| 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; |
| }; |
| } |