-- Lesson 5: Your Own Functions

local LAYERS          = 3
local STARS_PER_LAYER = 40
local ACROSS_STEP     = 37
local DOWN_STEP       = 61
local LAYER_SHIFT     = 13
local LAYER_SPEED     = 0.5
local LAYER_BRIGHT    = 55

local PLAYER_WIDTH  = 20
local PLAYER_HEIGHT = 6
local PLAYER_SPEED  = 3
local PLAYER_MARGIN = 6

local width  = overlayGetWidth()
local height = overlayGetHeight()

local drift       = 0
local playerX     = width // 2
local playerY     = height - PLAYER_HEIGHT - PLAYER_MARGIN
local movingLeft  = false
local movingRight = false


local function clamp(value, low, high)
    if value < low then
        return low
    end
    if value > high then
        return high
    end
    return value
end


local function shadeFor(layer)
    return math.random(60, 90) + layer * LAYER_BRIGHT
end


local function starAt(star, layer)
    local x = (star * ACROSS_STEP + layer * LAYER_SHIFT) % width
    local y = (star * DOWN_STEP + math.floor(drift * layer * LAYER_SPEED)) % height
    return x, y
end


local function drawStar(x, y, shade)
    colorForeground(shade, shade, shade)
    overlayPlot(x, y)
end


local function drawLayer(layer)
    for star = 1, STARS_PER_LAYER do
        local x, y = starAt(star, layer)
        drawStar(x, y, shadeFor(layer))
    end
end


local function drawStars()
    for layer = 1, LAYERS do
        drawLayer(layer)
    end
end


local function drawPlayer()
    colorForeground(80, 255, 120)
    overlayBox(playerX, playerY, playerX + PLAYER_WIDTH, playerY + PLAYER_HEIGHT)
end


local function movePlayer()
    if movingLeft then
        playerX = playerX - PLAYER_SPEED
    end
    if movingRight then
        playerX = playerX + PLAYER_SPEED
    end
    playerX = clamp(playerX, 0, width - PLAYER_WIDTH - 1)
end


function onInputPressed(what)
    if what == SWITCH_LEFT then
        movingLeft = true
    elseif what == SWITCH_RIGHT then
        movingRight = true
    end
end


function onInputReleased(what)
    if what == SWITCH_LEFT then
        movingLeft = false
    elseif what == SWITCH_RIGHT then
        movingRight = false
    end
end


function onOverlayUpdate()
    drift = drift + 1
    overlayClear()
    movePlayer()
    drawStars()
    drawPlayer()
    return OVERLAY_UPDATED
end