functional-data-structure/finger-tree

import assert from 'assert';

import {Split} from '../0-core/index.js';
import {node2, node3} from '../2-node/index.js';

import {Digit, One, Three} from './index.js';

export function Two(a, b) {
    this.a = a;
    this.b = b;
    this.v = null;
}

Two.prototype = new Digit();

Two.prototype.measure = function (M) {
    if (this.v === null) this.v = M.plus(M.measure(this.a), M.measure(this.b));
    return this.v;
};

Two.prototype.head = function () {
    return this.a;
};

Two.prototype.last = function () {
    return this.b;
};

Two.prototype.init = function () {
    return new One(this.a);
};

Two.prototype.tail = function () {
    return new One(this.b);
};

Two.prototype.push = function (value) {
    return new Three(this.a, this.b, value);
};

Two.prototype.cons = function (value) {
    return new Three(value, this.a, this.b);
};

Two.prototype.node = function (_M) {
    throw new Error(
        'Two should never be converted to Node2 with current implementation',
    );
};

/**
 * It is assumed that p(i+|this|) is true.
 */
Two.prototype.splitDigit = function (p, i, M) {
    assert(p(M.plus(i, this.measure(M)))); // /!\ Potential Heisenbug generator.
    i = M.plus(i, M.measure(this.a));
    if (p(i)) return new Split([], this.a, [this.b]);
    return new Split([this.a], this.b, []);
};

Two.prototype._nodes = function (M, other) {
    if (other instanceof One) return [node3(M, this.a, this.b, other.a)];
    if (other instanceof Two)
        return [node2(M, this.a, this.b), node2(M, other.a, other.b)];
    if (other instanceof Three)
        return [node3(M, this.a, this.b, other.a), node2(M, other.b, other.c)];
    assert(other instanceof Digit);
    return [
        node3(M, this.a, this.b, other.a),
        node3(M, other.b, other.c, other.d),
    ];
};

Two.prototype._list = function () {
    return [this.a, this.b];
};