---
title: "world-school-student-state-retained-codec — Mova code model"
description: "Mova code model of world/src/school.rs: components with their labels, obernants and the model text."
url: "https://groupants.elamur.ai/free/en/mova/world/src/school.rs.groupant/student-state.groupant/retained-codec.groupant/"
---
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# world-school-student-state-retained-codec

The groupant beside world/src/school.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

- **kind:** groupant_fragment
- **status:** partial source bound model
- **source:** [`world/src/school.rs`](https://github.com/ElamurAI/mova-free/blob/main/world/src/school.rs)
- **source SHA-256:** `06aaec6f562c2acc…`
- **covers:** Decode lines in order: ignore # and widths below five; parse four current columns with invalid numbers mapped to zero; widths >=9 also replace accumulator columns. A later five-column duplicate replaces w but leaves an earlier acc row. Encode sorted union keys; emit nonzero w or any explicit acc, and only emit accumulator columns when present. (bytes 17898–19673)
- **part of:** [world-school-student-state](https://md.groupants.elamur.ai/free/en/mova/world/src/school.rs.groupant/student-state.groupant/)
- **repository folder:** [`world/src/school.rs.groupant/student-state.groupant/retained-codec.groupant`](https://github.com/ElamurAI/mova-free/tree/main/world/src/school.rs.groupant/student-state.groupant/retained-codec.groupant)
- **repository commit:** `855da7bd4a7b`

> **Implementation material.** A Mova code model describes this code at one source revision: partial models, finite checks and their stated scope. It is not a reviewed general exposition, and its labels are shown as written, never raised.

## Components

| component | label | where |
|---|---|---|
| **approximant** | partial source model | [forward](#forward) |
| **ast** | partial | `../../syntax.json` |
| **composition** | partial source model | [composition](#composition) |
| **cost** | partial source model | [cost](#cost) |
| **descriptant** | partial source model | [epoch-memory](#epoch-memory) |
| **invariant** | partial source model | [invariant](#invariant) |
| **tests** | partial source model | [checks](#checks) |

## 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.

| id | type | label | where |
|---|---|---|---|
| `world-school-student-state-retained-codec-behavior-complement` | behavior_complement | partial source model | [behavior-complement](#behavior-complement) |
| `world-school-student-state-retained-codec-reverse-reconstruction` | reverse_reconstruction | partial source model | [reverse-reconstruction](#reverse-reconstruction) |
| `world-school-student-state-retained-codec-accumulator_activation` | state_transition | partial source model | [accumulator_activation](#accumulator-activation) |
| `world-school-student-state-retained-codec-duplicate_width` | order_relation | partial source model | [duplicate_width](#duplicate-width) |
| `world-school-student-state-retained-codec-canonical_projection` | identity_relation | partial source model | [canonical_projection](#canonical-projection) |
| `world-school-student-state-retained-codec-row_order` | observation_relation | partial source model | [row_order](#row-order) |

## Model text


Wave34, 2026-10-31. Source-bound helper model; no universal proof or approximation-error certificate. Historical parent epoch remains intact.

### forward

Decode lines in order: ignore # and widths below five; parse four current columns with invalid numbers mapped to zero; widths >=9 also replace accumulator columns. A later five-column duplicate replaces w but leaves an earlier acc row. Encode sorted union keys; emit nonzero w or any explicit acc, and only emit accumulator columns when present.

### invariant

For format-safe keys (no tab/newline and no leading #), numeric table selection survives canonical encoding. Current-only zero rows disappear; orphan acc rows gain a zero current row. This is projection, not arbitrary map identity.

### cost

R rows, K keys, B bytes: decoding O(B+R log K), encoding O(K log K+B), O(K) maps and O(B) output.

### composition

Student load invokes this decoder; save creates a directory then calls write_atomic with this encoder. The parent selects acc globally when nonempty. Presence is therefore a control bit shared with selection, not merely a numeric zero. Filesystem publication and global paths are unexecuted.

### behavior-complement

In the supplied TSV/map universe, exclude decoding a short duplicate as deletion of an existing accumulator; exclude encoding explicit zero acc as absent; exclude invented nonzero values for omitted or malformed numeric cells. Unsafe key strings are outside the roundtrip relation.

### reverse-reconstruction

Given decoded maps, antecedents are all ordered TSV rows whose last >=5-column occurrence sets each w and whose last >=9-column occurrence sets each acc; comments, malformed zero spellings and overridden rows remain free. Given encoded bytes, compatible input maps may additionally contain omitted current-only zero keys.

### accumulator_activation

The observation acc.is_empty after decode/encode equals its input value for format-safe maps, including explicit zero rows; w-only state and zero-acc state can choose different operations.

### duplicate_width

Two rows of the same key commute only if their effective current replacements agree and accumulator writes do not conflict. A five-column row cannot undo a prior nine-column accumulator.

### canonical_projection

Encode(decode(encode(s)))=encode(s) for format-safe representable rows. Decoding may insert zero w for orphan acc; equality is byte-canonical, not every original key-set identity.

### row_order

Encoder key union is a BTreeSet: output is lexical key order regardless of insertion order. Within a row current coordinates precede accumulator coordinates in Add/Sub/Mul/Div order.

### checks

Exact world library selections: school::wave12_models::retained_student_format_is_ordered_and_legacy_tolerant, school::wave12_models::legacy_retention_does_not_change_answer_table, school::wave12_models::zero_accumulator_activation_survives_retention. Current repeated debug/release outcomes and compiled controls are in the root sidecar wave34-evidence.md. Only these supplied-input tests are current evidence; older parent module-wide receipts are historical. Macro bodies remain opaque.

### epoch-memory

Prospective model saved before edits. This epoch deepens one existing node rather than renaming it. Production implementation unchanged; tests add helper-boundary observations where stated. No live lessons, frozen suites, model loading, teacher or network. Read-only symbolic relations are broader than the finite native domain.