Opened 7 years ago
Closed 7 years ago
#13213 closed enhancement (fixed)
Comparisons in quadratic number field
Reported by: | vdelecroix | Owned by: | vdelecroix |
---|---|---|---|
Priority: | major | Milestone: | sage-5.10 |
Component: | number fields | Keywords: | real number field, comparison |
Cc: | Merged in: | sage-5.10.beta4 | |
Authors: | Vincent Delecroix | Reviewers: | Burcin Erocal, Volker Braun |
Report Upstream: | N/A | Work issues: | |
Branch: | Commit: | ||
Dependencies: | Stopgaps: |
Description (last modified by )
The order of quadratic field with specified embedding is not induced from the order of RR and CC. More precisely we have
sage: K.<sqrt2> = NumberField(x^2 - 2, 'sqrt2', embedding=1.4142) sage: sqrt2 < 100 False sage: K.<s> = NumberField(x^3 - 2, 's', embedding=1.26) sage: s < 100 False
which is not compatible with the order of RR and
sage: K.<i> = QuadraticField(-1) sage: i > 1 True sage: 1 > i True
which is not compatible (!) with the order of CC.
The ticket implements the order for quadratic field (for which comparisons are made using only operations on integers).
Note:
- this patch is partly a duplicate because of #7160
- the modifications for quadratic field field modify the behavior of many commands (especially about output order).
- the patch introduces a new attribute to NumberFieldElement_quadratic called standard_embedding (of type bint)
There is no significant lost of speed as shown on the following timings.
sage: K.<sqrt2> = QuadraticField(2, 'sqrt2', embedding=1.4142) sage: a = (3*sqrt2 + 18)/7 sage: b = (5*sqrt2 + 14)/5 sage: %timeit a < b 1000000 loops, best of 3: 408 ns per loop sage: K.<s> = QuadraticField(-2) sage: a = 3*s + 2/4 sage: b = 5/7*s + 1/3 sage: %timeit a < b 1000000 loops, best of 3: 352 ns per loop
Timings without the patch
sage: sage: K.<sqrt2> = QuadraticField(2,'sqrt2',embedding=1.4142) sage: a = (3*sqrt2 + 18)/7 sage: b = (5*sqrt2 + 14)/5 sage: %timeit a < b 1000000 loops, best of 3: 393 ns per loop sage: K.<s> = QuadraticField(-2) sage: a = 3*s+2/4 sage: b = 5/7*s+1/3 sage: %timeit a < b 1000000 loops, best of 3: 344 ns per loop
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Attachments (1)
Change History (38)
comment:1 Changed 7 years ago by
comment:2 Changed 7 years ago by
- Description modified (diff)
comment:3 Changed 7 years ago by
- Description modified (diff)
comment:4 Changed 7 years ago by
- Description modified (diff)
comment:5 follow-up: ↓ 6 Changed 7 years ago by
- Reviewers set to Burcin Erocal
Thanks again for the patch. A few comments:
- reimplementing the comparison of quadratic number field elements is a well defined unit of change itself. I suggest moving
floor()
andceil()
implementations to a separate patch, even a different ticket. - Do you really need to use rich comparisons? Working only with
__cmp__
should simplify the case comparisons before the return statements and avoid the newpython_object.pxi
include. - I guess speed could be further improved by using an approximation (with
double
s say) first and falling back to the exact computation if the results are too close. left.D
is of typeInteger
. Cython tries to do the right thing for<unsigned int> left.D
, but it would be better to usempz_mul()
withleft.D.value
.mpz_sgn()
returns 0 if the argument is0
. You don't seem to cover this case, though I couldn't find any example that returns wrong results.- To avoid code duplication, it might be better change the if statement to:
if mpz_sgn(i)*mpz_sgn(j) == 1: # both i and j are positive or negative mpz_mul(i, i, i) mpz_mul(j, j, j) mpz_mul_ui(j, j, <unsigned int> left.D) test = mpz_cmp(i, j) if mpz_sgn(i) == -1: # both i and j are negative test *= -1
comment:6 in reply to: ↑ 5 Changed 7 years ago by
- Description modified (diff)
- reimplementing the comparison of quadratic number field elements is a well defined unit of change itself. I suggest moving
floor()
andceil()
implementations to a separate patch, even a different ticket.left.D
is of typeInteger
. Cython tries to do the right thing for<unsigned int> left.D
, but it would be better to usempz_mul()
withleft.D.value
.
Done and done.
- Do you really need to use rich comparisons? Working only with
__cmp__
should simplify the case comparisons before the return statements and avoid the newpython_object.pxi
include.
We loose speed (~1 micro sec) by doing this. I quickly look at the code of comparison for Integer. The comparison is directly implemented inside richcmp and avoid as much as possible the coercion machinery (ie the type of the input is check with the cython function PY_TYPE_CHECK). Do you think we should proceed that way ?
- I guess speed could be further improved by using an approximation (with
double
s say) first and falling back to the exact computation if the results are too close.
What do you mean by too close ? How do I certify that my float comparison is accurate enough ? Moreover, if we proceed that way we have to perform conversion from mpz_t to float (or double or real_mpfr). A way it may work is to look first if a,b, D and denom have reasonable values (recall that mpz_t have unlimited precision) and if yes, convert it to double and try to compare the arguments. Last, I'm not sure the gain of speed would be terrific. Comparison requires only 7 integer operations.
mpz_sgn()
returns 0 if the argument is0
. You don't seem to cover this case, though I couldn't find any example that returns wrong results.
Actually there is no problem since we compute the sign of an expression of the form a^{2 + b}2 D where a and b are both integers (and one of them is non null) and D is a square free integer. Hence the sign is never 0.
- To avoid code duplication, it might be better change the if statement to:
if mpz_sgn(i)*mpz_sgn(j) == 1: # both i and j are positive or negative mpz_mul(i, i, i) mpz_mul(j, j, j) mpz_mul_ui(j, j, <unsigned int> left.D) test = mpz_cmp(i, j) if mpz_sgn(i) == -1: # both i and j are negative test *= -1
Sure but that way we call twice mpz_sgn(i)... I may change it if you prefer.
Actually, the part where a lot of time is lost is the check for embedding
if not RDF.has_coerce_map_from(left._parent): ...
By replacing it by
if left.D < 0: ...
I pass from 4 micro second to 2 micro seconds. That way comparison without embedding for positive discriminant will be equivalent to comparison with the standard embedding. This is in the last version of the patch. What do you think about that ?
comment:7 Changed 7 years ago by
- Status changed from new to needs_review
comment:8 Changed 7 years ago by
The code for comparison introduces a small bug as it ask to the parent whether the embedding is standard or not (via the boolean attribute _standard_embedding). But the latter was not defined for Cyclotomic field of order 3,4,6 (which are the ones which are quadratic). In that case, the attribute _standard_embedding is created at the initialization of the cyclotomic field.
The new patch takes care of this.
comment:9 Changed 7 years ago by
- Description modified (diff)
- Status changed from needs_review to needs_work
comment:10 Changed 7 years ago by
Because there are many parents for quadratic elements, I was obliged to test whether the parent has a method '_standard_embedding' or not. I correct all tests (many output have changed) and waiting for patchbot and reviewer!
comment:11 Changed 7 years ago by
- Status changed from needs_work to needs_review
comment:12 Changed 7 years ago by
I corrected the two last tests that have failed...
comment:13 Changed 7 years ago by
How does this patch here relate to #7160?
comment:14 Changed 7 years ago by
In the ticket title "real number field", should it say "quadratic number field" instead?
comment:15 follow-up: ↓ 16 Changed 7 years ago by
comment:16 in reply to: ↑ 15 Changed 7 years ago by
Replying to vdelecroix:
From the last comment of Burcin on #7160, it seems to me that we will close #7160 as a duplicate. On the other hand, an other patch is coming for comparisons of general number field. Would you prefer to have it on a separate ticket ?
I don't care very much, but I think it should be clear how the various tickets relate to eachother.
Also #10064 is related, at least it has #7160 as dependency.
comment:17 Changed 7 years ago by
- Description modified (diff)
comment:18 Changed 7 years ago by
Just had a quick look at your patch. How about using @cached_method
as a decorator for _gen_approx
? Then you don't have to implement caching yourself. It makes the code a little bit easier to read.
comment:19 Changed 7 years ago by
Hello,
Actually, a lot of time is lost for asking questions to the parent. For quadratic number field, it is for asking about the attribute _standard_embedding. I build a new patch with a bint attribute added to NumberFieldElement_quadratic and the speed of comparison is divided by 10 (from 4 milliseconds to 400 nanoseconds). It will be updated as soon as possible. As modifying the structure of NumberFieldElement_quadratic is a major change, I will split the patch into two parts : quadratic number fields and general number fields.
Vincent
comment:20 Changed 7 years ago by
- Description modified (diff)
- Summary changed from Comparisons in real number field to Comparisons in quadratic number field
comment:21 Changed 7 years ago by
apply trac_13213-quadratic_field_comparison.patch
comment:22 follow-up: ↓ 23 Changed 7 years ago by
- Description modified (diff)
- Status changed from needs_review to needs_work
- Work issues set to rebase 5.10.beta2
This fails to apply on 5.10.beta2. Having a look right now.
comment:23 in reply to: ↑ 22 Changed 7 years ago by
Replying to jpflori:
This fails to apply on 5.10.beta2.
Yes, it is a minor rejection on sage/schemes/elliptic_curves/ell_number_field.py and a more serious problem with python_object.pxi that seems to not exist anymore.
Having a look right now.
Does it mean that I should wait for your modifications ?
comment:24 Changed 7 years ago by
No, if you identified the issues, please go ahead. I've just untarred 10.beta2 and are currently on something else, so you will definitely get faster.
comment:25 Changed 7 years ago by
For the second problem I simply removed the import of python_object.pxi which was not needed anymore.
apply trac_13213-quadratic_field_comparison.patch
comment:26 Changed 7 years ago by
- Status changed from needs_work to needs_review
comment:27 Changed 7 years ago by
Last minute modification: more tests, more docs.
apply trac_13213-quadratic_field_comparison.patch
comment:28 Changed 7 years ago by
#7545 is possibly a related ticket.
comment:29 follow-up: ↓ 30 Changed 7 years ago by
- Status changed from needs_review to needs_work
There is a strange problem
sage: K.<sqrt2> = QuadraticField(2) sage: 1/2 + sqrt2 > 0 False
comment:30 in reply to: ↑ 29 Changed 7 years ago by
Replying to vdelecroix:
There is a strange problem
sage: K.<sqrt2> = QuadraticField(2) sage: 1/2 + sqrt2 > 0 False
I got the problem... it was because there is a natural morphism implemented from the rational field to quadratic number field (sage.rings.number_field.number_field_element_quadratic.Q_to_quadratic_field_element
) which uses a non properly initialized instance of a quadratic element !
comment:31 Changed 7 years ago by
- Status changed from needs_work to needs_review
apply trac_13213-quadratic_field_comparison.patch
comment:32 follow-up: ↓ 33 Changed 7 years ago by
I get a doctest failure with sage-5.10.beta2:
File "devel/sage/doc/de/tutorial/tour_advanced.rst", line 483, in doc.de.tutorial.tour_advanced Failed example: M.T(2).charpoly('x').factor() Expected: (x - 2*zeta6 - 1) * (x - zeta6 - 2) * (x + zeta6 + 1)^2 Got: (x - zeta6 - 2) * (x - 2*zeta6 - 1) * (x + zeta6 + 1)^2 ********************************************************************** 1 item had failures: 1 of 136 in doc.de.tutorial.tour_advanced [118 tests, 1 failure, 1.44 s]
Changed 7 years ago by
comment:33 in reply to: ↑ 32 Changed 7 years ago by
Replying to vbraun:
I get a doctest failure with sage-5.10.beta2:
File "devel/sage/doc/de/tutorial/tour_advanced.rst", line 483, in doc.de.tutorial.tour_advanced Failed example: M.T(2).charpoly('x').factor() Expected: (x - 2*zeta6 - 1) * (x - zeta6 - 2) * (x + zeta6 + 1)^2 Got: (x - zeta6 - 2) * (x - 2*zeta6 - 1) * (x + zeta6 + 1)^2 ********************************************************************** 1 item had failures: 1 of 136 in doc.de.tutorial.tour_advanced [118 tests, 1 failure, 1.44 s]
Thanks for that. I got the same error on my computer and change the patch accordingly. Why was patchbot happy with the former patch ?
apply trac_13213-quadratic_field_comparison.patch
comment:34 Changed 7 years ago by
The patchbot apparently doesn't check German docs. Enttäuschend! ;-)
comment:35 Changed 7 years ago by
- Reviewers changed from Burcin Erocal to Burcin Erocal, Volker Braun
- Status changed from needs_review to positive_review
All tests pass.
comment:36 Changed 7 years ago by
- Work issues rebase 5.10.beta2 deleted
comment:37 Changed 7 years ago by
- Merged in set to sage-5.10.beta4
- Resolution set to fixed
- Status changed from positive_review to closed
I had a short look at your patch. Could you use Python 3 compatible syntax to raise errors. E.g.