aaac7fed
 liuqimichale
add
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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;
};
}
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