function export_comsol_slice(mphFile, outFile, z0, nx, ny)
%EXPORT_COMSOL_SLICE  Export |E| on z=z0 plane from COMSOL mph (LiveLink required).
%
% 不要无参调用本函数。请直接运行:  run_export_comsol.m
%
% 或手动带齐 3 个参数:
%   export_comsol_slice('D:\path\SBC.mph', 'data\comsol_z050.txt', 0.5);

if nargin < 3
    error(['输入参数不足。\n' ...
        '请运行 run_export_comsol.m（推荐），或：\n' ...
        '  export_comsol_slice(''完整路径\\SBC.mph'', ''data\\comsol_z050.txt'', 0.5);']);
end
if nargin < 4, nx = 121; end
if nargin < 5, ny = 121; end

assert(isfile(mphFile), 'COMSOL model not found: %s', mphFile);
assert(exist('mphload', 'file') == 2, ...
    'COMSOL LiveLink for MATLAB not found (mphload missing).');

model = mphload(mphFile);

x = linspace(-0.5, 0.5, nx);
y = linspace(-0.5, 0.5, ny);
[X, Y] = meshgrid(x, y);
Z = z0 * ones(size(X));
coords = [X(:)'; Y(:)'; Z(:)'];

expr = 'sqrt(realdot(ewfd.Ex,ewfd.Ex)+realdot(ewfd.Ey,ewfd.Ey)+realdot(ewfd.Ez,ewfd.Ez))';
val = mphinterp(model, expr, 'coord', coords, 'unit', 'V/m');
val = reshape(val, size(X));

outDir = fileparts(outFile);
if ~isempty(outDir) && ~isfolder(outDir)
    mkdir(outDir);
end

fid = fopen(outFile, 'w');
fprintf(fid, '%% x(m) y(m) z(m) normE(V/m)\n');
for i = 1:numel(X)
    fprintf(fid, '%.6e\t%.6e\t%.6e\t%.6e\n', X(i), Y(i), Z(i), val(i));
end
fclose(fid);
fprintf('Exported COMSOL slice to %s\n', outFile);
end