feat: a backwards wire re-sorts the graph instead of being refused

The evaluator walks the node list in order, which used to make the order the
user's problem -- an input of an earlier node was not even drawn as
connectable. `math_connect()` runs Kahn's algorithm over the trial edge set,
with ties broken by the current index so a graph already in order is not
permuted, and remaps every index the dialog holds in one block. A self wire
and a cycle are refused with a message and change nothing.

src/ui.cpp

@@ -3565,6 +3565,97 @@ MathWireRef math_wire_at_canvas(const MathDialog* d, double cx, double cy) {
   return math_wire_at(d, gx, gy, kWirePick / d->view_scale);
 }
 
+/// Wires `from`'s output into `to`'s input `which`. The evaluator walks the node list in order, so
+/// a reader must sit after what it reads -- and rather than refuse a wire drawn right to left (the
+/// old editor did not even draw such an input as connectable, which made the order the *user's*
+/// problem), the list is re-sorted so that it holds. Returns false and sets `why` for a wire that
+/// cannot be ordered at all, i.e. one that would close a cycle; nothing is written in that case.
+bool math_connect(MathDialog* d, int from, int to, int which, std::string* why) {
+  const int n = static_cast<int>(d->nodes.size());
+  if (from < 0 || from >= n || to < 0 || to >= n) return false;
+  if (from == to) {
+    if (why != nullptr) *why = "a node cannot read itself";
+    return false;
+  }
+  // Kahn's algorithm over the *trial* edge set, so a cycle is found before anything is written.
+  std::vector<int> in_a(static_cast<std::size_t>(n));
+  std::vector<int> in_b(static_cast<std::size_t>(n));
+  for (int i = 0; i < n; ++i) {
+    in_a[static_cast<std::size_t>(i)] = d->nodes[static_cast<std::size_t>(i)].in_a;
+    in_b[static_cast<std::size_t>(i)] = d->nodes[static_cast<std::size_t>(i)].in_b;
+  }
+  (which == 0 ? in_a : in_b)[static_cast<std::size_t>(to)] = from;
+
+  std::vector<int> indegree(static_cast<std::size_t>(n), 0);
+  const auto edges_of = [&](int i, int (&out)[2]) {
+    const MathNode& node = d->nodes[static_cast<std::size_t>(i)];
+    const int arity = math_op_arity(node.op);
+    out[0] = arity >= 1 ? in_a[static_cast<std::size_t>(i)] : -1;
+    out[1] = arity >= 2 ? in_b[static_cast<std::size_t>(i)] : -1;
+  };
+  for (int i = 0; i < n; ++i) {
+    int e[2] = {-1, -1};
+    edges_of(i, e);
+    for (int k = 0; k < 2; ++k) {
+      if (e[k] >= 0 && e[k] < n && e[k] != i) ++indegree[static_cast<std::size_t>(i)];
+    }
+  }
+  // Ties broken by the current list index, so a graph that is already in order is not permuted:
+  // the positions are the user's layout as much as the evaluator's order, and shuffling a valid
+  // graph on every wire would read as the editor rearranging itself.
+  std::vector<int> order;
+  order.reserve(static_cast<std::size_t>(n));
+  std::vector<bool> done(static_cast<std::size_t>(n), false);
+  for (int step = 0; step < n; ++step) {
+    int pick = -1;
+    for (int i = 0; i < n; ++i) {
+      if (!done[static_cast<std::size_t>(i)] && indegree[static_cast<std::size_t>(i)] == 0) {
+        pick = i;
+        break;
+      }
+    }
+    if (pick < 0) {
+      if (why != nullptr) *why = "that wire would make a loop";
+      return false;
+    }
+    done[static_cast<std::size_t>(pick)] = true;
+    order.push_back(pick);
+    for (int i = 0; i < n; ++i) {
+      if (done[static_cast<std::size_t>(i)]) continue;
+      int e[2] = {-1, -1};
+      edges_of(i, e);
+      for (int k = 0; k < 2; ++k) {
+        if (e[k] == pick) --indegree[static_cast<std::size_t>(i)];
+      }
+    }
+  }
+
+  // `order[p]` is the node that moves to position `p`; `where[old]` is where it ends up. Every
+  // index the dialog holds is in the old space and has to be remapped in one block -- a missed one
+  // silently renames an operation or re-points a wire.
+  std::vector<int> where(static_cast<std::size_t>(n), -1);
+  for (int p = 0; p < n; ++p) where[static_cast<std::size_t>(order[static_cast<std::size_t>(p)])] = p;
+  std::vector<MathNode> sorted;
+  sorted.reserve(static_cast<std::size_t>(n));
+  for (int p = 0; p < n; ++p) sorted.push_back(d->nodes[static_cast<std::size_t>(order[static_cast<std::size_t>(p)])]);
+  const auto remap = [&](int i) { return i >= 0 && i < n ? where[static_cast<std::size_t>(i)] : -1; };
+  for (int p = 0; p < n; ++p) {
+    MathNode& node = sorted[static_cast<std::size_t>(p)];
+    const int old = order[static_cast<std::size_t>(p)];
+    node.in_a = math_op_arity(node.op) >= 1 ? remap(in_a[static_cast<std::size_t>(old)]) : -1;
+    node.in_b = math_op_arity(node.op) >= 2 ? remap(in_b[static_cast<std::size_t>(old)]) : -1;
+  }
+  d->nodes = std::move(sorted);
+  d->output = remap(d->output);
+  d->selected = remap(d->selected);
+  d->drag_node = remap(d->drag_node);
+  d->wire_from = remap(d->wire_from);
+  d->wire_detach = remap(d->wire_detach);
+  d->selected_wire.node = remap(d->selected_wire.node);
+  if (why != nullptr) why->clear();
+  return true;
+}
+
 void math_canvas_draw(GtkDrawingArea*, cairo_t* cr, int width, int height, gpointer user) {
   MathDialog* d = static_cast<MathDialog*>(user);
   cairo_set_source_rgb(cr, 0.06, 0.078, 0.094);
@@ -3793,10 +3884,16 @@ void math_canvas_drag_end(GtkGestureDrag*, double dx, double dy, gpointer user)
   int which = kPortOut;
   const int target = math_port_at_canvas(d, d->press_cx + dx, d->press_cy + dy, which);
   if (target >= 0 && which != kPortOut && target != d->wire_from) {
-    MathNode& n = d->nodes[static_cast<std::size_t>(target)];
-    (which == 0 ? n.in_a : n.in_b) = d->wire_from;
-    d->selected = target;
-    math_dialog_sync(d);
+    std::string why;
+    if (math_connect(d, d->wire_from, target, which, &why)) {
+      // `math_connect()` may have permuted the list, so the target's index is no longer `target`.
+      math_dialog_sync(d);
+    } else if (d->wire_detach >= 0) {
+      // A refused wire must not also lose the one the drag detached to start with.
+      MathNode& back = d->nodes[static_cast<std::size_t>(d->wire_detach)];
+      (d->wire_detach_which == 0 ? back.in_a : back.in_b) = d->wire_from;
+      math_dialog_sync(d);
+    }
   }
   // A release over nothing leaves the input the drag emptied empty: that *is* the detach.
   d->drag_mode = MathDrag::None;
@@ -7343,6 +7440,60 @@ gboolean math_probe(gpointer user) {
       p->selected_wire = MathWireRef{-1, 0};
     }
   }
+  if (a->math_dialog != nullptr) {
+    auto* c = static_cast<MathDialog*>(
+        g_object_get_data(G_OBJECT(a->math_dialog), "udpscope-math-dialog"));
+    if (c != nullptr && c->nodes.size() >= 2) {
+      // A wire drawn right-to-left is legal: the editor re-sorts so that the reader still sits
+      // after what it reads. Node 0 is the source and node 1 the gain, so wiring the gain into
+      // the source is impossible (a source has no inputs) -- a second gain reading the first,
+      // placed *before* it, is the case.
+      c->nodes.clear();
+      MathNode src;
+      src.op = MathOp::Source;
+      src.source = name;
+      // The reader is placed *before* what it reads, which is the case the evaluator cannot run
+      // and the editor has to repair. Index 1 will be wired to read index 2, and index 2 reads
+      // the source.
+      MathNode reader;
+      reader.op = MathOp::Gain;
+      MathNode feeder;
+      feeder.op = MathOp::Gain;
+      c->nodes = {src, reader, feeder};
+      c->nodes[2].in_a = 0;
+      // The output is the reader, so the remap is visible in `output` as well as in the wiring.
+      c->output = 1;
+      c->selected = 1;
+      std::string why;
+      const bool ok = math_connect(c, 2, 1, 0, &why);
+      // After the sort node 1 must still be the one that reads 0, and the reader of it must sit
+      // after it. The op is the same for both, so the *wiring* is the reading.
+      std::string order;
+      for (std::size_t i = 0; i < c->nodes.size(); ++i) {
+        order += (i != 0 ? "," : "");
+        order += std::to_string(i) + ":" + std::to_string(c->nodes[i].in_a);
+      }
+      std::string valid_why;
+      const bool valid = math_channel_valid_nodes(c->nodes, c->output, &valid_why);
+      std::fprintf(stderr,
+                   "[math-probe] wire: backwards ok=%d order=[%s] output=%d valid=%d why=[%s] "
+                   "(want 1 ... 1 [])\n",
+                   ok ? 1 : 0, order.c_str(), c->output, valid ? 1 : 0, valid_why.c_str());
+
+      std::string self_why;
+      const bool self_ok = math_connect(c, 1, 1, 0, &self_why);
+      std::fprintf(stderr, "[math-probe] wire: self refused=%d why=[%s] (want 1)\n",
+                   self_ok ? 0 : 1, self_why.c_str());
+
+      // A cycle: the node that already feeds the other is asked to read it back.
+      const int reader_idx = c->nodes[1].in_a == 0 ? 2 : 1;
+      const int feeder_idx = reader_idx == 2 ? 1 : 2;
+      std::string cyc_why;
+      const bool cyc_ok = math_connect(c, reader_idx, feeder_idx, 0, &cyc_why);
+      std::fprintf(stderr, "[math-probe] wire: cycle refused=%d why=[%s] (want 1)\n",
+                   cyc_ok ? 0 : 1, cyc_why.c_str());
+    }
+  }
   std::fflush(stderr);
   return G_SOURCE_REMOVE;
 }