LFC2 circuit storage

LFC2 is a compact, canonical storage format for the existing Circuit object. It does not change circuit arithmetic, canonical circuit IDs, transcripts, or proof bytes. LFC1 remains the default writer format and is always accepted by the reader.

An LFC2 record starts with the four bytes 4c 46 43 32 (LFC2). The header values follow immediately, in order: field ID, nv, nc, public-input count, subfield boundary, input count, layer count, and constant count. They use minimally encoded unsigned LEB128 varints. Constants are fixed width canonical field encodings. Each layer stores logw, wire count, a deduplicated zig-zag delta table, a segment dictionary, and segment tokens. The record ends with the unchanged 32-byte canonical circuit ID.

Constants and terms are emitted in the existing Quad traversal order. A reader reconstructs the same compact delta-table representation, so normal evaluation continues to traverse compact terms without materializing an expanded EQuad.

All LFC1 dimensional and resource limits apply to LFC2: at most 10,000 layers, 5,000,000 wires/constants, 20,000,000 terms in a layer and in total, and the configured ByteCursor byte/allocation/element limits. The reader rejects truncated data, a bad magic, non-minimal or overflowing varints, delta underflow/overflow, invalid indices, noncanonical field elements, invalid dimensions, ID mismatches, and trailing bytes.

Read-only upstream Rust interop

The Google vendor/longfellow-zk submodule is read-only. Reciprocal fixture coverage lives in the first-party vendor/longfellow-zk-interop Cargo adapter, which uses local path dependencies to the published upstream crates without changing their source. make lfc2-cross-language-test writes the C++ fixture, has the adapter validate its ID and exact consumption, emits the canonical Rust fixture, and then has C++ validate the reciprocal file. The qualification matrix invokes this target.

If a development checkout retains an unpublished detached submodule object, restore the parent repository's published gitlink and run git submodule update --init --recursive vendor/longfellow-zk. Do not patch, commit, or publish the submodule to recover the test. The adapter's own build artifacts are separate; leave existing vendor/longfellow-zk/rust/target/ contents untouched.

Operators opt in by passing CircuitFormat::kLfc2 to CircuitWriter; the default is kLfc1. The magic is sufficient to identify files for metrics or rollback. Reverting that selection writes LFC1 again; no circuit conversion, proof conversion, or historical artifact migration is needed.