45 lines
1.9 KiB
Markdown
45 lines
1.9 KiB
Markdown
# ADR 005 — Branch-Preserving Story History
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**Status:** Accepted; implementation direction validated in Phase 0B
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## Decision
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Returning to an earlier story point preserves abandoned future history rather than destructively erasing it.
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The selected implementation model uses a **movable active head over retained lineage**:
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- Undo moves the head backward,
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- Redo moves it forward along the retained continuation,
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- accepted turns are not deleted by ordinary Undo,
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- a new write after moving backward creates a new continuation on first divergent write,
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- the displaced future remains retained/disposable,
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- normal UI presents Undo/Redo/Retry/Save Point rather than branch-management concepts.
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## Context
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The user must be able to recover from unwanted story developments and explore alternatives while retaining prior work.
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Phase 0B found that AI-DnD's shipped Undo was destructive even though its tree/take infrastructure was otherwise strong. A disposable spike demonstrated non-destructive head-cursor Undo/Redo using existing lineage chokepoints and branch creation while preserving branch-scoped memory isolation.
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## Alternatives Considered
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- destructive undo,
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- overwrite-in-place editing,
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- complete copy of campaigns for every retry,
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- branch-preserving turn graph with movable active head.
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## Reason
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The selected model provides recovery, experimentation, auditability, and Redo without unnecessary campaign duplication or a complicated user-facing branch workflow.
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## Consequences
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- turn identity/parentage remains first-class,
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- active head and retained tip are distinct concepts,
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- state restore is lineage-aware,
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- retry/edit/add-take must honor fork-if-behind-head behavior,
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- summaries/memories must respect active lineage/head,
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- named checkpoints point to durable story positions,
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- abandoned history is marked disposable but is not automatically cleaned up in v1,
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- export/import must preserve the active head coordinate as well as the retained history graph.
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