-- Love2d Mesh

function makeVertices(src, xc, yc)
  xyuv = {}
  local xi, yi, u, v
  local yi = 0
  for yi = 0, yc do
    v = yi / yc
    local sx0 = (src[4][1] - src[1][1]) * v + src[1][1]
    local sy0 = (src[4][2] - src[1][2]) * v + src[1][2]
    local sx1 = (src[3][1] - src[2][1]) * v + src[2][1]
    local sy1 = (src[3][2] - src[2][2]) * v + src[2][2]
    for xi = 0, xc do
      u = xi / xc
      local x = (sx1 - sx0) * u + sx0
      local y = (sy1 - sy0) * u + sy0
      table.insert(xyuv, {x, y, u, v})
    end
  end

  vidx = {}
  for y = 1, yc do
    for x = 1, xc do
      local i0 = x + ((xc + 1) * (y - 1))
      local i1 = i0 + 1
      local i2 = i0 + (xc + 1)
      local i3 = i2 + 1
      table.insert(vidx, i0)
      table.insert(vidx, i1)
      table.insert(vidx, i2)

      table.insert(vidx, i2)
      table.insert(vidx, i1)
      table.insert(vidx, i3)
    end
  end

  vt = {}
  for j = 1, #vidx do
    local i = vidx[j]
    local x, y, u, v
    x, y = xyuv[i][1], xyuv[i][2]
    u, v = xyuv[i][3], xyuv[i][4]
    table.insert(vt, {x, y, u, v, 1.0, 1.0, 1.0, 1.0})
  end

  return vt, xyuv
end

-- init
function love.load()
  -- get window width and height
  wdw_w, wdw_h = love.graphics.getDimensions()

  img = love.graphics.newImage("uvcheckermap01-512.png")

  divide = 3

  -- src = { {270, 40}, {370, 40}, {620, 440}, {20, 440} }
  -- src = { {270, 20}, {370, 60}, {620, 350}, {20, 440} }
  src = { {270, 240}, {370, 240}, {630, 460}, {10, 460} }

  vert, xyuv = makeVertices(src, divide, divide)

  mesh = love.graphics.newMesh(vert, "triangles")
  mesh:setTexture(img)

  guide = true
  mesh_refresh = false

  ang = 0
end

-- update
function love.update(dt)
  if mesh_refresh then
    vert, xyuv = makeVertices(src, divide, divide)
    mesh = love.graphics.newMesh(vert, "triangles")
    mesh:setTexture(img)
  end

  ang = ang + 90 * dt
  local d = 140 * math.sin(math.rad(ang))

  -- change x0, x1
  src[1][1] = (wdw_w / 2) - 150 + d
  src[2][1] = (wdw_w / 2) + 150 - d

  vert, xyuv = makeVertices(src, divide, divide)
  mesh:setVertices(vert, 1)
end

-- draw
function love.draw()
  -- fill BG color
  love.graphics.setColor(0.1, 0.2, 0.4)
  love.graphics.rectangle("fill", 0, 0, wdw_w, wdw_h)

  -- draw mesh
  love.graphics.setColor(1, 1, 1)
  love.graphics.draw(mesh, 0, 0)

  if guide then
    -- draw lines
    love.graphics.setColor(1, 1, 1, 0.5)
    for i=1, #vert, 3 do
      local vlst = {vert[i][1], vert[i][2], vert[i+1][1], vert[i+1][2], vert[i+2][1], vert[i+2][2]}
      love.graphics.polygon("line", vlst)
    end

    --draw points
    love.graphics.setColor(0, 1, 1, 0.5)
    for i = 1, #xyuv do
      local x = xyuv[i][1]
      local y = xyuv[i][2]
      love.graphics.ellipse("line", x, y, 3, 3)
    end
  end

  -- print FPS
  love.graphics.setColor(1, 1, 1)
  love.graphics.print("FPS: "..tostring(love.timer.getFPS()), 2, 2)
  love.graphics.print("Divide: "..tostring(divide), 2, 20)
  love.graphics.print("G key : Guide on/off", 2, 40)
  love.graphics.print("Up, Down : Divide +/-", 2, 60)
end

function love.keypressed(key, isrepeat)
  -- ESC to exit
  if key == "escape" then
    love.event.quit()
  end
  if key == "g" then
    guide = not guide
  end
  if key == "up" then
    if divide < 32 then
      divide = divide + 1
    end
    mesh_refresh = true
  end
  if key == "down" then
    if divide > 2 then
      divide = divide - 1
    end
    mesh_refresh = true
  end
end