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AGENTS.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working
+with code in this repository.
+
+## Project Overview
+
+cura-te-ipsum is a space-efficient incremental backup utility for Linux and
+macOS. It creates hardlink-based snapshots similar to Time Machine/rsnapshot,
+where unchanged files share inodes between backups to minimize storage.
+
+## Development Commands
+
+### Environment Setup
+Project uses `uv` for Python version and dependency management:
+```bash
+uv sync --all-extras --dev
+```
+
+### Testing
+```bash
+uv run pytest                    # run all tests
+uv run pytest -v                 # verbose output
+uv run pytest tests/test_fs.py   # run specific test file
+```
+
+### Installation
+```bash
+uv pip install .          # install from source
+```
+
+### Running the Tool
+```bash
+cura-te-ipsum -b /path/to/backups /path/to/source
+cura-te-ipsum -b /backups /src1 /src2 --dry-run --verbose
+```
+
+## Architecture
+
+### Core Modules
+
+**curateipsum/backup.py**: Main backup orchestration
+- `initiate_backup()`: Main entry point - coordinates hardlinking from
+  previous backup and rsync from sources
+- `cleanup_old_backups()`: Implements grandfather-father-son retention
+  (daily/weekly/monthly/yearly) with configurable thresholds
+- Lock management (`set_backups_lock`, `release_backups_lock`): Prevents
+  concurrent backups using PID-based lock file
+- Backup markers (`set_backup_marker`, `_is_backup`): Tracks completed vs
+  incomplete backups
+
+**curateipsum/fs.py**: Low-level filesystem operations
+- `rsync()`: Pure Python rsync implementation - syncs source to destination
+  with delete support
+- `rsync_ext()`: Wrapper for external rsync command
+- `hardlink_dir()`: Recursively hardlinks entire directory tree (Python or
+  external cp)
+- `nest_hardlink()`: Hardlinks individual files to `.backup_delta` directory
+  preserving structure
+- `scantree()`: Recursive directory iterator
+- `copy_direntry()`, `update_direntry()`, `rm_direntry()`: DirEntry
+  manipulation utilities
+
+**curateipsum/cli.py**: Command-line interface
+- Argument parsing and validation
+- Platform check (Linux/macOS only)
+- Logging setup and execution flow
+
+### Key Data Structures
+
+**PseudoDirEntry**: Emulates os.DirEntry for paths that don't exist yet or
+for consistency in handling both real and pseudo entries. Used extensively
+in backup operations.
+
+**Actions enum**: Tracks filesystem changes during rsync (NOTHING, DELETE,
+REWRITE, UPDATE_TIME, UPDATE_PERM, UPDATE_OWNER, CREATE, ERROR).
+
+### Backup Process Flow
+
+1. Acquire lock file (`.backups_lock`) with PID
+2. Clean up old backups per retention policy
+3. Hardlink entire previous backup directory to new timestamped directory
+4. Remove backup markers from hardlinked copy
+5. Delete `.backup_delta` directory from hardlinked copy
+6. Rsync each source to new backup (overwrites changed files, breaking
+   hardlinks)
+7. For each changed file, hardlink to `.backup_delta` directory
+8. Set backup marker (`.backup_finished_TIMESTAMP`)
+9. Release lock
+
+### Important Implementation Details
+
+**Retention policy**: `cleanup_old_backups()` implements threshold-based
+retention where each threshold is inclusive. For example, `keep_weekly=1`
+on Thursday keeps one backup from current week AND one from previous week.
+
+**Delta tracking**: Changed files are hardlinked to `.backup_delta/` within
+each backup to track what changed in that specific backup snapshot.
+
+**Hardlink breakage**: When rsync overwrites a hardlinked file, it breaks
+the hardlink, creating a new inode. This is how changed files get tracked
+while unchanged files continue sharing inodes.
+
+**External tools**: Can use external rsync (--external-rsync) or GNU cp/gcp
+for hardlinking (--external-hardlink) instead of Python implementations.
+macOS requires `gcp` (GNU cp) for external hardlinking.
+
+**Dry run behavior**: Creates backup directory and performs operations but
+removes directory at end without setting backup marker.
+
+## Platform Specifics
+
+- Linux: Uses standard `cp` command
+- macOS: Requires `gcp` (GNU coreutils) for external hardlinking
+- Only these platforms are supported (checked in cli.py)
+
+## Testing
+
+Tests use unittest.TestCase with mock.patch for datetime manipulation.
+Focus is on backup cleanup logic with various retention scenarios.
+Tests create temporary backup directories and verify correct backups are
+kept/removed.
+
+## Version Management
+
+Version is automatically derived from git tags using `setuptools_scm`.
+The version file `curateipsum/_version.py` is auto-generated during build
+and should not be committed to version control.
+
+To create a new release, tag the commit:
+```bash
+git tag -a v0.0.4 -m "Release 0.0.4"
+git push origin v0.0.4
+```

CLAUDE.md

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+AGENTS.md