***************************************************
***************************************************
THIS TODO HAS BEEN MOVED TO THE DIDEROT WIKI:

http://diderot-wiki.cs.uchicago.edu/index.php/Todo

PLEASE USE THAT PAGE TO UPDATE PROBLEMS AND PROGRESS
***************************************************
***************************************************

NOTE: GLK's approximate ranking of 8 most important tagged with
[GLK:1], [GLK:2], ...

========================
SHORT TERM ============= (*needed* for streamlines & tractography)
========================

[GLK:2] Add sequence types (needed for evals & evecs)
    syntax
	types: ty '{' INT '}'
	value construction: '{' e1 ',' … ',' en '}'
	indexing: e '{' e '}'

[GLK:3] evals & evecs for symmetric tensor[2,2] and
tensor[3,3] (requires sequences)

ability to emit/track/record variables into dynamically re-sized
runtime output buffer

[GLK:4] tensor fields from tensor images: Initially need at least
convolution on tensor[2,2] and tensor[3,3] (the same component-wise
convolution as for vectors).

========================
SHORT-ISH TERM ========= (to make using Diderot less annoying to 
========================  program in, and slow to execute)

Allow ".ddro" file extensions in addition to ".diderot"

Be able to output values of type tensor[2,2] and tensor[3,3];
(currently only scalars & vectors).  Want to add some regression tests
based on this and currently can't

[GLK:1] Proper handling of stabilize method

Convolution on general tensor images (order > 2)

allow "*" to represent "modulate": per-component multiplication of
vectors, and vectors only (not tensors of order 2 or higher).  Once
sequences are implemented this should be removed: the operation is not
invariant WRT basis so it is not a legit vector computation.

implicit type promotion of integers to reals where reals are
required (e.g. not exponentiation "^")

[Nick working on this] Save Diderot output to nrrd, instead of "mip.txt"
  For grid of strands, save to similarly-shaped array
  For list of strands, save to long 1-D (or 2-D for non-scalar output) list
  For ragged things (like tractography output), will need to save both
    complete list of values, as well as list of start indices and lengths
    to index into complete list

[GLK:6] ability to declare a field so that probe positions are
*always* "inside"; with various ways of mapping the known image values
to non-existant index locations.  One possible syntax emphasizes that
there is a index mapping function that logically precedes convolution:
  F = bspln3 ⊛ (img ◦ clamp)
  F = bspln3 ⊛ (img ◦ repeat)
  F = bspln3 ⊛ (img ◦ mirror)
where "◦" or "∘" is used to indicate function composition

Level of differentiability in field type should be statement about how
much differentiation the program *needs*, rather than what the kernel
*provides*.  The needed differentiability can be less than or equal to
the provided differentiability.

Use ∇⊗ etc. syntax
    syntax [DONE]
    typechecking
    IL and codegen

Add type aliases for color types
    rgb = real{3}
    rgba = real{4}

Revisit how images are created within the language.
The "load" operator should probably go away, and its strange
that strings are there only as a way to refer to nrrd filenames

==============================
MEDIUM TERM ================== (*needed* for particles)
==============================

[Lamont working on this] run-time birth of strands

"initially" supports lists

"initially" supports lists of positions output from different
initalization Diderot program (or output from the same program;
e.g. using output of iso2d.diderot for one isovalue to seed the input
to another invocation of the same program)

[Lamont working on this] Communication between strands: they have to
be able to learn each other's state (at the previous iteration).
Early version of this can have the network of neighbors be completely
static (for running one strand/pixel image computations).  Later
version with strands moving through the domain will require some
spatial data structure to optimize discovery of neighbors.

============================
MEDIUM-ISH TERM ============ (to make Diderot more useful/effective)
============================

[GLK:5] Want code-generation working for tensors of order three.
Order three matters for edge detection in scalar fields (to get
second derivatives of gradient magnitude), second derivatives
of vector fields (for some feature extraction), and first
derivatives of diffusion tensor fields.

Python/ctypes interface to run-time

support for Python interop and GUI

Allow integer exponentiation ("^2") to apply to square matrices,
to represent repeated matrix multiplication

Put small 1-D and 2-D fields, when reconstructed specifically by tent
and when differentiation is not needed, into faster texture buffers.
test/illust-vr.diderot is good example of program that uses multiple
such 1-D fields basically as lookup-table-based function evaluation

extend norm (|exp|) to all tensor types [DONE for vectors and matrices]

determinant ("det") for tensor[3,3]

add ":" for tensor dot product (contracts out two indices
instead of one like •), valid for all pairs of tensors with
at least two indices

test/uninit.diderot:
documents need for better compiler error messages when output variables
are not initialized; the current messages are very cryptic

want: warnings when "D" (reserved for differentiation) is declared as
a variable name (get confusing error messages now)

==============================
LONG TERM ==================== (make Diderot more interesting/attractive from 
==============================  a research standpoint)

IL support for higher-order tensor values (matrices, etc).
    tensor construction [DONE]
    tensor indexing [DONE]
    tensor slicing

Better handling of variables that determines the scope of a variable
based on its actual use, instead of where the user defined it.  So,
for example, we should lift strand-invariant variables to global
scope.  Also prune out useless variables, which should include field
variables after the translation to mid-il.

test/vr-kcomp2.diderot: Add support for code like
	(F1 if x else F2)@pos
This will require duplication of the continuation of the conditional
(but we should only duplicate over the live-range of the result of the
conditional.

[GLK:7] Want: non-trivial field expressions & functions.
scalar fields from scalar fields F and G:
  field#0(2)[] X = (sin(F) + 1.0)/2;
  field#0(2)[] X = F*G;
scalar field of vector field magnitude:
  image(2)[2] Vimg = load(...);
  field#0(2)[] Vlen = |Vimg ⊛ bspln3|;
field of normalized vectors (for LIC and vector field feature extraction)
  field#2(2)[2] F = ...
  field#0(2)[2] V = normalize(F);
scalar field of gradient magnitude (for edge detection))
  field#2(2)[] F = Fimg ⊛ bspln3;
  field#0(2)[] Gmag = |∇F|;    
scalar field of squared gradient magnitude (simpler to differentiate):
  field#2(2)[] F = Fimg ⊛ bspln3;
  field#0(2)[] Gmsq = ∇F•∇F;
There is value in having these, even if the differentiation of them is
not supported (hence the indication of "field#0" for these above)

Introduce region types (syntax region(d), where d is the dimension of the
region.  One useful operator would be
	dom : field#k(d)[s] -> region(d)
Then the inside test could be written as
	pos ∈ dom(F)
We could further extend this approach to allow geometric definitions of
regions.  It might also be useful to do inside tests in world space,
instead of image space.

co- vs contra- index distinction

Permit field composition:
  field#2(3)[3] warp = bspln3 ⊛ warpData;
  field#2(3)[] F = bspln3 ⊛ img;
  field#2(3)[] Fwarp = F ◦ warp;
So Fwarp(x) = F(warp(X)).  Chain rule can be used for differentation.
This will be instrumental for expressing non-rigid registration
methods (but those will require co-vs-contra index distinction)

Allow the convolution to be specified either as a single 1D kernel
(as we have it now):
  field#2(3)[] F = bspln3 ⊛ img;
or, as a tensor product of kernels, one for each axis, e.g.
  field#0(3)[] F = (bspln3 ⊗ bspln3 ⊗ tent) ⊛ img;
This is especially important for things like time-varying fields
and the use of scale-space in field visualization: one axis of the
must be convolved with a different kernel during probing.
What is very unclear is how, in such cases, we should notate the
gradient, when we only want to differentiate with respect to some
subset of the axes.  One ambitious idea would be:
  field#0(3)[] Ft = (bspln3 ⊗ bspln3 ⊗ tent) ⊛ img; // 2D time-varying field
  field#0(2)[] F = lambda([x,y], Ft([x,y,42.0]))     // restriction to time=42.0
  vec2 grad = ∇F([x,y]);                             // 2D gradient 

representation of tensor symmetry
(have to identify the group of index permutations that are symmetries)

dot works on all tensors

outer works on all tensors

Help for debugging Diderot programs: need to be able to uniquely
identify strands, and for particular strands that are known to behave
badly, do something like printf or other logging of their computations
and updates.

Permit writing dimensionally general code: Have some statement of the
dimension of the world "W" (or have it be learned from one particular
field of interest), and then able to write "vec" instead of
"vec2/vec3", and perhaps "tensor[W,W]" instead of
"tensor[2,2]/tensor[3,3]"

Traits: all things things that have boilerplate code (especially
volume rendering) should be expressed in terms of the unique
computational core.  Different kinds of streamline/tractography
computation will be another example, as well as particle systems.

Einstein summation notation

"tensor comprehension" (like list comprehension)

Fields coming from different sources of data: 
* triangular or tetrahedral meshes over 2D or 3D domains (of the
  source produced by finite-element codes; these will come with their
  own specialized kinds of reconstruction kernels, called "basis
  functions" in this context)
* Large point clouds, with some radial basis function around each point,
  which will be tuned by parameters of the point (at least one parameter
  giving some notion of radius)

======================
BUGS =================
======================

test/zslice2.diderot:
// HEY (bug) bspln5 leads to problems ...
//  uncaught exception Size [size]
//    raised at c-target/c-target.sml:47.15-47.19
//field#4(3)[] F = img ⊛ bspln5;