NotAlterra/docs/DECISIONS.md

Design Decisions

This file captures the rationale behind significant architecture and format choices, so the reasoning is preserved for future maintainers (including yourself six months from now).

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Sentinel File vs config.ini (v0.3.2)

Problem

config.ini persisted the save folder path to disk, including the user's filesystem-username. This is a privacy concern — paths are visible next to the binary.

Decision

Remove config.ini entirely. The save folder is session-only — set it each time via Set save folder. The disclaimer acceptance is tracked via a 0-byte sentinel file (NotAlterra_LICENSE_ACCEPTED) alongside the binary.

Rationale

Privacy — no paths written to disk. The save folder exists only in memory while the tool runs.

Simplicity — no config parsing, no INI format to maintain, no migration code for renamed keys.

Sentinel, not config — a 0-byte file communicates exactly one boolean (disclaimer accepted). It cannot grow into a configuration file over time. The format intentionally prevents scope creep.

What was removed:

• AppConfig struct (save_path, ini_path, save_scan, disclaimer_accepted)
• load_config() / save_config() with INI parsing
• Cached ini_path — now derived from save folder at runtime
• Four integration tests for config round-trips

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Manual Path Entry vs Auto-Discovery (v0.3.0)

Problem

Auto-discovery scanned user profiles and system directories for Subnautica 2 save folders. This is a privacy concern — it traverses /home/* (Linux) and C:\Users\* (Windows).

Decision

Replace full auto-discovery with manual path entry via Set save folder. Keep a lightweight quick_discover() that checks only the current user's default install paths at startup.

Rationale

Privacy — no scanning of other users' profiles or system drives.

Current-user convenience — quick_discover() checks 1 path on Windows, 3 paths on Linux, all within the current user's own directories. Returns the first match silently, no UI. If nothing is found, the user enters their path manually.

Discovery module retained — validate_custom_path() and derive_ini_path() still live in discovery.rs for the manual entry flow. The aggressive scan functions (discover_save_folders(), scan_other_users(), walk_for_subnautica()) are removed.

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tar.gz Backup Format (v0.4.0)

Problem

Directory-tree backups (NotAlterra_Backups/notalterra_copy_<timestamp>/) are messy, uncompressed, and have no integrity guarantees.

Decision

One tar.gz archive per backup event, stored in backups/saves/.

Rationale

No vendor lock-in — standard tar -xzf recovers data without the tool. If NotAlterra stops working, the user's backups are still accessible with standard system utilities.

Single file per event — reduces clutter. One backup = one file, not a directory tree with 15+ loose save files.

Compression — save files compress well (~75MB → ~20MB). Reduces disk usage without user effort.

Pure Rust implementation — tar + flate2 crates, 200M+ downloads combined. No system dependencies, no external tools.

Per-entry restore — extracting a single save file from the archive does not require decompressing the entire archive.

Safeguards

• Atomic write: backup written to .tmp file, then atomically renamed. Power loss during backup discards a temp file, not a real backup.
• Integrity check after creation: archive is read back and validated before reporting success.
• SHA256 manifest: a MANIFEST file inside each archive records the hash of every contained save file. On restore, each extracted file is verified against its expected hash — silent bit-rot detected before bad data reaches the save folder.
• Fuzz target: round-trip fuzzing (create archive from diverse inputs → restore → compare) catches logic bugs.

Migration

Existing NotAlterra_Backups/ directory-tree backups are detected and transparently imported on first run after upgrade. No manual migration required.