//Math是全局的
 
//Math.PI 數學里的3.1415926....
 
console.log(Math.PI);
 
//取隨機數
//js提供的隨機函數 Math.random() -->[0,1)范圍內的數
function random_int(start, end) {
 var num = start + (end - start) * Math.random();
 //小數---》整數,向下取整 Math.floor()
 return Math.floor(num);
}
 
console.log(random_int(5, 15));
console.log("=======================");
 
//數學的正弦，余弦，正切
//單位是數學的弧度,而不是度，方向是數學的正方向，逆時針方向
console.log(Math.sin(Math.PI / 4)); //sin 45°
console.log(Math.sin(Math.PI / 6)); //sin 45°
console.log(Math.cos(Math.PI / 6)); //cos 30°
console.log(Math.tan(Math.PI / 4)); //tan 45°
console.log("=======================");
 
//度[0,360)
//弧度[0,2*PI)
//度轉弧度
function degree_to_r(degree) {
 //PI-->180°
 return (degree / 180) * Math.PI;
}
 
//弧度轉度
function r_to_degree(r) {
 return (r / Math.PI) * 180;
}
 
var r = degree_to_r(90);
console.log(r);
console.log(r_to_degree(r));
 
console.log("=======================");
 
//sin 30°= 0.5 ,asin(0.5)對應多少度呢
//asin(0.5)算出的是弧度單位
//范圍在[-2PI,2PI]
r = Math.asin(0.5);
console.log(Math.floor(r_to_degree(r)));
r = Math.acos(0.5)
console.log(Math.floor(r_to_degree(r)));
 
console.log("=======================");
 
//返回一個坐標對應的角度,范圍[-PI,PI]
//Math.atan2(y,x);
r = Math.atan2(-1, 1);
console.log(r_to_degree(r));
r = Math.atan2(0, -1);
console.log(r_to_degree(r));
 
console.log("=======================");
//Math.sqrt 開平方
console.log(Math.sqrt(16));//16的平方根
console.log(Math.sqrt(2)); //2的平方根
console.log("=======================");
 
//計算兩點之間的距離
function vector_distance(lhs_x, lhs_y, rhs_x, rhs_y) {
 var len = (lhs_x - rhs_x) * (lhs_x - rhs_x) + (lhs_y - rhs_y) * (lhs_y - rhs_y);
 return Math.sqrt(len);
}
 
console.log(vector_distance(0, 0, 1, 1));
console.log("=======================");