The first milestone of the ext3 implementation was to have read support. Since ext2 read support is already implemented, the only missing feature (as far as I can tell) for ext3 read implementation was support for indexed directories. In ext3, indexed directories use a tree structured called HTree. This tree has a fixed depth and its keys are file name hashes. Each node of the tree is a file system block inside the directory file (ie. linked by the directory i-node).
This is a companion discussion topic for the original entry at https://www.haiku-os.org/blog/jvff/2010-06-14_ext3_indexed_directory_lookup/