Text area script

function expanded_print_text(sep, end, elmn) {
    for (var part = 3; part < arguments.length; part += 1)
        $(elmn).html($(elmn).html()+arguments[part].toString()+(part != arguments.length-1 ? sep : ''))
    $(elmn).html($(elmn).html()+end)
}

function print_text() {
    expanded_print_text.bind(null, " ", "<br/>", "body").apply(null, arguments)
}

function getCursorPos(input) {
    if ("selectionStart" in input && document.activeElement == input) {
        return {
            start: input.selectionStart,
            end: input.selectionEnd
        };
    }
    else if (input.createTextRange) {
        var sel = document.selection.createRange();
        if (sel.parentElement() === input) {
            var rng = input.createTextRange();
            rng.moveToBookmark(sel.getBookmark());
            for (var len = 0;
                     rng.compareEndPoints("EndToStart", rng) > 0;
                     rng.moveEnd("character", -1)) {
                len++;
            }
            rng.setEndPoint("StartToStart", input.createTextRange());
            for (var pos = { start: 0, end: len };
                     rng.compareEndPoints("EndToStart", rng) > 0;
                     rng.moveEnd("character", -1)) {
                pos.start++;
                pos.end++;
            }
            return pos;
        }
    }
    return -1;
}

function setCursorPos(input, start, end) {
    if (arguments.length < 3) end = start;
    if ("selectionStart" in input) {
        setTimeout(function() {
            input.selectionStart = start;
            input.selectionEnd = end;
        }, 1);
    }
    else if (input.createTextRange) {
        var rng = input.createTextRange();
        rng.moveStart("character", start);
        rng.collapse();
        rng.moveEnd("character", end - start);
        rng.select();
    }
}


AVAILABLE = 8

function init() {
    print_text("<textarea id='area'/>")
    $(document).on('keypress', '#area', area_handler)
}

function get_slots() {
    split_it = (string) => { return string.split("") }
    restrict_it = (array) => { return array.slice(0, AVAILABLE) }
    return $('#area').val().split(/(\n)/g).map(split_it).map(restrict_it)
}

function set_from_slots(slots) {
    n_function = (slot) => slot.indexOf("\n")==-1 ? slot.length>=AVAILABLE ? slot.concat(["\n"]) : slot : slot
    end_result = slots.map(n_function)
    end_result = [].concat.apply([], end_result)
    end_result = end_result.join("")
    $('#area').val(end_result)
}

function shift(direction, slots, line, col, filler='') {
    common_checks_fail = () => (
            line < 0 ||
            line >= slots.length ||
            col < 0 ||
            col >= AVAILABLE
            )

    non_empty_array = (elmn) => elmn.length > 0
    compose_result = (insertion) => Array.prototype.concat(slots.slice(0, line), [insertion], slots.slice(line+1)).filter(non_empty_array)
    right_check = () => slots[line].length >= AVAILABLE
    left_result = () => slots[line].slice(0, col).concat(slots[line].slice(col+1, AVAILABLE+1))
    right_result = () => slots[line].slice(0, col).concat(filler, slots[line].slice(col, AVAILABLE-1), slots[line].length >= AVAILABLE ? "\n" : "")

    if (common_checks_fail()) return slots

    switch(direction) {
        case 'left': return compose_result(left_result())
        case 'right': return right_check() ? slots : compose_result(right_result())
    }
}

function area_handler(event) {
    key = event.which
    slots = get_slots()
    cur_pos = getCursorPos($('#area')[0])["start"]
    cur_line = Math.floor(cur_pos / (AVAILABLE+1))
    cur_col = cur_pos % (AVAILABLE+1)

    // ENTER = 13, not allowed
    // BACKSPACE = 8: LSHIFT
    // DELETE = 46: LSHIFT FOR NEXT
    // ASSUME NO SELECTIONS

    if (key == 13) {
        event.preventDefault() // Forbidden key
        return
    }

    if (key == 8 || key == 46) {
        event.preventDefault()
        new_slots = shift('left', slots, cur_line, cur_col+(key == 8 ? -1 : 1))
        set_from_slots(new_slots)
        setCursorPos($('#area')[0], cur_pos-1, cur_pos-1)
        return
    }

    if (event.key.length == 1) { // Ultimate shitcode
        event.preventDefault()
        new_slots = shift('right', slots, cur_line, cur_col, String.fromCharCode(key))
        set_from_slots(new_slots)
        return
    }
}

$(document).ready(init)