Immediate-Mode GUI for ComputerCraft

local drawFuncs = {}
local maxid = 0

local defaults = {
    label = {
        bgColor = colors.black;
        textColor = colors.white;
    };
    button = {
        bgColor = colors.blue;
        textColor = colors.white;
        padding = {2, 1};
    };
    slider = {
        orientation = 'horizontal';
        length = 20;
        bgColor = colors.gray;
        fgColor = colors.blue;
        min = 0;
        max = 1;
    };
}

local mouse = {
    clicked = false;
    dragged = false;
    focus = nil;
    x = 1;
    y = 1;
}

local function copy(source, from)
    for k,v in pairs(from) do
        if type(v) == 'table' then
            copy(source[k], v)
        else
            source[k] = v
        end
    end
end

local function genid()
    maxid = maxid + 1
    return maxid
end

local function checkArea(mx, my, x, y, w, h)
    return mx >= x and mx < x + w and my >= y and my < y + h
end

local function checkClick(...)
    return mouse.clicked and checkArea(mouse.x, mouse.y, ...)
end

local function checkDrag(...)
    return mouse.dragged and checkArea(mouse.x, mouse.y, ...)
end

local function dimensions(text)
    return #text + (defaults.button.padding[1] or 2) * 2,
        1 + (defaults.button.padding[2] or 1) * 2
end

local function rect(x, y, width, height, color)
    local fill = string.rep(' ', width)
    if color then term.setBackgroundColor(color) end
    for i=1, height do
        term.setCursorPos(x, y + (i - 1))
        term.write(fill)
    end
end

local function checkInfo(info)
    if type(info) ~= 'table' then
        error("Bad argument: table expected", 3)
    end
end

local function drawButton(info)
    rect(info.x, info.y, info.width, info.height,
        info.bgColor or (defaults.button and defaults.button.bgColor))

    if info.text then
        textColor = info.textColor or defaults.button.textColor
        if textColor then term.setTextColor(textColor) end
        term.setCursorPos(
            info.x + math.ceil(info.width/2) - math.ceil(#info.text/2),
            info.y + math.ceil(info.height/2) - 1)
        term.write(tostring(info.text))
    end
end

local function drawSlider(info, width, height)
    rect(info.x, info.y, width, height, info.bgColor)

    -- i hate this math
    -- i really do
    if info.orientation == 'horizontal' then
        width = math.max(math.ceil(width * ((info.value - info.min) / (info.max - info.min))), 1)
    else
        height = math.max(math.ceil(height * ((info.value - info.min) / (info.max - info.min))), 1)
    end

    rect(info.x, info.y, width, height, info.fgColor)
end


function setDefaults(t)
    copy(defaults, t)
end

function clear(color)
    if color then term.setBackgroundColor(color) end
    term.clear()
end

function label(info)
    checkInfo(info)
    register(function()
        bgColor = info.bgColor or defaults.label.bgColor
        textColor = info.textColor or defaults.label.textColor
        if bgColor then term.setBackgroundColor(bgColor) end
        if textColor then term.setTextColor(textColor) end

        term.setCursorPos(info.x, info.y)
        term.write(info.text)
    end)
end

function button(info)
    checkInfo(info)

    local tw, th = dimensions(info.text)
    info.width = info.width or tw
    info.height = info.height or th
    info.bgColor = info.bgColor or defaults.button.bgColor
    info.textColor = info.textColor or defaults.button.textColor

    local clicked = checkClick(info.x, info.y, info.width, info.height)
    if clicked then
        mouse.clicked = false -- to make sure overlapping items don't receive clicks
    end

    info.text = tostring(info.text)
    register(drawButton, info)

    return clicked
end

function slider(info)
    checkInfo(info)

    setmetatable(info, { __index = defaults.slider })
    info.value = info.value or (info.max - info.min) / 2

    local width, height = info.length, 1
    if info.orientation == 'vertical' then
        width, height = height, width
    end

    local id = genid()
    local clicked = checkClick(info.x, info.y, width, height)
    local changed = false

    if clicked then
        mouse.clicked = false
        mouse.focus = id
    end

    if clicked or mouse.dragged and mouse.focus == id then
        local dist = (info.orientation == 'horizontal' and mouse.x - info.x or mouse.y - info.y)
        local ratio = math.min(dist / (info.length - 1), 1)
        info.value = ratio * (info.max - info.min) + info.min
        changed = true
    end

    register(drawSlider, info, width, height)
    return changed
end


function register(func, ...)
    table.insert(drawFuncs, {func, ...})
end

function draw()
    for i=1, #drawFuncs do
        local func, params = table.remove(drawFuncs[i], 1), drawFuncs[i]
        func(unpack(params))
    end
    drawFuncs = {}
end


function pullEvent()
    mouse.clicked = false
    mouse.dragged = false
    maxid = 0

    local params = {os.pullEvent()}
    local event = table.remove(params, 1)

    if event:match 'mouse' then
        mouse.x = params[2]
        mouse.y = params[3]
        if event == 'mouse_click' then
            mouse.clicked = true
        elseif event == 'mouse_drag' then
            mouse.dragged = true
        end
    end
    return event, unpack(params)
end