From 9c543ef65e4a091acb6f96023ef8d1d697ef20da Mon Sep 17 00:00:00 2001
From: Luc Grosheintz <luc.grosheintz@gmail.com>
Date: Mon, 14 Oct 2024 14:04:20 +0200
Subject: [PATCH] NONLINEAR requires `sympy --analytic`. (#1512)

---
 src/language/nmodl.yaml                |  6 +++---
 src/main.cpp                           |  2 ++
 test/usecases/CMakeLists.txt           |  1 +
 test/usecases/nonlinear/nonlin.mod     | 21 +++++++++++++++++++++
 test/usecases/nonlinear/test_nonlin.py | 15 +++++++++++++++
 5 files changed, 42 insertions(+), 3 deletions(-)
 create mode 100644 test/usecases/nonlinear/nonlin.mod
 create mode 100644 test/usecases/nonlinear/test_nonlin.py

diff --git a/src/language/nmodl.yaml b/src/language/nmodl.yaml
index 0f76b01a3..de598cec3 100644
--- a/src/language/nmodl.yaml
+++ b/src/language/nmodl.yaml
@@ -1140,15 +1140,15 @@
                             type: Expression
 
                   - NonLinEquation:
-                      brief: "TODO"
+                      brief: "One equation in a system of equations that collectively make a NONLINEAR block."
                       nmodl: "~ "
                       members:
                         - lhs:
-                            brief: "TODO"
+                            brief: "Left-hand-side of the equation."
                             type: Expression
                             suffix: {value: " = "}
                         - rhs:
-                            brief: "TODO"
+                            brief: "Right-hand-side of the equation."
                             type: Expression
 
                   - LinEquation:
diff --git a/src/main.cpp b/src/main.cpp
index 436ec2c4e..ba9de4a19 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -516,6 +516,8 @@ int run_nmodl(int argc, const char* argv[]) {
 
             enable_sympy(solver_exists(*ast, "derivimplicit"), "'SOLVE ... METHOD derivimplicit'");
             enable_sympy(node_exists(*ast, ast::AstNodeType::LINEAR_BLOCK), "'LINEAR' block");
+            enable_sympy(node_exists(*ast, ast::AstNodeType::NON_LINEAR_BLOCK),
+                         "'NONLINEAR' block");
             enable_sympy(solver_exists(*ast, "sparse"), "'SOLVE ... METHOD sparse'");
         }
 
diff --git a/test/usecases/CMakeLists.txt b/test/usecases/CMakeLists.txt
index 764b907cf..22a806a1c 100644
--- a/test/usecases/CMakeLists.txt
+++ b/test/usecases/CMakeLists.txt
@@ -18,6 +18,7 @@ set(NMODL_USECASE_DIRS
     net_receive
     net_send
     neuron_variables
+    nonlinear
     nonspecific_current
     parameter
     point_process
diff --git a/test/usecases/nonlinear/nonlin.mod b/test/usecases/nonlinear/nonlin.mod
new file mode 100644
index 000000000..ec0bed30f
--- /dev/null
+++ b/test/usecases/nonlinear/nonlin.mod
@@ -0,0 +1,21 @@
+NEURON {
+    SUFFIX nonlin
+}
+
+STATE { x }
+
+FUNCTION solve() {
+    : Iterative solvers need a starting guess.
+    x = 1.0
+    SOLVE eq
+
+    solve = x
+}
+
+NONLINEAR eq {
+    ~ x*x = 4.0
+}
+
+FUNCTION residual(x) {
+    residual = x - 2.0
+}
diff --git a/test/usecases/nonlinear/test_nonlin.py b/test/usecases/nonlinear/test_nonlin.py
new file mode 100644
index 000000000..eef75edbd
--- /dev/null
+++ b/test/usecases/nonlinear/test_nonlin.py
@@ -0,0 +1,15 @@
+from neuron import h, gui
+
+
+def test_nonlin():
+    s = h.Section()
+    s.insert("nonlin")
+    inst = s(0.5).nonlin
+
+    x = inst.solve()
+    error = inst.residual(x)
+    assert abs(error) < 1e-9, f"{x = }, {error = }"
+
+
+if __name__ == "__main__":
+    test_nonlin()