Usdc Value Decoder

usdc-value-decoder

Use this skill when implementing or verifying USDC Crate value representation decoding.

Spec Sources

• USD Core Spec Core File Formats section Binary
• USD Core Spec Document Data Model field and value representation sections
• contracts/value.schema.json
• docs/usdc-spec-errata.md — pinned interpretations where the AOUSD prose is ambiguous or diverges from real .usdc content. In particular: the Array / Inlined / Compressed flag bits live in the high bits of byte 7 of the 8-byte LE value-representation word (masks 0x80 / 0x40 / 0x20); a decoder must not also accept the low-bit layout as a fallback.

Pinned commit: 6b4f01f2d6d46b01507481524eb516b4ef358c31

Provides

• Crate value representation decoding
• Token, string, and path index resolution
• Inlined scalar values
• Offset-backed scalar and array values
• Dictionaries
• Vectors used by first fixtures
• Authored listOp values
• Specifier and variability values

Contract

This skill converts low-level Crate value representations into the existing canonical value shape used by contracts/value.schema.json.

It depends on usdc-binary-format for byte and buffer decoding, on usd-tokens for token identity, on usd-foundational-values for canonical value shapes, on usd-paths for namespace path strings, and on usd-listops-authored for listOp boundaries.

It may use decoded Crate token, string, and path tables supplied by usdc-spec-parser, but it must not own Crate section traversal or document spec storage.

Production-family obligations are factored into:

• contracts/usdc-productions/crate-value-representations.contract.json — wire format of the 8-byte value-representation word and the type table
• contracts/usdc-productions/attribute-spec-mapping.contract.json — shared ownership of TimeSamples (type 46), VariantSelectionMap (type 45), and LayerOffsetVector (type 49) decoding, called out as co_owners: ["usdc-value-decoder"] in that contract
• contracts/usdc-productions/parser-diagnostics-mapping.contract.json — mapping of value-level diagnostics (MalformedUsdcValueRepresentation → TypeMismatch, UnsupportedUsdcFeature → UnsupportedFeature) at the layer-open dispatch boundary
• contracts/document-model-productions/attribute-spec.contract.json — the format-neutral value-shape rules for TimeSamples (native time→FieldValue map; no string-keyed JSON), Spline (native Spline record; no Python dict payloads), default-vs-typeName agreement, and value-block sentinel semantics

Boundary Guards

Defer byte-level bounds, endian, bootstrap, TOC, compression, and integer-array decoding to usdc-binary-format.

Defer section-to-document mapping and spec creation to usdc-spec-parser.

Defer storage to usd-document-model through usdc-spec-parser.

Do not call USDA text lexers, USDA value parsers, or USDA spec parsers.

Do not implement composition, value resolution, schema fallback, clips, package opening, or USDC writing.

Test Obligations

Baseline (covered by goldens/unit/usdc-value-decoder/value-decoder.json)

• decode token and string values used by layer metadata and type names
• decode int, float, double, and double3 values used by attributes
• decode specifier and variability values
• decode dictionaries used by layer metadata fixtures
• decode relationship target PathListOp values
• preserve the canonical JSON value shape already used by USDA dumps
• decode one inlined scalar and one offset-backed scalar of the same target type so the inlined/offset distinction is exercised
• decode at least one offset-backed token[] array sourced from token indices in TOKENS (not inlined)

Document-model value-shape compliance

Each obligation references the format-neutral value-shape rule in contracts/document-model-productions/attribute-spec.contract.json plus the USDC-specific mapping in contracts/usdc-productions/attribute-spec-mapping.contract.json:

• TimeSamples (Crate type 46) — decode as a native time→FieldValue map keyed by numeric time ordinates. String-keyed JSON storage is a contract violation per attribute-spec.contract.json#fields.timeSamples. None-valued samples (Crate ValueBlock type 51) decode to value-block sentinels. Exercised by goldens/integration/usdc-single-layer/attribute-time-samples.json (pending Phase D fixture).
• VariantSelectionMap (Crate type 45) — decode the wire format (uint64 count + alternating uint32 string-index key + uint32 string-index value) as a native variantSelection map; empty-string values are preserved, not dropped. Exercised by goldens/integration/usdc-single-layer/variant-specs.json (pending Phase D fixture).
• LayerOffsetVector (Crate type 49) — decode as a sequence of native Retiming records (offset:double, scale:double), one per subLayer item; raw dictionaries / JSON payloads are forbidden per layer-spec.contract.json#fields.subLayerOffsets. Exercised by goldens/integration/usdc-single-layer/layer-metadata.json (pending Phase D).
• PathListOp (Crate type 34) subfield semantics — explicit / prepend / append / delete subfields are decoded independently from the listOp bitmask byte; the parser must not collapse a single-item explicit listOp to a bare path. Used by relationship targetPaths and attribute connectionPaths; format-neutral rule lives in common-metadata.contract.json#listop_subfields.
• Relocates (Crate type 58) — at Crate minor ≥ 11, decode as a native source-PathRef → target-PathRef map. At minor < 11, emit UnsupportedUsdcFeature (which reduces to UnsupportedFeature at dump boundary per parser-diagnostics-mapping.contract.json).
• default vs typeName agreement — when decoding a default field on an attribute spec, the value-representation type id must match the attribute's typeName per the type-id table in attribute-spec-mapping.contract.json#storage_mapping.default_field_type_table_reference. Mismatch emits MalformedUsdcValueRepresentation, which reduces to TypeMismatch.

Structural integrity (universal)

• read every value representation as a single 8-byte little-endian word and dispatch on the 1-byte type and 1-byte flags; multiple per-byte reads, per-field-name dispatch, or section byte string scans are contract violations
• the Array / Inlined / Compressed flag bits live in the HIGH bits of byte 7 (masks 0x80 / 0x40 / 0x20) per crate-value-representations.contract.json#flags and docs/usdc-spec-errata.md#1. Accepting the low-bit layout (0x01/0x02/0x04) as a fallback is a contract violation.
• emit a clear UnsupportedUsdcFeature diagnostic for value types introduced at Crate 0.10.0 or later (Splines, Relocates, PathExpression) when encountered under a lower minor; placeholder JSON is a contract violation
• must not dispatch values by field-name substrings or by section byte string scans
• the value-decoder source code must be a single shared implementation consumed by both usdc_parser.cpp and usdc_value_decoder_adapter.cpp; a parser-internal duplicate decoder is a contract violation enforced by the no_inline_value_decoder_in_parser lint rule in contracts/lint/usdc-single-layer.lint.json

Performance

The value-decoder unit must satisfy contracts/performance/usdc.performance.json. Value representations are read as a single 8-byte little-endian word and dispatched in O(1) on type and flags. Offset-backed scalar and array values are returned as non-owning views into the opened resource byte view until canonicalization runs.