1 files changed, 55 insertions, 0 deletions
diff --git a/third_party/python/fluent.migrate/fluent/migrate/merge.py b/third_party/python/fluent.migrate/fluent/migrate/merge.py
new file mode 100644
index 0000000000..c8bedc4583
--- /dev/null
+++ b/third_party/python/fluent.migrate/fluent/migrate/merge.py
@@ -0,0 +1,55 @@
+import fluent.syntax.ast as FTL
+
+from .errors import SkipTransform
+from .util import get_message, get_transform
+
+
+def merge_resource(ctx, reference, current, transforms, in_changeset):
+    """Transform legacy translations into FTL.
+
+    Use the `reference` FTL AST as a template.  For each en-US string in the
+    reference, first check for an existing translation in the current FTL
+    `localization` and use it if it's present; then if the string has
+    a transform defined in the migration specification and if it's in the
+    currently processed changeset, evaluate the transform.
+    """
+
+    def merge_body(body):
+        return [
+            entry
+            for entry in map(merge_entry, body)
+            if entry is not None
+        ]
+
+    def merge_entry(entry):
+        # All standalone comments will be merged.
+        if isinstance(entry, FTL.BaseComment):
+            return entry
+
+        # Ignore Junk
+        if isinstance(entry, FTL.Junk):
+            return None
+
+        ident = entry.id.name
+
+        # If the message is present in the existing localization, we add it to
+        # the resulting resource.  This ensures consecutive merges don't remove
+        # translations but rather create supersets of them.
+        existing = get_message(current.body, ident)
+        if existing is not None:
+            return existing
+
+        transform = get_transform(transforms, ident)
+
+        # Make sure this message is supposed to be migrated as part of the
+        # current changeset.
+        if transform is not None and in_changeset(ident):
+            if transform.comment is None:
+                transform.comment = entry.comment
+            try:
+                return ctx.evaluate(transform)
+            except SkipTransform:
+                return None
+
+    body = merge_body(reference.body)
+    return FTL.Resource(body)