Solexa quality scores.
A Solexa score is defined as -10*log(p/(1-p)) rounded to an integer, where p is a probability. Phred scores are far more widely used, and the Phred_score module supports converting Solexa scores to Phred scores.
For details see The Sanger FASTQ file format for sequences with quality scores, and the Solexa/Illumina FASTQ variants. This module supports what is called the fastq-solexa format in this paper, with one exception. We are more permissive here in allowing conversions from/to the entire range of visible ASCII characters (codes 33 - 126) instead of restricting to codes 59 - 126 as specified in this paper. The smaller range is apparently based on the original Solexa software returning minimum scores of -5, but there is no reason for this minimum based on the general definition of Solexa scores.
Note: this module is adapted from biocaml 0.8 released under LGPL
val of_ascii : char -> tof_ascii x returns the PHRED score encoded by ASCII character x.
Error if x's ASCII code is not between 33 - 126.
val to_ascii : t -> charto_ascii t encodes t as an ASCII character.
Error if t cannot be encoded as a visible ASCII character (codes 33 - 126).
val of_probability : ?f:(float -> int) -> float -> tof_probability ~f x returns -10 * log_10(x/(1-x)), which is the definition of Solexa scores.
Solexa scores are integral, and it is unclear what convention is used to convert the resulting float value to an integer. Thus, the optional f is provided to dictate this. The default is to round the computed score to the closest integer.
Error if x is not between 0.0 - 1.0.
val to_probability : t -> floatto_probablity x converts x to a probablity score. Note this is not the inverse of of_probability due to the rounding done by the latter.