ModelRouter/internal/lua/fastvalue_test.go

package lua

import (
	"encoding/json"
	"reflect"
	"testing"
)

// The hook payload used to be JSON round-tripped on every call. It no longer is,
// so the fast path and the old JSON path must be indistinguishable — otherwise
// a plugin silently sees a different payload than before, which is the worst
// kind of change: it compiles, passes a smoke test, and misprices traffic.
//
// These tests therefore compare the two paths on the SAME inputs rather than
// asserting the fast path's output in isolation.

func viaJSON(t *testing.T, payload map[string]interface{}) interface{} {
	t.Helper()
	b, err := json.Marshal(payload)
	if err != nil {
		t.Fatalf("marshal: %v", err)
	}
	var decoded interface{}
	if err := json.Unmarshal(b, &decoded); err != nil {
		t.Fatalf("unmarshal: %v", err)
	}
	return decoded
}

// payloads that exercise every branch of the fast converter.
func payloadCases() []map[string]interface{} {
	return []map[string]interface{}{
		{"model": "deepseek-v4.1-flash", "source": "commandcode", "ok": true},
		// Numbers of every width: a converter that only knows float64 turns
		// ints into something else, and a token count that arrives as a string
		// makes a Lua hook do arithmetic on nil.
		{"i": 42, "i8": int8(8), "i16": int16(16), "i32": int32(32), "i64": int64(1 << 40),
			"u": uint(7), "u64": uint64(1 << 50), "f32": float32(1.5), "f64": 2.25},
		// Zero, negative, and very large values must stay numbers.
		{"zero": 0, "neg": -17, "huge": 1e308, "tiny": 1e-308},
		// Slices and string maps: common in payloads and absent from a switch
		// that only knows []interface{} / map[string]interface{}.
		{"msgs": []interface{}{"a", "b"}, "tags": []string{"x", "y"}},
		{"kv": map[string]string{"a": "1", "b": "2"}},
		// Nesting, which is where a shallow converter silently drops a level.
		{"usage": map[string]interface{}{
			"prompt_tokens": 2048, "cache_hit_tokens": 1024,
			"nested": map[string]interface{}{"deep": []interface{}{1, "two", true, nil}},
		}},
		{"nil_field": nil, "empty_map": map[string]interface{}{}, "empty_slice": []interface{}{}},
		// A value the fast path does NOT model: it must fall back to JSON and
		// still arrive, not disappear.
		{"weird": struct {
			A int    `json:"a"`
			B string `json:"b"`
		}{1, "x"}},
	}
}

func TestFastPathMatchesJSONPath(t *testing.T) {
	for i, p := range payloadCases() {
		want := viaJSON(t, p)
		got := plainForLua(p)
		if !reflect.DeepEqual(want, got) {
			t.Errorf("case %d: fast path differs from JSON path\n payload: %#v\n  json: %#v\n  fast: %#v",
				i, p, want, got)
		}
	}
}

// TestFastPathIsActuallyUsed guards against the fast path silently degrading to
// JSON for the payload the gateway really sends. If a future payload gains a
// type the converter does not model, this still works (it falls back) but the
// optimization is gone — and the next person measuring the hook would be
// measuring the old cost without knowing why.
func TestFastPathIsActuallyUsed(t *testing.T) {
	// This mirrors the real request_end payload shape from the gateway.
	realistic := map[string]interface{}{
		"stage": "request_end", "kind": "end", "model": "deepseek-v4.1-flash",
		"source": "commandcode", "key": "stress-key", "ok": true,
		"status": 200, "duration_ms": 1234,
		"usage": map[string]interface{}{
			"prompt_tokens": float64(2048), "completion_tokens": float64(512),
			"cache_hit_tokens": float64(1024), "total_tokens": float64(3584),
		},
		"walk": []interface{}{
			map[string]interface{}{"kind": "tier_skip", "tier": 1, "source": "", "model": "", "reason": "cooldown"},
			map[string]interface{}{"kind": "selected", "tier": 2, "source": "commandcode", "model": "m", "reason": ""},
		},
		"ts": float64(1700000000),
	}
	if _, ok := fastToPlain(realistic); !ok {
		t.Errorf("★ the realistic request payload does NOT take the fast path — " +
			"the JSON round-trip is still on the hot path for real traffic")
	}
}

// TestFastPathDoesNotAliasInput: the converter builds a new tree. If it ever
// returned the caller's map directly, a Lua hook's writes could not reach Go —
// but worse, a later mutation of the payload would race with the snapshot the
// persistence saver is holding.
func TestFastPathDoesNotAliasInput(t *testing.T) {
	src := map[string]interface{}{
		"usage": map[string]interface{}{"prompt_tokens": float64(1)},
		"list":  []interface{}{"a"},
	}
	out, ok := fastToPlain(src)
	if !ok {
		t.Fatal("fast path declined a plain payload")
	}
	m := out.(map[string]interface{})
	m["new"] = "added"
	src["also_new"] = "must not appear"

	if _, leaked := m["also_new"]; leaked {
		t.Error("output map aliases the input map")
	}
	if _, leaked := src["new"]; leaked {
		t.Error("writing to the output mutated the input")
	}
	// And the nested maps must be copies too.
	inner := m["usage"].(map[string]interface{})
	inner["prompt_tokens"] = float64(999)
	if src["usage"].(map[string]interface{})["prompt_tokens"] != float64(1) {
		t.Error("nested map is shared, not copied — a hook could mutate the payload")
	}
	list := m["list"].([]interface{})
	list[0] = "changed"
	if src["list"].([]interface{})[0] != "a" {
		t.Error("nested slice is shared, not copied")
	}
}