Wip errors reporting

internal/unsafe/unsafe.go

@@ -0,0 +1,35 @@
+package unsafe
+
+import (
+	"fmt"
+	"reflect"
+	"unsafe"
+)
+
+const maxInt = uintptr(int(^uint(0) >> 1))
+
+func SubsliceOffset(data []byte, subslice []byte) int {
+	datap := (*reflect.SliceHeader)(unsafe.Pointer(&data))
+	hlp := (*reflect.SliceHeader)(unsafe.Pointer(&subslice))
+
+	if hlp.Data < datap.Data {
+		panic(fmt.Errorf("subslice address (%d) is before data address (%d)", hlp.Data, datap.Data))
+	}
+	offset := hlp.Data - datap.Data
+
+	if offset > maxInt {
+		panic(fmt.Errorf("slice offset larger than int (%d)", offset))
+	}
+
+	intoffset := int(offset)
+
+	if intoffset >= datap.Len {
+		panic(fmt.Errorf("slice offset (%d) is farther than data length (%d)", intoffset, datap.Len))
+	}
+
+	if intoffset+hlp.Len > datap.Len {
+		panic(fmt.Errorf("slice ends (%d+%d) is farther than data length (%d)", intoffset, hlp.Len, datap.Len))
+	}
+
+	return intoffset
+}