function main_export()
%MAIN_EXPORT  二阶单周期散射：导出 Ab 矩阵与 normE（格式同 3D 一阶散射 OutFile）。
%
% 输出目录：./OutFile/
%   Ai.txt, Aj.txt, Av.txt, Bv_real.txt, Bv_imag.txt  — 投影后线性系统 P'*A*P, P'*b
%   Ex, Ey, Ez, normE                                 — 求解后全场（每顶点一行）
%
% 另导出 ./OutFile_asm/：PBC 投影前的装配矩阵（与一阶散射 main.m 时机一致）。
%
% 依赖：case_config.m 中 cfg.meshMatFile（默认 c6.mat）须在本目录。

clc;
[phy, cfg] = case_config();
root = fileparts(mfilename('fullpath'));
outDir = fullfile(root, 'OutFile');
outAsm = fullfile(root, 'OutFile_asm');

meshMat = fullfile(root, cfg.meshMatFile);
if ~isfile(meshMat)
    error('main_export:NoMesh', '找不到网格 %s，请从 COMSOL 导出 c6.mat 到本目录。', meshMat);
end
load(meshMat);

mesh.incIndex = findTri(phy.inc, mesh);
mesh.outIndex = findTri(phy.out, mesh);
[mesh.PBCIndex, mesh.origIndex] = findPBCIndex(phy.src, phy.dst, cfg.pbcAngle, mesh);
mesh.PBCphi = cfg.pbcPhi;

solver.Ai = [];
solver.Aj = [];
solver.Av = [];
solver.dof = mesh.NbrEdge * 2 + mesh.NbrFace * 2;
solver.b = zeros(solver.dof, 1);
solver = assembly_equ(phy, mesh, solver);
solver = assembly_out(phy, mesh, solver);
solver = assembly_inc(phy, mesh, solver);
solver.A = sparse(solver.Ai, solver.Aj, solver.Av);

% 装配矩阵（无 PBC 投影），与一阶散射 export_ab_matrix 时机一致
export_ab_matrix(solver, outAsm);

% Bloch 周期边界 + 投影
solver = assembly_rbc(mesh, solver);
solver.A = solver.P' * solver.A * solver.P;
solver.b = solver.P' * solver.b;

% 投影后线性系统（实际求解的 Ab）
export_ab_matrix(solver, outDir);

% 求解（与 main_RBC.m 相同）
r = symamd(solver.A);
solver.A = solver.A(r, r);
solver.b = solver.b(r);
solver.x = solver.A \ solver.b;
q = zeros(length(r), 1);
for i = 1:length(r)
    q(r(i)) = i;
end
solver.x = solver.x(q);
solver.x = solver.P * solver.x;

[Ex, Ey, Ez, normE] = get_ele_vertices(mesh, solver);
export_fields(Ex, Ey, Ez, normE, outDir);

fprintf('\n完成。投影后系统已写入 %s\n', outDir);
fprintf('装配矩阵（无 PBC）已写入 %s\n', outAsm);
end