Problem I cannot solve, should be simple math for someone...

cheery.js

function oscillator(low, high, inc) {

    // basic test for illegal parameters
    if (low > high || inc < 0 ||  2 * (high - low) < inc) 
        return function() { return NaN; };

    var curr = low;
    return function() {
        var ret = curr;
        curr += inc;

        if (curr > high || curr < low) 
        {   
            curr = inc>0 ? 2 * high - curr: 2 * low - curr;  
            inc = -inc;
        };

        return ret;
    }; 
}

ori-via-stackoverflow.js

function oscillator(low, high, increment) {

    // basic test for illegal parameters
    if (low > high || increment < 0) 
        return function() { return NaN; };

    var curr = low;
    return function() {
        var ret = curr;
        curr += increment;

        // if the next number will exceed the boundaries, reverse the increment
        if (curr + increment > high || curr + increment < low) 
            increment = -increment;

        return ret;
    }; 
}

oscillating-iterator.coffee

# Pseudo Coffeescript class

Class OscillatingIterator
	constructor: ( low, high, increment )->
		this.low = low
                this.high = high
		this.i = increment
        iterate: ->
                ### can haz magical math code plz? ###

# Usage

oi = new OscillatingIterator( 1, 10 , 1 )

oi.iterate() #=> 1
oi.iterate() #=> 2
oi.iterate() #=> 3
oi.iterate() #=> 4
oi.iterate() #=> 5
oi.iterate() #=> 6
oi.iterate() #=> 7
oi.iterate() #=> 8
oi.iterate() #=> 9
oi.iterate() #=> 10
oi.iterate() #=> 9
oi.iterate() #=> 8
oi.iterate() #=> 7
oi.iterate() #=> 6
oi.iterate() #=> 5
oi.iterate() #=> 4
oi.iterate() #=> 3
oi.iterate() #=> 2
oi.iterate() #=> 1
oi.iterate() #=> 2
oi.iterate() #=> 3
oi.iterate() #=> etc...

solution1.coffee

# V1

class OscillatingIterator 
  constructor: ( @min, @max, @incr )-> 
    @index = 0
  iterate: ->
    @index+=@incr*( @dir = if @index >= @max and @dir isnt -1 then -1 else if @index<=@min and @dir isnt +1 then +1 else @dir )
      

@ph No big rush, was just poking you in a very complex way! I'm actually logging all this for meself

for more on ori-via-stackoverflow.js see http://stackoverflow.com/a/9236626/261114

I ended up using cheery's tweaked version of Ori's http://stackoverflow.com/a/9238879/261114