utils.js

function lerp(A, B, t) {
    return A + (B-A) * t
}

//https://www.youtube.com/watch?v=fHOLQJo0FjQ
function getIntersection(A, B, C, D) {
    const tTop = (D.x - C.x) * (A.y - C.y) - (D.y-C.y) * (A.x-C.x)
    const uTop = (C.y - A.y) * (A.x - B.x) - (C.x -A.x) * (A.y - B.y)
    const bottom  = (D.y - C.y) * (B.x - A.x) -(D.x-C.x) * (B.y - A.y)

    if (bottom != 0) {
        const t = tTop/bottom
        const u = uTop/bottom

        if (t>=0 && t<=1 && u>=0 && u<=1) {
            return {
                x: lerp(A.x, B.x, t),
                y: lerp(A.y, B.y, t),
                offset: t
            }
        }
    }

    return null
}

function polysIntersect(poly1, poly2) {
    for (let i = 0; i<poly1.length; i++) {
        for (let j = 0; j<poly2.length; j++) {
            const touch = getIntersection(
                poly1[i],
                poly1[(i+1) % poly1.length], //so we do this because if the i+1 value (the next side of the polygon) is equal to the total number of sides, it will just go back to the first point (point 0)
                poly2[j],
                poly2[(j+1) % poly2.length]
            )

            if (touch) {
                return true
            }
        }
    }

    return false
}