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import { Expr } from 'https://deno.land/x/pgsql_ast_parser@12.0.1/mod.ts';import { buildValue } from '../parser/expression-builder.ts';import { IValue, Stats, _Explainer, _ISelection, _SelectExplanation, _Transaction } from '../interfaces-private.ts';import { FilterBase } from './transform-base.ts';import objectHash from 'https://deno.land/x/object_hash@2.0.3.1/mod.ts';import { withSelection } from '../parser/context.ts';
export function buildDistinct(on: _ISelection, exprs?: Expr[]) {    return withSelection(on, () => {        const vals = exprs && exprs.length > 0            ? exprs.map(v => buildValue(v))            : on.columns        return new Distinct(on, vals);    });}

// todo: use indices to optimize this (avoid iterating everything)
class Distinct<T> extends FilterBase<any> {
    get index() {        return null;    }
    entropy(t: _Transaction) {        // cant foresight how many items will be filtered        //  => just asumme nothing will be.        return this.base.entropy(t);    }
    hasItem(raw: T, t: _Transaction): boolean {        return this.base.hasItem(raw, t);    }
    constructor(selection: _ISelection, private exprs: ReadonlyArray<IValue>) {        super(selection);    }
    stats(t: _Transaction): Stats | null {        return this.base.stats(t);    }
    *enumerate(t: _Transaction): Iterable<T> {        const got = new Set();        for (const i of this.base.enumerate(t)) {            const vals = this.exprs.map(v => v.type.hash(v.get(i, t)));            const hash = vals.length === 1 ? vals[0] : objectHash(vals);            if (got.has(hash)) {                continue;            }            got.add(hash);            yield i;        }    }
    explain(e: _Explainer): _SelectExplanation {        return {            id: e.idFor(this),            _: 'distinct',            of: this.base.explain(e),        };    }}