import { QuestionnaireResponse, QuestionnaireResponseItem } from 'fhir/r4';

/**
 * WHAT A QUESTIONNAIRE WITH NOTHING IN IT MUST NOT DO - send itself.
 *
 * Watched in a browser 2026-08-20: a patient opened the questionnaire her practice had assigned her,
 * answered nothing, pressed **Submit**, and confirmed the "this cannot be undone" dialog. The server
 * refused it with a 400. The screen behaved exactly as it does on success - the dialog closed and
 * nothing objected.
 *
 * That is worse than an ordinary unhandled error. The patient has every reason to believe the form
 * went in; the practice never receives it; and the task stays open against someone who is sure they have
 * already done it. The next thing that happens is a phone call at the front desk.
 *
 * THE ANSWER IS TO NOT SEND IT, not to explain the refusal afterwards. The server is right to refuse,
 * and there is nothing useful to tell a patient about a FHIR array-cardinality error. Surfacing a
 * refusal is a separate and also-necessary fix - see `serverRefusalMessage.ts` - but it is what catches
 * the failures nobody has hit yet, not this one.
 *
 * THE FENCE, and it is the thing an obvious fix breaks. **The bar is AT LEAST ONE answer, never all
 * of them.** A partial answer must stay submittable: the form has its own *Save for Later* button for
 * the incomplete case, and a PHQ deliberately allows unanswered items. "Empty submissions are blocked"
 * must not be satisfied by blocking everything a patient can realistically send.
 */

/** What the person pressed. The bar is identical either way; only the sentence differs. */
export type QuestionnaireSubmissionAction = 'submit' | 'saveForLater';

/**
 * Does this response carry a single answer anywhere in it?
 *
 * Asked of the TRANSFORMED FHIR resource rather than of the form state, on purpose. The resource is
 * what the server sees, so this cannot disagree with it about what "answered" means - and form state
 * shape varies per item type (a boolean, a coding, a quantity and an attachment are all "a value"),
 * which is exactly the kind of per-type test that goes stale as item types are added.
 *
 * Walks the nested form too: a grouped item carries its children under `answer[].item[]`, so a response
 * whose only answer sits inside a group is answered.
 */
export const hasAnyQuestionnaireAnswer = (response: QuestionnaireResponse | undefined): boolean => {
    if (!response) return false;

    const itemHasAnswer = (item: QuestionnaireResponseItem): boolean => {
        const answers = item.answer ?? [];

        for (const answer of answers) {
            // A group's `answer` entry is a container, not a value: it carries only `id` and `item`.
            // A value of any kind at this level means the question was genuinely answered.
            const isContainerOnly =
                Object.keys(answer).every((key) => key === 'id' || key === 'item');

            if (!isContainerOnly) return true;
            if ((answer.item ?? []).some(itemHasAnswer)) return true;
        }

        return (item.item ?? []).some(itemHasAnswer);
    };

    return (response.item ?? []).some(itemHasAnswer);
};

/**
 * The sentence to show when there is nothing to send, or `undefined` when the submission may proceed.
 *
 * Returns a reason rather than a boolean so the caller cannot invent its own wording, and so the two
 * actions cannot drift apart into one being explained and the other silently doing nothing.
 */
export const emptyQuestionnaireSubmissionRefusal = (
    response: QuestionnaireResponse | undefined,
    action: QuestionnaireSubmissionAction,
): string | undefined => {
    if (hasAnyQuestionnaireAnswer(response)) return undefined;

    // Says what to do, not what went wrong. "At least one" is the literal rule, and stating it is what
    // stops the message reading as "you must answer everything" - which is the behaviour this must not
    // acquire.
    return action === 'submit'
        ? 'Answer at least one question before submitting this questionnaire.'
        : 'Answer at least one question before saving this questionnaire for later.';
};