pow10.go

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package math
     6  
     7  // pow10tab stores the pre-computed values 10**i for i < 32.
     8  var pow10tab = [...]float64{
     9  	1e00, 1e01, 1e02, 1e03, 1e04, 1e05, 1e06, 1e07, 1e08, 1e09,
    10  	1e10, 1e11, 1e12, 1e13, 1e14, 1e15, 1e16, 1e17, 1e18, 1e19,
    11  	1e20, 1e21, 1e22, 1e23, 1e24, 1e25, 1e26, 1e27, 1e28, 1e29,
    12  	1e30, 1e31,
    13  }
    14  
    15  // pow10postab32 stores the pre-computed value for 10**(i*32) at index i.
    16  var pow10postab32 = [...]float64{
    17  	1e00, 1e32, 1e64, 1e96, 1e128, 1e160, 1e192, 1e224, 1e256, 1e288,
    18  }
    19  
    20  // pow10negtab32 stores the pre-computed value for 10**(-i*32) at index i.
    21  var pow10negtab32 = [...]float64{
    22  	1e-00, 1e-32, 1e-64, 1e-96, 1e-128, 1e-160, 1e-192, 1e-224, 1e-256, 1e-288, 1e-320,
    23  }
    24  
    25  // Pow10 returns 10**n, the base-10 exponential of n.
    26  //
    27  // Special cases are:
    28  //
    29  //	Pow10(n) =    0 for n < -323
    30  //	Pow10(n) = +Inf for n > 308
    31  func Pow10(n int) float64 {
    32  	if 0 <= n && n <= 308 {
    33  		return pow10postab32[uint(n)/32] * pow10tab[uint(n)%32]
    34  	}
    35  
    36  	if -323 <= n && n <= 0 {
    37  		return pow10negtab32[uint(-n)/32] / pow10tab[uint(-n)%32]
    38  	}
    39  
    40  	// n < -323 || 308 < n
    41  	if n > 0 {
    42  		return Inf(1)
    43  	}
    44  
    45  	// n < -323
    46  	return 0
    47  }
    48
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