08-27-周三_17-09-29

node_modules/nth-check/compile.js

@@ -0,0 +1,40 @@
+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;
+	};
+}