// 获取小数部分
const fractional = function(x) {
	x = Math.abs(x);
	return x - Math.floor(x);
}
const formatInt = function(x, len) {
	let result = '' + x;
	len = len - result.length;
	while (len > 0) {
		result = '0' + result;
		len--;
	}
	return result;
}

/**
 * 创建标准分幅网格
 * @param west,south,east,north 传入要创建的标准分幅网格的经纬度范围
 * @param scalem 表示比例尺的分母（例如 10000 表示 1:1万）
 * @returns 返回一个 geojson 对象
 */
export function makeStandardMapGrids(west, south, east, north, scalem) {
	let lngDiff = 0;
	let latDiff = 0;
	let scaleCode = '';
	switch (scalem) {
		case 1000000:
			lngDiff = 6;
			latDiff = 4;
			break;
		case 500000:
			lngDiff = 3;
			latDiff = 2;
			scaleCode = 'B';
			break;
		case 250000:
			lngDiff = 1.5;
			latDiff = 1;
			scaleCode = 'C';
			break;
		case 100000:
			lngDiff = 0.5;
			latDiff = 1 / 3;
			scaleCode = 'D';
			break;
		case 50000:
			lngDiff = 0.25;
			latDiff = 1 / 6;
			scaleCode = 'E';
			break;
		case 25000:
			lngDiff = 0.125;
			latDiff = 1 / 12;
			scaleCode = 'F';
			break;
		case 10000:
			lngDiff = 0.0625;
			latDiff = 1 / 24;
			scaleCode = 'G';
			break;
		case 5000:
			lngDiff = 0.03125;
			latDiff = 1 / 48;
			scaleCode = 'H';
		case 2000:
			lngDiff = 37.5 / 3600.0;
			latDiff = 25.0 / 3600.0;
			scaleCode = 'I';
			break;
		case 1000:
			lngDiff = 18.75 / 3600.0;
			latDiff = 12.5 / 3600.0;
			scaleCode = 'J';
			break;
		case 500:
			lngDiff = 9.375 / 3600.0;
			latDiff = 6.25 / 3600.0;
			scaleCode = 'K';
			break;
		default:
			return null;
	}
	const GridX0 = -180;
	const GridX1 = 180;
	const GridY0 = -88;
	const GridY1 = 88;
	let x0 = Math.max(GridX0, west);
	let y0 = Math.max(GridY0, south);
	let x1 = Math.min(GridX1, east);
	let y1 = Math.min(GridY1, north);
	if (((x1 - x0) < lngDiff) || ((y1 - y0) < latDiff)) {
		return null;
	}

	let features = []; // 存储生成的面要素
	let coordinates = [] // 存储生成的面要素坐标对
	// 计算标准分幅网格行列范围
	const col0 = parseInt((x0 - GridX0) / lngDiff);
	const col1 = parseInt((x1 - GridX0) / lngDiff);
	const row0 = parseInt((y0 - GridY0) / latDiff);
	const row1 = parseInt((y1 - GridY0) / latDiff);
	const millionRowCode = 'ABCDEFGHIJKLMNOPQRSTUV';
	for (let row = row0; row <= row1; row++) {
		let gy0 = GridY0 + row * latDiff;
		let gy1 = gy0 + latDiff;
		let gcy = (gy0 + gy1) * 0.5; // 分幅中心点 y 坐标
		let millionRow = parseInt((gy0 - 0) / 4); // 1:100分幅行号
		let Hemisphere = ''; // 北半球标志
		if (millionRow < 0) {
			millionRow = -1 - millionRow;
			Hemisphere = 'S'; // 南半球标志
		}
		for (let col = col0; col <= col1; col++) {
			let gx0 = GridX0 + col * lngDiff;
			let gx1 = gx0 + lngDiff;
			let gcx = (gx0 + gx1) * 0.5; // 分幅中心点 x 坐标
			let millionCol = parseInt((gcx - GridX0) / 6) + 1; // 1:100分幅列号(从1开始)
			coordinates = [
				[
					[gx0, gy0],
					[gx1, gy0],
					[gx1, gy1],
					[gx0, gy1],
					[gx0, gy0]
				]
			];
			millionCol = (millionCol < 10) ? ('0' + millionCol) : millionCol;
			let gridID = Hemisphere + millionRowCode[millionRow] + millionCol;
			if (scaleCode != '') {
				// 计算当前分幅在 1:100万 分幅内的行列号(注意，这里行列号从左向右，从北向南，从1开始编号)
				let colID = parseInt((fractional((gcx - GridX0) / 6) * 6) / lngDiff) + 1;
				let rowID = parseInt((fractional((GridY1 - gcy) / 4) * 4) / latDiff) + 1;
				gridID += scaleCode + formatInt(rowID, 3) + formatInt(colID, 3);
			}
			// 生成矢量要素(几何信息+属性信息)
			let feature = {
				type: "Feature",
				geometry: {
					type: "Polygon",
					coordinates: coordinates
				},
				properties: {
					ID: gridID,
					extend: '西:' + gx0 + ' 东:' + gx1 + ' 南:' + gy0 + ' 北:' + gy1,
					lat: gcy.toFixed(6),
					lng: gcx.toFixed(6)
				},
				location: {
					latitude: Number(gcy.toFixed(6)),
					longitude: Number(gcx.toFixed(6)),
				}
			};
			features.push(feature);
		}
	}
	return {
		type: "FeatureCollection",
		features: features
	};
}

 我国把 1:1 万、1:2.5 万、1:5 万、1:10 万、1:25 万、1:50 万、1:100 万 7 种比例尺作为国家基本地图的比例尺系列。根据国家标准GB/13989-92 《国家基本比例尺地形图分幅和编号》规定，我国基本比例尺地形图均1:100 万地形图为基础，按规定的经差和纬差划分图幅。下表为地形图的经纬差、行列数及图幅数。