1 files changed, 38 insertions, 0 deletions
diff --git a/format/commitgraph/read/position.go b/format/commitgraph/read/position.go
new file mode 100644
index 00000000..b2e1138b
--- /dev/null
+++ b/format/commitgraph/read/position.go
@@ -0,0 +1,38 @@
+package read
+
+import (
+	"fmt"
+
+	"codeberg.org/lindenii/furgit/internal/intconv"
+)
+
+// Position identifies one commit record by layer and row index.
+type Position struct {
+	Graph uint32
+	Index uint32
+}
+
+func (reader *Reader) globalToPosition(global uint32) (Position, error) {
+	for i := range reader.layers {
+		layer := &reader.layers[i]
+		from := layer.globalFrom
+
+		to := from + layer.numCommits
+		if global >= from && global < to {
+			graph, err := intconv.IntToUint32(i)
+			if err != nil {
+				return Position{}, err
+			}
+
+			return Position{
+				Graph: graph,
+				Index: global - from,
+			}, nil
+		}
+	}
+
+	return Position{}, &MalformedError{
+		Path:   "commit-graph",
+		Reason: fmt.Sprintf("parent global position out of range: %d", global),
+	}
+}