/**
 * 压缩图片到指定大小
 * @param baseImg base64图片
 * @param maxSize 单位kb
 */
export function compressImgBySize(baseImg, maxSize = 200) {
	return new Promise((resolve) => {
		// 计算图片大小
		const strLength = baseImg.length
		const fileLength = parseInt(strLength - (strLength / 8) * 2)
		let size = parseInt((fileLength / 1024).toFixed(2))

		// 判断图片是否符合指定大小要求
		if (size <= maxSize) {
			resolve(baseImg)
			return
		}

		// 创建image对象
		const img = new Image()
		if (baseImg.indexOf('data:image/') !== -1) {
			img.src = baseImg
		} else {
			img.src = 'data:image/jpeg;base64,' + baseImg
		}

		img.onload = () => {
			// 把image对象 转换为 canvas对象
			const canvas = imgToCanvas(img)
			let resUrl = ''
			// 图片质量，范围：0 ~ 1
			let quality = 0.9

			// 当图片大小大于指定maxSize，图片质量大于0时继续通过降低图片资料来压缩
			while (size > maxSize && quality > 0) {
				// 在canvas上绘制该HTMLImageElement，得到图片base64
				resUrl = canvas.toDataURL('image/jpeg', quality).replace(/^data:image\/\w+;base64,/, '')
				const resLength = resUrl.length
				// 计算绘制出的base64图片大小
				const resFileLength = parseInt(resLength - (resLength / 8) * 2)
				size = parseInt((resFileLength / 1024).toFixed(2))
				// 降低图片质量
				quality = (quality - 0.1).toFixed(1)
			}
			resolve(resUrl)
		}
		img.onerror = () => {
			resolve(baseImg)
		}
	})
}

// 把image 转换为 canvas对象
export function imgToCanvas(image) {
	var canvas = document.createElement('canvas')
	canvas.width = image.width
	canvas.height = image.height
	canvas.getContext('2d').drawImage(image, 0, 0)
	return canvas
}