/**
 * 
 * @param {ImageData} imageData 
 * @param {Number} size 峰值代表背景色，背景色偏移多少个的值0
 * @returns 
 */
module.exports = function otsu(imageData, size = 2) {
    const grayPixelInfo = []; // 每个灰度值的信息的集合 {size: 像素总数， ratio: 概论}
    let threshold;
    for (i = 0; i < 256; i++) {
        grayPixelInfo[i] = { pixelSize: 0, pixelRatio: 0 };
    }
    const data = imageData.data;
    const pixelTotal = data.length;
    for (i = 0; i < pixelTotal; i += 4) {
        //获取r的像素值，因为灰度图像，r=g=b，所以取第一个即可
        const key = data[i];
        grayPixelInfo[key].pixelSize++;
    }
    let imageSize = imageData.width * imageData.height;
    let maxPixel = 0;
    let maxGray = 0;
    for (let i = 0; i < 256; i++) {
        if( grayPixelInfo[i].pixelSize > maxPixel){
            maxPixel = grayPixelInfo[i].pixelSize;
            maxGray = i;
        }
    }
    for(let i = -size ; i <= size; i++){
       if(maxGray + i >= 0 && maxGray +  i <= 255){
        imageSize-=grayPixelInfo[maxGray + i].pixelSize;
        grayPixelInfo[maxGray + i].pixelSize = 0;
       }
    }
    for (i = 0; i < 256; i++) {
        grayPixelInfo[i].pixelRatio = grayPixelInfo[i].pixelSize / imageSize;
    }
    // 去掉波峰的值，
    let maxVariance = 0;
    for (let i = 0; i < 256; i++) {
        let probabilityFront = 0;
        let probabilityBack = 0;
        let avagerGrayFront = 0;
        let avagerGrayBack = 0;
        grayPixelInfo.slice(0, i).forEach(el => {
            probabilityFront += el.pixelRatio;
            avagerGrayFront += el.pixelRatio * el.pixelSize
        })
        grayPixelInfo.slice(i + 1).forEach(el => {
            probabilityBack += el.pixelRatio;
            avagerGrayBack += el.pixelRatio * el.pixelSize
        })
        if (probabilityFront === 0 || probabilityBack === 0) {
            continue;
        }
        avagerGrayFront = avagerGrayFront / probabilityFront;
        avagerGrayBack = avagerGrayBack / probabilityBack;
        const variance = probabilityFront * probabilityBack * (avagerGrayFront - avagerGrayBack) * (avagerGrayFront - avagerGrayBack)
        if (variance > maxVariance) {
            maxVariance = variance;
            threshold = i;
        }
    }

    return{
        threshold: threshold,
        variance: maxVariance,
        histogram: grayPixelInfo,
    }
}