import { createHash } from 'node:crypto'

/**
 * A short, stable fingerprint of a file body.
 *
 * Used to answer one question at mutation admission: is the file still what
 * the agent read? An `edit` or full replacement is based on a remembered
 * body, and before the write begins that body can move — a person editing in
 * an editor, another process, a second agent's turn. The in-process mutation
 * lock serializes this runtime's own writers and is blind to every one of
 * those.
 *
 * Without this the drift is not merely undetected, it is actively
 * misreported: an `old_string` that no longer matches comes back as "not
 * found in file — make sure the string matches exactly", which tells the
 * agent its input was wrong when the file changed underneath it. The agent
 * then retries the same edit against the same moved file.
 *
 * Truncated deliberately. This distinguishes two bodies; it is not a
 * security boundary, and a full digest per read costs bytes in a structure
 * that lives for the whole turn.
 */
export function fingerprintContent(content: string): string {
	return createHash('sha256').update(content, 'utf-8').digest('hex').slice(0, 16)
}

/** What a mutation tool tells the agent when admission sees a moved file. */
export function staleFileError(path: string, operation: 'edit' | 'write' = 'edit'): string {
	return `${path} changed on disk after you read it, so this ${operation} was based on a stale copy and was not applied. Read the file again and redo the ${operation} against its current contents. Nothing was written.`
}