RZero

RZero: Return by Death

RZero is a mod inspired by the "Return by Death" concept from Re:Zero. It replaces the standard death and respawn mechanics with a world rollback: upon death, the game state returns to the last saved checkpoint.

How it works

The mod records the state of loaded chunks, entities, and players. When a rollback is triggered upon death or manually, the world state is rewound.

To prevent divergence from the original timeline, internal random number generator (RNG) logic is adjusted. Repeating the exact same actions after a rollback causes all probabilistic events (mob behavior, projectile paths, loot generation, or entity spawn coordinates) to occur identically to the initial timeline.

What is restored

• Terrain and blocks: Placed, broken, or exploded blocks (including TNT and creeper explosions) revert to their previous state. Block entities (chests, furnaces), scheduled block and fluid ticks, block events (piston movements, bell ringing), and villager workstation points of interest (POI) are restored.
• Entities and mobs: Position coordinates, health, attack targets, navigation paths, damage timers, passenger chains (jockeys, mobs in boats and minecarts), and AI RAM state (villager memory, trades and schedules, active behavior tasks) are restored.
• Players: Inventory, ender chest contents, coordinates, view angles, health, hunger, experience, potion effects, item cooldowns, abilities, recipe book, spawn point, score, open menu windows, and the item held on the cursor.
• Global world state: World time, weather, active raids, Ender Dragon fight phases, scoreboard data, custom boss bars, world borders, and force-loaded chunks.

Determinism

• Reproducibility of combat and random events: Chest loot generation, mob drops, knockback directions, and raid group spawn positions are fully reproducible.
• Natural spawning: Mob spawning algorithms are aligned with the world seed and time, so repeating the same actions spawns the same creatures.

Client optimizations

• Client chunk cache: The client displays locally cached chunks and lighting while server data synchronizes over the network.
• GPU geometry cache: Compiled chunk geometry is stored in video memory (VRAM), reducing render pipeline stalls during rollbacks.
• Visual adjustments: Smooths camera transitions, resets damage tilt and item-switching animations, and preserves search queries and interface tabs.

Auto-save modes

• Interval mode: Creates checkpoints on a fixed timer or at random intervals within a configured range.
• Adaptive mode: Analyzes surrounding threat levels (player health, attacking mobs, nearby hazards, available escape routes) to create checkpoints before critical danger or after combat ends.
• Boss defeat saves: Automatically saves progress upon defeating the Ender Dragon or Wither.
• Hazard protection: Prevents checkpoint creation in unrecoverable fatal conditions (such as falling into the void or burning in lava).

Multiplayer: Key player system

In multiplayer, rollbacks apply globally across the server. The key player configuration determines whose death triggers a rollback:

• Fixed: Rollback is tied to a single designated player.
• Multi: A list of multiple designated key players.
• Everyone: Death of any online player initiates a global rollback.
• Rotating: The key player role cycles periodically among online players.
• Rollback cooldown: A configurable cooldown between rollbacks to prevent death loops.
• Offline players: If a player disconnects before a rollback, data changes are written directly to their .dat file on disk, placing them at the checkpoint state upon rejoining.

Commands and controls

Commands:

• /rzero set — Manually create a checkpoint.
• /rzero anchor status — Displays the active key player mode, current anchors, and cooldown status.
• /rzero anchor cooldown <seconds> — Sets the rollback cooldown duration after death.
• /rzero anchor mode fixed [player] — Sets a single key player.
• /rzero anchor mode multi <players> — Sets a list of key players.
• /rzero anchor mode everyone — Enables rollbacks for any player death.
• /rzero anchor mode rotating <seconds> — Enables periodic rotation of the key player.

Hotkeys:
Keybindings for manual saving and manual rollback can be configured in the standard game controls menu.

Game rules:

• rzeroAutoSave — Enables or disables automatic checkpoints.
• rzeroUseAdaptiveMode — Toggles adaptive save mode.
• rzeroAdaptiveInterval — Required safety duration before auto-saving in adaptive mode (in seconds).
• rzeroFixedInterval — Interval between auto-saves in seconds.
• rzeroUseRandomInterval — Enables random intervals between saves.
• rzeroRandomIntervalMin / rzeroRandomIntervalMax — Range bounds for random intervals.
• rzeroPlayRollbackSound — Plays a sound effect during time rewind.

Configuration

Rollback parameters, determinism settings, and client rendering options can be configured via the Cloth Config interface (supporting both standard and advanced display modes).