Elias gamma code is a universal code encoding positive integers. It is used most commonly when coding integers whose upper-bound cannot be determined beforehand. To code a number:
- Write it in binary.
- Subtract 1 from the number of bits written in step 1 and prepend that many zeros.
An equivalent way to express the same process:
- Separate the integer into the highest power of 2 it contains (2N) and the remaining N binary digits of the integer.
- Encode N in unary; that is, as N zeroes followed by a one.
- Append the remaining N binary digits to this representation of N.
The code begins:
1 = 20 + 0 = 1 2 = 21 + 0 = 010 3 = 21 + 1 = 011 4 = 22 + 0 = 00100 5 = 22 + 1 = 00101 6 = 22 + 2 = 00110 7 = 22 + 3 = 00111 8 = 23 + 0 = 0001000 9 = 23 + 1 = 0001001 10 = 23 + 2 = 0001010 11 = 23 + 3 = 0001011 12 = 23 + 4 = 0001100 13 = 23 + 5 = 0001101 14 = 23 + 6 = 0001110 15 = 23 + 7 = 0001111 16 = 24 + 0 = 000010000 17 = 24 + 1 = 000010001
To decode an Elias gamma-coded integer:
- Read and count 0s from the stream until you reach the first 1. Call this count of zeroes N.
- Considering the one that was reached to be the first digit of the integer, with a value of 2N, read the remaining N digits of the integer.
Gamma coding is used in applications where the largest encoded value is not known ahead of time, or to compress data in which small values are much more frequent than large values.
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Generalizations
Gamma coding does not code zero or negative integers. One way of handling zero is to add 1 before coding and then subtract 1 after decoding. Another way is to prefix each nonzero code with a 1 and then code zero as a single 0. One way to code all integers is to set up a bijection, mapping integers (0, 1, -1, 2, -2, 3, -3, ...) to (1, 2, 3, 4, 5, 6, 7, ...) before coding.
Example code
Encode <source lang="c"> void eliasGammaEncode(char* source, char* dest) {
IntReader intreader(source);
BitWriter bitwriter(dest);
while(intreader.hasLeft())
{
int num = intreader.getInt();
int l = log2(num);
for (int a=0; a < l; a++)
{
bitwriter.putBit(false); //put 0s to indicate how much bits that will follow
}
bitwriter.putBit(true);//mark the end of the 0s
for (int a=0; a < l; a++) //Write the bits as plain binary
{
if (num & 1 << a)
bitwriter.putBit(true);
else
bitwriter.putBit(false);
}
}
intreader.close();
bitwriter.close();
} </source> Decode <source lang="c"> void eliasGammaDecode(char* source, char* dest) {
BitReader bitreader(source);
BitWriter bitwriter(dest);
int numberBits = 0;
while(bitreader.hasLeft())
{
while(!bitreader.getBit() || bitreader.hasLeft())numberBits++; //keep on reading until we fetch a one...
int current = 0;
for (int a=0; a < numberBits; a++) //Read numberBits bits
{
if (bitreader.getBit())
current += 1 << a;
}
//write it as a 32 bit number
current= current | 1 ; //last bit isn't encoded!
for (int a=0; a < 32; a++) //Read numberBits bits
{
if (current & (1 << a))
bitwriter.putBit(true);
else
bitwriter.putBit(false);
}
}
} </source>
