function h = dinterptable(X, Y, u)
% n = length(X);
% DYDX = diff(Y) ./ diff(X);
% DYDX = DYDX([1, 1:end, end]);
% [~, b] = histc(u, [-inf;X;inf]);
% b = b - 1;
% b(b == 0) = 1;
% b(b == n) = n - 1;
% h = DYDX(b);
X = X(:);
Y = Y(:);
u = u(:);

if numel(X) ~= numel(Y)
    error('dinterptable:SizeMismatch', ...
        'X and Y must have the same length. Got %d and %d.', numel(X), numel(Y));
end
if any(~isfinite(X)) || any(~isfinite(Y))
    error('dinterptable:InvalidTable', ...
        'Interpolation table contains NaN or Inf.');
end
if any(diff(X) < 0)
    error('dinterptable:NonMonotonicX', ...
        'Interpolation table X must be nondecreasing.');
end

DYDX = diff(Y) ./ diff(X);
DYDX = DYDX([1, 1:end, end]);
[~, b] = histc(u, [-inf;X;inf]);

invalid_mask = ~isfinite(u) | b < 1 | b > numel(DYDX);
if any(invalid_mask)
    bad_values = u(invalid_mask);
    bad_preview = bad_values(1:min(5, numel(bad_values)));
    error('dinterptable:InvalidQuery', ...
        ['Interpolation query contains invalid values. ' ...
         'num_bad=%d, sample=%s, X_range=[%g, %g], Y_range=[%g, %g].'], ...
        numel(bad_values), mat2str(bad_preview.'), X(1), X(end), min(Y), max(Y));
end

h = reshape(DYDX(b), [], 1);