# CSharp - double.NaN(倍精度浮動小数のNaN) のバイト列判定

###### tags: `C#`, `.net`

~~~cs
using System.IO;
using System.Runtime.InteropServices;

[StructLayout(LayoutKind.Explicit)]
public struct NaNData
{
[FieldOffset(0)]
public float floatValue;
[FieldOffset(0)]
public double doubleValue;

[FieldOffset(0)]
public byte byte0;
[FieldOffset(1)]
public byte byte1;
[FieldOffset(2)]
public byte byte2;
[FieldOffset(3)]
public byte byte3;
[FieldOffset(4)]
public byte byte4;
[FieldOffset(5)]
public byte byte5;
[FieldOffset(6)]
public byte byte6;
[FieldOffset(7)]
public byte byte7;
}

class Program
{
static void Main(string[] args)
{
NaNData nan = new NaNData();
nan.byte0 = 0; // f!=0
nan.byte1 = 0; // f!=0
nan.byte2 = 1; // f!=0
nan.byte3 = 0; // f!=0
nan.byte4 = 0; // f!=0
nan.byte5 = 0; // f!=0
nan.byte6 = 0b1111 << 4; //BIT52-55 e=2047:bit0-4
nan.byte7 = 0b0 << 7 | 0b1111 << 3 | 0b111; //BIT56-63 s=x, e=2047:bit5-11

byte[] bytes = new byte[]{
nan.byte0,
nan.byte1,
nan.byte2,
nan.byte3,
nan.byte4,
nan.byte5,
nan.byte6,
nan.byte7,
};

System.Console.WriteLine($"{IsNaN(bytes)} / {nan.doubleValue}");
}

static public bool IsNaN(byte[] binary)
{
// see also: https://docs.oracle.com/cd/E19957-01/806-4847/ncg_math.html
/*
0b 0 11111111111 xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxx
+ -----+----- -----------------------+----------------------------------
| | |
| | +----------------- f: 最低一つのビットが1
| +----------------------------------------------- e: 2047
+------------------------------------------------------ s: 符号
*/
return
// e == 2047
(binary[7] & 0b01111111) == 0b01111111
&& (binary[6] & 0b11110000) == 0b11110000
&&
// f != 0
(
(binary[6] & 0b00001111) != 0
|| (binary[5] & 0b11111111) != 0
|| (binary[4] & 0b11111111) != 0
|| (binary[3] & 0b11111111) != 0
|| (binary[2] & 0b11111111) != 0
|| (binary[1] & 0b11111111) != 0
|| (binary[0] & 0b11111111) != 0
);

}
}

~~~