Update README.md

README.md

@@ -85,6 +85,47 @@ Each scenario contains:
 - **Index files**: `hf_dataset/{split}_index_{type}.json` — maps sample indices to `(sim_id, time_id)`
 - **test_mode metadata**: `{in_dist,out_dist,remain}_params_{type}.json`
 
+### Arrow shard files vs. trajectory counts
+
+> **Important clarification**: The number of `.arrow` shard files does **not** equal the number of trajectories. HuggingFace Arrow format packs multiple rows into one shard up to a size limit (~500 MB by default), but never splits a single row across shards. Real trajectories are smaller (fewer channels, ~130–260 MB each), so 2–4 trajectories are packed per shard, resulting in fewer shards than trajectories. Numerical trajectories are larger (extra channels such as pressure or 15 simulated fields, ~1.5–2.1 GB each), so each one already exceeds the shard limit on its own, resulting in a 1:1 mapping between shards and trajectories.
+
+Dataset version: **`2.0.0`** (see `version.json` at repo root; released 2026-01-24, format `lazy_slicing_v2`).
+
+| Scenario | Trajectories (real / numerical) | Arrow shards (real / numerical) |
+|---|---:|---:|
+| cylinder | 92 / 92 | 73 / 92 |
+| controlled_cylinder | 96 / 96 | 51 / 96 |
+| fsi | 51 / 51 | 51 / 51 |
+| foil | 98 / 99 | 98 / 99 |
+| combustion | 30 / 30 | 8 / 30 |
+
+**Notes:**
+- The **Trajectories** column is the ground-truth count (each row in the Arrow dataset = one complete trajectory). The **Arrow shards** column is the number of `.arrow` files on disk — a storage-level detail that depends on per-trajectory size.
+- The `{remain,in_dist_test,out_dist_test}_params_{real,numerical}.json` files partition trajectories by **physical parameter regime**. Each entry is keyed by HDF5 filename and maps to its parameter tuple (e.g., `(Re, control_freq)`). The three groups sum to the total trajectory count:
+  - `in_dist_test_params`: trajectories with in-distribution parameters, **entirely** reserved for testing.
+  - `out_dist_test_params`: trajectories with out-of-distribution (edge/extreme) parameters, **entirely** reserved for testing.
+  - `remain_params`: all other trajectories — **part** of each trajectory's time axis is used for training, the rest for validation/testing.
+
+  At evaluation time, `test_mode` can be set to `"seen"` (remain), `"in_dist"`, `"out_dist"`, `"unseen"` (in\_dist + out\_dist), or `"all"`.
+
+  <details>
+  <summary>Per-scenario split counts</summary>
+
+  | Scenario | Type | remain | in\_dist\_test | out\_dist\_test | Total |
+  |---|---|---:|---:|---:|---:|
+  | cylinder | real | 72 | 10 | 10 | 92 |
+  | cylinder | numerical | 92 | 0 | 0 | 92 |
+  | controlled\_cylinder | real | 76 | 10 | 10 | 96 |
+  | controlled\_cylinder | numerical | 96 | 0 | 0 | 96 |
+  | fsi | real | 39 | 0 | 12 | 51 |
+  | fsi | numerical | 51 | 0 | 0 | 51 |
+  | foil | real | 78 | 10 | 10 | 98 |
+  | foil | numerical | 99 | 0 | 0 | 99 |
+  | combustion | real | 30 | 0 | 0 | 30 |
+  | combustion | numerical | 30 | 0 | 0 | 30 |
+
+  </details>
+
 **Repository layout**:
 
 ```
@@ -242,7 +283,7 @@ If you find our work and/or our code useful, please cite us via:
 
 ```bibtex
 @inproceedings{hu2026realpdebench,
-      title={RealPDEBench: A Benchmark for Complex Physical Systems with Real-World Data}, 
+      title={RealPDEBench: A Benchmark for Complex Physical Systems with Real-World Data},
       author={Peiyan Hu and Haodong Feng and Hongyuan Liu and Tongtong Yan and Wenhao Deng and Tianrun Gao and Rong Zheng and Haoren Zheng and Chenglei Yu and Chuanrui Wang and Kaiwen Li and Zhi-Ming Ma and Dezhi Zhou and Xingcai Lu and Dixia Fan and Tailin Wu},
       booktitle={The Fourteenth International Conference on Learning Representations},
       year={2026},
@@ -253,6 +294,6 @@ If you find our work and/or our code useful, please cite us via:
 
 ## Contact
 
-AI for Scientific Simulation and Discovery Lab, Westlake University  
-Maintainer: `westlake-ai4s` (Hugging Face)  
+AI for Scientific Simulation and Discovery Lab, Westlake University
+Maintainer: `westlake-ai4s` (Hugging Face)
 Org: `AI4Science-WestlakeU`