"""Separate normalized embedded MTP by byte-preserving safetensors repack.

Avoids mlx-lm's presence-keyed RMSNorm shift on a converted trunk. No inference,
quantization, normalization, provider selection or source mutation is performed.
"""
from pathlib import Path
import argparse,hashlib,json,os,shutil,struct


def header(path):
 with path.open('rb') as f:
  length=struct.unpack('<Q',f.read(8))[0]
  if length>16*1024**2:raise ValueError('Header size limit')
  data=json.loads(f.read(length))
 return data,8+length


def repack(output,entries,heads):
 metadata={'__metadata__':{'format':'mlx'}};cursor=0
 for newname,src,old,base,key in entries:
  start,end=old['data_offsets'];metadata[newname]={**old,'data_offsets':[cursor,cursor+end-start]};cursor+=end-start
 raw=json.dumps(metadata,separators=(',',':')).encode();raw+=b' '*(-len(raw)%8);digests={}
 with output.open('xb') as out:
  out.write(struct.pack('<Q',len(raw)));out.write(raw)
  for newname,src,old,base,key in entries:
   start,end=old['data_offsets'];count=end-start;digest=hashlib.sha256()
   with src.open('rb') as source:
    source.seek(base+start)
    while count:
     chunk=source.read(min(count,8*1024**2))
     if not chunk:raise ValueError('Truncated tensor')
     out.write(chunk);digest.update(chunk);count-=len(chunk)
   digests[newname]={'source_tensor':key,'payload_sha256':digest.hexdigest(),'payload_bytes':end-start}
 return digests,cursor


def prepare(source,output):
 source,output=Path(source).resolve(),Path(output).resolve()
 if output.exists() or source==output or source in output.parents:raise ValueError('Output exists or inside source')
 index=json.loads((source/'model.safetensors.index.json').read_text())['weight_map'];heads=[k for k in index if k.startswith('language_model.mtp.') or k.startswith('mtp.')]
 if not heads:raise ValueError('No embedded MTP tensors')
 schemas={}
 for filename in sorted(set(index.values())):
  if Path(filename).name!=filename:raise ValueError('Unsafe source shard')
  h,base=header(source/filename);keys={k for k in h if k!='__metadata__'}
  if keys!={k for k,v in index.items() if v==filename}:raise ValueError('Index/header mismatch')
  if h.get('__metadata__',{}).get('format')!='mlx':raise ValueError('Source not marked normalized MLX format')
  schemas[filename]=(h,base)
 output.mkdir();mapping={};allhead=[];payloads={};total=0
 for filename,(h,base) in schemas.items():
  selected=[k for k,v in index.items() if v==filename];trunk=[k for k in selected if k not in heads]
  for key in selected:
   if key in heads:
    newname=key.removeprefix('language_model.')
    allhead.append((newname,source/filename,h[key],base,key))
  if not trunk:continue
  if len(trunk)==len(selected):
   os.link(source/filename,output/filename)
   total+=sum(h[k]['data_offsets'][1]-h[k]['data_offsets'][0] for k in trunk)
  else:
   entries=[(k,source/filename,h[k],base,k) for k in trunk]
   d,size=repack(output/filename,entries,heads);payloads.update(d);total+=size
  mapping.update({k:filename for k in trunk})
 d,size=repack(output/'mtp.safetensors',allhead,heads);payloads.update(d)
 for p in source.iterdir():
  if p.is_file() and p.suffix in ['.json','.jinja','.txt','.md'] and p.name not in ['model.safetensors.index.json']:
   shutil.copy2(p,output/p.name)
 (output/'model.safetensors.index.json').write_text(json.dumps({'metadata':{'total_size':total},'weight_map':mapping},indent=2)+'\n')
 # Verify every rewritten payload independently against the copied digest.
 for filename in [*{index[k] for k in heads if index[k] in mapping.values()},'mtp.safetensors']:
  p=output/filename
  if not p.exists():continue
  h,base=header(p)
  with p.open('rb') as f:
   for name,record in h.items():
    if name=='__metadata__':continue
    start,end=record['data_offsets'];f.seek(base+start);remaining=end-start;digest=hashlib.sha256()
    while remaining:
     block=f.read(min(remaining,8*1024**2));digest.update(block);remaining-=len(block)
    if digest.hexdigest()!=payloads[name]['payload_sha256']:raise ValueError('Repacked tensor hash mismatch')
 receipt={'method':'exact tensor-payload repack only; no numerical change',
          'source':str(source),'base_tensors':len(mapping),'mtp_tensors':len(heads),'payload_hashes':payloads,
          'norm_shift_applied':False,'quantization_changed':False,'source_unchanged':True}
 (output/'mtp-separation-manifest.json').write_text(json.dumps(receipt,indent=2)+'\n');return receipt

if __name__=='__main__':
 p=argparse.ArgumentParser();p.add_argument('--source',type=Path,required=True);p.add_argument('--output',type=Path,required=True);a=p.parse_args();r=prepare(a.source,a.output);print({k:r[k] for k in ['base_tensors','mtp_tensors','norm_shift_applied','quantization_changed']})