Create System Facet

1 files changed, 87 insertions, 0 deletions

diff --git a/pint/facets/system/definitions.py b/pint/facets/system/definitions.py
new file mode 100644
index 0000000..368dd71
--- /dev/null
+++ b/pint/facets/system/definitions.py
@@ -0,0 +1,87 @@
+"""
+    pint.facets.systems.definitions
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+    :copyright: 2022 by Pint Authors, see AUTHORS for more details.
+    :license: BSD, see LICENSE for more details.
+"""
+
+
+from __future__ import annotations
+
+import re
+from dataclasses import dataclass
+from typing import Tuple
+
+from ...util import SourceIterator
+
+
+@dataclass(frozen=True)
+class SystemDefinition:
+    """Definition of a System:
+
+        @system <name> [using <group 1>, ..., <group N>]
+            <rule 1>
+            ...
+            <rule N>
+        @end
+
+    The syntax for the rule is:
+
+        new_unit_name : old_unit_name
+
+    where:
+        - old_unit_name: a root unit part which is going to be removed from the system.
+        - new_unit_name: a non root unit which is going to replace the old_unit.
+
+    If the new_unit_name and the old_unit_name, the later and the colon can be omitted.
+    """
+
+    #: Regex to match the header parts of a context.
+    _header_re = re.compile(r"@system\s+(?P<name>\w+)\s*(using\s(?P<used_groups>.*))*")
+
+    name: str
+    unit_replacements: Tuple[Tuple[int, str, str], ...]
+    using_group_names: Tuple[str, ...]
+
+    @classmethod
+    def from_lines(cls, lines, non_int_type=float):
+        lines = SourceIterator(lines)
+
+        lineno, header = next(lines)
+
+        r = cls._header_re.search(header)
+
+        if r is None:
+            raise ValueError("Invalid System header syntax '%s'" % header)
+
+        name = r.groupdict()["name"].strip()
+        groups = r.groupdict()["used_groups"]
+
+        # If the systems has no group, it automatically uses the root group.
+        if groups:
+            group_names = tuple(a.strip() for a in groups.split(","))
+        else:
+            group_names = ("root",)
+
+        unit_replacements = []
+        for lineno, line in lines:
+            line = line.strip()
+
+            # We would identify a
+            #  - old_unit: a root unit part which is going to be removed from the system.
+            #  - new_unit: a non root unit which is going to replace the old_unit.
+
+            if ":" in line:
+                # The syntax is new_unit:old_unit
+
+                new_unit, old_unit = line.split(":")
+                new_unit, old_unit = new_unit.strip(), old_unit.strip()
+
+                unit_replacements.append((lineno, new_unit, old_unit))
+            else:
+                # The syntax is new_unit
+                # old_unit is inferred as the root unit with the same dimensionality.
+                unit_replacements.append((lineno, line, None))
+
+        return cls(name, tuple(unit_replacements), group_names)