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 `Biocaml_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.

`module Biocaml_solexa_score: `

`sig`

`exception Error of ``string`

type`t =`

`int`

`val of_ascii : ``char -> t`

`of_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 -> char`

`to_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 -> t`

`of_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.

**Raises** `Error`

if `x`

is not between 0.0 - 1.0.

`val to_probability : ``t -> float`

`to_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.`end`