1 files changed, 110 insertions, 0 deletions
diff --git a/pint/delegates/txt_defparser/system.py b/pint/delegates/txt_defparser/system.py
new file mode 100644
index 0000000..b21fd7a
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+++ b/pint/delegates/txt_defparser/system.py
@@ -0,0 +1,110 @@
+"""
+    pint.delegates.txt_defparser.system
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+    :copyright: 2022 by Pint Authors, see AUTHORS for more details.
+    :license: BSD, see LICENSE for more details.
+"""
+
+from __future__ import annotations
+
+import re
+import typing as ty
+from dataclasses import dataclass
+
+from ..._vendor import flexparser as fp
+from ...facets.system import definitions
+from . import block, common, plain
+
+
+@dataclass(frozen=True)
+class BaseUnitRule(fp.ParsedStatement, definitions.BaseUnitRule):
+    @classmethod
+    def from_string(cls, s: str) -> fp.FromString[BaseUnitRule]:
+        if ":" not in s:
+            return cls(s.strip())
+        parts = [p.strip() for p in s.split(":")]
+        if len(parts) != 2:
+            return common.DefinitionSyntaxError(
+                f"Exactly two terms expected for rule, not {len(parts)} (`{s}`)"
+            )
+        return cls(*parts)
+
+
+@dataclass(frozen=True)
+class BeginSystem(fp.ParsedStatement):
+    """Being of a system directive.
+
+    @system <name> [using <group 1>, ..., <group N>]
+    """
+
+    #: 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
+    using_group_names: ty.Tuple[str, ...]
+
+    @classmethod
+    def from_string(cls, s: str) -> fp.FromString[BeginSystem]:
+        if not s.startswith("@system"):
+            return None
+
+        r = cls._header_re.search(s)
+
+        if r is None:
+            raise ValueError("Invalid System header syntax '%s'" % s)
+
+        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",)
+
+        return cls(name, group_names)
+
+
+@dataclass(frozen=True)
+class SystemDefinition(block.DirectiveBlock):
+    """Definition of a System:
+
+        @system <name> [using <group 1>, ..., <group N>]
+            <rule 1>
+            ...
+            <rule N>
+        @end
+
+    See Rule and Comment for more parsing related information.
+
+    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.
+    """
+
+    opening: fp.Single[BeginSystem]
+    body: fp.Multi[ty.Union[plain.CommentDefinition, BaseUnitRule]]
+
+    def derive_definition(self):
+        return definitions.SystemDefinition(
+            self.name, self.using_group_names, self.rules
+        )
+
+    @property
+    def name(self):
+        return self.opening.name
+
+    @property
+    def using_group_names(self):
+        return self.opening.using_group_names
+
+    @property
+    def rules(self):
+        return tuple(el for el in self.body if isinstance(el, BaseUnitRule))