//椭球相关参数计算
const a = 6378137; //长半轴 a
const f = 1 / 298.257223563; //扁率 (a-b)/a
const b = a * (1 - f);//短半轴 b
const e = Math.sqrt(a * a - b * b) / a;//椭球第一偏心率

// console.log(a);
// console.log(f);
// console.log(b);
// console.log(e);
// console.log(e*e);

function degToRad(deg) {
    return deg / 180 * Math.PI;
}

function radToDeg(rad) {
    return rad / Math.PI * 180;
}

function wgs84ToWebMercator(lat, lon) {
    let x = a * lon;
    let y = a * Math.log(Math.tan(Math.PI / 4 + lat / 2));
    return { x: x, y: y };
}

function webMercatorToWgs84(x, y) {
    let lon = x / a;
    let lat = 2 * Math.atan(Math.pow(Math.E, y / a)) - Math.PI / 2;
    return { lon: lon, lat: lat }
}

// console.log(wgs84ToWebMercator(degToRad(50), degToRad(50)));
// console.log(wgs84ToWebMercator(degToRad(49.99999999999999), degToRad(49.99999999999999)));
// let wgs = webMercatorToWgs84(5565974.539663678, 6446275.841017158);
// console.log(wgs);
// console.log('lat: ' + radToDeg(wgs.lat) + ','
//     + 'lon:' + radToDeg(wgs.lon));


// console.log(2 * Math.atan(Math.pow(Math.E, 0 / a)) - Math.PI / 2);
// console.log(radToDeg(webMercatorToWgs84(5565974.539663678, 0).lat));

function lon2tile(lon, z) {
    return (Math.floor((lon + 180) / 360 * Math.pow(2, z)));
}

function lat2tile(lat, z) {
    return (Math.floor((1 - Math.log(Math.tan(lat * Math.PI / 180)
        + 1 / Math.cos(lat * Math.PI / 180)) / Math.PI)
        / 2 * Math.pow(2, z)));
}

function lon2x(lon, z) {
    return ((lon + 180) / 360 * Math.pow(2, z));
}

function lat2y(lat, z) {
    return ((1 - Math.log(Math.tan(lat * Math.PI / 180)
        + 1 / Math.cos(lat * Math.PI / 180)) / Math.PI)
        / 2 * Math.pow(2, z));
}

function tile2long(x, z) {
    return (x / Math.pow(2, z) * 360 - 180);
}
function tile2lat(y, z) {
    var n = Math.PI - 2 * Math.PI * y / Math.pow(2, z);
    return (180 / Math.PI * Math.atan(0.5 * (Math.exp(n) - Math.exp(-n))));
}

//test
//22.7490073, 114.3781213  z=15
//let mappiece = Math.pow(2, zoom_level); //一个方向的地图块数 1块256个像素

// console.log(lon2tile(long, zoom_level));
// console.log(lat2tile(lat, zoom_level));
// console.log(lon2x(long, zoom_level));
// console.log(lat2y(lat, zoom_level));

// console.log('----------------------');
// console.log(tile2long(26794, zoom_level));
// console.log(tile2long(26794 + 1 / 256, zoom_level));

//https://tile.openstreetmap.org/z/x/y.png
//https://tile.openstreetmap.org/15/26794/14256.png

let zoom_level = 15;
let lat = 22.7490073;
let long = 114.3781213;
let x = lon2x(long, zoom_level);
let y = lat2y(lat, zoom_level);

let tilex = lon2tile(long, zoom_level);
let tiley = lat2tile(lat, zoom_level);

let offsetx = Math.floor((x - tilex) * 256);
let offsety = Math.floor((y - tiley) * 256);

//计算出瓦片地图的基准坐标,假设显示的宽高为1600*900
let width = 1600;
let height = 900;
let minx = width / 2 - offsetx;
let maxx = minx + 256;
let miny = height / 2 - offsety;
let maxy = miny + 256;

//计算出四个方向各需要的块数
//p1 p2 p3 p4 上 下 左 右
let p1 = Math.floor(maxy / 256 + 0.5);
let p2 = Math.floor((height - miny) / 256 + 0.5);
let p3 = Math.floor(maxx / 256 + 0.5);
let p4 = Math.floor((width - minx) / 256 + 0.5);

//基于已知条件计算需要的各个tile
let tiles = new Array();
tiles.push({ x: tilex, y: tiley });
for (let i = 1; i < p4; i++) {
    tiles.push({ x: tilex + i, y: tiley });
}
for (let i = 1; i < p3; i++) {
    tiles.push({ x: tilex - i, y: tiley });
}
for (let i = 1; i < p1; i++) {
    for (let j = 0; j < p3 + p4 - 1; j++) {
        tiles.push({ x: tilex - p3 + 1 + j, y: tiley - i });
    }
}
for (let i = 1; i < p2; i++) {
    for (let j = 0; j < p3 + p4 - 1; j++) {
        tiles.push({ x: tilex - p3 + 1 + j, y: tiley + i });
    }
}
console.log(tiles);
console.log(tiles.length);

//渲染问题
//通过与基准之间的位置关系计算渲染的坐标
//minx miny 即为基准tile当前渲染位置

for (let i = 0; i < tiles.length; i++) {
    const element = tiles[i];
    let renderx = (element.x - tilex) * 256 + minx;
    let rendery = (element.y - tiley) * 256 + miny;
    console.log({ x: renderx, y: rendery });
}