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;
    };
}