/** Max accepted base64 delta, in characters (~8 MiB). */
export declare const MAX_DELTA_B64_CHARS: number;
/** Chars of provenance label kept — a diff carries no text, so the label is its only description. */
export declare const DELTA_LABEL_MAX_CHARS = 800;
/** A validated delta contribution — the exact body a producer sends. */
export interface DeltaLearnPayload {
    /** base64 serialized RowDelta (`diffCheckpoints(base, adapted)`). */
    diff: string;
    /** The model version the delta was diffed against. */
    baseVersion: number;
    /** FedAvg sample weight (> 0). Absent means the writer's default (1). */
    weight?: number;
    /** Provenance for the merged row (e.g. the run's task). */
    label?: string;
}
/** Why a body was refused: `invalid` (malformed) or `too-large` (over {@link MAX_DELTA_B64_CHARS}). */
export type DeltaParseResult = {
    ok: true;
    payload: DeltaLearnPayload;
} | {
    ok: false;
    reason: "invalid" | "too-large";
    error: string;
};
/**
 * Parse + validate a delta body. Pure — a front door calls it to refuse a bad body
 * early, and the single writer calls it again because it trusts no caller.
 */
export declare function parseDeltaLearnPayload(raw: unknown): DeltaParseResult;
/**
 * Build the payload for one computed diff. The caller owns the size guard (compare
 * `payload.diff.length` with {@link MAX_DELTA_B64_CHARS}) so it can choose a fallback.
 */
export declare function buildDeltaLearnPayload(diff: ArrayBuffer, baseVersion: number, weight?: number, label?: string): DeltaLearnPayload;
/** Decode a payload's base64 `diff` to the serialized RowDelta bytes. Throws on malformed base64. */
export declare function decodeDeltaPayload(payload: Pick<DeltaLearnPayload, "diff">): ArrayBuffer;
/**
 * Structural check of ONE delta against the base it claims to diff: it must
 * deserialize, be element-granular, index only elements the base has, and carry
 * finite values. Returns why it is unusable, or null when it can join a merge.
 */
export declare function deltaUnusableReason(delta: ArrayBuffer, baseCheckpoint: ArrayBuffer): string | null;
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