Establishes the kb-embed trait crate so concrete embedding adapters
(p3-2 fastembed, future ollama-embed/candle) target a stable surface.
Pure re-export of kb_core::{Embedder, EmbeddingInput, EmbeddingKind,
EmbeddingModelId, EmbeddingVersion} plus a feature-gated deterministic
mock for downstream tests.
MockEmbedder (cfg(feature = "mock"), default OFF):
- Per-component hash recipe: blake3(seed_le8 || kind_byte ||
text_len_le8 || text || i_le8). Length-prefixed text avoids the
domain-separation ambiguity where two (text, i) pairs could shift
bytes between text tail and the i field.
- Document = 0u8, Query = 1u8 — same text different kind yields
different vectors (mirrors e5 prefix behaviour).
- Per component: blake3 first 8 bytes → u64 → reinterpret as i64 →
f64/i64::MAX → f32. i64::MIN gives -1.0000000000000002 which f32
rounds to -1.0; range [-1, 1] holds.
- L2 unit-normalised. Norm sums in f64 (avoid catastrophic precision
loss) before f32 cast. Zero-norm guard skips the divide.
- with_seed(...) constructor lets two embedders share identity but
produce different vectors — useful for downstream parametric tests.
Helpers:
- assert_vector_shape(vecs, dims) — len + finite check.
- assert_unit_norm(vecs, tolerance) — caller-supplied tolerance;
5e-4 documented as safe for dims=384 under f32 epsilon × √dims.
Tests:
- cargo test -p kb-embed (no features): 2 reexport/dyn-dispatch tests.
- cargo test -p kb-embed --features mock: 7 tests including 100-case
proptest asserting len == dims, all finite, ‖v‖ ≈ 1.0 within
tolerance, Doc(text) byte-equal Doc(text), Doc(text) ≠ Query(text),
Doc(text1) ≠ Doc(text2).
- All 220 workspace tests pass; clippy clean for both default and
mock-on feature configurations.
Symbol gating: nm on the release rlib confirms zero MockEmbedder
symbols under default features; three trait impl symbols under
--features mock. Spec invariant "release builds MUST NOT include
MockEmbedder" verified at the symbol level.
Allowed deps respected: kb-core, kb-config, serde, thiserror, tracing,
plus anyhow (forced by trait return type) and blake3 (justified by
the determinism contract; already in workspace lockfile via kb-core).
No fastembed/ort/tokenizers anywhere.
Out of scope: real adapter (p3-2), reranker traits (P+).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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