engine: Config, scenario runner, golden regression tests; demo CLI

Config is the single source of truth for parameters (TS types will be
generated from these structs in milestone 2); all randomness flows from
its three seeds, so identical configs give identical results. Golden
test pins the default world: none=99/120 (82%), random=70/120 (58%),
most-connected=7/120 (6%). Same story as the prototype's 85/64/8 with
different dice; the ordering and the collapse are asserted explicitly,
exact Python numbers are out of scope by design.
'go run ./cmd/spreadlab' prints the three-scenario comparison.
This completes milestone 1 (engine ported, parameterised, tested).

cmd/spreadlab/main.go

@@ -1,9 +1,35 @@
-// Command spreadlab will serve the spreadlab dashboard. For now it only
+// Command spreadlab will serve the spreadlab dashboard. Until the HTTP
-// proves the module builds; the HTTP server arrives in milestone 2.
+// server lands (milestone 2), it runs the prototype's three scenarios in
+// the default world and prints the comparison.
 package main
 
-import "fmt"
+import (
+	"fmt"
+	"os"
+
+	"github.com/JustinZeus/spreadlab/internal/engine"
+)
 
 func main() {
-	fmt.Println("spreadlab: engine under construction (milestone 1)")
+	config := engine.DefaultConfig()
+	fmt.Printf("spreadlab: %d students, educate %d, forwarding probability %.2f\n",
+		config.NumStudents, config.NumEducated, config.ForwardProb)
+	fmt.Println("(illustrative, not validated)")
+	fmt.Println()
+
+	strategies := []engine.Strategy{
+		engine.StrategyNone,
+		engine.StrategyRandom,
+		engine.StrategyMostConnected,
+	}
+	for _, strategy := range strategies {
+		result, err := engine.RunScenario(config, strategy)
+		if err != nil {
+			fmt.Fprintln(os.Stderr, "spreadlab:", err)
+			os.Exit(1)
+		}
+		fmt.Printf("%-15s educated=%3d reached=%3d/%d (%2.0f%%) in %d rounds\n",
+			result.Strategy, len(result.Educated), result.NumReached,
+			config.NumStudents, result.ReachedPct, result.NumRounds)
+	}
 }

internal/engine/scenario.go

@@ -0,0 +1,99 @@
+package engine
+
+import "fmt"
+
+// Strategy selects who gets educated.
+type Strategy string
+
+const (
+	StrategyNone          Strategy = "none"
+	StrategyRandom        Strategy = "random"
+	StrategyMostConnected Strategy = "most-connected"
+)
+
+// Config fully describes one simulation world. It is the single source of
+// truth for parameters: API and frontend types will be generated from the
+// structs in this file, never redefined by hand. Identical configs produce
+// identical results; all randomness flows from the three seeds.
+type Config struct {
+	NumStudents  int     `json:"numStudents"`
+	EdgesPerNode int     `json:"edgesPerNode"` // attachment edges per new student (network density)
+	TriangleProb float64 `json:"triangleProb"` // chance to close a friend-of-a-friend triangle
+	ForwardProb  float64 `json:"forwardProb"`  // chance a student forwards the fake along an edge
+	NumEducated  int     `json:"numEducated"`  // students the education program reaches
+	Origin       int     `json:"origin"`       // student who first posts the fake
+
+	GraphSeed     uint64 `json:"graphSeed"`
+	ThresholdSeed uint64 `json:"thresholdSeed"`
+	EducationSeed uint64 `json:"educationSeed"` // used by the random strategy only
+}
+
+// DefaultConfig mirrors the Python prototype: a school year of 120
+// students, educate 30% of them, forwarding probability 0.38.
+func DefaultConfig() Config {
+	return Config{
+		NumStudents:   120,
+		EdgesPerNode:  3,
+		TriangleProb:  0.45,
+		ForwardProb:   0.38,
+		NumEducated:   36,
+		Origin:        0,
+		GraphSeed:     17,
+		ThresholdSeed: 2,
+		EducationSeed: 1,
+	}
+}
+
+// Result is the outcome of one scenario run.
+type Result struct {
+	Strategy       Strategy `json:"strategy"`
+	Educated       []int    `json:"educated"`
+	ReachedAtRound []int    `json:"reachedAtRound"` // per node; -1 means never reached
+	NumReached     int      `json:"numReached"`
+	NumRounds      int      `json:"numRounds"`
+	ReachedPct     float64  `json:"reachedPct"`
+}
+
+// RunScenario builds the world the config describes (network plus edge
+// thresholds), picks the educated students per strategy, and runs the
+// cascade. Scenarios with the same config share the same world, so
+// comparing strategies compares only the lever.
+func RunScenario(config Config, strategy Strategy) (Result, error) {
+	if config.ForwardProb < 0 || config.ForwardProb > 1 {
+		return Result{}, fmt.Errorf("scenario: need 0 <= forwardProb <= 1, got %v", config.ForwardProb)
+	}
+	if config.Origin < 0 || config.Origin >= config.NumStudents {
+		return Result{}, fmt.Errorf("scenario: origin %d outside 0..%d", config.Origin, config.NumStudents-1)
+	}
+	if config.NumEducated < 0 {
+		return Result{}, fmt.Errorf("scenario: need numEducated >= 0, got %d", config.NumEducated)
+	}
+
+	graph, err := HolmeKim(config.NumStudents, config.EdgesPerNode, config.TriangleProb, newRand(config.GraphSeed))
+	if err != nil {
+		return Result{}, err
+	}
+	thresholds := NewEdgeThresholds(graph, newRand(config.ThresholdSeed))
+
+	var educated []int
+	switch strategy {
+	case StrategyNone:
+		// nobody educated; the baseline
+	case StrategyRandom:
+		educated = EducateRandom(graph, config.Origin, config.NumEducated, newRand(config.EducationSeed))
+	case StrategyMostConnected:
+		educated = EducateMostConnected(graph, config.Origin, config.NumEducated)
+	default:
+		return Result{}, fmt.Errorf("scenario: unknown strategy %q", strategy)
+	}
+
+	cascade := RunCascade(graph, config.Origin, config.ForwardProb, educated, thresholds)
+	return Result{
+		Strategy:       strategy,
+		Educated:       educated,
+		ReachedAtRound: cascade.ReachedAtRound,
+		NumReached:     cascade.NumReached,
+		NumRounds:      cascade.NumRounds,
+		ReachedPct:     100 * float64(cascade.NumReached) / float64(config.NumStudents),
+	}, nil
+}

internal/engine/scenario_test.go

@@ -0,0 +1,87 @@
+package engine
+
+import "testing"
+
+// Golden regression tests, per the handoff: exact reach values are pinned
+// for the default (fixed-seed) config so any change to engine behaviour
+// shows up as a diff, and the qualitative ordering vs the prototype is
+// asserted explicitly. The exact numbers intentionally differ from the
+// Python figure (different RNG); the SHAPE of the result must match:
+// no program >> random >> most-connected, with most-connected collapsing.
+
+func runAllStrategies(t *testing.T) map[Strategy]Result {
+	t.Helper()
+	results := make(map[Strategy]Result)
+	for _, strategy := range []Strategy{StrategyNone, StrategyRandom, StrategyMostConnected} {
+		result, err := RunScenario(DefaultConfig(), strategy)
+		if err != nil {
+			t.Fatalf("RunScenario(%q): %v", strategy, err)
+		}
+		results[strategy] = result
+	}
+	return results
+}
+
+func TestRunScenarioGoldenReachValues(t *testing.T) {
+	// Pinned from the first verified run (2026-06-10). The prototype's
+	// figure showed 102/77/10 out of 120 (85%/64%/8%); ours is the same
+	// story with different dice.
+	wantReached := map[Strategy]int{
+		StrategyNone:          99, // 82%
+		StrategyRandom:        70, // 58%
+		StrategyMostConnected: 7,  // 6%
+	}
+	results := runAllStrategies(t)
+	for strategy, result := range results {
+		t.Logf("%-15s educated=%2d reached=%3d/120 (%.0f%%) rounds=%d",
+			strategy, len(result.Educated), result.NumReached, result.ReachedPct, result.NumRounds)
+		if want, pinned := wantReached[strategy]; pinned && result.NumReached != want {
+			t.Errorf("%s: NumReached = %d, want %d", strategy, result.NumReached, want)
+		}
+	}
+}
+
+func TestRunScenarioQualitativeOrdering(t *testing.T) {
+	results := runAllStrategies(t)
+	noProgram := results[StrategyNone].NumReached
+	random := results[StrategyRandom].NumReached
+	mostConnected := results[StrategyMostConnected].NumReached
+
+	if noProgram <= random || random <= mostConnected {
+		t.Errorf("want noProgram > random > mostConnected, got %d, %d, %d",
+			noProgram, random, mostConnected)
+	}
+	if noProgram < 60 { // an unchecked cascade must reach most of the school
+		t.Errorf("no program reached only %d/120, expected a majority", noProgram)
+	}
+	if mostConnected > 30 { // educating the hubs must collapse the spread
+		t.Errorf("most-connected still reached %d/120, expected a collapse", mostConnected)
+	}
+}
+
+func TestRunScenarioRejectsInvalidInput(t *testing.T) {
+	tests := []struct {
+		name   string
+		mutate func(*Config)
+	}{
+		{name: "forward probability above one", mutate: func(c *Config) { c.ForwardProb = 1.5 }},
+		{name: "origin out of range", mutate: func(c *Config) { c.Origin = 120 }},
+		{name: "negative educated count", mutate: func(c *Config) { c.NumEducated = -1 }},
+		{name: "edges per node too large", mutate: func(c *Config) { c.EdgesPerNode = 120 }},
+	}
+	for _, testCase := range tests {
+		t.Run(testCase.name, func(t *testing.T) {
+			config := DefaultConfig()
+			testCase.mutate(&config)
+			if _, err := RunScenario(config, StrategyNone); err == nil {
+				t.Error("RunScenario accepted an invalid config")
+			}
+		})
+	}
+
+	t.Run("unknown strategy", func(t *testing.T) {
+		if _, err := RunScenario(DefaultConfig(), Strategy("telepathy")); err == nil {
+			t.Error("RunScenario accepted an unknown strategy")
+		}
+	})
+}