📐mova-json-merge

The groupant beside tools/mova-json/src/merge.rs in the Mova repository: what the code computes, what it never does, how to go back from its outputs, its invariants, cost and checks.

Facts

Components

Obernants

The typed relations around the forward model: the behaviour complement (what the code never does), the reverse reconstruction (every input compatible with an output) and specialised views.

Model text

domain

A git repository with git on PATH; JSON files in layouts the value library reproduces; for merge3 a merge in progress (MERGE_HEAD present); for rebind-groupant a base commit holding the previous source bytes and sidecar manifests under FILE.groupant/.

forward

matching_blocks(a, b): longest common subsequence after trimming common prefix/suffix, O(n*m) table on the middle bounded by 40M cells, returned as maximal blocks. Merger.merge(base, ours, theirs): equal sides or one unchanged side resolve directly; objects merge key-wise (one-side delete of an unchanged value applies, delete vs change conflicts, theirs additions go to the sorted place in sorted objects, else after their predecessor); arrays whose elements all have unique (path,id,section,type,name) keys merge per key (path-sorted arrays keep sorted insertion); other arrays merge by diff3 hunks against base: disjoint hunks apply, identical hunks apply once, two pure insertions at one point concatenate ours-then-theirs only when the whole result keeps unique elements; anything else is a conflict and ours stands in. A both-sides change of sha256 in an object with a path is settled by hashing the working-tree file (resolve_bound: .//../ relative to the manifest directory, others repo root then manifest directory). merge3_file reads base/HEAD/MERGE_HEAD through git show, takes the layout of HEAD (else MERGE_HEAD), writes the working-tree file only with zero conflicts, optionally refreshes every {path, sha256} binding. rebind_manifest refreshes listed paths in source_bindings and with add inserts missing ones at the sorted place. rebind_groupant maps every sidecar coverage.byte_range through the line diff of base -> current source (exact inside unchanged lines; a start moves to the end of the previous unchanged block, an end to the start of the next), rehashes covered bytes and the source, and rebinds tracked *.groupant/**/manifest.json objects {path: FILE, sha256: old}.

invariant

• merge3 never writes a file with a conflict; exit status 3 marks conflicts.
• Arrays never gain a repeated element by joining two insertions (argv safety: two different --add-dir X additions conflict).
• rebind_manifest changes only bindings of listed paths; unlisted stale hashes stay stale (no silent refresh); a listed bound file that is missing is an error.
• rebind_groupant reports every fragment whose covered text changed (TEXT CHANGED) instead of claiming the model still holds; obernant revision/subject_revision fields are not touched.

cost

Merge is O(size of the three values) plus diff3 tables O(n*m) per unkeyed array (bounded; over the bound the array is a conflict). git show is three subprocesses per file. rebind_groupant is one git show, one line diff (bounded), one pass per sidecar manifest and a git ls-files scan reading only manifests whose text contains the old hash.

behavior-complement

Not done: staging (git add), committing, resolving conflicts by preference, merging non-JSON files, rebinding revision/subject_revision, mapping ranges across intra-line edits exactly (an edited line maps to block boundaries), detecting moved code, checking that a model still describes changed text, touching contracts.json hashes.

reverse-reconstruction

A clean merge output is compatible with many (base, ours, theirs) triples; with the three inputs retained in git, the result is reproducible (deterministic order rules). A rebound byte range does not recover the old range without the base commit; with it, the old range is the input and the report line records both.

diff3-insertion-join

Type: partial_exclusion. Domain: unkeyed arrays changed on both sides at one insertion point. Excluded output: a merged array with a repeated element. Negative control: dropping the uniqueness check (filtering theirs by ours) merged --add-dir A and --add-dir B into --add-dir A B and turned merge3_true_conflicts_are_red red (2026-11-05).

checks

Unit tests: clean merges (keys, keyed nodes with delete, path-sorted bindings, disjoint sequence hunks, unique joint appends), true conflicts (scalar, delete vs change, keyed delete vs change, argv joint insertion, overlapping replacement) with a same-change control; offset mapping with an identity-map red control; rebind with stale-unlisted, missing-file and non-canonical red controls; rebind_groupant on a temporary git repo with TEXT CHANGED control; merge3_file inside a real conflicting git merge (hash conflict settled from the working tree, joint list insertion). Mutation controls run 2026-11-05: raw escapes dropped, unsorted insert, delete-vs-change ignored, identity map, no rehash: each turned at least one test red.

epoch-memory

2026-11-05: replaces scratch merge3.py, mergejson.py, bind.py and rebind.py. merge3.py was asymmetric and resolved true conflicts silently to one side; this version reports them. difflib's SequenceMatcher is replaced by an exact LCS, so ambiguous line matches can map differently from the Python helper.

This page in Markdown → https://md.groupants.elamur.ai/free/en/mova/tools/mova-json/src/merge.rs.groupant/