element-web/src/utils/stringOrderField.ts

/*
Copyright 2024 New Vector Ltd.
Copyright 2021 The Matrix.org Foundation C.I.C.

SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only
Please see LICENSE files in the repository root for full details.
*/

import { alphabetPad, baseToString, stringToBase, DEFAULT_ALPHABET } from "matrix-js-sdk/src/utils";

import { moveElement } from "./arrays";

export function midPointsBetweenStrings(
    a: string,
    b: string,
    count: number,
    maxLen: number,
    alphabet = DEFAULT_ALPHABET,
): string[] {
    const padN = Math.min(Math.max(a.length, b.length), maxLen);
    const padA = alphabetPad(a, padN, alphabet);
    const padB = alphabetPad(b, padN, alphabet);
    const baseA = stringToBase(padA, alphabet);
    const baseB = stringToBase(padB, alphabet);

    if (baseB - baseA - BigInt(1) < count) {
        if (padN < maxLen) {
            // this recurses once at most due to the new limit of n+1
            return midPointsBetweenStrings(
                alphabetPad(padA, padN + 1, alphabet),
                alphabetPad(padB, padN + 1, alphabet),
                count,
                padN + 1,
                alphabet,
            );
        }
        return [];
    }

    const step = (baseB - baseA) / BigInt(count + 1);
    const start = BigInt(baseA + step);
    return Array(count)
        .fill(undefined)
        .map((_, i) => baseToString(start + BigInt(i) * step, alphabet));
}

interface IEntry {
    index: number;
    order?: string;
}

export const reorderLexicographically = (
    orders: Array<string | undefined>,
    fromIndex: number,
    toIndex: number,
    maxLen = 50,
): IEntry[] => {
    // sanity check inputs
    if (fromIndex < 0 || toIndex < 0 || fromIndex > orders.length || toIndex > orders.length || fromIndex === toIndex) {
        return [];
    }

    // zip orders with their indices to simplify later index wrangling
    const ordersWithIndices: IEntry[] = orders.map((order, index) => ({ index, order }));
    // apply the fundamental order update to the zipped array
    const newOrder = moveElement(ordersWithIndices, fromIndex, toIndex);

    // check if we have to fill undefined orders to complete placement
    const orderToLeftUndefined = newOrder[toIndex - 1]?.order === undefined;

    let leftBoundIdx = toIndex;
    let rightBoundIdx = toIndex;

    let canMoveLeft = true;
    const nextBase =
        newOrder[toIndex + 1]?.order !== undefined
            ? stringToBase(newOrder[toIndex + 1].order!)
            : BigInt(Number.MAX_VALUE);

    // check how far left we would have to mutate to fit in that direction
    for (let i = toIndex - 1, j = 1; i >= 0; i--, j++) {
        if (newOrder[i]?.order !== undefined && nextBase - stringToBase(newOrder[i].order!) > j) break;
        leftBoundIdx = i;
    }

    // verify the left move would be sufficient
    const firstOrderBase = newOrder[0].order === undefined ? undefined : stringToBase(newOrder[0].order);
    const bigToIndex = BigInt(toIndex);
    if (
        leftBoundIdx === 0 &&
        firstOrderBase !== undefined &&
        nextBase - firstOrderBase <= bigToIndex &&
        firstOrderBase <= bigToIndex
    ) {
        canMoveLeft = false;
    }

    const canDisplaceRight = !orderToLeftUndefined;
    let canMoveRight = canDisplaceRight;
    if (canDisplaceRight) {
        const prevBase =
            newOrder[toIndex - 1]?.order !== undefined
                ? stringToBase(newOrder[toIndex - 1].order!)
                : BigInt(Number.MIN_VALUE);

        // check how far right we would have to mutate to fit in that direction
        for (let i = toIndex + 1, j = 1; i < newOrder.length; i++, j++) {
            if (newOrder[i]?.order === undefined || stringToBase(newOrder[i].order!) - prevBase > j) break;
            rightBoundIdx = i;
        }

        // verify the right move would be sufficient
        if (
            rightBoundIdx === newOrder.length - 1 &&
            (newOrder[rightBoundIdx]?.order ? stringToBase(newOrder[rightBoundIdx].order!) : BigInt(Number.MAX_VALUE)) -
                prevBase <=
                rightBoundIdx - toIndex
        ) {
            canMoveRight = false;
        }
    }

    // pick the cheaper direction
    const leftDiff = canMoveLeft ? toIndex - leftBoundIdx : Number.MAX_SAFE_INTEGER;
    const rightDiff = canMoveRight ? rightBoundIdx - toIndex : Number.MAX_SAFE_INTEGER;
    if (orderToLeftUndefined || leftDiff < rightDiff) {
        rightBoundIdx = toIndex;
    } else {
        leftBoundIdx = toIndex;
    }

    const prevOrder = newOrder[leftBoundIdx - 1]?.order ?? "";
    const nextOrder =
        newOrder[rightBoundIdx + 1]?.order ??
        DEFAULT_ALPHABET.charAt(DEFAULT_ALPHABET.length - 1).repeat(prevOrder.length || 1);

    const changes = midPointsBetweenStrings(prevOrder, nextOrder, 1 + rightBoundIdx - leftBoundIdx, maxLen);

    return changes.map((order, i) => ({
        index: newOrder[leftBoundIdx + i].index,
        order,
    }));
};