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3

1

This is the naive algorithm that calculates integer log2(n),

 function ilog2(n) // n is a positive non-zero BigInt
 const C1 = BigInt(1)
 const C2 = BigInt(2)
 for(var count=0; n>C1; count++) n = n/C2
 return count
 // example ilog2(16n)==4

I am testing optimizations (see example below) but they are not "for log2", as commented here.

There are something better using WebAssembly?

PS: if it is not possible a generic solution, to solve my problem I can use functions like ilog2_64bits(n), with WebAssembly truncating or ignoring some bits of input BigInt.

---

non-WebAssembly

Example of not so optimized in pure Javascript (ideal is WebAssembly!)... Trying adaptation of integerLogarithm() of BigInteger.js:

 function ilog2_pe(value, base=2n) 
 // example: ilog2_pe(255n) returns p: 128n, e: 7n 
 if (base <= value) 
 let i = ilog2_pe(value, base**2n);
 let t = i.p*base
 return (t <= value)
 ? p: t, e: i.e*2n + 1n 
 : p: i.p, e: i.e*2n 
 
 return p: 1n, e: 0n ;
 

 function ilog2_str(n) // 2*faster!
 return n.toString(2).length - 1
 

PS: it is running in modern browsers and last versions of NodeJS.

---

About performance... ilog2_pe(x64bits) is 4 times faster tham the naive ilog2(), ilog2_pe(x1024bits) is ~9 times faster... Was expected... But:

Any non-WebAssembly solution has so ugly performance, even for 512, 1024, 2048 bits... counting string digits with ilog_str() is 2 times faster!
For less tham 64 bits is 3 or more times faster.

javascript logarithm

share|improve this question

edited Mar 26 at 23:01

Peter Krauss

asked Mar 26 at 10:43

Peter KraussPeter Krauss

5,9431111 gold badges9090 silver badges193193 bronze badges

add a comment | 

3

1

This is the naive algorithm that calculates integer log2(n),

 function ilog2(n) // n is a positive non-zero BigInt
 const C1 = BigInt(1)
 const C2 = BigInt(2)
 for(var count=0; n>C1; count++) n = n/C2
 return count
 // example ilog2(16n)==4

I am testing optimizations (see example below) but they are not "for log2", as commented here.

There are something better using WebAssembly?

PS: if it is not possible a generic solution, to solve my problem I can use functions like ilog2_64bits(n), with WebAssembly truncating or ignoring some bits of input BigInt.

---

non-WebAssembly

Example of not so optimized in pure Javascript (ideal is WebAssembly!)... Trying adaptation of integerLogarithm() of BigInteger.js:

 function ilog2_pe(value, base=2n) 
 // example: ilog2_pe(255n) returns p: 128n, e: 7n 
 if (base <= value) 
 let i = ilog2_pe(value, base**2n);
 let t = i.p*base
 return (t <= value)
 ? p: t, e: i.e*2n + 1n 
 : p: i.p, e: i.e*2n 
 
 return p: 1n, e: 0n ;
 

 function ilog2_str(n) // 2*faster!
 return n.toString(2).length - 1
 

PS: it is running in modern browsers and last versions of NodeJS.

---

About performance... ilog2_pe(x64bits) is 4 times faster tham the naive ilog2(), ilog2_pe(x1024bits) is ~9 times faster... Was expected... But:

Any non-WebAssembly solution has so ugly performance, even for 512, 1024, 2048 bits... counting string digits with ilog_str() is 2 times faster!
For less tham 64 bits is 3 or more times faster.

javascript logarithm

share|improve this question

edited Mar 26 at 23:01

Peter Krauss

asked Mar 26 at 10:43

Peter KraussPeter Krauss

5,9431111 gold badges9090 silver badges193193 bronze badges

add a comment | 

This is the naive algorithm that calculates integer log2(n),

 function ilog2(n) // n is a positive non-zero BigInt
 const C1 = BigInt(1)
 const C2 = BigInt(2)
 for(var count=0; n>C1; count++) n = n/C2
 return count
 // example ilog2(16n)==4

I am testing optimizations (see example below) but they are not "for log2", as commented here.

There are something better using WebAssembly?

PS: if it is not possible a generic solution, to solve my problem I can use functions like ilog2_64bits(n), with WebAssembly truncating or ignoring some bits of input BigInt.

---

non-WebAssembly

Example of not so optimized in pure Javascript (ideal is WebAssembly!)... Trying adaptation of integerLogarithm() of BigInteger.js:

 function ilog2_pe(value, base=2n) 
 // example: ilog2_pe(255n) returns p: 128n, e: 7n 
 if (base <= value) 
 let i = ilog2_pe(value, base**2n);
 let t = i.p*base
 return (t <= value)
 ? p: t, e: i.e*2n + 1n 
 : p: i.p, e: i.e*2n 
 
 return p: 1n, e: 0n ;
 

 function ilog2_str(n) // 2*faster!
 return n.toString(2).length - 1
 

PS: it is running in modern browsers and last versions of NodeJS.

---

About performance... ilog2_pe(x64bits) is 4 times faster tham the naive ilog2(), ilog2_pe(x1024bits) is ~9 times faster... Was expected... But:

Any non-WebAssembly solution has so ugly performance, even for 512, 1024, 2048 bits... counting string digits with ilog_str() is 2 times faster!
For less tham 64 bits is 3 or more times faster.

javascript logarithm

share|improve this question

edited Mar 26 at 23:01

Peter Krauss

asked Mar 26 at 10:43

Peter KraussPeter Krauss

5,9431111 gold badges9090 silver badges193193 bronze badges

 function ilog2(n) // n is a positive non-zero BigInt
 const C1 = BigInt(1)
 const C2 = BigInt(2)
 for(var count=0; n>C1; count++) n = n/C2
 return count
 // example ilog2(16n)==4

 function ilog2_pe(value, base=2n) 
 // example: ilog2_pe(255n) returns p: 128n, e: 7n 
 if (base <= value) 
 let i = ilog2_pe(value, base**2n);
 let t = i.p*base
 return (t <= value)
 ? p: t, e: i.e*2n + 1n 
 : p: i.p, e: i.e*2n 
 
 return p: 1n, e: 0n ;
 

 function ilog2_str(n) // 2*faster!
 return n.toString(2).length - 1
 

share|improve this question

edited Mar 26 at 23:01

Peter Krauss

asked Mar 26 at 10:43

Peter KraussPeter Krauss

5,9431111 gold badges9090 silver badges193193 bronze badges

share|improve this question

edited Mar 26 at 23:01

Peter Krauss

asked Mar 26 at 10:43

Peter KraussPeter Krauss

5,9431111 gold badges9090 silver badges193193 bronze badges

asked Mar 26 at 10:43

Peter KraussPeter Krauss

5,9431111 gold badges9090 silver badges193193 bronze badges

asked Mar 26 at 10:43

Peter KraussPeter Krauss

5,9431111 gold badges9090 silver badges193193 bronze badges