"""index.py - the engram's key table: a faiss IVF (inner product, SQ8 codes) over every static passage key in emb_static/,
with its inverted lists on Optane (OnDiskInvertedLists) so only the 16k centroids sit in RAM (10-19).
Steps: train on a TRAIN-vector sample spread over all shards -> per-shard sub-indexes (index/shard_*.index) -> merge into
index/engram.index + index/lists.ivfdata. Resumable per step. Env: EMB (emb_static/), OUT (index/), NLIST (16384), TRAIN (1000000), THREADS (4),
NSHARDS (optional cap; default all shards - the old 999 default silently dropped health's last 19 shards)."""
import os, glob, time
import numpy as np, faiss
from faiss.contrib.ondisk import merge_ondisk
E = "/mnt/optane/engram"; OUT = os.environ.get("OUT", f"{E}/index"); os.makedirs(OUT, exist_ok=True)
NLIST = int(os.environ.get("NLIST", 16384)); NTRAIN = int(os.environ.get("TRAIN", 1_000_000))
faiss.omp_set_num_threads(int(os.environ.get("THREADS", 4)))
log = lambda *a: print(time.strftime("%H:%M:%S"), *a, flush=True)
shards = sorted(f for f in glob.glob(f"{os.environ.get('EMB', E + '/emb_static')}/*.npy") if not f.endswith(".pid.npy") and ".tmp" not in f)[:int(os.environ.get("NSHARDS", 10**9))]
mm = [np.load(f, mmap_mode="r") for f in shards]; D = mm[0].shape[1]; N = sum(len(m) for m in mm)
log(f"{len(shards)} shards, {N} keys, dim {D}")
tr = f"{OUT}/trained.index"
if not os.path.exists(tr):
    rng = np.random.default_rng(0); per = NTRAIN // len(mm)
    X = np.concatenate([m[np.sort(rng.choice(len(m), min(per, len(m)), replace=False))] for m in mm]).astype(np.float32)
    ix = faiss.index_factory(D, f"IVF{NLIST}_HNSW32,SQ8", faiss.METRIC_INNER_PRODUCT)
    t0 = time.time(); ix.train(X); faiss.write_index(ix, tr); log(f"trained on {len(X)} in {time.time() - t0:.0f} s"); del X
for f, m in zip(shards, mm):
    o = f"{OUT}/shard_{os.path.basename(f)[:-4]}.index"
    if os.path.exists(o):
        continue
    ix = faiss.read_index(tr); pid = np.load(f[:-4] + ".pid.npy"); t0 = time.time()
    for i in range(0, len(m), 500_000):
        ix.add_with_ids(np.ascontiguousarray(m[i:i + 500_000], dtype=np.float32), pid[i:i + 500_000])
    faiss.write_index(ix, o + ".tmp"); os.replace(o + ".tmp", o); log(f"{o}: {ix.ntotal} in {time.time() - t0:.0f} s")
if not os.path.exists(f"{OUT}/engram.index"):
    ix = faiss.read_index(tr); t0 = time.time()
    merge_ondisk(ix, sorted(glob.glob(f"{OUT}/shard_*.index")), f"{OUT}/lists.ivfdata")
    faiss.write_index(ix, f"{OUT}/engram.index"); log(f"merged {ix.ntotal} keys in {time.time() - t0:.0f} s")
    for f in glob.glob(f"{OUT}/shard_*.index"):
        os.remove(f)   # their lists now live in lists.ivfdata