swaf/src/Pagination.ts

import {WrappingError} from "./Utils";

export default class Pagination {
    public readonly page: number;
    public readonly perPage: number;
    public readonly totalCount: number;

    public constructor(page: number, perPage: number, totalCount: number) {
        if (perPage <= 0) throw new Error('perPage must be >= 1');
        if (totalCount < 0) throw new Error('totalCount must be >= 0');

        this.page = page;
        this.perPage = perPage;
        this.totalCount = totalCount;

        if (this.page < 1 || this.page > 1 && this.page > this.lastPage) {
            throw new PageNotFoundError(this.page);
        }
    }

    public hasPrevious(): boolean {
        return this.page > 1;
    }

    public hasNext(): boolean {
        return this.page < this.lastPage;
    }

    public get lastPage(): number {
        return Math.max(Math.ceil(this.totalCount / this.perPage), 1);
    }

    public previousPages(contextSize: number): number[] {
        const pages = [];

        let i = 1;

        // Leftmost context
        while (i < this.page && i <= contextSize) {
            pages.push(i);
            i++;
        }

        // Ellipsis
        if (i < this.page - contextSize) {
            pages.push(-1);
        }

        // Middle left context
        i = Math.max(i, this.page - contextSize);
        while (i < this.page) {
            pages.push(i);
            i++;
        }

        return pages;
    }

    public nextPages(contextSize: number): number[] {
        const pages = [];

        let i = this.page + 1;
        // Middle right context
        while (i <= this.lastPage && i <= this.page + contextSize) {
            pages.push(i);
            i++;
        }

        // Ellipsis
        if (this.page + contextSize + 1 < this.lastPage - contextSize + 1) {
            pages.push(-1);
        }

        // Rightmost context
        i = Math.max(i, this.lastPage - contextSize + 1);
        while (i <= this.lastPage) {
            pages.push(i);
            i++;
        }

        return pages;
    }
}

export class PageNotFoundError extends WrappingError {
    public constructor(
        public readonly page: number,
    ) {
        super(`Page ${page} not found.`);
    }
}