compile.js

module.exports = compile;

var BaseFuncs = require("boolbase"),
    trueFunc  = BaseFuncs.trueFunc,
    falseFunc = BaseFuncs.falseFunc;

/*
	returns a function that checks if an elements index matches the given rule
	highly optimized to return the fastest solution
*/
function compile(parsed){
	var a = parsed[0],
	    b = parsed[1] - 1;

	//when b <= 0, a*n won't be possible for any matches when a < 0
	//besides, the specification says that no element is matched when a and b are 0
	if(b < 0 && a <= 0) return falseFunc;

	//when a is in the range -1..1, it matches any element (so only b is checked)
	if(a ===-1) return function(pos){ return pos <= b; };
	if(a === 0) return function(pos){ return pos === b; };
	//when b <= 0 and a === 1, they match any element
	if(a === 1) return b < 0 ? trueFunc : function(pos){ return pos >= b; };

	//when a > 0, modulo can be used to check if there is a match
	var bMod = b % a;
	if(bMod < 0) bMod += a;

	if(a > 1){
		return function(pos){
			return pos >= b && pos % a === bMod;
		};
	}

	a *= -1; //make `a` positive

	return function(pos){
		return pos <= b && pos % a === bMod;
	};
}