def _get_solid_objects(model_data):
    """Safely extracts unique 3D concrete solid meshes from model_data['objects']."""
    objs = []
    seen = set()
    raw = model_data.get('objects', {})
    if 'culvert' in raw and isinstance(raw['culvert'], dict):
        c = raw['culvert']
        objs.append(c)
        seen.add(c.get('id', 'CULVERT'))
    if 'wingwalls' in raw and isinstance(raw['wingwalls'], list):
        for w in raw['wingwalls']:
            if isinstance(w, dict) and w.get('id') not in seen and 'vertices' in w:
                objs.append(w)
                seen.add(w['id'])
    else:
        for k, v in raw.items():
            if isinstance(v, dict) and v.get('id') and v['id'] not in seen and 'vertices' in v:
                objs.append(v)
                seen.add(v['id'])
    return objs


def export_obj(model_data, include_rebars=True):
    """
    Exports 3D model data (exterior concrete solids + 3D rebars) to Wavefront OBJ format.
    """
    lines = ["# Wingwall & Culvert 3D Solid & Rebar Model", "# Generated by Antigravity CAD/BIM Modeler", ""]
    v_offset = 1

    solid_objects = _get_solid_objects(model_data)
    for obj in solid_objects:
        lines.append(f"o {obj['id']}")
        lines.append(f"g {obj['name']}")

        for v in obj['vertices']:
            lines.append(f"v {v[0]:.4f} {v[1]:.4f} {v[2]:.4f}")

        for f in obj['faces']:
            lines.append(f"f {f[0] + v_offset} {f[1] + v_offset} {f[2] + v_offset}")

        v_offset += len(obj['vertices'])
        lines.append("")

    if include_rebars and 'rebars' in model_data:
        for ww_id, ww_rebar in model_data['rebars'].get('wingwalls', {}).items():
            for grp in ww_rebar.get('groups', []):
                lines.append(f"o REBAR_{ww_id}_{grp['mark']}_{grp['dia']}")
                for poly in grp.get('polylines', []):
                    if len(poly) < 2:
                        continue
                    for pt in poly:
                        lines.append(f"v {pt[0]:.4f} {pt[1]:.4f} {pt[2]:.4f}")
                    idx_str = " ".join(str(v_offset + i) for i in range(len(poly)))
                    lines.append(f"l {idx_str}")
                    v_offset += len(poly)
                lines.append("")

    return "\n".join(lines)


def export_stl(model_data):
    """
    Exports 3D model data to ASCII STL format.
    """
    lines = ["solid WingwallCulvertModel"]
    for obj in _get_solid_objects(model_data):
        v = obj['vertices']
        for f in obj['faces']:
            p1 = v[f[0]]
            p2 = v[f[1]]
            p3 = v[f[2]]
            v1 = [p2[0]-p1[0], p2[1]-p1[1], p2[2]-p1[2]]
            v2 = [p3[0]-p1[0], p3[1]-p1[1], p3[2]-p1[2]]
            nx = v1[1]*v2[2] - v1[2]*v2[1]
            ny = v1[2]*v2[0] - v1[0]*v2[2]
            nz = v1[0]*v2[1] - v1[1]*v2[0]
            mag = (nx*nx + ny*ny + nz*nz)**0.5
            if mag > 1e-8:
                nx, ny, nz = nx/mag, ny/mag, nz/mag
            else:
                nx, ny, nz = 0.0, 0.0, 1.0

            lines.append(f"  facet normal {nx:.4e} {ny:.4e} {nz:.4e}")
            lines.append("    outer loop")
            lines.append(f"      vertex {p1[0]:.4f} {p1[1]:.4f} {p1[2]:.4f}")
            lines.append(f"      vertex {p2[0]:.4f} {p2[1]:.4f} {p2[2]:.4f}")
            lines.append(f"      vertex {p3[0]:.4f} {p3[1]:.4f} {p3[2]:.4f}")
            lines.append("    endloop")
            lines.append("  endfacet")
    lines.append("endsolid WingwallCulvertModel")
    return "\n".join(lines)


def export_dxf_3d(model_data, include_rebars=True):
    """
    Exports 3D model data to DXF R2000 ASCII format containing 3DFACE entities for concrete
    and 3D LINE entities for internal rebars.
    """
    lines = [
        "0", "SECTION",
        "2", "ENTITIES"
    ]
    for obj in _get_solid_objects(model_data):
        layer = obj['id']
        v = obj['vertices']
        for f in obj['faces']:
            p1 = v[f[0]]
            p2 = v[f[1]]
            p3 = v[f[2]]
            lines.extend([
                "0", "3DFACE",
                "8", layer,
                "10", f"{p1[0]:.4f}", "20", f"{p1[1]:.4f}", "30", f"{p1[2]:.4f}",
                "11", f"{p2[0]:.4f}", "21", f"{p2[1]:.4f}", "31", f"{p2[2]:.4f}",
                "12", f"{p3[0]:.4f}", "22", f"{p3[1]:.4f}", "32", f"{p3[2]:.4f}",
                "13", f"{p3[0]:.4f}", "23", f"{p3[1]:.4f}", "33", f"{p3[2]:.4f}"
            ])

    if include_rebars and 'rebars' in model_data:
        color_by_dia = {'H19': '1', 'H16': '4', 'H13': '3'}
        for ww_id, ww_rebar in model_data['rebars'].get('wingwalls', {}).items():
            for grp in ww_rebar.get('groups', []):
                layer = f"REBAR_{ww_id}_{grp['mark']}_{grp['dia']}"
                c_code = color_by_dia.get(grp['dia'], '2')
                for poly in grp.get('polylines', []):
                    for i in range(len(poly) - 1):
                        p1 = poly[i]
                        p2 = poly[i + 1]
                        lines.extend([
                            "0", "LINE",
                            "8", layer,
                            "62", c_code,
                            "10", f"{p1[0]:.4f}", "20", f"{p1[1]:.4f}", "30", f"{p1[2]:.4f}",
                            "11", f"{p2[0]:.4f}", "21", f"{p2[1]:.4f}", "31", f"{p2[2]:.4f}"
                        ])

    lines.extend([
        "0", "ENDSEC",
        "0", "EOF"
    ])
    return "\n".join(lines)


def export_rebar_bom_csv(model_data):
    """
    Exports 3단계 철근 재료표 (Bar Bending Schedule) to UTF-8 BOM CSV format.
    """
    rows = [
        "\ufeff구분(날개벽),기호(Mark),직경(Dia),배치부위,역할,간격,절곡치수,1본길이(m),수량(개),총길이(m),단위중량(kg/m),총중량(TON)"
    ]
    if 'rebars' in model_data:
        for ww_id, ww in model_data['rebars'].get('wingwalls', {}).items():
            ww_name = ww.get('wingwall_name', ww_id)
            for g in ww.get('groups', []):
                shape_str = " / ".join(f"{k}={v}" for k, v in g.get('shape_params', {}).items())
                rows.append(
                    f'"{ww_name}",{g["mark"]},{g["dia"]},"{g["zone"]}","{g["role"]}","{g["spacing"]}",'
                    f'"{shape_str}",{g["length_m"]:.3f},{g["count"]},{g["total_length_m"]:.3f},'
                    f'{g["unit_weight"]:.3f},{g["weight_ton"]:.4f}'
                )
            s = ww.get('summary', {})
            rows.append(
                f'"{ww_name} 소계",-,전체,-,-,-,-,-,{s.get("total_bars",0)},'
                f'{s.get("total_length_m",0):.3f},-,{s.get("total_weight_ton",0):.3f} (할증3%: {s.get("total_surcharge_ton",0):.3f})'
            )
    return "\n".join(rows)