diff --git a/arbor/iexpr.cpp b/arbor/iexpr.cpp
index f2fe81acac734ed5b82f7e07ed57ed9cf89a1a5c..339e4e0dcf01591c99a695f783f171812c7e5659 100644
--- a/arbor/iexpr.cpp
+++ b/arbor/iexpr.cpp
@@ -281,6 +281,20 @@ struct exp: public iexpr_interface {
iexpr_ptr value;
};
+struct step: public iexpr_interface {
+ step(iexpr_ptr v): value(std::move(v)) {}
+
+ double eval(const mprovider& p, const mcable& c) const override {
+ double x = value->eval(p, c);
+ // x < 0: 0
+ // x == 0: 0.5
+ // x > 0: 1
+ return 0.5*((0. < x) - (x < 0.) + 1);
+ }
+
+ iexpr_ptr value;
+};
+
struct log: public iexpr_interface {
log(iexpr_ptr v): value(std::move(v)) {}
@@ -402,6 +416,8 @@ iexpr iexpr::div(iexpr left, iexpr right) {
iexpr iexpr::exp(iexpr value) { return iexpr(iexpr_type::exp, std::make_tuple(std::move(value))); }
+iexpr iexpr::step(iexpr value) { return iexpr(iexpr_type::step, std::make_tuple(std::move(value))); }
+
iexpr iexpr::log(iexpr value) { return iexpr(iexpr_type::log, std::make_tuple(std::move(value))); }
iexpr iexpr::named(std::string name) {
@@ -488,6 +504,9 @@ iexpr_ptr thingify(const iexpr& expr, const mprovider& m) {
case iexpr_type::exp:
return iexpr_ptr(new iexpr_impl::exp(
thingify(std::get<0>(std::any_cast<const std::tuple<iexpr>&>(expr.args())), m)));
+ case iexpr_type::step:
+ return iexpr_ptr(new iexpr_impl::step(
+ thingify(std::get<0>(std::any_cast<const std::tuple<iexpr>&>(expr.args())), m)));
case iexpr_type::log:
return iexpr_ptr(new iexpr_impl::log(
thingify(std::get<0>(std::any_cast<const std::tuple<iexpr>&>(expr.args())), m)));
@@ -581,6 +600,10 @@ std::ostream& operator<<(std::ostream& o, const iexpr& e) {
o << "exp " << std::get<0>(std::any_cast<const std::tuple<iexpr>&>(e.args()));
break;
}
+ case iexpr_type::step: {
+ o << "step " << std::get<0>(std::any_cast<const std::tuple<iexpr>&>(e.args()));
+ break;
+ }
case iexpr_type::log: {
o << "log " << std::get<0>(std::any_cast<const std::tuple<iexpr>&>(e.args()));
break;
diff --git a/arbor/include/arbor/iexpr.hpp b/arbor/include/arbor/iexpr.hpp
index 6921b777ed912fe6582da404830fba57f2bacc27..3164c53d975e1c071f7d8ede4ef7124d74932428 100644
--- a/arbor/include/arbor/iexpr.hpp
+++ b/arbor/include/arbor/iexpr.hpp
@@ -29,6 +29,7 @@ enum class iexpr_type {
mul,
div,
exp,
+ step,
log,
named
};
@@ -97,6 +98,8 @@ struct ARB_SYMBOL_VISIBLE iexpr {
static iexpr exp(iexpr value);
+ static iexpr step(iexpr value);
+
static iexpr log(iexpr value);
static iexpr named(std::string name);
diff --git a/arborio/label_parse.cpp b/arborio/label_parse.cpp
index 6700295c830e23ef9c15077e3824bbb7dbcd97e2..02df7dad8912f24bde490c9fe2e621a606e1e0af 100644
--- a/arborio/label_parse.cpp
+++ b/arborio/label_parse.cpp
@@ -163,6 +163,9 @@ std::unordered_multimap<std::string, evaluator> eval_map {
{"exp", make_call<arb::iexpr>(arb::iexpr::exp, "iexpr with 1 argument: (value:iexpr)")},
{"exp", make_call<double>(arb::iexpr::exp, "iexpr with 1 argument: (value:double)")},
+ {"step", make_call<arb::iexpr>(arb::iexpr::step, "iexpr with 1 argument: (value:iexpr)")},
+ {"step", make_call<double>(arb::iexpr::step, "iexpr with 1 argument: (value:double)")},
+
{"log", make_call<arb::iexpr>(arb::iexpr::log, "iexpr with 1 argument: (value:iexpr)")},
{"log", make_call<double>(arb::iexpr::log, "iexpr with 1 argument: (value:double)")},
diff --git a/doc/concepts/labels.rst b/doc/concepts/labels.rst
index 7027275384f0ef5e2d53143da560c1ecae4e3fbd..2997d38a61daa3be9fe98f0246b83e444eb196eb 100644
--- a/doc/concepts/labels.rst
+++ b/doc/concepts/labels.rst
@@ -762,6 +762,10 @@ Inhomogeneous Expressions
The exponential function of the inhomogeneous expression or real ``value``.
+.. label:: (step value:(iexpr | real))
+
+ The Heaviside step function of the inhomogeneous expression or real ``value``, with `(step 0.0)` evaluating to 0.5.
+
.. label:: (log value:(iexpr | real))
The logarithm of the inhomogeneous expression or real ``value``.
diff --git a/test/unit/test_iexpr.cpp b/test/unit/test_iexpr.cpp
index 9043381a0ad7a94380ea70d581a28904ed34c52a..f399fa785a502206c2f93aec756ab190d8d1f4d7 100644
--- a/test/unit/test_iexpr.cpp
+++ b/test/unit/test_iexpr.cpp
@@ -398,6 +398,17 @@ TEST(iexpr, exp) {
EXPECT_DOUBLE_EQ(e->eval(prov, {0, 0.0, 1.0}), std::exp(3.0 * 10.0));
}
+TEST(iexpr, step) {
+ segment_tree tree;
+ tree.append(mnpos, {0, 0, 0, 10}, {0, 0, 10, 10}, 1);
+
+ arb::mprovider prov(arb::morphology(std::move(tree)));
+ auto root_dist = arb::iexpr::distance(1.0, arb::mlocation{0, 0.0});
+ auto e = thingify(arb::iexpr::step(root_dist-5.0), prov);
+ EXPECT_DOUBLE_EQ(e->eval(prov, {0, 0.0, 0.5}), 0.0f); /* step(2.5-5) == 0 */
+ EXPECT_DOUBLE_EQ(e->eval(prov, {0, 0.5, 1.0}), 1.0f); /* step(7.5-5) == 1 */
+}
+
TEST(iexpr, log) {
segment_tree tree;
tree.append(mnpos, {0, 0, 0, 10}, {0, 0, 10, 10}, 1);
diff --git a/test/unit/test_s_expr.cpp b/test/unit/test_s_expr.cpp
index 9920ee86b100ad0ad60030f9f0d376d27dcb56c7..85ed36d59a60faaa8cdf69e6e1896a5666197dbb 100644
--- a/test/unit/test_s_expr.cpp
+++ b/test/unit/test_s_expr.cpp
@@ -269,6 +269,7 @@ TEST(iexpr, round_tripping) {
"(radius 2.1)",
"(diameter 2.1)",
"(exp (scalar 2.1))",
+ "(step (scalar 2.1))",
"(log (scalar 2.1))",
"(add (scalar 2.1) (radius 3.2))",
"(sub (scalar 2.1) (radius 3.2))",
@@ -281,7 +282,7 @@ TEST(iexpr, round_tripping) {
}
// check double values for input instead of explicit scalar iexpr
- auto mono_iexpr = {std::string("exp"), std::string("log")};
+ auto mono_iexpr = {std::string("exp"), std::string("step"), std::string("log")};
auto duo_iexpr = {std::string("add"), std::string("sub"), std::string("mul"), std::string("div")};
constexpr auto v1 = "1.2";
constexpr auto v2 = "1.2";