import { Result, assertNever } from '@shared/kernel/fp';

/** The three actions the beoordeling screen offers a behandelaar — mirrors the
    backend's `Besluit` enum member names 1:1 (the wire convention: a string, not a raw
    enum — see `RecordBesluitRequest`). */
const BESLUIT_TAGS = ['Goedkeuren', 'Afwijzen', 'MeerInfoOpvragen'] as const;
export type BesluitTag = (typeof BESLUIT_TAGS)[number];

function isBesluitTag(v: string): v is BesluitTag {
  return (BESLUIT_TAGS as readonly string[]).includes(v);
}

/** What the user picked (raw, possibly empty while nothing is selected yet). */
export interface Draft {
  besluit: string;
  toelichting: string;
}

/** After parsing — besluit is the narrow tag; toelichting is present only when given
    (required for Afwijzen/MeerInfoOpvragen, optional for Goedkeuren — enforced by validate). */
export interface Valid {
  besluit: BesluitTag;
  toelichting?: string;
}

export type Errors = Partial<Record<keyof Draft, string>>;

/** The decision form as one tagged union — same idiom as every other form in this
    house (form-machine skill), single-step. draft/errors exist only while Editing. */
export type BesluitState =
  | { tag: 'Editing'; draft: Draft; errors: Errors }
  | { tag: 'Submitting'; data: Valid }
  | { tag: 'Submitted'; data: Valid }
  | { tag: 'Failed'; data: Valid; error: string };

export const initial: BesluitState = {
  tag: 'Editing',
  draft: { besluit: '', toelichting: '' },
  errors: {},
};

function validate(draft: Draft): Result<Errors, Valid> {
  if (!isBesluitTag(draft.besluit)) {
    return {
      ok: false,
      error: { besluit: $localize`:@@besluit.error.verplicht:Kies een besluit.` },
    };
  }
  const toelichting = draft.toelichting.trim();
  if (draft.besluit !== 'Goedkeuren' && toelichting === '') {
    return {
      ok: false,
      error: {
        toelichting: $localize`:@@besluit.error.toelichtingVerplicht:Geef een toelichting.`,
      },
    };
  }
  return { ok: true, value: { besluit: draft.besluit, toelichting: toelichting || undefined } };
}

export type BesluitMsg =
  | { tag: 'SetField'; key: keyof Draft; value: string }
  | { tag: 'Submit' }
  | { tag: 'Retry' }
  | { tag: 'SubmitConfirmed' }
  | { tag: 'SubmitFailed'; error: string }
  | { tag: 'Reset' }
  | { tag: 'Seed'; state: BesluitState }; // mount a specific state (stories/tests)

export function reduce(s: BesluitState, m: BesluitMsg): BesluitState {
  switch (m.tag) {
    case 'SetField':
      return s.tag === 'Editing' ? { ...s, draft: { ...s.draft, [m.key]: m.value } } : s;
    case 'Submit': {
      if (s.tag !== 'Editing') return s;
      const r = validate(s.draft);
      return r.ok ? { tag: 'Submitting', data: r.value } : { ...s, errors: r.error };
    }
    case 'Retry':
      return s.tag === 'Failed' ? { tag: 'Submitting', data: s.data } : s;
    case 'SubmitConfirmed':
      return s.tag === 'Submitting' ? { tag: 'Submitted', data: s.data } : s;
    case 'SubmitFailed':
      return s.tag === 'Submitting' ? { tag: 'Failed', data: s.data, error: m.error } : s;
    case 'Reset':
      return initial;
    case 'Seed':
      return m.state;
    default:
      return assertNever(m);
  }
}