# GOL-015: Rewind Life

Each position keeps a private occupancy history: local crowding can send it backward through its own past until another neighbor count, the oldest retained state, or a duration cap releases it.

[Open simulation](https://100gol.com/experiments/015) · [Directory](https://100gol.com/experiments)

## What to explore

The experiment should make local time direction visible. Forward cells should
still form a recognizable configurable Life field, while rewind triggers produce
coherent flashes and trails of cells replaying states that no longer agree with
their present neighborhood.

A correct implementation is not sufficient if rewind almost never occurs, if it
immediately releases everywhere, or if history behaves like a decorative color
instead of determining occupancy. The authored default must sustain both forward
and rewinding positions and must exercise more than one release path.

## Rules, update order and replay

The world is a square grid. Every position reads the eight-cell Moore neighborhood
from one frozen current board, with either toroidal or bounded edges. Updates are
fully synchronous and make no random calls after construction.

Each position owns visible occupancy plus a private stack of its last 32 forward
states, a forward/rewinding mode, the number of history-controlled steps in the
current rewind, and last-transition evidence. The stack includes the current
occupancy. Once full, a new forward result discards the oldest retained state.

On each Step:

1. A forward position whose current neighbor count matches the reverse-trigger
   mask and whose history contains an earlier state immediately enters rewind.
   It removes its current history entry and emits the new top entry. Ordinary
   Life is not evaluated for that position on that Step.
2. A forward position without an actionable reverse trigger evaluates the
   configured birth or survival mask and appends that result to its history.
   A trigger at the oldest retained state cannot rewind and therefore falls
   through to ordinary Life.
3. A rewinding position first checks the configured forward-release mask. A
   match releases it immediately; ordinary Life determines and appends that
   Step's result.
4. If no forward condition matches but the position has already consumed the
   configured maximum number of rewind steps, it is released through the same
   ordinary-Life path.
5. Otherwise it removes one history entry and emits the newly exposed prior
   occupancy. Reaching the oldest retained entry releases the position for the
   following Step.

The release checks use the current visible neighbors even though a rewinding
position ignores ordinary Life. Reverse and forward masks may overlap: their
meaning depends on the position's current direction, so an overlap can create a
one-step rewind followed by release when the count persists. The duration is a
maximum number of history-controlled Steps, not a guaranteed minimum.

Step advances this contract exactly once. Reset reconstructs the selected board,
sets every private history to its one-entry initial occupancy, places every cell
forward, clears rewind counters/evidence, and returns generation to zero. Exact
replay requires the initial-condition ID or random seed/density, field profile,
topology, four masks, duration, generation, occupancy, history bits and lengths,
direction modes, rewind counters, and last-event evidence.

## Reading the simulation

The normal color view uses lime for forward life, violet for rewinding life,
indigo for a rewinding dead position, cream for condition release, blue for
duration release, pale mint for arrival at the oldest history entry, and coral
when reverse playback exposes a dead state. A simplified view
shows only dead and alive.

Inspection pauses simulation, outlines the selected cell and its eight frozen
neighbors, and reports current occupancy, current direction, neighbor count,
history depth, rewind steps, the relevant trigger/release match, ordinary Life's
hypothetical result, the exact next result, and the reason for it. The canvas
label, legend, instructions, and written inspector repeat these meanings so
color is never the only explanation.

## Reproduce a starting configuration

[Download example JSON](https://100gol.com/api/experiments/015/configuration). Import it using Share & reproduce in the simulation controls. It starts paused at Step zero. Files contain rules and initial conditions, not evolved boards or manual edits. See [format, limitations and automated access](https://100gol.com/agents).