169 lines
7.2 KiB
TypeScript
169 lines
7.2 KiB
TypeScript
/**
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* Lossless-JSON validation and snapshot materialization for session data.
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*
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* The session event log is the durable source of truth (the event-sourcing / session-persistence RFCs): every
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* `event.data` must round-trip losslessly through JSON so any persistence
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* backend can store and reload it byte-identically. This invariant belongs to
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* the log itself — `Session.append` enforces it at the source, so a
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* non-serializable event never enters `session.events` and the live log can
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* never diverge from what a backend can persist. Other public boundaries use
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* {@link snapshotJsonValue} when they must validate and detach in one pass;
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* {@link isJsonValue} remains the non-copying structural predicate.
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*
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* @module @deepseek-ai/dsh-session/json
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*/
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/**
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* A value that round-trips losslessly through JSON: `null`, a boolean, a finite
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* number other than negative zero, a string, an array of such values, or a
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* plain object whose values are such values. TypeScript cannot distinguish
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* `-0` from `number`, so {@link isJsonValue} and {@link snapshotJsonValue}
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* enforce that last numeric detail at runtime. Use this type for a payload that
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* must survive session-log persistence and replay byte-identically — e.g. a
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* tool's private presentation `meta`.
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*/
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export type JsonValue = null | boolean | number | string | JsonValue[] | { [key: string]: JsonValue }
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/**
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* Materialize one detached lossless-JSON snapshot in a SINGLE recursive pass.
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* Each array slot or own enumerable string-keyed object value is read exactly
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* once, validated, and copied immediately. This is intentionally not
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* `isJsonValue(value)` followed by `structuredClone(value)`: a stateful getter
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* could return plain JSON to the check and an exotic class instance to the
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* clone, whose prototype `structuredClone` would erase before a later check.
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*
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* Accepts the same scalar/object vocabulary as {@link isJsonValue}: arrays use
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* the ordinary `Array.prototype` (subclass instances are not plain JSON
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* containers), while null-prototype objects are accepted and normalized to
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* ordinary plain objects. Sparse arrays, cycles, negative zero, non-finite
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* numbers, unsupported scalar types, and exotic object or array shells return
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* `undefined`. A throwing getter is a caller failure and propagates unchanged.
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*
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* @param value - the candidate value to validate and detach.
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* @returns the detached snapshot, or `undefined` when the value is not
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* losslessly JSON-serializable.
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*/
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export function snapshotJsonValue<T>(value: T): T | undefined {
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const ancestors = new Set<object>()
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const visit = (current: unknown): JsonValue | undefined => {
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if (current === null) return null
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switch (typeof current) {
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case 'boolean':
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case 'string':
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return current
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case 'number':
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return Number.isFinite(current) && !Object.is(current, -0) ? current : undefined
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case 'bigint':
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case 'function':
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case 'symbol':
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case 'undefined':
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return undefined
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case 'object':
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break
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}
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if (ancestors.has(current)) return undefined
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ancestors.add(current)
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try {
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if (Array.isArray(current)) {
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if (Object.getPrototypeOf(current) !== Array.prototype) return undefined
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const length = current.length
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const snapshot: JsonValue[] = []
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for (let index = 0; index < length; index++) {
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if (!Object.prototype.hasOwnProperty.call(current, index)) return undefined
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const item = visit(current[index])
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if (item === undefined) return undefined
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snapshot.push(item)
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}
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return snapshot
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}
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const prototype = Object.getPrototypeOf(current) as unknown
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if (prototype !== Object.prototype && prototype !== null) return undefined
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const snapshot: { [key: string]: JsonValue } = {}
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for (const key of Object.keys(current)) {
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const item = visit((current as Record<string, unknown>)[key])
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if (item === undefined) return undefined
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// Define the key as data so a JSON field literally named "__proto__"
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// cannot mutate the snapshot's prototype through ordinary assignment.
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Object.defineProperty(snapshot, key, {
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value: item,
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enumerable: true,
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configurable: true,
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writable: true,
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})
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}
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return snapshot
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} finally {
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ancestors.delete(current)
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}
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}
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return visit(value) as T | undefined
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}
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/**
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* Whether `value` is losslessly JSON-serializable: only `null`, finite numbers
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* other than negative zero, booleans, strings, plain arrays, and plain objects
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* of such values. Rejects `BigInt`, function, symbol, `undefined`, `-0` (which
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* JSON rewrites to `0`), non-finite numbers (`NaN`/`Infinity`, which JSON turns
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* into `null`), and exotic objects (`Map`/`Set`/`Date`/class instances) —
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* anything `JSON.stringify` would drop, throw on, or convert lossily. Sparse
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* arrays are rejected too: a hole serializes to `null`, so `[1, , 3]` would not
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* round-trip. Detects circular references (which would throw) and reports them
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* as non-serializable rather than propagating the throw.
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*
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* Scope — this is a structural plain-data predicate, not an invocation of
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* `JSON.stringify`: only an object's OWN ENUMERABLE STRING-keyed properties are
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* inspected (`Object.values`). Symbol-keyed and non-enumerable properties are
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* omitted from the durable data surface. Custom `toJSON` behavior is not
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* executed; boundaries that persist a value first materialize a new plain-data
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* record with {@link snapshotJsonValue}. Getters are invoked during this check,
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* so callers that need a stable detached value use that one-pass materializer
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* instead of checking and then rereading a side-effecting record.
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* @param value - the candidate event data to test.
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* @param seen - objects on the current descent path, for circular-reference
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* detection; the recursion threads it — callers omit it.
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* @returns true when `value` survives a JSON round-trip losslessly.
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*/
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export function isJsonValue(value: unknown, seen: Set<object> = new Set()): boolean {
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if (value === null) return true
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switch (typeof value) {
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case 'boolean':
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case 'string':
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return true
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case 'number':
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return Number.isFinite(value) && !Object.is(value, -0)
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case 'bigint':
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case 'function':
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case 'symbol':
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case 'undefined':
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return false
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case 'object':
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break // handled below
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}
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// object
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if (seen.has(value)) return false // circular
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seen.add(value)
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try {
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if (Array.isArray(value)) {
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if (Object.getPrototypeOf(value) !== Array.prototype) return false
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// Reject sparse arrays: a hole is skipped by `every`/`forEach` but
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// JSON.stringify writes it as `null`, so `[1, , 3]` would round-trip
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// lossily. Require every index 0..length-1 to be an OWN property.
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for (let i = 0; i < value.length; i++) {
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if (!Object.prototype.hasOwnProperty.call(value, i)) return false
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if (!isJsonValue(value[i], seen)) return false
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}
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return true
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}
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// Plain object only (reject Map/Set/Date/class instances).
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const proto = Object.getPrototypeOf(value) as unknown
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if (proto !== Object.prototype && proto !== null) return false
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return Object.values(value).every(v => isJsonValue(v, seen))
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} finally {
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seen.delete(value)
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}
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}
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