package contract

import (
	"encoding/json"
	"os"
	"path/filepath"
	"reflect"
	"testing"
)

// fixtureDir holds JSON copies of the spec-derived YAML fixtures, written by
// conformance/validate_schemas.py.
const fixtureDir = "../../conformance/fixtures/json"

// roundTrip decodes a fixture into T, re-encodes it, and reports any field the
// generated types dropped on the way through.
//
// This is the drift check that matters: a schema change that schemagen did not
// pick up shows here as a field that vanishes.
func roundTrip[T any](t *testing.T, name string) {
	t.Helper()

	raw, err := os.ReadFile(filepath.Join(fixtureDir, name+".json"))
	if err != nil {
		t.Fatalf("read fixture: %v (run `make validate` first)", err)
	}

	var typed T
	dec := json.NewDecoder(bytesReader(raw))
	dec.DisallowUnknownFields()
	if err := dec.Decode(&typed); err != nil {
		t.Fatalf("decode into %T: %v", typed, err)
	}

	encoded, err := json.Marshal(typed)
	if err != nil {
		t.Fatalf("re-encode: %v", err)
	}

	var before, after map[string]any
	if err := json.Unmarshal(raw, &before); err != nil {
		t.Fatalf("unmarshal fixture: %v", err)
	}
	if err := json.Unmarshal(encoded, &after); err != nil {
		t.Fatalf("unmarshal re-encoded: %v", err)
	}

	if missing := missingKeys(before, after, ""); len(missing) > 0 {
		t.Errorf("round trip dropped fields: %v", missing)
	}
}

// missingKeys walks want and reports paths absent from got.
//
// Explicit nulls and empty collections are skipped. The schemas treat
// "end: null" and an absent "end" as the same statement (the window is still
// open), so omitempty dropping them on re-encode is correct rather than drift.
// Genuine drift -- a fixture field the generated types have no home for -- is
// caught by DisallowUnknownFields on the way in.
func missingKeys(want, got map[string]any, prefix string) []string {
	var missing []string
	for k, wv := range want {
		path := k
		if prefix != "" {
			path = prefix + "." + k
		}
		if isEmptyValue(wv) {
			continue
		}
		gv, ok := got[k]
		if !ok {
			missing = append(missing, path)
			continue
		}
		wm, wok := wv.(map[string]any)
		gm, gok := gv.(map[string]any)
		if wok && gok {
			missing = append(missing, missingKeys(wm, gm, path)...)
		}
	}
	return missing
}

// isEmptyValue reports values the schema treats as equivalent to absent.
func isEmptyValue(v any) bool {
	switch t := v.(type) {
	case nil:
		return true
	case []any:
		return len(t) == 0
	case map[string]any:
		return len(t) == 0
	}
	return false
}

func TestFixturesRoundTrip(t *testing.T) {
	t.Run("pressure", func(t *testing.T) { roundTrip[PressureDocument](t, "pressure") })
	t.Run("hypothesis", func(t *testing.T) { roundTrip[HypothesisDocument](t, "hypothesis") })
	t.Run("revision", func(t *testing.T) { roundTrip[RevisionDocument](t, "revision") })
	t.Run("experiment", func(t *testing.T) { roundTrip[ExperimentDocument](t, "experiment") })
	t.Run("event", func(t *testing.T) { roundTrip[EventDocument](t, "event") })
	t.Run("feedback", func(t *testing.T) { roundTrip[FeedbackDocument](t, "feedback") })
	t.Run("backend-requirement", func(t *testing.T) {
		roundTrip[BackendRequirementDocument](t, "backend-requirement")
	})
	t.Run("revision-descriptor", func(t *testing.T) {
		roundTrip[RevisionDescriptorDocument](t, "revision-descriptor")
	})
	t.Run("routing-policy", func(t *testing.T) { roundTrip[RoutingPolicyDocument](t, "routing-policy") })
	t.Run("telemetry-envelope", func(t *testing.T) {
		roundTrip[TelemetryEnvelopeDocument](t, "telemetry-envelope")
	})
}

func TestKindOf(t *testing.T) {
	cases := map[string]EntityKind{
		"H-000184":  KindHypothesis,
		"R-000221":  KindRevision,
		"E-000093":  KindExperiment,
		"P-1831":    KindPressure,
		"BR-0041":   KindBackendRequirement,
		"D-2811":    KindDecision,
		"EV-990281": KindEvent,
		"F-9821":    KindFeedback,
		"C-17":      KindCohort,
	}
	for id, want := range cases {
		got, ok := KindOf(id)
		if !ok || got != want {
			t.Errorf("KindOf(%q) = %q, %v; want %q", id, got, ok, want)
		}
	}

	// BR- and EV- must not be swallowed by the single-letter prefixes.
	if k, _ := KindOf("BR-1"); k != KindBackendRequirement {
		t.Errorf("BR- prefix mis-classified as %q", k)
	}
	if k, _ := KindOf("EV-1"); k != KindEvent {
		t.Errorf("EV- prefix mis-classified as %q", k)
	}

	if _, ok := KindOf("something"); ok {
		t.Error("unprefixed identifier should not classify")
	}
}

func TestRequireKind(t *testing.T) {
	if err := RequireKind("R-1", KindRevision); err != nil {
		t.Errorf("valid revision id rejected: %v", err)
	}
	err := RequireKind("H-1", KindRevision)
	if err == nil {
		t.Fatal("hypothesis id accepted where a revision id was required")
	}
	var wrong *ErrWrongKind
	if !errorsAs(err, &wrong) {
		t.Fatalf("expected *ErrWrongKind, got %T", err)
	}
	if wrong.Got != KindHypothesis || wrong.Want != KindRevision {
		t.Errorf("unexpected error detail: %+v", wrong)
	}
}

// Small helpers keep the test file free of imports the generated code does not
// already require.
func bytesReader(b []byte) *jsonReader { return &jsonReader{b: b} }

type jsonReader struct {
	b []byte
	i int
}

func (r *jsonReader) Read(p []byte) (int, error) {
	if r.i >= len(r.b) {
		return 0, errEOF
	}
	n := copy(p, r.b[r.i:])
	r.i += n
	return n, nil
}

var errEOF = errorString("EOF")

type errorString string

func (e errorString) Error() string { return string(e) }

func errorsAs(err error, target any) bool {
	tv := reflect.ValueOf(target).Elem()
	for err != nil {
		if reflect.TypeOf(err).AssignableTo(tv.Type()) {
			tv.Set(reflect.ValueOf(err))
			return true
		}
		u, ok := err.(interface{ Unwrap() error })
		if !ok {
			return false
		}
		err = u.Unwrap()
	}
	return false
}