import { Reader, Annotated, Dictionary, is, KeyedDictionary, peel, preserves, Record, strip, Tuple } from '@preserves/core';
import { Input, NamedPattern, Pattern, Schema } from './meta';
import * as M from './meta';

function splitBy<T>(items: Array<T>, separator: T): Array<Array<T>> {
    const groups: Array<Array<T>> = [];
    let group: Array<T> = [];
    function finish() {
        if (group.length > 0) {
            groups.push(group);
            group = [];
        }
    }
    for (const item of items) {
        if (is(item, separator)) {
            finish();
        } else {
            group.push(item);
        }
    }
    finish();
    return groups;
}

function invalidClause(clause: Input): never {
    throw new Error(preserves`Invalid Schema clause: ${clause}`);
}

function invalidPattern(name: symbol, item: Input): never {
    throw new Error(preserves`Invalid pattern in ${name}: ${item}`);
}

export function readSchema(source: string): Schema {
    const toplevelTokens = new Reader<never>(source, { includeAnnotations: true }).readToEnd();
    return parseSchema(toplevelTokens);
}

export function parseSchema(toplevelTokens: Array<Input>): Schema {
    const toplevelClauses = splitBy(peel(toplevelTokens) as Array<Input>, M.DOT);
    let version: number | undefined = void 0;
    let definitions = new KeyedDictionary<symbol, Pattern, never>();
    for (const clause0 of toplevelClauses) {
        const clause = clause0.map(peel);
        if (!Array.isArray(clause)) {
            invalidClause(clause);
        } else if (clause.length >= 2 && is(clause[1], M.EQUALS)) {
            if (typeof clause[0] !== 'symbol') invalidClause(clause);
            const name = clause[0];
            if (!M.isValidToken(name.description!)) {
                throw new Error(preserves`Invalid definition name: ${name}`);
            }
            if (definitions.has(name)) {
                throw new Error(preserves`Duplicate definition: ${clause}`);
            }
            definitions.set(name, parseDefinition(name, clause.slice(2).map(peel)));
        } else if (clause.length === 2 && is(clause[0], M.___version)) {
            if (typeof clause[1] !== 'number') invalidClause(clause);
            version = clause[1];
        } else {
            invalidClause(clause);
        }
    }
    if (version === void 0) {
        throw new Error("Schema: missing version declaration.");
    }
    if (version !== 1) {
        throw new Error("Schema: unsupported version " + version);
    }
    return Record(M.___schema, [version, definitions]);
}

function parseDefinition(name: symbol, body: Array<Input>): Pattern {
    return parseOp(body, M.ORSYM, p => parseOp(p, M.ANDSYM, p => parseBase(name, p)));
}

function parseOp(body: Array<Input>, op: Input, k: (p: Array<Input>) => Pattern): Pattern {
    const pieces = splitBy(body, op);
    if (pieces.length === 1) return k(pieces[0]);
    switch (op) {
        case M.ORSYM: return Record(M.___or, [pieces.map(k)]);
        case M.ANDSYM: return Record(M.___and, [pieces.map(k)]);
        default: throw new Error("Internal error: unexpected operator");
    }
}

function findName(x: Input): symbol | false {
    if (!Annotated.isAnnotated<never>(x)) return false;
    for (const a0 of x.annotations) {
        const a = peel(a0);
        if (typeof a === 'symbol') return a;
    }
    return false;
}

function parseBase(name: symbol, body: Array<Input>): Pattern {
    body = peel(body) as Array<Input>;
    if (body.length !== 1) invalidPattern(name, body);
    const item = peel(body[0]);
    const walk = (b: Input): Pattern => parseBase(name, [b]);
    const namedwalk = (b: Input): NamedPattern => {
        const name = findName(b);
        if (name === false) return walk(b);
        return Record(M.___named, [name, walk(b)]);
    };
    const walkitems = (b: Input): Pattern[] => {
        b = peel(b);
        if (!Array.isArray(b)) complain();
        return b.map(walk);
    };

    function complain(): never {
        invalidPattern(name, item);
    }

    if (typeof item === 'symbol') {
        const s = item.description;
        if (s === void 0) complain();
        if (s[0] === '=') return Record(M.___lit, [Symbol.for(s.slice(1))]);
        return Record(M.___ref, [item]);
    } else if (Record.isRecord<Input, Tuple<Input>, never>(item)) {
        const label = item.label;
        if (Record.isRecord<Input, [], never>(label)) {
            if (label.length !== 0) complain();
            switch (label.label) {
                case M.___lit:
                    if (item.length !== 1) complain();
                    return Record(M.___lit, [item[0]]);
                case M.___or:
                    if (item.length !== 1) complain();
                    return Record(M.___or, [walkitems(item[0])]);
                case M.___and:
                    if (item.length !== 1) complain();
                    return Record(M.___and, [walkitems(item[0])]);
                case M.___rec:
                    if (item.length !== 2) complain();
                    return Record(M.___rec, [walk(item[0]), walk(item[1])]);
                default:
                    complain();
            }
        } else {
            return Record(M.___rec, [Record(M.___lit, [label]), Record(M.___tuple, [item.map(namedwalk)])]);
        }
    } else if (Array.isArray(item)) {
        if (is(item[item.length - 1], M.DOTDOTDOT)) {
            if (item.length < 2) complain();
            return Record(M.___tuple_STAR_, [
                item.slice(0, item.length - 2).map(namedwalk),
                namedwalk(item[item.length - 2]),
            ]);
        } else {
            return Record(M.___tuple, [item.map(namedwalk)]);
        }
    } else if (Dictionary.isDictionary<Input, never>(item)) {
        if (item.size === 2 && item.has(M.DOTDOTDOT)) {
            const v = item.clone();
            v.delete(M.DOTDOTDOT);
            const [[kp, vp]] = v.entries();
            return Record(M.___dictof, [walk(kp), walk(vp)]);
        } else {
            return Record(M.___dict, [item.mapEntries<Pattern, Input, never>(([k, vp]) =>
                [strip(k), walk(vp)])]);
        }
    } else if (Set.isSet<never>(item)) {
        if (item.size !== 1) complain();
        const [vp] = item.entries();
        return Record(M.___setof, [walk(vp)]);
    } else {
        return Record(M.___lit, [strip(item)]);
    }
}