local ffi = require "ffi"

ffi.cdef[[
double hypot(double x, double y);
]]
local hypot = ffi.C.hypot

local function dashed(points, pattern)
    local x0, y0, x1, y1
    local cx, cy
    local i, j
    local d, h
    local polylines = {}
    local polyline
    local draw = true
    x0, y0 = unpack(points[1])
    polyline = {{x0, y0}}
    i = 2
    x1, y1 = unpack(points[i])
    j = 1
    d = pattern[j]
    while true do
        h = hypot(x1-x0, y1-y0)
        if d < h then
            cx = x0 + (x1-x0)*d/h
            cy = y0 + (y1-y0)*d/h
            if draw then
                table.insert(polyline, {cx, cy})
                table.insert(polylines, polyline)
            else
                polyline = {{cx, cy}}
            end
            x0, y0 = cx, cy
            draw = not draw
            if j < #pattern then j = j + 1 else j = 1 end
            d = pattern[j]
        else
            if draw then
                table.insert(polyline, {x1, y1})
            end
            d = d - h
            if i < #points then i = i + 1 else break end
            x0, y0 = x1, y1
            x1, y1 = unpack(points[i])
        end
    end
    if draw then
        table.insert(polyline, points[i])
        table.insert(polylines, polyline)
    end
    return polylines
end

return {dashed=dashed}