Building and using the library

Building

From the repository root:

cmake -S . -B build
cmake --build build

This builds libyaml_c_wrapper (.dylib/.so), a shared library that other languages can load. CMake fetches the dependencies — rapidyaml, PCRE2, and AtomicAndPhysicalConstantsCLib — automatically. Rebuild after changing any source file, and before running the tests. Put lattice files under lattice_files/.

Expanding a lattice

parse_and_expand_PALS reads a lattice file, resolves its includes and loads, expands the selected lattice, and returns a lattices struct with five independent views — original, combined, expanded, full_expanded, and leftover (see The five trees). Each is a YAMLTreeHandle and must be freed with delete_tree.

#include "yaml_c_wrapper.h"

struct lattices lat = parse_and_expand_PALS("ex.pals.yaml", nullptr);

char* s = tree_to_string(lat.full_expanded);
std::puts(s);
yaml_free_string(s);

// Report any problems met while expanding (see below), then free everything.
for (size_t i = 0; i < lat.problems.count; ++i)
    std::fprintf(stderr, "  - %s\n", lat.problems.items[i]);
free_lattice_problems(lat.problems);

delete_tree(lat.original);
delete_tree(lat.combined);
delete_tree(lat.expanded);
delete_tree(lat.full_expanded);
delete_tree(lat.leftover);

The second argument names the lattice to expand. Pass nullptr (or an empty string) to expand the lattice named by the last use statement, or — if there is none — the last lattice defined in the file.

Problems found during expansion

Expansion does not abort on a recoverable problem. It leaves the offending value as it found it, carries on with the rest of the lattice, and appends a human‑readable message to lat.problems, an owning string_list. The list is empty when expansion was clean. The library never prints — the caller decides whether to report, save, or ignore the messages — and must release the list with free_lattice_problems.

Expansion is therefore always worth reading: a lattice with problems still comes back expanded as far as it could be, with the trees around the fault intact. The one exception is a top‑level file that is not valid YAML, where there is nothing to expand: all five handles come back NULL and the parse error, with its location, is the single problem reported.

What gets reported falls into a few groups.

Structure. A lattice named in the call (or in use) that does not exist, a line reference to an undefined element or line, a missing inherit/repeat/Fork target, an invalid repeat count, a Fork whose to_line cannot be resolved, a branch that never got expanded, and a load or include that cannot be read or that conflicts with what it is merged into.

Expressions. A value that cannot be evaluated — an unknown constant or species, a dangling element‑parameter reference, a reference cycle, a syntax error — is left as text and reported. Only values that look like expressions (an operator, a parenthesis, a > reference, or an explicit expr(...)) are flagged, so a plain name or a label is not mistaken for broken math. See Evaluating expressions.

Controllers. A control_type that is neither ABSOLUTE nor RELATIVE; a controls entry with no parameter or no expression; a control target that is malformed, names no element parameter, or matches nothing in the expanded lattice; a reference to a variable a named controller does not have; a controller variable whose initial value is not a constant expression; a circular control hierarchy; a parameter driven by both an ABSOLUTE and a RELATIVE controller; and a parameter that is both controlled and assigned a delayed (expr(...)) expression.

Sets. A set with no parameter or no value; a target that is malformed, names no element parameter, or matches nothing (for a pre‑expand_lattice set, nothing defined before it); a value that cannot be evaluated; and a value that reads a parameter whose value is still to be derived during expansion.

Bookkeeping. A branch whose first element gives neither a reference species nor a reference energy, so the reference parameters cannot be computed; and a parameter the author wrote that is inconsistent with what the rest of its family or the bend geometry implies — the authored value is kept and the disagreement reported rather than silently overwritten.

Vocabulary. An element kind or parameter group name that is not one the standard defines — a FlorP group is valid YAML and would otherwise go unrecognised in silence — reported with a “did you mean” suggestion where a near match exists.

Deliberately not computed. Where the standard fixes a value’s magnitude but not how to produce it, this library reports rather than guesses: an absolute_error/relative_error on a set (the error distribution is unspecified, so the deterministic value is written and the error is not), and a Foil element’s downstream species change (the stripping model is undefined, so the species is left as the upstream one).

Reading the expanded lattice

Which of the five trees answers a given question, and what the expansion adds to full_expanded that appears nowhere else — element_index, s_position, ReferenceP, FloorP, the derived parameter families, the branch_end cap — is covered in The five trees.

Navigating and editing trees

A tree is addressed by an opaque YAMLTreeHandle plus integer YAMLNodeIds. Walk it with get_root, get_child_by_key, get_child_by_index, get_parent, and get_size; inspect nodes with is_map, is_sequence, is_scalar, get_node_key, and as_string; and edit with add_scalar, add_map, add_sequence, set_scalar, set_node_key, remove_node, deep_copy_node, and deep_copy_children. Serialize with node_to_string / tree_to_string, or write_file. Every one of these is listed in the API Reference.

Three more entry points build on the expanded tree:

• build_correspondence_map links a node across the derivation chain — original, combined, full_expanded and leftover (their provenance is recorded as the trees are derived from one another). expanded takes no part: it is a pruned copy of full_expanded, so its nodes are found by path.

• match_names finds every named construct that a PALS Name Matching string refers to.

• get_parameter_value looks up a single parameter value by a Name Matching string — an element parameter (with a path) or, as a bare name, a constant or variable. It returns a param_value tagged as a number, a string, or missing. The value is returned as stored, not evaluated: a plain number comes back as PARAM_VALUE_NUMBER, while an expression (e.g. 0.3 * 5), a species name such as #3He, or other text comes back verbatim as a string (evaluating is the job of expansion — the expanded trees already hold numbers). An unset element parameter yields its default (0 for now); and a string that identifies no single value — no matching element/constant/variable, a bare element name, a path stopping on a whole group, or matches that disagree — yields PARAM_VALUE_MISSING. When the tag is PARAM_VALUE_STRING, free the returned string with yaml_free_string.

  struct param_value v = get_parameter_value(full_expanded, "lat1>>>B1a>BendP.e1");
  if (v.kind == PARAM_VALUE_NUMBER) printf("%g\n", v.number);
  else if (v.kind == PARAM_VALUE_STRING) { puts(v.string); yaml_free_string(v.string); }
  

• get_lattice_parameter_value is the whole-lattice form: pass the full_expanded and leftover handles from one parse_and_expand_PALS() result and it looks in full_expanded first (element parameters), then leftover (constants, variables, and unused definitions) — never in the raw original/combined trees. Values therefore come back already evaluated. Pass full_expanded rather than expanded: a dependent parameter is a legitimate thing to ask for, and only the former carries one.

Examples

The examples/ directory contains runnable programs:

• examples/example_rw.cpp — read a lattice, do basic manipulations (add, remove, rename), print, and write it back out.

  ./example_rw
  

• examples/print_lattices — expand a lattice and print all five views. It takes a file name and an optional -lat <name> flag naming the lattice to expand:

  ./print_lattices ex.pals.yaml -lat lat2
  

  The file name is resolved under ../lattice_files/, so run it from a directory that sits beside lattice_files/ — build/, say — and put the lattice in lattice_files/.

Extensions

PALSJulia extends pals-cpp with a Julia interface to the C API and translators from PALS to Bmad and SciBmad lattice files.