Calculate a image size to fit different platforms

calculator.ts

type Width = number;
type Height = number;
type AspectRatio = [Width, Height];
type LowerUpperBounds = [number, number];

function unreachable(): never {
    throw new Error("unreachable!");
}

function calculate(width: number, height: number, options: { ratioRange: LowerUpperBounds }): AspectRatio {
    const ratio = width / height;
    const { ratioRange: [lowerBound, upperBound] } = options;
    
    if(ratio >= lowerBound && ratio <= upperBound) {
        return [width, height];
    }

    if(ratio < lowerBound) {
        // 不满的条件只有竖图小于lowerBound，增加宽度即可，lowerBound * 高度 = 新宽度
        const newWidth = Math.ceil(lowerBound * height);
        return [newWidth, height];
    }

    if(ratio > upperBound) {
        // 不满足的条件只有横图大于upperBound，增加高度即可，宽度 / upperBound = 新高度
        const newHeight = Math.ceil(width / upperBound);
        return [width, newHeight];
    }

    unreachable();
}

function calculateInstagram(width: number, height: number): AspectRatio {
    return calculate(width, height, { ratioRange: [0.8, 1.91] });
}