#!/usr/bin/env python3
"""PyTorch parity check for the xtrain tiny transformer (Phase T5).

Loads the weights/ids dumped by tests/parity_dump.rs, rebuilds the IDENTICAL
model in PyTorch (same x@W convention, same RoPE rotate_half + position=row,
same RMSNorm, SwiGLU, causal mask, per-head SDPA), runs forward + one backward,
and compares the forward logits and every parameter's gradient against the Rust
values within a relative tolerance.

Usage:  python3 parity.py /tmp/xtrain_parity
"""
import sys
import os
import math
import torch

DIR = sys.argv[1] if len(sys.argv) > 1 else "/tmp/xtrain_parity"


def read_vec(name):
    path = os.path.join(DIR, name)
    shape = None
    vals = []
    with open(path) as f:
        for line in f:
            line = line.strip()
            if line.startswith("# shape"):
                shape = [int(x) for x in line.split()[2].split(",") if x]
            elif line:
                vals.append(float(line))
    t = torch.tensor(vals, dtype=torch.float64)
    if shape:
        t = t.reshape(shape)
    return t


def read_cfg():
    cfg = {}
    with open(os.path.join(DIR, "config.txt")) as f:
        for line in f:
            k, v = line.split()
            cfg[k] = v
    return cfg


def read_ids(name):
    with open(os.path.join(DIR, name)) as f:
        return [int(x) for x in f.read().split()]


cfg = read_cfg()
DIM = int(cfg["dim"])
NL = int(cfg["n_layers"])
NH = int(cfg["n_heads"])
HD = int(cfg["head_dim"])
EPS = float(cfg["eps"])
THETA = float(cfg["rope_theta"])

ids = read_ids("ids.txt")
targets = read_ids("targets.txt")
SEQ = len(ids)

# Load params as leaf tensors requiring grad (float64 for a clean reference).
P = {}


def load(name):
    t = read_vec(f"w_{name}.txt").clone().requires_grad_(True)
    P[name] = t
    return t


def rms_norm(x, gamma):
    # y = x / sqrt(mean(x^2)+eps) * gamma   (no mean subtraction)
    ms = x.pow(2).mean(dim=-1, keepdim=True)
    return x * torch.rsqrt(ms + EPS) * gamma


def rope(x):  # x: [seq, nh, hd], position = token index, matching the kernel
    half = HD // 2
    out = torch.empty_like(x)
    i = torch.arange(half, dtype=torch.float64)
    freq = THETA ** (-(2.0 * i) / HD)  # [half]
    pos = torch.arange(SEQ, dtype=torch.float64).reshape(SEQ, 1)  # [seq,1]
    ang = pos * freq  # [seq, half]
    c = torch.cos(ang).reshape(SEQ, 1, half)
    s = torch.sin(ang).reshape(SEQ, 1, half)
    x0 = x[..., :half]
    x1 = x[..., half:]
    out[..., :half] = x0 * c - x1 * s
    out[..., half:] = x1 * c + x0 * s
    return out


emb = load("embed")
final_norm = load("final_norm")
lm_head = load("lm_head")
layers = []
for l in range(NL):
    layers.append({p: load(f"l{l}_{p}") for p in
                   ["attn_norm", "wq", "wk", "wv", "wo",
                    "ffn_norm", "w_gate", "w_up", "w_down"]})

idx = torch.tensor(ids, dtype=torch.long)
mask = torch.triu(torch.full((SEQ, SEQ), -1.0e9, dtype=torch.float64), diagonal=1)

h = emb[idx]  # [seq, dim]
for L in layers:
    # Attention
    x = rms_norm(h, L["attn_norm"])
    q = (x @ L["wq"]).reshape(SEQ, NH, HD)
    k = (x @ L["wk"]).reshape(SEQ, NH, HD)
    v = (x @ L["wv"]).reshape(SEQ, NH, HD)
    q = rope(q).transpose(0, 1)  # [nh, seq, hd]
    k = rope(k).transpose(0, 1)
    v = v.transpose(0, 1)
    scale = 1.0 / math.sqrt(HD)
    scores = (q @ k.transpose(-1, -2)) * scale + mask  # [nh, seq, seq]
    probs = torch.softmax(scores, dim=-1)
    out = probs @ v  # [nh, seq, hd]
    out = out.transpose(0, 1).reshape(SEQ, DIM)  # [seq, dim]
    attn = out @ L["wo"]
    h = h + attn
    # MLP
    x = rms_norm(h, L["ffn_norm"])
    gate = x @ L["w_gate"]
    up = x @ L["w_up"]
    act = torch.nn.functional.silu(gate) * up
    mlp = act @ L["w_down"]
    h = h + mlp

h = rms_norm(h, final_norm)
logits = h @ lm_head  # [seq, vocab]

loss = torch.nn.functional.cross_entropy(
    logits, torch.tensor(targets, dtype=torch.long), reduction="mean")
loss.backward()

# ---- Compare ----
def relerr(a, b):
    a = a.double()
    b = b.double()
    denom = b.abs().clamp(min=1e-6)
    return ((a - b).abs() / denom).max().item()


ref_logits = read_vec("logits.txt")
ref_loss = read_vec("loss.txt").item()

print(f"loss: rust={ref_loss:.6e} torch={loss.item():.6e} "
      f"relerr={abs(loss.item()-ref_loss)/max(abs(ref_loss),1e-6):.2e}")
le = relerr(logits.detach(), ref_logits)
print(f"logits: max relerr = {le:.2e}")

RTOL = 2e-2
worst = le
worst_name = "logits"
fails = []
if le > RTOL:
    fails.append(("logits", le))

for name, t in P.items():
    ref_g = read_vec(f"g_{name}.txt")
    ge = relerr(t.grad, ref_g)
    if ge > worst:
        worst, worst_name = ge, f"grad[{name}]"
    if ge > RTOL:
        fails.append((f"grad[{name}]", ge))

print(f"params checked: {len(P)}  worst = {worst_name} @ {worst:.2e}  (rtol={RTOL})")
if fails:
    print("FAIL:")
    for n, e in fails:
        print(f"  {n}: relerr={e:.3e}")
    sys.exit(1)
print("PARITY OK: forward logits + all param grads within rtol")