/**
 * A write refused because the record is no longer the version it was made against (412), or because the page
 * named no version at all (428, an old page). Nothing was saved either way; the user's choice is still theirs.
 */
export class RecordChangedError extends Error {
    // A protected member makes the class nominal: no other error, whatever its fields, passes as this one.
    protected readonly ownedNotice!: true;
    constructor(
        message: string,
        /** The page is too old to name a version: reload, rather than review and save again. */
        public readonly outOfDate = false,
        /** The record as it is now, when the refusal knows it. */
        public readonly current?: unknown,
    ) {
        super(message);
        this.name = 'RecordChangedError';
    }
}

export const isRecordChanged = (error: unknown): error is RecordChangedError =>
    error instanceof RecordChangedError || (error as { name?: unknown } | null)?.name === 'RecordChangedError';

// The standard refusal: nothing is merged or retried over another person's change. The person reloads the record
// (which discards their unsaved edit, and is said before they do) or leaves.
export const RECORD_CHANGED_MESSAGE = 'This record changed elsewhere. Your changes were not saved. Reload the record before editing again.';
export const PAGE_OUT_OF_DATE_MESSAGE = 'This page is out of date. Reload before saving.';
export const RELOAD_DISCARDS_MESSAGE = 'Reload record discards the changes you have not saved.';

/** The precondition a conditional write names: the version it was built on. */
export const ifMatch = (version: string): Record<string, string> => ({ 'If-Match': `W/"${version}"` });

/** A refused conditional write, in the words above; anything else passes through. */
export const asRecordChanged = (error: unknown): unknown => {
    const status = (error as { status?: unknown } | null)?.status;
    if (status === 412) return new RecordChangedError(RECORD_CHANGED_MESSAGE);
    if (status === 428) return new RecordChangedError(PAGE_OUT_OF_DATE_MESSAGE, true);
    return error;
};