Remove README.jaydiff.md and now-empty utils/ and tests/ packages

jay_diff docs, tests, and data live in jayfield/jaypy; the __init__.py files for the empty local packages are no longer needed. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
2026-05-27 14:00:32 +02:00
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 # jay_diff — JSON diff / patch library
 `utils/jay_diff.py` is a lightweight, pure-Python library for computing and
 applying structural diffs between two JSON-compatible dicts.  It is used by
 `collect.py` and `gui_client.py` to compress the push-server stream: instead
 of retransmitting full snapshots on every poll, the server sends only the
 fields that changed since the previous snapshot.
 ---
 ## Concepts
 A **diff** is itself a plain dict.  It describes what changed from an **old**
 state to a **new** state using one or more of these keys:
 | Key | Meaning |
 |-----|---------|
 | `update_add` | Fields whose value changed *or* that are new in `new` |
 | `update` | Fields whose value changed (must already exist in `old`) |
 | `add` | Fields that are new in `new` (did not exist in `old`) |
 | `delete` | Fields that were removed or whose sub-keys were pruned |
 `update_add` combines `update` and `add` into a single key (the mode used
 everywhere in this project).  The separate `update` / `add` form is
 available via `combine_upd_add=False` for completeness.
 An **empty dict `{}`** returned by `jay_diff_full` means the two states are
 identical.
 ### Delete-delta encoding
 Values inside the `delete` sub-dict carry a specific meaning:
 | Value in delete delta | Effect when merged |
 |-----------------------|--------------------|
 | `None` | The **entire key** is deleted from `old` |
 | A **non-empty dict** | Recurse into the sub-dict and delete those specific sub-keys; the parent key is kept (possibly as `{}`) |
 | A **scalar** (string, int, …) | The key is deleted from `old` |
 | `{}` (empty dict) | The key is deleted from `old` |
 This distinction matters when the new state has an empty dict `{}` at a
 location where the old state had a populated dict.  In that case the delete
 delta carries the populated sub-keys as a non-empty dict — after merging,
 the parent key is preserved as `{}`.  If the key is entirely absent from
 the new state, `None` is stored in the delta, and the merge removes the key
 completely.
 ---
 ## API
 ### Diff functions
 ```python
 jay_diff_full(old, new, combine_upd_add=True) -> dict
 ```
 The main entry point.  Computes a complete diff from `old` to `new`.
 - `combine_upd_add=True` (default) — produces `update_add` + `delete`
 - `combine_upd_add=False` — produces `update` + `add` + `delete`
 Returns `{}` when `old == new`.
 ```python
 jay_diff_add(old, new) -> dict
 ```
 Returns keys present in `new` but absent from `old` (additions only).
 ```python
 jay_diff_del(old, new) -> dict
 ```
 Returns keys present in `old` but absent from `new`, or sub-keys that were
 removed from a shared dict.  Uses `None` as sentinel for entirely-absent
 dict keys.
 ```python
 jay_diff_upd(old, new) -> dict
 ```
 Returns keys present in both `old` and `new` whose values differ (updates
 only; new keys are ignored).
 ```python
 jay_diff_upd_add(old, new) -> dict
 ```
 Like `jay_diff_upd` but also includes keys new in `new` — equivalent to
 `jay_diff_full(..., combine_upd_add=True)` minus the delete section.
 ---
 ### Merge functions
 ```python
 jay_merge_full(old, diff) -> dict
 ```
 The main entry point.  Applies a diff produced by `jay_diff_full` to `old`
 and returns the reconstructed new state.
 **Bootstrap passthrough:** if `old` is `{}` and `diff` contains none of the
 recognised diff keys, `diff` is returned as-is.  This is how the first
 message from the push server is handled: the server sends
 `jay_diff_full({}, record[0])`, which looks like a plain snapshot, and the
 client stores it directly.
 Applies operations in order: `update` → `update_add` → `add` → `delete`.
 ```python
 jay_merge_update(old, new) -> dict   # apply an update/update_add delta
 jay_merge_add(old, new)    -> dict   # apply an add delta
 jay_merge_delete(old, new) -> dict   # apply a delete delta
 ```
 Low-level helpers; prefer `jay_merge_full` unless you need fine-grained
 control.
 ---
 ## Round-trip guarantee
 ```
 jay_merge_full(old, jay_diff_full(old, new)) == new
 ```
 This holds for all inputs where values are JSON-compatible scalars (strings,
 numbers, booleans, `None`, lists) or nested dicts thereof.  It is verified
 by `tests/test_jay_diff_real_data.py` against 674 consecutive pairs of real
 vehicle snapshots.
 ---
 ## Wire format
 `jay_diff_full` / `jay_merge_full` are used by the push server when
 `--diff` is enabled.  The diff dict is serialised as a newline-delimited
 JSON line.
 - The **first** message to a new client is `jay_diff_full({}, record[0])`,
  which always contains `update_add` with every field of the first record.
 - Subsequent messages contain only the changed fields.
 - The format is self-describing: a message with none of the diff keys is
  treated as a full snapshot by `jay_merge_full` (bootstrap passthrough).
 `null` in JSON round-trips to Python `None`, so the delete sentinel survives
 serialisation transparently.
 ---
 ## Tests
 ```
 tests/test_jay_diff.py           — 104 unit tests (all diff/merge functions,
                                    corner cases: type changes, empty dicts,
                                    deep nesting, round-trips)
 tests/test_jay_diff_real_data.py — parametrised over 675 real VW snapshots
                                    in tests/data/ (674 consecutive pairs):
                                    round-trip, identical diff = {}, and
                                    empty-diff-is-noop checks
 ```
 Run with:
 ```bash
 pytest tests/
 ```
 ---
 ## Bug history
 ### `jay_merge_delete`: crash on aliased dicts (fixed)
 `jay_diff_rev` (used internally by the old `jay_diff_del`) stored references
 to sub-dicts of `_old` directly in the delete delta without copying them.
 When `jay_merge_full(old, jay_diff_full(old, new))` was called, some
 sub-dicts in the delete delta were the same Python objects as sub-dicts in
 `old`.  Iterating `_new.items()` while deleting from `_old` (the same
 object) raised `RuntimeError: dictionary changed size during iteration`.
 **Fix:** `list(_new.items())` snapshots the items before the loop.  Also,
 `jay_diff_del` was rewritten as a standalone recursive function that never
 stores aliases to its inputs.
 ### `jay_merge_delete`: empty dict wrongly pruned (fixed)
 When the new state had an empty dict `{}` at a location where the old state
 had a non-empty dict, `jay_merge_delete` would delete all sub-keys and then
 prune the now-empty parent key entirely.  The result differed from `new`,
 which retained the parent key as `{}`.
 Root cause: the old delta format stored the full old sub-dict for both "key
 absent from new" and "key present but empty in new" — the two cases were
 indistinguishable at merge time.
 **Fix:** `jay_diff_del` now stores `None` for dict keys entirely absent from
 `new`, and a non-empty sub-dict for partial deletions.  `jay_merge_delete`
 no longer prunes: a `None` (or scalar) value in the delta is the explicit
 signal to remove the parent key.