/**
 * Expand a value set the terminology server refuses as the pieces it is composed of, and merge.
 *
 * Snowstorm will not expand a value set spanning SNOMED CT and another code system - it declines to
 * COUNT across two systems, not to answer - and will not apply a `descendent-of` filter to SNOMED.
 * Both refusals are about the PARENT, never the content: every child below expands 200 on its own.
 * Without this, the bound fields render as empty pickers, which look identical to "no matches".
 *
 * THIS IS NOT A RE-BINDING. Re-binding a clinical field to a narrower value set changes what a
 * clinician can record, which is a terminology decision rather than an engineering one. Every
 * substitution here reproduces the published value set's OWN definition - the same concepts, from
 * the same sources, computed a way this server implements. The meaning is unchanged.
 *
 * Each entry records the server's refusal verbatim and the parent version it was derived from.
 */

export type ExpansionSubstitution = {
    /** What the published value set is called on the server, so a reader can find it. */
    readonly name: string;
    /** The version observed when this was derived - the thing to re-check if VSAC republishes. */
    readonly observedVersion: string;
    /** The server's own words when asked to expand the parent. */
    readonly refusal: string;
    /** Expand these instead, in order, and merge. */
    readonly expand: readonly string[];
};

const VSAC = 'http://cts.nlm.nih.gov/fhir/ValueSet/';

/**
 * THE ONE WAY THIS GOES WRONG, stated so it is not discovered instead: these decompositions were read
 * off each parent's own `compose` at the version recorded below. **If VSAC republishes a parent with
 * different children, this table keeps expanding the OLD ones and nothing says so** - the field still
 * fills, with a stale set. There is no automated guard for that, because checking it means calling the
 * terminology server, and a test that reaches the network is a test that fails on a bad morning.
 * Re-read each parent's `compose` when US Core or VSAC moves version.
 */
export const EXPANSION_SUBSTITUTIONS: Readonly<Record<string, ExpansionSubstitution>> = {
    // The "interpreter required" field, and the advance-directive documentation field
    //
    // The parent is a plain union of two enumerations: two SNOMED codes (Yes / No) and two
    // data-absent-reason codes (Unknown / Asked but unknown). Four concepts in total, and the server
    // expands each half without complaint - it simply will not put them in one bundle.
    // THE REFUSAL IS ABOUT PAGINATION, NOT ABOUT MEANING, which is why merging is faithful rather than
    // a shortcut: the server is declining to COUNT across two code systems, not declining to answer.
    [`${VSAC}2.16.840.1.113762.1.4.1267.16`]: {
        name: 'AnswerSetWithYesNoAndUnknowns',
        observedVersion: '20241005',
        refusal:
            'This server does not yet support ValueSet$expand on ValueSets with multiple code systems '
            + 'if any are SNOMED CT, because of the complexities around pagination and result totals.',
        expand: [
            `${VSAC}2.16.840.1.113762.1.4.1267.15`, // YesOrNoSCT           - 2 concepts, SNOMED
            `${VSAC}2.16.840.1.113762.1.4.1267.14`, // UnknownAndAskedButUnknownDAR - 2, data-absent-reason
        ],
    },

    // The encounter TYPE field
    //
    // A DIFFERENT CAUSE FROM THE ONE ABOVE, and the two are easy to conflate. This parent does not
    // fail on multiple code systems; its SNOMED half fails on a FILTER OPERATOR:
    // "Unexpected operation 'descendent-of' for SNOMED CT ValueSet 'concept' filter."
    // `descendent-of` is valid FHIR R4 and this server does not implement it for SNOMED. What it does
    // implement is ECL, so the same definition is expressed as ECL and expands 200 with 98 concepts.
    //
    // `<` AND NOT `<<`, AND THE DIFFERENCE IS THE POINT. FHIR's `descendent-of` EXCLUDES the root
    // concept; `is-a` includes it. ECL `<` excludes, `<<` includes. Using `is-a`/`<<` would have been the
    // obvious translation and would silently add "Patient encounter procedure" and "Indirect encounter"
    // themselves to a clinician's picker as selectable encounter types, which the published value set
    // deliberately does not offer.
    //
    // THE HONEST GAP, and it is NOT caused by this substitution: the parent also unions
    // `EncounterTypeCPT`, and that child expands 200 with a total of ZERO - CPT is licensed content and
    // is not loaded on our server. So this field offers the SNOMED half whether or not the parent ever
    // expands, and it did so before this change too. Recorded here rather than left to be rediscovered.
    [`${VSAC}2.16.840.1.113762.1.4.1267.23`]: {
        name: 'EncounterType',
        observedVersion: '20250510',
        refusal: "Unexpected operation 'descendent-of' for SNOMED CT ValueSet 'concept' filter.",
        expand: [
            // EncounterSCT (2.16.840.1.113883.11.20.9.52) states exactly this: descendants of
            // 308335008 |Patient encounter procedure| and of 185316007 |Indirect encounter|.
            'http://snomed.info/sct?fhir_vs=ecl/(<308335008 OR <185316007)',
        ],
    },

    [`${VSAC}2.16.840.1.113762.1.4.1099.30`]: {
        name: 'CareTeamMemberFunction',
        observedVersion: '20240605',
        refusal:
            'This server does not yet support ValueSet$expand on ValueSets with multiple code systems '
            + 'if any are SNOMED CT, because of the complexities around pagination and result totals.',
        expand: [
            `${VSAC}2.16.840.1.113883.1.11.10267`, // v3-ParticipationFunction - 49
            `${VSAC}2.16.840.1.113762.1.4.1099.27`, // SNOMED half - 633, via the ECL override
        ],
    },

    // The allergy substance picker. THE BIGGEST ONE HERE: 826 concepts a clinician could not reach.
    [`${VSAC}2.16.840.1.113762.1.4.1186.8`]: {
        name: 'AllergyIntoleranceSubstance',
        observedVersion: '20240221',
        refusal:
            'This server does not yet support ValueSet$expand on ValueSets with multiple code systems '
            + 'if any are SNOMED CT, because of the complexities around pagination and result totals.',
        expand: [
            `${VSAC}2.16.840.1.113762.1.4.1114.14`, // 107
            `${VSAC}2.16.840.1.113762.1.4.1186.5`, // 7
            `${VSAC}2.16.840.1.113762.1.4.1186.4`, // 15
            `${VSAC}2.16.840.1.113762.1.4.1186.1`, // 563 - RxNorm, the bulk of it
            `${VSAC}2.16.840.1.113762.1.4.1186.3`, // 134
        ],
    },

    // Pregnancy status. 3 + 1.
    [`${VSAC}2.16.840.1.113762.1.4.1240.12`]: {
        name: 'PregnancyStatus',
        observedVersion: '20240221',
        refusal:
            'This server does not yet support ValueSet$expand on ValueSets with multiple code systems '
            + 'if any are SNOMED CT, because of the complexities around pagination and result totals.',
        expand: [
            `${VSAC}2.16.840.1.113762.1.4.1099.24`, // 3
            `${VSAC}2.16.840.1.113883.1.11.10612`, // 1
        ],
    },

    // Sexual orientation. 4 + 1 + 2.
    [`${VSAC}2.16.840.1.113762.1.4.1240.11`]: {
        name: 'SexualOrientation',
        observedVersion: '20240221',
        refusal:
            'This server does not yet support ValueSet$expand on ValueSets with multiple code systems '
            + 'if any are SNOMED CT, because of the complexities around pagination and result totals.',
        expand: [
            `${VSAC}2.16.840.1.113762.1.4.1021.33`, // 4
            `${VSAC}2.16.840.1.113762.1.4.1240.4`, // 1
            `${VSAC}2.16.840.1.113762.1.4.1240.10`, // 2
        ],
    },

    // Smoking status codes. 2 + 2. Distinct from usCoreSmokingstatusOptions, which the terminology
    // server answers on its own through an ECL override - these are different value sets.
    [`${VSAC}2.16.840.1.113762.1.4.1267.6`]: {
        name: 'SmokingStatusCodes',
        observedVersion: '20240222',
        refusal:
            'This server does not yet support ValueSet$expand on ValueSets with multiple code systems '
            + 'if any are SNOMED CT, because of the complexities around pagination and result totals.',
        expand: [
            `${VSAC}2.16.840.1.113762.1.4.1267.4`, // 2 - LOINC
            `${VSAC}2.16.840.1.113762.1.4.1267.5`, // 2 - SNOMED
        ],
    },

    // The sex field. 4 SNOMED + 1 data-absent-reason.
    [`${VSAC}2.16.840.1.113762.1.4.1240.3`]: {
        name: 'Sex',
        observedVersion: '20230628',
        refusal:
            'This server does not yet support ValueSet$expand on ValueSets with multiple code systems '
            + 'if any are SNOMED CT, because of the complexities around pagination and result totals.',
        expand: [
            `${VSAC}2.16.840.1.113762.1.4.1099.53`, // 4 - SNOMED
            `${VSAC}2.16.840.1.113762.1.4.1240.4`, // 1 - data-absent-reason, shared with gender identity
        ],
    },

    // The gender identity field. 6 SNOMED + 1 data-absent-reason + 2 v3-NullFlavor.
    //
    // THREE HALVES, NOT TWO, and the third is easy to lose: v3-NullFlavor supplies `OTH` and `UNK`,
    // which is how a clinician records "other" and "unknown" here. Dropping it because two children
    // already cover the named identities would remove both, and the field would look complete.
    [`${VSAC}2.16.840.1.113762.1.4.1021.32`]: {
        name: 'GenderIdentity',
        observedVersion: '20240215',
        refusal:
            'This server does not yet support ValueSet$expand on ValueSets with multiple code systems '
            + 'if any are SNOMED CT, because of the complexities around pagination and result totals.',
        expand: [
            `${VSAC}2.16.840.1.113762.1.4.1021.101`, // 6 - SNOMED
            `${VSAC}2.16.840.1.113762.1.4.1240.4`, // 1 - data-absent-reason
            `${VSAC}2.16.840.1.113762.1.4.1240.10`, // 2 - v3-NullFlavor
        ],
    },

    // The route-of-administration field. 161 SNOMED + 125 NCI Thesaurus.
    //
    // The SNOMED half needed a server-side ECL override before it would expand at all - the
    // published child filters on `descendent-of`, which this server does not implement - and the
    // NCI half needed its code system loaded, because VSAC enumerates 125 NCI codes without
    // shipping anything that can validate them. Both are done; this makes the parent whole.
    [`${VSAC}2.16.840.1.113762.1.4.1267.22`]: {
        name: 'USCoreRouteCodes',
        observedVersion: '20241015',
        refusal:
            'This server does not yet support ValueSet$expand on ValueSets with multiple code systems '
            + 'if any are SNOMED CT, because of the complexities around pagination and result totals.',
        expand: [
            `${VSAC}2.16.840.1.113762.1.4.1099.12`, // 161 - SNOMED, via the ECL override
            `${VSAC}2.16.840.1.113883.3.88.12.3221.8.7`, // 125 - NCI Thesaurus
        ],
    },

    // Why an immunization was not given. 4 ActReason + 24 SNOMED.
    ['http://hl7.org/fhir/ValueSet/immunization-status-reason']: {
        name: 'ImmunizationStatusReason',
        observedVersion: '20190815',
        refusal:
            'This server does not yet support ValueSet$expand on ValueSets with multiple code systems '
            + 'if any are SNOMED CT, because of the complexities around pagination and result totals.',
        expand: [
            // Published by our own terminology server precisely so this half can be requested.
            'http://akashichealthrecords.com/fhir/ValueSet/immunization-status-reason-actreason',
            // is-a 310376006 -> ECL '<<' INCLUDES the root, which is what is-a means. Using '<'
            // here would silently drop the root concept the published value set does offer.
            'http://snomed.info/sct?fhir_vs=ecl/<<310376006',
        ],
    },

    // What a reference range applies to. v3-Race + three SNOMED concepts.
    ['http://hl7.org/fhir/ValueSet/referencerange-appliesto']: {
        name: 'ReferenceRangeAppliesTo',
        observedVersion: '20190815',
        refusal:
            'This server does not yet support ValueSet$expand on ValueSets with multiple code systems '
            + 'if any are SNOMED CT, because of the complexities around pagination and result totals.',
        expand: [
            'http://terminology.hl7.org/ValueSet/v3-Race', // 921 - already a real URL
            // Three enumerated concepts, expressed as an ECL disjunction rather than published as
            // a value set of our own: Male, Female, Pregnancy. Exactly the parent's three.
            'http://snomed.info/sct?fhir_vs=ecl/(248153007 OR 248152002 OR 77386006)',
        ],
    },
};

/**
 * The URLs to actually ask for. Normally the one you were given.
 *
 * DELIBERATELY NOT A RETRY-ON-FAILURE. Falling back after a 400 would fire the doomed request on every
 * single load of the encounter form, and - worse - it would quietly paper over the NEXT value set that
 * starts failing for an unrelated reason. A declared substitution is one request path and one thing to
 * read.
 */
export const resolveExpansionUrls = (valueSetUrl: string): readonly string[] =>
    EXPANSION_SUBSTITUTIONS[valueSetUrl]?.expand ?? [valueSetUrl];

export const isSubstitutedValueSet = (valueSetUrl: string): boolean =>
    valueSetUrl in EXPANSION_SUBSTITUTIONS;

type Concept = { system?: string; code?: string; display?: string };

/**
 * Merge the pieces back into one expansion, in the order they were requested.
 *
 * DEDUPED ON `system|code`, never on `code` alone. These unions cross code systems on purpose, and
 * two systems can and do use the same code string for different things - collapsing on the code would
 * silently drop a concept and leave a picker one option short with nothing to notice it.
 */
export const mergeExpansionContains = (
    contains: ReadonlyArray<readonly Concept[] | undefined>,
): Concept[] => {
    const seen = new Set<string>();
    const merged: Concept[] = [];

    for (const list of contains) {
        for (const concept of list ?? []) {
            const key = `${concept.system ?? ''}|${concept.code ?? ''}`;
            if (seen.has(key)) continue;
            seen.add(key);
            merged.push(concept);
        }
    }

    return merged;
};