Bug in IncidenceStructure.is_block_design

[nathanncohen]

This bug has been noticed by Frederic in #15285 :

sage: IncidenceStructure([0,1,2],[[0,1],[0,2],[1,2]]*15).is_block_design()
(True, [1, 3, 2, 15])
sage: IncidenceStructure([0,1,2],[[0,1],[0,2],[1,2]]*15).parameters()     
(2, 3, 2, 15)

Turns out that the code of is_block_design was rather unclear, and rather... Costly. Here is a simpler, faster and more correct version in which the bug in solved.

Before:

sage: D = IncidenceStructure(range(44),designs.steiner_quadruple_system(44))
sage: time D.is_block_design()                                              
CPU times: user 99.26 s, sys: 0.06 s, total: 99.32 s
Wall time: 99.40 s
(True, [3, 44, 4, 1])

After:

sage: D = IncidenceStructure(range(44),designs.steiner_quadruple_system(44))
sage: time D.is_block_design()                                              
CPU times: user 0.06 s, sys: 0.00 s, total: 0.06 s
Wall time: 0.06 s
(True, [3, 44, 4, 1])

Secondly, IncidenceStructure.parameter has a very tricky behaviour :

sage: D = IncidenceStructure(range(44),designs.steiner_quadruple_system(44))
sage: D.is_block_design()                                                   
(True, [3, 44, 4, 1])
sage: D.parameters()                                                        
(2, 44, 4, 21)

This is because parameters() take t as a parameter, and sets it to 2 by default. That's.... surprising :-P

This patch adds a deprecation warning when this parameter is not set, and we will make it mandatory... in a while.

Here it is !

Nathann

(this ticket took roughly 3hours of solid work)

Depends on #15285

CC: @fchapoton @sagetrac-Stefan @wdjoyner

Component: combinatorics

Author: Nathann Cohen

Branch/Commit: u/ncohen/15664 @ 2aa7e18

Reviewer: Frédéric Chapoton

Issue created by migration from https://trac.sagemath.org/ticket/15664

[nathanncohen]

Branch: u/ncohen/15664

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. Last 10 new commits:

ee6d412	Projective Plane designs constructor
363badb	trac 15107 -- reviewer's comments
9fcfb13	Rename the method from ProjectivePlaneDesign to DesarguesianProjectivePlaneDesign
cf71d58	Rebase on 5.13.beta0
2ec76c7	Bug in AffineGeometryDesign
2750b79	trac 15285: rebase on 6.1.beta4
e2935fe	Trac #15107: Rebase on 6.1.beta3
6f247f6	trac #15107: back to the first name with a new argument
8468131	trac #15285: Rebase on updated #15107
6ff6a06	trac #15285: Doc of AffineGeometryDesign

[sagetrac-git]

Commit: e84f4ee

[fchapoton]

comment:7

Hello,

Nice job !

Do you still need to use self.blocks() instead of self.blcks ? You have replaced some of them, but there remains one.

[sagetrac-git]

Changed commit from e84f4ee to 77c7d44

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. Last 10 new commits:

ee6d412	Projective Plane designs constructor
363badb	trac 15107 -- reviewer's comments
9fcfb13	Rename the method from ProjectivePlaneDesign to DesarguesianProjectivePlaneDesign
cf71d58	Rebase on 5.13.beta0
2ec76c7	Bug in AffineGeometryDesign
2750b79	trac 15285: rebase on 6.1.beta4
e2935fe	Trac #15107: Rebase on 6.1.beta3
6f247f6	trac #15107: back to the first name with a new argument
8468131	trac #15285: Rebase on updated #15107
6ff6a06	trac #15285: Doc of AffineGeometryDesign

[nathanncohen]

comment:9

Fixed !

Nathann

[fchapoton]

comment:10

Looks good to me.

One last detail : there lacks a space after "2" in "2is used when none is provided"

Then you can set a positive review.

[fchapoton]

Reviewer: Frédéric Chapoton

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. Last 10 new commits:

ee6d412	Projective Plane designs constructor
363badb	trac 15107 -- reviewer's comments
9fcfb13	Rename the method from ProjectivePlaneDesign to DesarguesianProjectivePlaneDesign
cf71d58	Rebase on 5.13.beta0
2ec76c7	Bug in AffineGeometryDesign
2750b79	trac 15285: rebase on 6.1.beta4
e2935fe	Trac #15107: Rebase on 6.1.beta3
6f247f6	trac #15107: back to the first name with a new argument
8468131	trac #15285: Rebase on updated #15107
6ff6a06	trac #15285: Doc of AffineGeometryDesign

[sagetrac-git]

Changed commit from 77c7d44 to 2aa7e18

[nathanncohen]

comment:13

Done ! Thanks for the review ;-)

Nathann