"""Skin a baked-pose GLB's hand region with Godot-exact LBS and write it as a plain OBJ.
usage: godot_skin_hand_obj.py posed.glb side(l|r) out.obj"""
import json, struct, sys

def read_glb(path):
    d = open(path, "rb").read()
    length = struct.unpack_from("<I", d, 8)[0]
    off = 12; g = None; b = None
    while off < length:
        clen, ct = struct.unpack_from("<II", d, off); off += 8
        if ct == 0x4E4F534A: g = json.loads(d[off:off+clen])
        else: b = d[off:off+clen]
        off += clen
    return g, b

def acc(g, b, i):
    a = g["accessors"][i]; bv = g["bufferViews"][a["bufferView"]]
    nc = {"SCALAR":1,"VEC2":2,"VEC3":3,"VEC4":4,"MAT4":16}[a["type"]]
    fmt = {5121:"B",5123:"H",5125:"I",5126:"f"}[a["componentType"]]
    size = struct.calcsize(fmt)*nc; stride = bv.get("byteStride") or size
    off = bv.get("byteOffset",0)+a.get("byteOffset",0)
    return [struct.unpack_from("<%d%s"%(nc,fmt), b, off+k*stride) for k in range(a["count"])], a["componentType"]

def quat_mat(q):
    x,y,z,w = q
    return [[1-2*(y*y+z*z),2*(x*y-z*w),2*(x*z+y*w)],
            [2*(x*y+z*w),1-2*(x*x+z*z),2*(y*z-x*w)],
            [2*(x*z-y*w),2*(y*z+x*w),1-2*(x*x+y*y)]]

def node_local(nd):
    t = nd.get("translation",[0,0,0]); r = nd.get("rotation",[0,0,0,1]); s = nd.get("scale",[1,1,1])
    R = quat_mat(r)
    M = [[R[i][j]*s[j] for j in range(3)]+[t[i]] for i in range(3)]
    return M+[[0,0,0,1]]

def matmul(A,B):
    return [[sum(A[i][k]*B[k][j] for k in range(4)) for j in range(4)] for i in range(4)]

glb, side, out = sys.argv[1:4]
g, b = read_glb(glb)
names = [nd.get("name","") for nd in g["nodes"]]
loc = [node_local(nd) for nd in g["nodes"]]
parent = {}
for i,nd in enumerate(g["nodes"]):
    for c in nd.get("children",[]): parent[c] = i
memo = {}
def gm(i):
    if i in memo: return memo[i]
    m = loc[i] if i not in parent else matmul(gm(parent[i]), loc[i])
    memo[i] = m; return m
G = [gm(i) for i in range(len(g["nodes"]))]

skin = g["skins"][0]; joints = skin["joints"]
ibm,_ = acc(g,b,skin["inverseBindMatrices"])
def m16(row):
    return [[row[c*4+r] for c in range(4)] for r in range(4)]
JM = [matmul(G[joints[j]], m16(ibm[j])) for j in range(len(joints))]

REGION = ("hand_", "thumb_", "index_", "middle_", "ring_", "pinky_", "lowerarm_")
region_j = {j for j in range(len(joints))
            if any(names[joints[j]].startswith(p) for p in REGION)
            and names[joints[j]].endswith("_"+side)}

prim = g["meshes"][0]["primitives"][0]
P,_ = acc(g,b,prim["attributes"]["POSITION"])
J,_ = acc(g,b,prim["attributes"]["JOINTS_0"])
W,wt = acc(g,b,prim["attributes"]["WEIGHTS_0"])
I,_ = acc(g,b,prim["indices"])
wsc = 1.0 if wt==5126 else (1/255 if wt==5121 else 1/65535)

keep = {}
for vi,(p,jr,wr) in enumerate(zip(P,J,W)):
    if not any(j in region_j and w>0 for j,w in zip(jr,wr)): continue
    x=y=z=0.0
    for j,w in zip(jr,wr):
        w*=wsc
        if w<=0: continue
        M=JM[j]
        x+=w*(M[0][0]*p[0]+M[0][1]*p[1]+M[0][2]*p[2]+M[0][3])
        y+=w*(M[1][0]*p[0]+M[1][1]*p[1]+M[1][2]*p[2]+M[1][3])
        z+=w*(M[2][0]*p[0]+M[2][1]*p[1]+M[2][2]*p[2]+M[2][3])
    keep[vi]=(x,y,z)

remap = {vi:k+1 for k,vi in enumerate(keep)}
tris = []
flat = [ix[0] for ix in I]
for t in range(0, len(flat), 3):
    a1,a2,a3 = flat[t], flat[t+1], flat[t+2]
    if a1 in remap and a2 in remap and a3 in remap:
        tris.append((remap[a1], remap[a2], remap[a3]))

with open(out, "w") as f:
    for vi in keep:
        x,y,z = keep[vi]
        f.write(f"v {x:.6f} {y:.6f} {z:.6f}\n")
    for t in tris:
        f.write(f"f {t[0]} {t[1]} {t[2]}\n")
print(f"[skinobj] {out}: {len(keep)} verts, {len(tris)} tris")