Element Selection Syntax

[ax3l]

I designed and then vibe-coded an implementation of a element selection syntax that I like in: BLAST-ImpactX/impactx#1182, BLAST-ImpactX/impactx#1183 and follow-ups.

lattice = [...]  # your PALS lattice, e.g., read from a file or generated by a function

# Filter by element type
drift_elements = lattice.select(kind="Drift")
quad_elements = lattice.select(kind=elements.Quad)

# Filter by regex pattern
all_quads = lattice.select(kind=r".*Quad")  # matches Quad, ChrQuad, ExactQuad

# Filter by name
specific_elements = lattice.select(name="quad1")

# Chain filters (AND logic)
drift_named_d1 = lattice.select(kind="Drift").select(name="drift1")

# Position filtering (overlap logic)
early_elements = lattice.select(s=(None, 2.0))  # Elements overlapping s <= 2.0

# Chaining: s always calculated from original lattice
drift_then_early = lattice.select(kind="Drift").select(s=(1.0, 3.0))  # Drift AND overlapping s=[1.0,3.0]

# Modify original elements through references
drift_elements[0].ds = 2.0  # modifies original lattice

We could and should add something like this to PALS Python to make this library user-friendly for lattice manipulations.

[ax3l]

Also needed for PALS from MetaP:

• alias (short keyword string)
• description (long, verbose string / text)
• label
• ID

https://pals-project.readthedocs.io/en/latest/element-parameters.html#metap-metadata-parameters

[ax3l]

Scott Berg: Range selection: because s can move a lot in design, a range selection by first/last element would be more useful.

David Sagan: allow that the last element is before the first element, and wrap around (for channels, turns).

Range by s: less often useful for design/control, but good for visualization. Overlapping with range makes sense.

[nikitakuklev]

To ensure same elements are returned after lattice changes, even with Kahan summation used to compute s, I use this call to fudge the definition of what is considered at a position. Maybe a version of this with controllable tolerance would be a good idea to use for checking start/end points.

  def at(self, s: float) -> List[Element]:
      """
      Get all elements at position s. Element is considered to be present if it is thick and intersects
      the location, or if element starts there (regardless of length). Results are in sequence order.
      """
      assert 0.0 <= s <= self.lattice.totalLen
      spans = {el: (el.s_start, el.s_end) for el in self.lattice.sequence}
      selection = []
      for k, (s1, s2) in spans.items():
          # Select thin elements exactly at s, and any thick ones that are on top or start there
          if (np.isclose(s1, s) or (s1 <= s < s2 and not np.isclose(s, s2))):
              selection.append(k)
      return selection

(from here)