mirror of https://github.com/jkjoy/sunpeiwen.git
40 lines
1.1 KiB
JavaScript
40 lines
1.1 KiB
JavaScript
module.exports = compile;
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var BaseFuncs = require("boolbase"),
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trueFunc = BaseFuncs.trueFunc,
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falseFunc = BaseFuncs.falseFunc;
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/*
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returns a function that checks if an elements index matches the given rule
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highly optimized to return the fastest solution
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*/
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function compile(parsed){
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var a = parsed[0],
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b = parsed[1] - 1;
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//when b <= 0, a*n won't be possible for any matches when a < 0
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//besides, the specification says that no element is matched when a and b are 0
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if(b < 0 && a <= 0) return falseFunc;
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//when a is in the range -1..1, it matches any element (so only b is checked)
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if(a ===-1) return function(pos){ return pos <= b; };
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if(a === 0) return function(pos){ return pos === b; };
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//when b <= 0 and a === 1, they match any element
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if(a === 1) return b < 0 ? trueFunc : function(pos){ return pos >= b; };
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//when a > 0, modulo can be used to check if there is a match
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var bMod = b % a;
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if(bMod < 0) bMod += a;
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if(a > 1){
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return function(pos){
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return pos >= b && pos % a === bMod;
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};
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}
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a *= -1; //make `a` positive
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return function(pos){
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return pos <= b && pos % a === bMod;
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};
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} |