import json, struct
import numpy as np
import mlx.core as mx

def _load(path, key):
    with open(path, 'rb') as f:
        n = struct.unpack('<Q', f.read(8))[0]
        hdr = json.loads(f.read(n))
        e = hdr[key]
        start, end = e['data_offsets']
        f.seek(8 + n + start)
        raw = f.read(end - start)
    return e, raw

def get_tensor(path, key, dtype=None):
    """Bit-accurate tensor reader. BF16 is bit-reinterpreted (uint16 bits -> bf16)."""
    e, raw = _load(path, key)
    t = e['dtype']
    if t == 'BF16':
        u16 = np.frombuffer(raw, dtype=np.uint16)
        f32 = np.left_shift(u16.astype(np.uint32), 16).view(np.float32).copy()
        a = mx.array(f32)
        if dtype in (None, 'bf16'):
            # exact bf16 round-trip: bf16 -> f32 is lossless
            return a.reshape(e['shape']) if dtype is None else a.astype(mx.bfloat16).reshape(e['shape'])
        return a.reshape(e['shape'])
    if t == 'F16':
        return mx.array(np.frombuffer(raw, dtype=np.float16).copy()).reshape(e['shape'])
    if t == 'F32':
        return mx.array(np.frombuffer(raw, dtype=np.float32).copy()).reshape(e['shape'])
    if t in ('U32', 'I32'):
        return mx.array(np.frombuffer(raw, dtype=np.uint32).copy()).reshape(e['shape'])
    raise ValueError(t)

def header(path):
    with open(path, 'rb') as f:
        n = struct.unpack('<Q', f.read(8))[0]
        return json.loads(f.read(n))