[dv8]

I was bored today... monkeying with JavaScript.

Code:

// JavaScript
function getIndex(input) {
	var index = -1, i = 0, found = false;
	while (i < input.form.length && index == -1)
	if (input.form[i] == input) index = i;
	else i++;
	return index;
}

function check (input) {
	if (input.form[getIndex(input)+1] != undefined) {
		if (input.value.length >= 30) {
			input.form[(getIndex(input)+1) % input.form.length].focus();
		}
	} else {
		if (input.value.length >= 30) {
			input.form[(getIndex(input)) % input.form.length].blur();
		}
	}
}

Usage:
<input type="text" name="field1" onKeyUp="check(this)">