# Detailed Decoding Trace Information

The -T option to `decode` causes a detailed trace of information on the process of decoding to be written to standard output, in a multi-column format, with the first line containing the headers for each column. This format is suitable for reading into S-Plus or R. The first column is always the number of the block being decoded; several lines may be output for each block. The other columns vary with the decoding method used, as described below.

## Enum-bit and Enum-block decoding methods

For source messages with K bits, 2K lines are output for each block, containing the following information:

> block | The number of the block, from zero
> decoding | A possible decoding for the message bits, expressed as a hexadecimal number. The other bits are determined by the message bits.
> likelihood | The likelihood for this decoding (ie, the probability of obtaining the data received if this was the message sent).

For these methods, the number of "iterations" (output with the -t option) is always 2K.

## Prprp decoding method

Each block results in one line of output for the initial state (based on individual likelihood ratios), and one line for each subsequent iteration, containing the following information:

> block | The number of the block, from zero
> iter | The number of an iteration, zero for the initial state.
> changed | The number of bits in the decoding that differ from the bit that would be chosen based just on the likelihood ratio for that bit. Bits whose likelihood ratios are exactly one contribute 0.5 to this count. (Likelihood ratios of exactly one can arise when the output of an AWGN channel rounds to exactly 0.00.)
> perrs | The number of parity check errors in the current tentative decoding.
> loglik | The log likelihood of the current tentative decoding.
> Eperrs | The expected number of parity check errors in a decoding found by randomly picking a value for each bit, independently, according to the current bit probabilities.
> Eloglik | The expected log likelihood of a decoding found by randomly picking a value for each bit, independently, according to the current bit probabilities.
> entropy | The entropy (in bits) of the distribution defined by the current bit probablities, assumed to apply to bits independently.

The number of "iterations" (output with the -t option) is the obvious count of probability propagation iterations. The initial state does not count as an iteration.

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