RoApp/Assets/Src/ThirdPlugins/MiscUtil/Conversion/DoubleConverter.cs

using System;
using System.Globalization;

namespace MiscUtil.Conversion
{
	/// <summary>
	/// A class to allow the conversion of doubles to string representations of
	/// their exact decimal values. The implementation aims for readability over
	/// efficiency.
	/// </summary>
	public class DoubleConverter
	{    
		/// <summary>
		/// Converts the given double to a string representation of its
		/// exact decimal value.
		/// </summary>
		/// <param name="d">The double to convert.</param>
		/// <returns>A string representation of the double's exact decimal value.</returns>
		public static string ToExactString (double d)
		{
			if (double.IsPositiveInfinity(d))
				return "+Infinity";
			if (double.IsNegativeInfinity(d))
				return "-Infinity";
			if (double.IsNaN(d))
				return "NaN";

			// Translate the double into sign, exponent and mantissa.
			long bits = BitConverter.DoubleToInt64Bits(d);
			bool negative = (bits < 0);
			int exponent = (int) ((bits >> 52) & 0x7ffL);
			long mantissa = bits & 0xfffffffffffffL;

			// Subnormal numbers; exponent is effectively one higher,
			// but there's no extra normalisation bit in the mantissa
			if (exponent==0)
			{
				exponent++;
			}
			// Normal numbers; leave exponent as it is but add extra
			// bit to the front of the mantissa
			else
			{
				mantissa = mantissa | (1L<<52);
			}
	        
			// Bias the exponent. It's actually biased by 1023, but we're
			// treating the mantissa as m.0 rather than 0.m, so we need
			// to subtract another 52 from it.
			exponent -= 1075;
	        
			if (mantissa == 0) 
			{
				return "0";
			}
	        
			/* Normalize */
			while((mantissa & 1) == 0) 
			{    /*  i.e., Mantissa is even */
				mantissa >>= 1;
				exponent++;
			}
	        
			// Construct a new decimal expansion with the mantissa
			ArbitraryDecimal ad = new ArbitraryDecimal (mantissa);
	        
			// If the exponent is less than 0, we need to repeatedly
			// divide by 2 - which is the equivalent of multiplying
			// by 5 and dividing by 10.
			if (exponent < 0) 
			{
				for (int i=0; i < -exponent; i++)
					ad.MultiplyBy(5);
				ad.Shift(-exponent);
			} 
				// Otherwise, we need to repeatedly multiply by 2
			else
			{
				for (int i=0; i < exponent; i++)
					ad.MultiplyBy(2);
			}
	        
			// Finally, return the string with an appropriate sign
			if (negative)
				return "-"+ad.ToString();
			else
				return ad.ToString();
		}
	    
		/// <summary>
		/// Private class used for manipulating sequences of decimal digits.
		/// </summary>
		class ArbitraryDecimal
		{
			/// <summary>Digits in the decimal expansion, one byte per digit</summary>
			byte[] digits;
			/// <summary> 
			/// How many digits are *after* the decimal point
			/// </summary>
			int decimalPoint=0;

			/// <summary> 
			/// Constructs an arbitrary decimal expansion from the given long.
			/// The long must not be negative.
			/// </summary>
			internal ArbitraryDecimal (long x)
			{
				string tmp = x.ToString(CultureInfo.InvariantCulture);
				digits = new byte[tmp.Length];
				for (int i=0; i < tmp.Length; i++)
					digits[i] = (byte) (tmp[i]-'0');
				Normalize();
			}
	        
			/// <summary>
			/// Multiplies the current expansion by the given amount, which should
			/// only be 2 or 5.
			/// </summary>
			internal void MultiplyBy(int amount)
			{
				byte[] result = new byte[digits.Length+1];
				for (int i=digits.Length-1; i >= 0; i--)
				{
					int resultDigit = digits[i]*amount+result[i+1];
					result[i]=(byte)(resultDigit/10);
					result[i+1]=(byte)(resultDigit%10);
				}
				if (result[0] != 0)
				{
					digits=result;
				}
				else
				{
					Array.Copy (result, 1, digits, 0, digits.Length);
				}
				Normalize();
			}
	        
			/// <summary>
			/// Shifts the decimal point; a negative value makes
			/// the decimal expansion bigger (as fewer digits come after the
			/// decimal place) and a positive value makes the decimal
			/// expansion smaller.
			/// </summary>
			internal void Shift (int amount)
			{
				decimalPoint += amount;
			}

			/// <summary>
			/// Removes leading/trailing zeroes from the expansion.
			/// </summary>
			internal void Normalize()
			{
				int first;
				for (first=0; first < digits.Length; first++)
					if (digits[first]!=0)
						break;
				int last;
				for (last=digits.Length-1; last >= 0; last--)
					if (digits[last]!=0)
						break;
	            
				if (first==0 && last==digits.Length-1)
					return;
	            
				byte[] tmp = new byte[last-first+1];
				for (int i=0; i < tmp.Length; i++)
					tmp[i]=digits[i+first];
	            
				decimalPoint -= digits.Length-(last+1);
				digits=tmp;
			}

			/// <summary>
			/// Converts the value to a proper decimal string representation.
			/// </summary>
			public override String ToString()
			{
				char[] digitString = new char[digits.Length];            
				for (int i=0; i < digits.Length; i++)
					digitString[i] = (char)(digits[i]+'0');
	            
				// Simplest case - nothing after the decimal point,
				// and last real digit is non-zero, eg value=35
				if (decimalPoint==0)
				{
					return new string (digitString);
				}
	            
				// Fairly simple case - nothing after the decimal
				// point, but some 0s to add, eg value=350
				if (decimalPoint < 0)
				{
					return new string (digitString)+
						new string ('0', -decimalPoint);
				}
	            
				// Nothing before the decimal point, eg 0.035
				if (decimalPoint >= digitString.Length)
				{
					return "0."+
						new string ('0',(decimalPoint-digitString.Length))+
						new string (digitString);
				}

				// Most complicated case - part of the string comes
				// before the decimal point, part comes after it,
				// eg 3.5
				return new string (digitString, 0, 
					digitString.Length-decimalPoint)+
					"."+
					new string (digitString,
					digitString.Length-decimalPoint, 
					decimalPoint);
			}
		}
	}
}