From 12e4620c419f35932130560870fb9505cd8aade0 Mon Sep 17 00:00:00 2001 From: Jens Ahrensfeld Date: Wed, 27 May 2026 12:29:51 +0200 Subject: [PATCH] =?UTF-8?q?Add=20README.jaydiff.md=20=E2=80=94=20jay=5Fdif?= =?UTF-8?q?f=20library=20documentation?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Covers API reference, diff/delete-delta encoding semantics, round-trip guarantee, wire format usage, test overview, and bug history from the recent aliasing crash and empty-dict pruning fixes. Co-Authored-By: Claude Sonnet 4.6 --- README.jaydiff.md | 195 ++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 195 insertions(+) create mode 100644 README.jaydiff.md diff --git a/README.jaydiff.md b/README.jaydiff.md new file mode 100644 index 0000000..2976b5c --- /dev/null +++ b/README.jaydiff.md @@ -0,0 +1,195 @@ +# 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. \ No newline at end of file