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, Two, Three} from './index.js';

export function Four(a, b, c, d) {
    this.a = a;
    this.b = b;
    this.c = c;
    this.d = d;
    this.v = null;
}

Four.prototype = new Digit();

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

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

Four.prototype.last = function () {
    return this.d;
};

Four.prototype.init = function () {
    return new Three(this.a, this.b, this.c);
};

Four.prototype.tail = function () {
    return new Three(this.b, this.c, this.d);
};

Four.prototype.push = function (_value) {
    throw new Error('cannot push digit Four');
};

Four.prototype.cons = function (_value) {
    throw new Error('cannot cons digit Four');
};

Four.prototype.node = function (_M) {
    throw new Error('cannot convert Four to node');
};

/**
 * It is assumed that p(i+|this|) is true.
 */
Four.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, this.c, this.d]);
    i = M.plus(i, M.measure(this.b));
    if (p(i)) return new Split([this.a], this.b, [this.c, this.d]);
    i = M.plus(i, M.measure(this.c));
    if (p(i)) return new Split([this.a, this.b], this.c, [this.d]);
    return new Split([this.a, this.b, this.c], this.d, []);
};

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

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