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diff --git a/list-objects-filter.h b/list-objects-filter.h
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+#ifndef LIST_OBJECTS_FILTER_H
+#define LIST_OBJECTS_FILTER_H
+
+/*
+ * During list-object traversal we allow certain objects to be
+ * filtered (omitted) from the result.  The active filter uses
+ * these result values to guide list-objects.
+ *
+ * _ZERO      : Do nothing with the object at this time.  It may
+ *              be revisited if it appears in another place in
+ *              the tree or in another commit during the overall
+ *              traversal.
+ *
+ * _MARK_SEEN : Mark this object as "SEEN" in the object flags.
+ *              This will prevent it from being revisited during
+ *              the remainder of the traversal.  This DOES NOT
+ *              imply that it will be included in the results.
+ *
+ * _DO_SHOW   : Show this object in the results (call show() on it).
+ *              In general, objects should only be shown once, but
+ *              this result DOES NOT imply that we mark it SEEN.
+ *
+ * Most of the time, you want the combination (_MARK_SEEN | _DO_SHOW)
+ * but they can be used independently, such as when sparse-checkout
+ * pattern matching is being applied.
+ *
+ * A _MARK_SEEN without _DO_SHOW can be called a hard-omit -- the
+ * object is not shown and will never be reconsidered (unless a
+ * previous iteration has already shown it).
+ *
+ * A _DO_SHOW without _MARK_SEEN can be used, for example, to
+ * include a directory, but then revisit it to selectively include
+ * or omit objects within it.
+ *
+ * A _ZERO can be called a provisional-omit -- the object is NOT shown,
+ * but *may* be revisited (if the object appears again in the traversal).
+ * Therefore, it will be omitted from the results *unless* a later
+ * iteration causes it to be shown.
+ */
+enum list_objects_filter_result {
+	LOFR_ZERO      = 0,
+	LOFR_MARK_SEEN = 1<<0,
+	LOFR_DO_SHOW   = 1<<1,
+};
+
+enum list_objects_filter_situation {
+	LOFS_BEGIN_TREE,
+	LOFS_END_TREE,
+	LOFS_BLOB
+};
+
+typedef enum list_objects_filter_result (*filter_object_fn)(
+	enum list_objects_filter_situation filter_situation,
+	struct object *obj,
+	const char *pathname,
+	const char *filename,
+	void *filter_data);
+
+typedef void (*filter_free_fn)(void *filter_data);
+
+/*
+ * Constructor for the set of defined list-objects filters.
+ * Returns a generic "void *filter_data".
+ *
+ * The returned "filter_fn" will be used by traverse_commit_list()
+ * to filter the results.
+ *
+ * The returned "filter_free_fn" is a destructor for the
+ * filter_data.
+ */
+void *list_objects_filter__init(
+	struct oidset *omitted,
+	struct list_objects_filter_options *filter_options,
+	filter_object_fn *filter_fn,
+	filter_free_fn *filter_free_fn);
+
+#endif /* LIST_OBJECTS_FILTER_H */