fix)
             {
-                rct_sprite * spr = get_sprite(gSpriteSpatialIndex[i]);
-
                 // There may be a better solution than simply setting this to 0xFFFF
-                spr->unknown.next_in_quadrant = SPRITE_INDEX_NULL;
+                cycle_start->unknown.next_in_quadrant = SPRITE_INDEX_NULL;
             }
             return true;
         }

From 07faba7aa538062311b82563b38c6827a283eafd Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Janiszewski?= <janisozaur@gmail.com>
Date: Thu, 6 Jul 2017 22:17:02 +0200
Subject: [PATCH 07/13] Re-add sprites that got disconnected in a looped list

---
 src/openrct2/world/sprite.c | 70 +++++++++++++++++++++++++++++++++++--
 1 file changed, 67 insertions(+), 3 deletions(-)

diff --git a/src/openrct2/world/sprite.c b/src/openrct2/world/sprite.c
index 8f3b835a95..107dab6c5e 100644
--- a/src/openrct2/world/sprite.c
+++ b/src/openrct2/world/sprite.c
@@ -901,16 +901,65 @@ static rct_sprite * find_sprite_quadrant_cycle(uint16 sprite_idx)
     return cycle_start;
 }
 
+static bool index_is_in_list(uint16 index, enum SPRITE_LIST sl)
+{
+    uint16 sprite_index = gSpriteListHead[sl];
+    while (sprite_index != SPRITE_INDEX_NULL)
+    {
+        if (sprite_index == index)
+        {
+            return true;
+        }
+        sprite_index = get_sprite(sprite_index)->unknown.next;
+    }
+    return false;
+}
+
+static bool index_is_in_spatial_list(uint16 index, sint32 quadrant)
+{
+    uint16 sprite_index = gSpriteSpatialIndex[quadrant];
+    while (sprite_index != SPRITE_INDEX_NULL)
+    {
+        if (sprite_index == index)
+        {
+            return true;
+        }
+        sprite_index = get_sprite(sprite_index)->unknown.next_in_quadrant;
+    }
+    return false;
+}
+
 bool check_for_sprite_list_cycles(bool fix)
 {
     for (sint32 i = 0; i < NUM_SPRITE_LISTS; i++) {
         rct_sprite * cycle_start = find_sprite_list_cycle(gSpriteListHead[i]);
-        if (cycle_start != NULL) 
+        if (cycle_start != NULL)
         {
             if (fix)
             {
-                // There may be a better solution than simply setting this to 0xFFFF
+                // Fix head list, but only in reverse order
+                // This is likely not needed, but just in case
+                get_sprite(gSpriteListHead[i])->unknown.previous = SPRITE_INDEX_NULL;
+
+                // Store the leftover part of cycle to be fixed
+                uint16 cycle_next = cycle_start->unknown.next;
+
+                // Break the cycle
                 cycle_start->unknown.next = SPRITE_INDEX_NULL;
+
+                // Now re-add remainder of the cycle back to list, safely.
+                // Add each sprite to the list until we encounter one that is already part of the list.
+                while (!index_is_in_list(cycle_next, i))
+                {
+                    rct_sprite * spr = get_sprite(cycle_next);
+
+                    cycle_start->unknown.next = cycle_next;
+                    spr->unknown.previous = cycle_start->unknown.sprite_index;
+                    cycle_next = spr->unknown.next;
+                    spr->unknown.next = SPRITE_INDEX_NULL;
+                    cycle_start = spr;
+                }
+
             }
             return true;
         }
@@ -926,8 +975,23 @@ bool check_for_spatial_index_cycles(bool fix)
         {
             if (fix)
             {
-                // There may be a better solution than simply setting this to 0xFFFF
+                // Store the leftover part of cycle to be fixed
+                uint16 cycle_next = cycle_start->unknown.next_in_quadrant;
+
+                // Break the cycle
                 cycle_start->unknown.next_in_quadrant = SPRITE_INDEX_NULL;
+
+                // Now re-add remainder of the cycle back to list, safely.
+                // Add each sprite to the list until we encounter one that is already part of the list.
+                while (!index_is_in_list(cycle_next, i))
+                {
+                    rct_sprite * spr = get_sprite(cycle_next);
+
+                    cycle_start->unknown.next_in_quadrant = cycle_next;
+                    cycle_next = spr->unknown.next_in_quadrant;
+                    spr->unknown.next_in_quadrant = SPRITE_INDEX_NULL;
+                    cycle_start = spr;
+                }
             }
             return true;
         }

From bed085d329e9e706e7e906b233fb1689861ba066 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Janiszewski?= <janisozaur@gmail.com>
Date: Thu, 6 Jul 2017 22:18:05 +0200
Subject: [PATCH 08/13] Remove unneeded sprite reset

This is done in all exports now
---
 src/openrct2/rct2/S6Exporter.cpp | 1 -
 1 file changed, 1 deletion(-)

diff --git a/src/openrct2/rct2/S6Exporter.cpp b/src/openrct2/rct2/S6Exporter.cpp
index 086456d20f..6e2951c61b 100644
--- a/src/openrct2/rct2/S6Exporter.cpp
+++ b/src/openrct2/rct2/S6Exporter.cpp
@@ -449,7 +449,6 @@ extern "C"
         }
 
         map_reorganise_elements();
-        sprite_clear_all_unused();
         viewport_set_saved_view();
 
         bool result     = false;

From 029aea0fc252995c521abe5bb2b35fa841929d23 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Janiszewski?= <janisozaur@gmail.com>
Date: Thu, 6 Jul 2017 22:20:07 +0200
Subject: [PATCH 09/13] Remove reset_empty_sprites

Leave in sprite_clear_all_unused as the safer version
---
 src/openrct2/rct2/S6Exporter.cpp |  4 +---
 src/openrct2/world/sprite.c      | 16 ----------------
 src/openrct2/world/sprite.h      |  1 -
 3 files changed, 1 insertion(+), 20 deletions(-)

diff --git a/src/openrct2/rct2/S6Exporter.cpp b/src/openrct2/rct2/S6Exporter.cpp
index 6e2951c61b..45c8dda18d 100644
--- a/src/openrct2/rct2/S6Exporter.cpp
+++ b/src/openrct2/rct2/S6Exporter.cpp
@@ -184,9 +184,7 @@ void S6Exporter::Export()
     // Might as well reset them in here to zero out the space and improve
     // compression ratios. Especially useful for multiplayer servers that
     // use zlib on the sent stream.
-    {
-        reset_empty_sprites();
-    }
+    sprite_clear_all_unused();
     for (sint32 i = 0; i < MAX_SPRITES; i++)
     {
         memcpy(&_s6.sprites[i], get_sprite(i), sizeof(rct_sprite));
diff --git a/src/openrct2/world/sprite.c b/src/openrct2/world/sprite.c
index 107dab6c5e..f153d3ef6d 100644
--- a/src/openrct2/world/sprite.c
+++ b/src/openrct2/world/sprite.c
@@ -296,22 +296,6 @@ void sprite_clear_all_unused()
     }
 }
 
-// Resets all sprites in SPRITE_LIST_NULL list
-void reset_empty_sprites()
-{
-    uint16 spriteIndex;
-    spriteIndex = gSpriteListHead[SPRITE_LIST_NULL];
-    while (spriteIndex != SPRITE_INDEX_NULL)
-    {
-        rct_unk_sprite *sprite = &(get_sprite(spriteIndex))->unknown;
-        spriteIndex = sprite->next;
-        if (sprite->sprite_identifier == SPRITE_IDENTIFIER_NULL)
-        {
-            sprite_reset(sprite);
-        }
-    }
-}
-
 /*
 * rct2: 0x0069EC6B
 * bl: if bl & 2 > 0, the sprite ends up in the MISC linked list.
diff --git a/src/openrct2/world/sprite.h b/src/openrct2/world/sprite.h
index 1cbe3415d0..5f0f4c7848 100644
--- a/src/openrct2/world/sprite.h
+++ b/src/openrct2/world/sprite.h
@@ -419,7 +419,6 @@ extern uint16 *gSpriteListCount;
 extern uint16 gSpriteSpatialIndex[0x10001];
 
 rct_sprite *create_sprite(uint8 bl);
-void reset_empty_sprites();
 void reset_sprite_list();
 void reset_sprite_spatial_index();
 void sprite_clear_all_unused();

From ab95988c661e388c84066c4c805fbe602412201a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Janiszewski?= <janisozaur@gmail.com>
Date: Thu, 6 Jul 2017 22:28:10 +0200
Subject: [PATCH 10/13] Improve error reporting for detected cycles

---
 src/openrct2/rct2/S6Exporter.cpp |  5 ++++-
 src/openrct2/world/sprite.c      | 12 ++++++------
 src/openrct2/world/sprite.h      |  4 ++--
 3 files changed, 12 insertions(+), 9 deletions(-)

diff --git a/src/openrct2/rct2/S6Exporter.cpp b/src/openrct2/rct2/S6Exporter.cpp
index 45c8dda18d..91d4018af1 100644
--- a/src/openrct2/rct2/S6Exporter.cpp
+++ b/src/openrct2/rct2/S6Exporter.cpp
@@ -152,7 +152,10 @@ void S6Exporter::Save(IStream * stream, bool isScenario)
 
 void S6Exporter::Export()
 {
-    openrct2_assert(!(check_for_spatial_index_cycles(false) || check_for_sprite_list_cycles(false)), "A sprite cycle exists.");
+    sint32 spatial_cycle = check_for_spatial_index_cycles(false);
+    sint32 regular_cycle = check_for_sprite_list_cycles(false);
+    openrct2_assert(spatial_cycle == -1, "Sprite cycle exists in spatial list %d", spatial_cycle);
+    openrct2_assert(regular_cycle == -1, "Sprite cycle exists in regular list %d", regular_cycle);
     _s6.info = gS6Info;
     uint32 researchedTrackPiecesA[128];
     uint32 researchedTrackPiecesB[128];
diff --git a/src/openrct2/world/sprite.c b/src/openrct2/world/sprite.c
index f153d3ef6d..98f80778c9 100644
--- a/src/openrct2/world/sprite.c
+++ b/src/openrct2/world/sprite.c
@@ -913,7 +913,7 @@ static bool index_is_in_spatial_list(uint16 index, sint32 quadrant)
     return false;
 }
 
-bool check_for_sprite_list_cycles(bool fix)
+sint32 check_for_sprite_list_cycles(bool fix)
 {
     for (sint32 i = 0; i < NUM_SPRITE_LISTS; i++) {
         rct_sprite * cycle_start = find_sprite_list_cycle(gSpriteListHead[i]);
@@ -945,13 +945,13 @@ bool check_for_sprite_list_cycles(bool fix)
                 }
 
             }
-            return true;
+            return i;
         }
     }
-    return false;
+    return -1;
 }
 
-bool check_for_spatial_index_cycles(bool fix)
+sint32 check_for_spatial_index_cycles(bool fix)
 {
     for (sint32 i = 0; i < SPATIAL_INDEX_LOCATION_NULL; i++) {
         rct_sprite * cycle_start = find_sprite_quadrant_cycle(gSpriteSpatialIndex[i]);
@@ -977,8 +977,8 @@ bool check_for_spatial_index_cycles(bool fix)
                     cycle_start = spr;
                 }
             }
-            return true;
+            return i;
         }
     }
-    return false;
+    return -1;
 }
diff --git a/src/openrct2/world/sprite.h b/src/openrct2/world/sprite.h
index 5f0f4c7848..ba132ff8f7 100644
--- a/src/openrct2/world/sprite.h
+++ b/src/openrct2/world/sprite.h
@@ -477,7 +477,7 @@ const char *sprite_checksum();
 
 void sprite_set_flashing(rct_sprite *sprite, bool flashing);
 bool sprite_get_flashing(rct_sprite *sprite);
-bool check_for_sprite_list_cycles(bool fix);
-bool check_for_spatial_index_cycles(bool fix);
+sint32 check_for_sprite_list_cycles(bool fix);
+sint32 check_for_spatial_index_cycles(bool fix);
 #endif
 

From 12500dd802caf45aebcc08546c25e0817f21f3d3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Janiszewski?= <janisozaur@gmail.com>
Date: Fri, 7 Jul 2017 00:12:54 +0200
Subject: [PATCH 11/13] Fixup the disjoint null sprites

---
 src/openrct2/rct2/S6Exporter.cpp |  6 +++++
 src/openrct2/rct2/S6Importer.cpp |  6 +++++
 src/openrct2/world/sprite.c      | 44 ++++++++++++++++++++++++++++++++
 src/openrct2/world/sprite.h      |  1 +
 4 files changed, 57 insertions(+)

diff --git a/src/openrct2/rct2/S6Exporter.cpp b/src/openrct2/rct2/S6Exporter.cpp
index 91d4018af1..bbf977651f 100644
--- a/src/openrct2/rct2/S6Exporter.cpp
+++ b/src/openrct2/rct2/S6Exporter.cpp
@@ -154,8 +154,14 @@ void S6Exporter::Export()
 {
     sint32 spatial_cycle = check_for_spatial_index_cycles(false);
     sint32 regular_cycle = check_for_sprite_list_cycles(false);
+    sint32 disjoint_sprites_count = fix_disjoint_sprites();
     openrct2_assert(spatial_cycle == -1, "Sprite cycle exists in spatial list %d", spatial_cycle);
     openrct2_assert(regular_cycle == -1, "Sprite cycle exists in regular list %d", regular_cycle);
+    // This one is less harmful, no need to assert for it ~janisozaur
+    if (disjoint_sprites_count > 0)
+    {
+        log_error("Found %d disjoint null sprites", disjoint_sprites_count);
+    }
     _s6.info = gS6Info;
     uint32 researchedTrackPiecesA[128];
     uint32 researchedTrackPiecesB[128];
diff --git a/src/openrct2/rct2/S6Importer.cpp b/src/openrct2/rct2/S6Importer.cpp
index d12ea1071e..3f71867f5c 100644
--- a/src/openrct2/rct2/S6Importer.cpp
+++ b/src/openrct2/rct2/S6Importer.cpp
@@ -417,6 +417,12 @@ public:
         // We try to fix the cycles on import, hence the 'true' parameter
         check_for_sprite_list_cycles(true);
         check_for_spatial_index_cycles(true);
+        sint32 disjoint_sprites_count = fix_disjoint_sprites();
+        // This one is less harmful, no need to assert for it ~janisozaur
+        if (disjoint_sprites_count > 0)
+        {
+            log_error("Found %d disjoint null sprites", disjoint_sprites_count);
+        }
     }
 
     void Initialise()
diff --git a/src/openrct2/world/sprite.c b/src/openrct2/world/sprite.c
index 98f80778c9..67058f0cb0 100644
--- a/src/openrct2/world/sprite.c
+++ b/src/openrct2/world/sprite.c
@@ -951,6 +951,50 @@ sint32 check_for_sprite_list_cycles(bool fix)
     return -1;
 }
 
+/**
+ * Finds and fixes null sprites that are not reachable via SPRITE_LIST_NULL list.
+ *
+ * @return count of disjoint sprites found
+ */
+sint32 fix_disjoint_sprites()
+{
+    // Find reachable sprites
+    bool reachable[MAX_SPRITES] = {};
+    uint16 sprite_idx = gSpriteListHead[SPRITE_LIST_NULL];
+    rct_sprite * null_list_tail = NULL;
+    while (sprite_idx != SPRITE_INDEX_NULL)
+    {
+        reachable[sprite_idx] = true;
+        // cache the tail, so we don't have to walk the list twice
+        null_list_tail = get_sprite(sprite_idx);
+        sprite_idx = null_list_tail->unknown.next;
+    }
+
+    sint32 count = 0;
+
+    // Find all null sprites
+    for (sprite_idx = 0; sprite_idx < MAX_SPRITES; sprite_idx++)
+    {
+        rct_sprite * spr = get_sprite(sprite_idx);
+        if (spr->unknown.sprite_identifier == SPRITE_IDENTIFIER_NULL)
+        {
+            openrct2_assert(null_list_tail != NULL, "Null list is empty, yet found null sprites");
+            spr->unknown.sprite_index = sprite_idx;
+            if (!reachable[sprite_idx])
+            {
+                // Add the sprite directly to the list
+                null_list_tail->unknown.next = sprite_idx;
+                spr->unknown.next = SPRITE_INDEX_NULL;
+                spr->unknown.previous = null_list_tail->unknown.sprite_index;
+                null_list_tail = spr;
+                count++;
+                reachable[sprite_idx] = true;
+            }
+        }
+    }
+    return count;
+}
+
 sint32 check_for_spatial_index_cycles(bool fix)
 {
     for (sint32 i = 0; i < SPATIAL_INDEX_LOCATION_NULL; i++) {
diff --git a/src/openrct2/world/sprite.h b/src/openrct2/world/sprite.h
index ba132ff8f7..44929c96ca 100644
--- a/src/openrct2/world/sprite.h
+++ b/src/openrct2/world/sprite.h
@@ -479,5 +479,6 @@ void sprite_set_flashing(rct_sprite *sprite, bool flashing);
 bool sprite_get_flashing(rct_sprite *sprite);
 sint32 check_for_sprite_list_cycles(bool fix);
 sint32 check_for_spatial_index_cycles(bool fix);
+sint32 fix_disjoint_sprites();
 #endif
 

From a3414502a6ec67580139d162a1620b8f83f5bd2e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Janiszewski?= <janisozaur@gmail.com>
Date: Fri, 7 Jul 2017 00:17:36 +0200
Subject: [PATCH 12/13] Update network version

---
 src/openrct2/network/network.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/openrct2/network/network.h b/src/openrct2/network/network.h
index 7404efc04d..3efb600404 100644
--- a/src/openrct2/network/network.h
+++ b/src/openrct2/network/network.h
@@ -55,7 +55,7 @@ extern "C" {
 // This define specifies which version of network stream current build uses.
 // It is used for making sure only compatible builds get connected, even within
 // single OpenRCT2 version.
-#define NETWORK_STREAM_VERSION "29"
+#define NETWORK_STREAM_VERSION "30"
 #define NETWORK_STREAM_ID OPENRCT2_VERSION "-" NETWORK_STREAM_VERSION
 
 #ifdef __cplusplus

From 8d02ce37b56fda04c25ac9b2fdd0d82918cf3f8d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Janiszewski?= <janisozaur@gmail.com>
Date: Fri, 7 Jul 2017 07:56:45 +0200
Subject: [PATCH 13/13] Fix compilation on MSVC

---
 src/openrct2/world/sprite.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/openrct2/world/sprite.c b/src/openrct2/world/sprite.c
index 67058f0cb0..53b88d3ec0 100644
--- a/src/openrct2/world/sprite.c
+++ b/src/openrct2/world/sprite.c
@@ -959,7 +959,7 @@ sint32 check_for_sprite_list_cycles(bool fix)
 sint32 fix_disjoint_sprites()
 {
     // Find reachable sprites
-    bool reachable[MAX_SPRITES] = {};
+    bool reachable[MAX_SPRITES] = { false };
     uint16 sprite_idx = gSpriteListHead[SPRITE_LIST_NULL];
     rct_sprite * null_list_tail = NULL;
     while (sprite_idx != SPRITE_INDEX_NULL)