radio: a preset keeps its coding rate, and a placed node joins the mesh's modem

docs/verb-reference.md

@@ -1060,7 +1060,7 @@ Put one node down at a position, which is how a repeater a feed never carried ge
 | `height_m` | number | optional | antenna height above ground in metres; absent is 10 |
 | `tx_dbm` | number | optional | transmit power in decibel-milliwatts; absent is 22 |
 
-**Answers** `placed`, `kind`, `regions`, `board`, `nodes`. `regions` and the firmware build are inherited from the neighbours rather than left empty: a node holding a region its neighbours do not is as silent as one holding none. `nodes` is the whole network's count afterwards. The links are not in the answer, because measuring them is a worker's job and this returns before it.
+**Answers** `placed`, `kind`, `regions`, `board`, `nodes`. `regions` and the firmware build are inherited from the neighbours rather than left empty: a node holding a region its neighbours do not is as silent as one holding none. So is the radio: the modem every node already shares, or EU/UK (Narrow) where they do not agree, since a node on a channel nobody else uses hears nobody. `nodes` is the whole network's count afterwards. The links are not in the answer, because measuring them is a worker's job and this returns before it.
 
 **Example** - put a repeater on the hill and see what it wins
 

docs/verbs.json

@@ -3649,7 +3649,7 @@
         "board",
         "nodes"
       ],
-      "answers": "`regions` and the firmware build are inherited from the neighbours rather than left empty: a node holding a region its neighbours do not is as silent as one holding none. `nodes` is the whole network's count afterwards. The links are not in the answer, because measuring them is a worker's job and this returns before it.",
+      "answers": "`regions` and the firmware build are inherited from the neighbours rather than left empty: a node holding a region its neighbours do not is as silent as one holding none. So is the radio: the modem every node already shares, or EU/UK (Narrow) where they do not agree, since a node on a channel nobody else uses hears nobody. `nodes` is the whole network's count afterwards. The links are not in the answer, because measuring them is a worker's job and this returns before it.",
       "example": {
         "params": {
           "board": "Heltec_v3",

internal/app/session/presetplace.go

@@ -31,12 +31,11 @@ func registerPresetsAndPlace(st *state.Store, s *Sim) {
 			if only != "" && s.nodes[i].Name != only {
 				continue
 			}
-			s.nodes[i].Radio = scenario.RadioConfig{
-				CentreHz:     preset.FreqMHz * 1e6,
-				BandwidthHz:  preset.BwKHz * 1e3,
-				SpreadFactor: preset.SF,
-				CodingRate:   preset.CR,
-			}
+			// The preset's own conversion. The table holds MeshCore's
+			// denominator (8 for 4/8) and a node's radio holds it minus four;
+			// copying the table across made every preset 4/12, half again the
+			// airtime, and a companion told CR 12 refuses it.
+			s.nodes[i].Radio = preset.Config()
 			n++
 		}
 		if n > 0 && len(s.nodes) > 0 {
@@ -73,6 +72,7 @@ func registerPresetsAndPlace(st *state.Store, s *Sim) {
 		}
 		node.HeightAGLm = 10
 		node.TxPowerDBm = 22
+		node.Radio = placedRadio(s.nodes)
 		if v, ok := namedNum(p, "height_m"); ok {
 			node.HeightAGLm = v
 		}
@@ -191,3 +191,32 @@ func regionsOfNeighbours(nodes []scenario.Node, lat, lon float64) []string {
 	}
 	return out
 }
+
+// placedRadio is the modem a placed node joins on: the one every node already
+// here shares, or the documented default where they do not agree or there are
+// none.
+//
+// Left empty, a placed node fell back to the engine's own figures - SF10 at
+// 250 kHz - while its firmware booted on its build's SF8 at 62.5 kHz, so on any
+// narrow mesh it was on a channel nothing else used and never heard anybody,
+// with nothing to say why.
+func placedRadio(nodes []scenario.Node) scenario.RadioConfig {
+	var shared *scenario.RadioConfig
+	for i := range nodes {
+		r := nodes[i].Radio
+		if r.CentreHz == 0 {
+			continue
+		}
+		if shared == nil {
+			shared = &r
+			continue
+		}
+		if *shared != r {
+			return scenario.DefaultRadio()
+		}
+	}
+	if shared == nil {
+		return scenario.DefaultRadio()
+	}
+	return *shared
+}

internal/app/session/presetplace.verbs.json

@@ -49,7 +49,7 @@
       "board",
       "nodes"
     ],
-    "answers": "`regions` and the firmware build are inherited from the neighbours rather than left empty: a node holding a region its neighbours do not is as silent as one holding none. `nodes` is the whole network's count afterwards. The links are not in the answer, because measuring them is a worker's job and this returns before it.",
+    "answers": "`regions` and the firmware build are inherited from the neighbours rather than left empty: a node holding a region its neighbours do not is as silent as one holding none. So is the radio: the modem every node already shares, or EU/UK (Narrow) where they do not agree, since a node on a channel nobody else uses hears nobody. `nodes` is the whole network's count afterwards. The links are not in the answer, because measuring them is a worker's job and this returns before it.",
     "example": {
       "params": {
         "board": "Heltec_v3",

internal/app/session/presetradio_test.go

@@ -0,0 +1,73 @@
+package session
+
+import (
+	"context"
+	"testing"
+
+	"github.com/MeshBench/meshbench/internal/app/state"
+	"github.com/MeshBench/meshbench/internal/world/scenario"
+)
+
+func presetSim(t *testing.T, nodes []scenario.Node) (*state.Store, *Sim, context.Context, func()) {
+	t.Helper()
+	st := state.New(10)
+	s := &Sim{gpuAsked: true}
+	Register(st, s)
+	ctx, cancel := context.WithCancel(context.Background())
+	go st.Run(ctx)
+	s.build(nodes, 869.618)
+	return st, s, ctx, cancel
+}
+
+// A preset puts a node on the preset's coding rate. The table holds MeshCore's
+// denominator, and copying it straight into the node's radio made EU/UK
+// (Narrow) 4/12.
+func TestAPresetKeepsItsCodingRate(t *testing.T) {
+	st, s, ctx, cancel := presetSim(t, []scenario.Node{repeaterNode("Alpha")})
+	defer cancel()
+	if _, err := st.Do(ctx, "radio.preset", map[string]any{"preset": "EU/UK (Narrow)"}); err != nil {
+		t.Fatal(err)
+	}
+	got := s.nodes[0].Radio
+	want, _ := scenario.PresetByLabel("EU/UK (Narrow)")
+	if !want.Matches(got) {
+		t.Fatalf("after the preset the node's radio is %+v, which is not EU/UK (Narrow)", got)
+	}
+	if got.CodingRate != 4 {
+		t.Fatalf("coding rate field %d, want 4 (4/8)", got.CodingRate)
+	}
+}
+
+// A placed node joins on the modem the mesh already shares, rather than on
+// the engine's fallback that nothing else is using.
+func TestAPlacedNodeJoinsTheMeshItIsPlacedIn(t *testing.T) {
+	nl, _ := scenario.PresetByLabel("Netherlands")
+	a, b := repeaterNode("Alpha"), repeaterNode("Bravo")
+	a.Radio, b.Radio = nl.Config(), nl.Config()
+	st, s, ctx, cancel := presetSim(t, []scenario.Node{a, b})
+	defer cancel()
+	if _, err := st.Do(ctx, "nodes.place", map[string]any{
+		"name": "Charlie", "kind": "companion", "lat": 56.31, "lon": -3.21,
+	}); err != nil {
+		t.Fatal(err)
+	}
+	for _, n := range s.nodes {
+		if n.Name == "Charlie" && n.Radio != nl.Config() {
+			t.Fatalf("Charlie was placed on %+v, the mesh is on %+v", n.Radio, nl.Config())
+		}
+	}
+}
+
+// With nothing to join, or a mesh that does not agree with itself, a placed
+// node gets the default the presets table documents.
+func TestAPlacedNodeOnNoAgreedModemGetsTheDefault(t *testing.T) {
+	if got := placedRadio(nil); got != scenario.DefaultRadio() {
+		t.Fatalf("an empty mesh placed on %+v", got)
+	}
+	nl, _ := scenario.PresetByLabel("Netherlands")
+	uk, _ := scenario.PresetByLabel("EU/UK (Narrow)")
+	mixed := []scenario.Node{{Radio: nl.Config()}, {Radio: uk.Config()}}
+	if got := placedRadio(mixed); got != scenario.DefaultRadio() {
+		t.Fatalf("a mixed mesh placed on %+v", got)
+	}
+}

internal/world/scenario/presets.go

@@ -7,8 +7,8 @@ type RadioPreset struct {
 	FreqMHz float64
 	BwKHz   float64
 	SF      int
-	// CR is the denominator minus four, as MeshCore's CLI takes it: 5..8 on the
-	// wire means 4/5..4/8.
+	// CR is the coding rate's denominator, as MeshCore's CLI takes it: 5..8
+	// means 4/5..4/8. A node's RadioConfig holds it minus four; Config converts.
 	CR int
 }