Unit tests for DERIVATIVE prevent change.

[1uc]

Even simple changes like:

• making J row-major
• (X - X_old)/dt = dX -> X = X_old + dt*dX

are practically almost impossible because they require changing the hard coded reference across 1000s of lines of code. For example,

nmodl/test/unit/visitor/sympy_solver.cpp

Lines 1848 to 2055 in 6d3b830

	GIVEN("12 state-var LINEAR solve block") {
	std::string nmodl_text = R"(
	STATE {
	C1 C2 C3 C4 C5 I1 I2 I3 I4 I5 I6 O
	}
	LINEAR seqinitial {
	~ I1*bi1 + C2*b01 - C1*( fi1+f01) = 0
	~ C1*f01 + I2*bi2 + C3*b02 - C2*(b01+fi2+f02) = 0
	~ C2*f02 + I3*bi3 + C4*b03 - C3*(b02+fi3+f03) = 0
	~ C3*f03 + I4*bi4 + C5*b04 - C4*(b03+fi4+f04) = 0
	~ C4*f04 + I5*bi5 + O*b0O - C5*(b04+fi5+f0O) = 0
	~ C5*f0O + I6*bin0 - O*(b0O+fin0) = 0
	~ C1*fi1 + I2*b11 - I1*( bi1+f11) = 0
	~ I1*f11 + C2*fi2 + I3*b12 - I2*(b11+bi2+f12) = 0
	~ I2*f12 + C3*fi3 + I4*bi3 - I3*(b12+bi3+f13) = 0
	~ I3*f13 + C4*fi4 + I5*b14 - I4*(b13+bi4+f14) = 0
	~ I4*f14 + C5*fi5 + I6*b1n - I5*(b14+bi5+f1n) = 0
	~ C1 + C2 + C3 + C4 + C5 + O + I1 + I2 + I3 + I4 + I5 + I6 = 1
	})";
	std::string expected_text = R"(
	LINEAR seqinitial {
	EIGEN_LINEAR_SOLVE[12]{
	}{
	}{
	nmodl_eigen_x[0] = C1
	nmodl_eigen_x[1] = C2
	nmodl_eigen_x[2] = C3
	nmodl_eigen_x[3] = C4
	nmodl_eigen_x[4] = C5
	nmodl_eigen_x[5] = I1
	nmodl_eigen_x[6] = I2
	nmodl_eigen_x[7] = I3
	nmodl_eigen_x[8] = I4
	nmodl_eigen_x[9] = I5
	nmodl_eigen_x[10] = I6
	nmodl_eigen_x[11] = O
	nmodl_eigen_f[0] = 0
	nmodl_eigen_f[1] = 0
	nmodl_eigen_f[2] = 0
	nmodl_eigen_f[3] = 0
	nmodl_eigen_f[4] = 0
	nmodl_eigen_f[5] = 0
	nmodl_eigen_f[6] = 0
	nmodl_eigen_f[7] = 0
	nmodl_eigen_f[8] = 0
	nmodl_eigen_f[9] = 0
	nmodl_eigen_f[10] = 0
	nmodl_eigen_f[11] = -1.0
	nmodl_eigen_j[0] = f01+fi1
	nmodl_eigen_j[12] = -b01
	nmodl_eigen_j[24] = 0
	nmodl_eigen_j[36] = 0
	nmodl_eigen_j[48] = 0
	nmodl_eigen_j[60] = -bi1
	nmodl_eigen_j[72] = 0
	nmodl_eigen_j[84] = 0
	nmodl_eigen_j[96] = 0
	nmodl_eigen_j[108] = 0
	nmodl_eigen_j[120] = 0
	nmodl_eigen_j[132] = 0
	nmodl_eigen_j[1] = -f01
	nmodl_eigen_j[13] = b01+f02+fi2
	nmodl_eigen_j[25] = -b02
	nmodl_eigen_j[37] = 0
	nmodl_eigen_j[49] = 0
	nmodl_eigen_j[61] = 0
	nmodl_eigen_j[73] = -bi2
	nmodl_eigen_j[85] = 0
	nmodl_eigen_j[97] = 0
	nmodl_eigen_j[109] = 0
	nmodl_eigen_j[121] = 0
	nmodl_eigen_j[133] = 0
	nmodl_eigen_j[2] = 0
	nmodl_eigen_j[14] = -f02
	nmodl_eigen_j[26] = b02+f03+fi3
	nmodl_eigen_j[38] = -b03
	nmodl_eigen_j[50] = 0
	nmodl_eigen_j[62] = 0
	nmodl_eigen_j[74] = 0
	nmodl_eigen_j[86] = -bi3
	nmodl_eigen_j[98] = 0
	nmodl_eigen_j[110] = 0
	nmodl_eigen_j[122] = 0
	nmodl_eigen_j[134] = 0
	nmodl_eigen_j[3] = 0
	nmodl_eigen_j[15] = 0
	nmodl_eigen_j[27] = -f03
	nmodl_eigen_j[39] = b03+f04+fi4
	nmodl_eigen_j[51] = -b04
	nmodl_eigen_j[63] = 0
	nmodl_eigen_j[75] = 0
	nmodl_eigen_j[87] = 0
	nmodl_eigen_j[99] = -bi4
	nmodl_eigen_j[111] = 0
	nmodl_eigen_j[123] = 0
	nmodl_eigen_j[135] = 0
	nmodl_eigen_j[4] = 0
	nmodl_eigen_j[16] = 0
	nmodl_eigen_j[28] = 0
	nmodl_eigen_j[40] = -f04
	nmodl_eigen_j[52] = b04+f0O+fi5
	nmodl_eigen_j[64] = 0
	nmodl_eigen_j[76] = 0
	nmodl_eigen_j[88] = 0
	nmodl_eigen_j[100] = 0
	nmodl_eigen_j[112] = -bi5
	nmodl_eigen_j[124] = 0
	nmodl_eigen_j[136] = -b0O
	nmodl_eigen_j[5] = 0
	nmodl_eigen_j[17] = 0
	nmodl_eigen_j[29] = 0
	nmodl_eigen_j[41] = 0
	nmodl_eigen_j[53] = -f0O
	nmodl_eigen_j[65] = 0
	nmodl_eigen_j[77] = 0
	nmodl_eigen_j[89] = 0
	nmodl_eigen_j[101] = 0
	nmodl_eigen_j[113] = 0
	nmodl_eigen_j[125] = -bin0
	nmodl_eigen_j[137] = b0O+fin0
	nmodl_eigen_j[6] = -fi1
	nmodl_eigen_j[18] = 0
	nmodl_eigen_j[30] = 0
	nmodl_eigen_j[42] = 0
	nmodl_eigen_j[54] = 0
	nmodl_eigen_j[66] = bi1+f11
	nmodl_eigen_j[78] = -b11
	nmodl_eigen_j[90] = 0
	nmodl_eigen_j[102] = 0
	nmodl_eigen_j[114] = 0
	nmodl_eigen_j[126] = 0
	nmodl_eigen_j[138] = 0
	nmodl_eigen_j[7] = 0
	nmodl_eigen_j[19] = -fi2
	nmodl_eigen_j[31] = 0
	nmodl_eigen_j[43] = 0
	nmodl_eigen_j[55] = 0
	nmodl_eigen_j[67] = -f11
	nmodl_eigen_j[79] = b11+bi2+f12
	nmodl_eigen_j[91] = -b12
	nmodl_eigen_j[103] = 0
	nmodl_eigen_j[115] = 0
	nmodl_eigen_j[127] = 0
	nmodl_eigen_j[139] = 0
	nmodl_eigen_j[8] = 0
	nmodl_eigen_j[20] = 0
	nmodl_eigen_j[32] = -fi3
	nmodl_eigen_j[44] = 0
	nmodl_eigen_j[56] = 0
	nmodl_eigen_j[68] = 0
	nmodl_eigen_j[80] = -f12
	nmodl_eigen_j[92] = b12+bi3+f13
	nmodl_eigen_j[104] = -bi3
	nmodl_eigen_j[116] = 0
	nmodl_eigen_j[128] = 0
	nmodl_eigen_j[140] = 0
	nmodl_eigen_j[9] = 0
	nmodl_eigen_j[21] = 0
	nmodl_eigen_j[33] = 0
	nmodl_eigen_j[45] = -fi4
	nmodl_eigen_j[57] = 0
	nmodl_eigen_j[69] = 0
	nmodl_eigen_j[81] = 0
	nmodl_eigen_j[93] = -f13
	nmodl_eigen_j[105] = b13+bi4+f14
	nmodl_eigen_j[117] = -b14
	nmodl_eigen_j[129] = 0
	nmodl_eigen_j[141] = 0
	nmodl_eigen_j[10] = 0
	nmodl_eigen_j[22] = 0
	nmodl_eigen_j[34] = 0
	nmodl_eigen_j[46] = 0
	nmodl_eigen_j[58] = -fi5
	nmodl_eigen_j[70] = 0
	nmodl_eigen_j[82] = 0
	nmodl_eigen_j[94] = 0
	nmodl_eigen_j[106] = -f14
	nmodl_eigen_j[118] = b14+bi5+f1n
	nmodl_eigen_j[130] = -b1n
	nmodl_eigen_j[142] = 0
	nmodl_eigen_j[11] = -1.0
	nmodl_eigen_j[23] = -1.0
	nmodl_eigen_j[35] = -1.0
	nmodl_eigen_j[47] = -1.0
	nmodl_eigen_j[59] = -1.0
	nmodl_eigen_j[71] = -1.0
	nmodl_eigen_j[83] = -1.0
	nmodl_eigen_j[95] = -1.0
	nmodl_eigen_j[107] = -1.0
	nmodl_eigen_j[119] = -1.0
	nmodl_eigen_j[131] = -1.0
	nmodl_eigen_j[143] = -1.0
	}{
	C1 = nmodl_eigen_x[0]
	C2 = nmodl_eigen_x[1]
	C3 = nmodl_eigen_x[2]
	C4 = nmodl_eigen_x[3]
	C5 = nmodl_eigen_x[4]
	I1 = nmodl_eigen_x[5]
	I2 = nmodl_eigen_x[6]
	I3 = nmodl_eigen_x[7]
	I4 = nmodl_eigen_x[8]
	I5 = nmodl_eigen_x[9]
	I6 = nmodl_eigen_x[10]
	O = nmodl_eigen_x[11]
	}{
	}
	})";

requires changing well over 100 lines of code.