public static decimal GetValueFromBytes(byte[] bytes, StCoordinatesSystem coordinatesSystem, StCoordType type)
{
if (type == StCoordType.M)
{
throw new NotSupportedException("M-Coordinates are not supported yet");
}
decimal val = 0;
// The first byte will give the sign
byte b;
bool isNegative = IsNegative(bytes, coordinatesSystem, out b);
val += b * coordinatesSystem.ByteFactors[0];
// Next bytes
for (int i = 1; i < bytes.Length; ++i)
{
b = bytes[i];
if (b >= coordinatesSystem.ByteCoeff[i + 1])
{
b = (byte)(b - coordinatesSystem.RestartValue);
}
val += b * coordinatesSystem.ByteFactors[i];
}
if (isNegative)
{
return(coordinatesSystem.GetMin(type) - val);
}
return(val + coordinatesSystem.GetMin(type));
}
public static byte[] GetBytesFromValue(decimal val, StCoordinatesSystem coordinatesSystem, StCoordType type)
{
if (type == StCoordType.M)
{
throw new NotSupportedException("M Coordiante is not supported yet");
}
bool isNegative = false;
decimal valFromMin = val - coordinatesSystem.GetMin(type);
if (valFromMin < 0)
{
// We have to encode a negative number
isNegative = true;
valFromMin = Math.Abs(valFromMin);
}
// Search how many bytes have to be encoded
int maxIndex = coordinatesSystem.ByteFactors.Length - 1;
while (maxIndex > 0 && valFromMin < coordinatesSystem.ByteFactors[maxIndex])
{
maxIndex--;
}
var bytes = new List <byte>();
// Encode utlimate byte first
decimal div = valFromMin / coordinatesSystem.ByteFactors[maxIndex];
var byteValue = (byte)Math.Floor(div);
decimal restVal = valFromMin - (byteValue * coordinatesSystem.ByteFactors[maxIndex]);
bytes.Insert(0, byteValue);
// Encode bytes from penultimate to first
for (int i = maxIndex - 1; i >= 0; --i)
{
div = restVal / coordinatesSystem.ByteFactors[i];
byteValue = (byte)Math.Floor(div);
restVal -= byteValue * coordinatesSystem.ByteFactors[i];
// byteValue cannot be < 128 (RestartValue)
byteValue += coordinatesSystem.RestartValue;
if (byteValue < coordinatesSystem.RestartValue || byteValue > 255)
{
throw new IndexOutOfRangeException();
}
bytes.Insert(0, byteValue);
}
if (isNegative)
{
SetNegative(bytes, coordinatesSystem);
}
return(bytes.ToArray());
}
public static byte[] GetDifferenceFromBytes(byte[] initialBytes, byte[] previousBytes, StCoordinatesSystem coordinatesSystem, StCoordType type)
{
// We mut encode the difference with the previous coordinate instead of the real value
decimal initialRealValue = GetValueFromBytes(initialBytes, coordinatesSystem, type);
// Get The previous value
decimal previousValue = GetValueFromBytes(previousBytes, coordinatesSystem, type);
// Get the difference
decimal difference = initialRealValue - previousValue;
// Get the value to encode
decimal valueToEncode = coordinatesSystem.GetMin(type) + difference;
return(GetBytesFromValue(valueToEncode, coordinatesSystem, type));
}
public static byte[] GetBytesFromDifference(byte[] initialBytes, byte[] previousBytes, StCoordinatesSystem coordinatesSystem, StCoordType type)
{
// Exemple : LINESTRING (-1 -1, 0 0)
// - initial bytes : { 208 156 1 } ==> the value of the point is 0
// but the difference with the previous point is encoded => 1
// - previous bytes : { 176 139 197 186 141 17 } ==> this is the encoding for -1
//
// Here we get the value decoded from the bytes representing the difference
decimal encodedDifference = GetValueFromBytes(initialBytes, coordinatesSystem, type);
// Get The previous value
decimal valuePrevious = GetValueFromBytes(previousBytes, coordinatesSystem, type);
decimal encodedPrevious = valuePrevious - coordinatesSystem.GetMin(type);
// Calculate new value
decimal value = encodedPrevious + encodedDifference;
return(GetBytesFromValue(value, coordinatesSystem, type));
}