local backingList = {}
local subFrames = {}
local length = 0
local frame = term.current()
local drawEntry = nil
local debugLog = fs.open("listframes.debug", "w")

-- pagination start line (not number of entries!)
local paginationPos = 1


local function setDrawEntryFunc(drawLineFunc)
    drawEntry = drawLineFunc
end

local function repositionEntriesAfter(idx)
    local tw, th = frame.getSize()
    local entryFrame = subFrames[idx]
    assert(entryFrame ~= nil, "provided entry at idx=" .. idx .. " has no frame")
    local xPos, yPos = entryFrame.getPosition()
    local xSize, ySize = entryFrame.getSize()
    local yStartNext = yPos + ySize
    while yStartNext <= th do
        idx = idx + 1
        entryFrame = subFrames[idx]
        if entryFrame == nil then
            break
        end
        xPos, yPos = entryFrame.getPosition()
        xSize, ySize = entryFrame.getSize()
        if yPos == yStartNext then
            break
        else
            entryFrame.reposition(1, yStartNext, tw, ySize, frame)
            entryFrame.setVisible(true)
        end
        yStartNext = yPos + ySize
    end
    idx = idx + 1
    entryFrame = subFrames[idx]
    while entryFrame ~= nil and entryFrame.isVisible() do
        entryFrame.setVisible(false)
        idx = idx + 1
        entryFrame = subFrames[idx]
    end
end

local function redrawEntry(idx)
    local entryFrame = subFrames[idx]
    if entryFrame ~= nil then
        entryFrame.setCursorPos(1,1)
        if drawEntry then
            drawEntry(entryFrame, backingList[idx])
        else
            entryFrame.setBackgroundColor(colors.red)
            entryFrame.clearLine()
            entryFrame.setBackgroundColor(colors.black)
        end
        local tw, th = frame.getSize()
        local xPos, yPos = entryFrame.getPosition()
        local xSize, ySize = entryFrame.getSize()
        local visible = (yPos+ySize-1) >= paginationPos and yPos <= th
        debugLog.writeLine(string.format("[redrawEntry] idx=%d, yPos=%d, visible=%s", idx, yPos, tostring(visible)))
        debugLog.flush()
        entryFrame.setVisible(visible)
    end
end

local function repositionOrCreateEntryFrame(idx)
    local previousFrame = subFrames[idx-1]
    local yPos = 2-paginationPos
    if previousFrame ~= nil then
        local xPos
        xPos, yPos = previousFrame.getPosition()
        local xSize, ySize = previousFrame.getSize()
        yPos = yPos + ySize
    end
    local tw, th = frame.getSize()
    local entryFrame = subFrames[idx]
    if entryFrame == nil then
        subFrames[idx] = window.create(frame, 1, yPos, tw, 1, false)
    else
        entryFrame.setVisible(false)
        local xSize, ySize = entryFrame.getSize()
        entryFrame.reposition(1, yPos, tw, ySize, frame)
    end
end

local function updateItemAt(idx, item)
    backingList[idx] = item
    repositionOrCreateEntryFrame(idx)
    if idx > length then
        length = idx
    end
    redrawEntry(idx)
    repositionEntriesAfter(idx)
end

local function clearFrom(idx)
    local tw, th = frame.getSize()
    for j = length, idx, -1 do
        backingList[j] = nil
        if subFrames[j] ~= nil then
            subFrames[j].setVisible(false)
        end
    end
    length = idx-1
    for y = idx-paginationPos+1, th, 1 do
        frame.setCursorPos(1, y)
        frame.clearLine()
    end
end

local function redraw()
    if drawEntry then
        local tw, th = frame.getSize()
        local yPos = 2-paginationPos
        for idx = 1, length, 1 do
            assert(backingList[idx] ~= nil, "nil element at idx="..idx)
            if yPos <= th then
                repositionOrCreateEntryFrame(idx)
                redrawEntry(idx)
                local xSize, ySize = subFrames[idx].getSize()
                yPos = yPos + ySize
            elseif subFrames[idx] ~= nil then
                subFrames[idx].setVisible(false)
            end
        end
    else
        frame.setBackgroundColor(colors.red)
        frame.clear()
        frame.setBackgroundColor(colors.black)
    end
end

local function setTerm(termlike)
    frame = termlike
    -- reposition visible subFrames
    redraw()
end

local function updatePage(scrollPos)
    if scrollPos ~= paginationPos then
        paginationPos = scrollPos
        redraw()
    end
end

local function estimatedHeight()
    local firstEntryFrame = subFrames[1]
    local lastEntryFrame = subFrames[length]
    if firstEntryFrame ~= nil and lastEntryFrame ~= nil then
        local firstXPos, firstYPos = firstEntryFrame.getPosition()
        local xPos, yPos = lastEntryFrame.getPosition()
        local xSize, ySize = lastEntryFrame.getSize()
        return yPos-firstYPos+ySize
    else
        return length
    end
end


return {
    setTerm=setTerm,
    setDrawEntryFunc=setDrawEntryFunc,
    updateItemAt=updateItemAt,
    clearFrom=clearFrom,
    redraw=redraw,
    updatePage=updatePage,
    estimatedHeight=estimatedHeight,
}