operator minBy.ts

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  import {map} from './map';
import {pipe} from 'fnxt/pipe';
import {Seq, Tuple, UnaryFunction} from 'fnxt/fnxt-types';
import {reduce} from './reduce';
 
type T<E> = Tuple<E, number>
const first = <E>(v: Tuple<E, unknown>): E => v[0];
 
 
export const minBy = <E>(mapping: UnaryFunction<E, number>): UnaryFunction<Seq<E>, E> =>
  pipe(
    map<E, T<E>>(e => [e, mapping(e)]),
    reduce<T<E>>((p: T<E>, c: T<E>) => p[1] < c[1] ? p : c),
    first
  );