--constants
local TIMER_INTERVAL = .1 --updates the screen this often (in seconds)
local X_MIN, Y_MIN = 5, 1 --min values for file display
local X_MAX, Y_MAX = term.getSize() --max values for file display

--flags
local bDebug = true --display debugging info if true
local bInsert = true --insert text if true, override text if false

--variables
local tFile = { "" } --table of lines for the file being editted
local xFile, yFile = 1, 1 --the location of the cursor in the text file
local xScroll, yScroll = 0, 0 --amount file is scrolled on screen
local sDebugText = "" --debug text shown if debugging is on

--intialization
function intialize()
    term.setCursorPos(X_MIN,Y_MIN)
    term.setCursorBlink(true)
    draw()
end

--moves cursor around
function shiftCursor(xShift,yShift)
    local x,y = term.getCursorPos()

    --handle left/right movement
    if xFile == 1 and xShift < 0 then --if at start of line and moving left, move to end of above line (if exists)
        if yFile ~= 1 then
            xFile = #tFile[yFile-1] + 1
            yFile = yFile - 1
            x = X_MIN + xFile - 1
            y = y - 1
        end
    elseif xFile > #tFile[yFile] and xShift > 0 then --if at end of line and moving right, move to start of next line (if exists)
        if yFile ~= #tFile then
            xFile = 1
            yFile = yFile + 1
            x = X_MIN
            y = y + 1
        end
    else --regular movement
        xFile = xFile + xShift
        x = x + xShift
    end

    --handle up/down movement
    if yFile == 1 and yShift < 0 then --if at first line and moving up, move to start of line
        xFile = 1
        x = X_MIN
    elseif yFile == #tFile and yShift > 0 then --if at last line and moving down, move to end of line
        xFile = #tFile[yFile] + 1
        x = X_MIN + xFile - 1
    else --regular movement
        yFile = yFile + yShift
        y = y + yShift
        --yScroll = yScroll + yShift

        --if the cursor is past the end of the last character after having moved, move it to the last character
        if xFile > #tFile[yFile] then
            xFile = 1 + #tFile[yFile]
            x = X_MIN + #tFile[yFile]
        end
    end

    --update visual cursor
    --[[
    if x > X_MAX then
        xScroll = xScroll + x - X_MAX
        x = X_MAX
    elseif x < X_MIN then
        xScroll = xScroll - X_MIN - x
        x = X_MIN
    end
    ]]
    if y > Y_MAX then
        yScroll = yScroll + y - Y_MAX
        y = Y_MAX
    elseif y < Y_MIN then
        yScroll = yScroll - Y_MIN - y
        y = Y_MIN
    end
    sDebugText = sDebugText .. " Scroll:"..xScroll..","..yScroll
    term.setCursorPos(x, y)
end

--move cursor to the start of the line
function shiftCursorHome()
    local x, y = term.getCursorPos()
    xFile = 1
    term.setCursorPos(X_MIN, y)
end

--move cursor to the end of the line (end of text)
function shiftCursorEnd()
    local x, y = term.getCursorPos()
    xFile = 1 + #tFile[yFile]
    term.setCursorPos(X_MIN + #tFile[yFile], y)
end

--save editor's text to a file
function writeToFile()
    file = fs.open("test", "w")
    for i,line in ipairs(tFile) do
        file.writeLine(line) end
    file.close()
end

function write(sText)
    tFile[yFile] = string.sub(tFile[yFile], 1, xFile-1) .. sText .. string.sub(tFile[yFile], xFile)
    shiftCursor(1,0)
end

--display debug info in bottom right
function drawDebug(sText)
    local x,y = term.getCursorPos()

    term.setCursorPos(X_MAX + 1 - #sText, Y_MAX) --set cursor at bottom rightmost while still showing all text
    term.write(sText)

    term.setCursorPos(x, y)
end

--redraws entire screen
function draw()
    local x,y = term.getCursorPos()
    term.clear()

    --draw line numbers
    for i=Y_MIN,Y_MAX do
        if tFile[i+yScroll] ~= nil then
            term.setCursorPos(1, i)
            term.write(i + yScroll)
        end
    end

    --draw line number seperator
    for i=Y_MIN,Y_MAX do
        term.setCursorPos(X_MIN-1, i)
        term.write("|")
    end

    --draw file text
    --TODO stop draw at last visible line for code effeciency
    for i=Y_MIN,Y_MAX do
        if tFile[i+yScroll] ~= nil then
            term.setCursorPos(X_MIN, i)
            term.write(string.sub(tFile[i+yScroll], 1 + xScroll, X_MAX - X_MIN + xScroll))
        end
    end

    --[[
    for i=yScroll+1, #tFile do
        term.setCursorPos(X_MIN + xScroll, i)
        term.write(tFile[i])
    end
    ]]

    --draw debug
    if bDebug then
        drawDebug(sDebugText) end
    term.setCursorPos(x, y)
end

--main loop
function run()
    while true do
        sDebugText = ""
        --wait for key input
        local e, p1 = os.pullEvent()

        if e == "char" then --handles character input
            write(p1)
        elseif e == "key" then -- handles non-character input
            --DONT USE F2, F10, F11, ALT, ESC

            --functional keys
            if p1 == keys.f1 then -- F1 - menu

            elseif p1 == keys.f3 then -- F3 - save
                writeToFile()
                sDebugText = "SAVED"
            elseif p1 == keys.f4 then -- F4 - exit
                term.clear()
                term.setCursorPos(1,1)
                return

            --arrow keys
            elseif p1 == keys.up then
                shiftCursor(0, -1)
            elseif p1 == keys.down then
                shiftCursor(0, 1)
            elseif p1 == keys.left then
                shiftCursor(-1, 0)
            elseif p1 == keys.right then
                shiftCursor(1, 0)

            --backspace, delete
            elseif p1 == keys.backspace then
                if not (yFile == 1 and xFile == 1) then
                    if xFile == 1 then --if at the start of a line, concatenate line above with this line
                        shiftCursor(0,-1)
                        shiftCursorEnd()

                        tFile[yFile] = tFile[yFile] .. tFile[yFile+1] --concat lines
                        table.remove(tFile, yFile+1) --remove second line
                    else --otherwise just delete one character
                        shiftCursor(-1,0)
                        tFile[yFile] = string.sub(tFile[yFile], 1, xFile-1) .. string.sub(tFile[yFile], xFile+1)
                    end
                end
            elseif p1 == keys.delete then --delete
                if not (yFile == #tFile and xFile == #tFile[yFile] + 1) then
                    if xFile > #tFile[yFile] then --if cursor is at the end of the line, concatenate with line below
                        tFile[yFile] = tFile[yFile] .. tFile[yFile+1] --concat lines
                        table.remove(tFile, yFile+1) --remove second line
                    else --otherwise just delete one character
                        tFile[yFile] = string.sub(tFile[yFile], 1, xFile-1) .. string.sub(tFile[yFile], xFile+1)
                    end
                end

            --enter, home, end
            elseif p1 == keys.enter then
                if xFile <= #tFile[yFile] then --if cursor is between characters
                    table.insert(tFile, yFile+1, string.sub(tFile[yFile], xFile)) --move remainder of characters to a new line
                    tFile[yFile] = string.sub(tFile[yFile], 1, xFile-1) --and delete the moved characters from the previous line
                else --cursor at the end of the line
                    table.insert(tFile, yFile+1, "")
                end

                shiftCursorHome()
                shiftCursor(0,1)
            elseif p1 == keys.home then --home
                shiftCursorHome()
            elseif p1 == keys["end"] then --end
                shiftCursorEnd()
            end
        end

        local x,y = term.getCursorPos()
        sDebugText = sDebugText .. " File:"..xFile..","..yFile .." Screen:"..x..","..y
        draw()
    end
end

intialize()
run()