ListFrames = {
    frame = term.current(),
    drawEntry = nil,
    backingList = {},
    subFrames = {},
    length = 0,
    estimatedTotalHeight = 0,
    -- pagination start line (not number of entries!)
    paginationPos = 1,
}

function ListFrames:new(o)
    o = o or {}
    setmetatable(o, self)
    self.__index = self
    return o
end

function ListFrames:repositionEntriesAfter(idx)
    local tw, th = self.frame.getSize()
    local entryFrame = self.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 = self.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, self.frame)
            entryFrame.setVisible(true)
        end
        yStartNext = yPos + ySize
    end
    idx = idx + 1
    entryFrame = self.subFrames[idx]
    while entryFrame ~= nil and entryFrame.isVisible() do
        entryFrame.setVisible(false)
        idx = idx + 1
        entryFrame = self.subFrames[idx]
    end
end

function ListFrames:redrawEntry(idx)
    local entryFrame = self.subFrames[idx]
    if entryFrame ~= nil then
        entryFrame.setCursorPos(1,1)
        if self.drawEntry then
            self.drawEntry(entryFrame, self.backingList[idx])
        else
            entryFrame.setBackgroundColor(colors.red)
            entryFrame.clearLine()
            entryFrame.setBackgroundColor(colors.black)
        end
        local tw, th = self.frame.getSize()
        local xPos, yPos = entryFrame.getPosition()
        local xSize, ySize = entryFrame.getSize()
        local yEnd = yPos+ySize-1
        entryFrame.setVisible(yEnd > 0 and yPos <= th)
        yEnd = yEnd+self.paginationPos-1
        if yEnd > self.estimatedTotalHeight then
            self.estimatedTotalHeight = yEnd
        end
    end
end

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

function ListFrames:updateItemAt(idx, item)
    self.backingList[idx] = item
    self:repositionOrCreateEntryFrame(idx)
    if idx > self.length then
        self.length = idx
    end
    self:redrawEntry(idx)
    self:repositionEntriesAfter(idx)
end

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

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

function ListFrames:updatePage(scrollPos)
    if scrollPos ~= self.paginationPos then
        self.paginationPos = scrollPos
        self:redraw()
    end
end

function ListFrames:estimatedHeight()
    local lastEntryFrame = self.subFrames[self.length]
    if lastEntryFrame ~= nil and lastEntryFrame.isVisible() then
        local xPos, yPos = lastEntryFrame.getPosition()
        local xSize, ySize = lastEntryFrame.getSize()
        self.estimatedTotalHeight = yPos+ySize-1+self.paginationPos-1
    end
    return self.estimatedTotalHeight
end


return ListFrames