[Distutils] Requirent specifiers specified? :)

[Phillip J. Eby]

At 03:20 PM 6/22/2006 -0400, Jim Fulton wrote:

>>The implementation scans from left to right until it's *sure* that
>>the version is either accepted or rejected.  Each condition is a
>>point, a bound, or a point and a bound.  If the version matches the
>>"point" part of condition, it's an exact match and you are *sure*
>>of an accept, unless it's a "!=", in which case you're *sure* it's
>>a reject.
>>
>>If the version falls below an upper bound (< or <=), you are also
>>*sure* that it's accepted.  If it is above an upper bound, it is
>>*tentatively* rejected.
>
>So given:    >1, <3, >5, <7
>You are sure that 4 is accepted?

Scanning left to right, >1 is a lower bound and 4 is above it, so that's a 
tentative accept.  Next we see <3, and it is an upper bound, and we fall 
below it, so it is a sure accept at that point, and we stop scanning.


>Given:  <2, <5
>Is 3 accepted or rejected?

<2, upper bound, 3 is above, tentative reject.  <5, upper bound, 3 is below 
it, sure accept.  (Intuitively -- at least for me -- <5 is paired 
with >-infinity here.)


>>If the version falls below a lower bound (> or >=), you are *sure*
>>that it's rejected.  If it is above a lower bound, it is
>>*tentatively* accepted.
>
>So given:    >1, <3, >5, <7
>You are sure that 2 is rejected?

 >1, lower bound, 3 is above, tentative accept.  <3, upper bound, 2 is 
below, sure accept, scanning stops.


>>A simple state machine is used to implement this:
>>
>>state_machine = {
>>     #       =><
>>     '<' :  '--T',
>>     '<=':  'T-T',
>>     '>' :  'F+F',
>>     '>=':  'T+F',
>>     '==':  'T..',
>>     '!=':  'F++',
>>}
>>
>>cmp() is used to determine whether the version is =, >, or < than
>>the condition's version, and the appropriate row and column is
>>pulled from the above table.  "T" means "sure accept", "F" means
>>"sure reject", "+" means "tentative accept", and "-" means
>>"tentative reject".  ("." means "don't care".)  The state machine
>>simply compares versions until its sure or there are no more to
>>compare.
>
>I don't understand the meaning of the values in the dictionary above.
>Do the character positions reflect states somehow?

It's a truth table: the rows are condition operators, and the columns are 
cmp() results.  It is simply a transcription of the rules about points and 
bounds that I spelled out verbally, reduced to a table lookup on the 
condition and the comparison results.