package authprojection

import (
	"bytes"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"io"
	"strings"
)

type manifest struct {
	Version           int            `json:"version"`
	Generation        string         `json:"generation"`
	Mode              string         `json:"mode"`
	CanonicalRevision string         `json:"canonicalRevision"`
	Files             []manifestFile `json:"files"`
}

type manifestFile struct {
	Name   string `json:"name"`
	Size   int    `json:"size"`
	SHA256 string `json:"sha256"`
}

func NewSnapshot(mode string, auth, users []byte) (Snapshot, error) {
	if (mode != "local" && mode != "oidc") || len(auth) > maximumAuthBytes || len(users) > maximumUsersBytes {
		return Snapshot{}, ErrIntegrity
	}
	if mode == "local" && users == nil {
		return Snapshot{}, ErrIntegrity
	}
	if mode == "oidc" && users != nil {
		return Snapshot{}, ErrIntegrity
	}

	authCopy := append([]byte(nil), auth...)
	usersCopy := append([]byte(nil), users...)
	authDigest := sha256.Sum256(authCopy)
	usersDigest := sha256.Sum256(usersCopy)
	authSHA256 := hex.EncodeToString(authDigest[:])
	usersSHA256 := "-"
	if mode == "local" {
		usersSHA256 = hex.EncodeToString(usersDigest[:])
	}
	record := "thothii-auth-projection-v1\nmode=" + mode + "\nauth=" + authSHA256 + "\nusers=" + usersSHA256 + "\n"
	generationDigest := sha256.Sum256([]byte(record))
	generation := hex.EncodeToString(generationDigest[:])
	return Snapshot{
		Mode:              mode,
		Auth:              authCopy,
		Users:             usersCopy,
		AuthSHA256:        authSHA256,
		UsersSHA256:       usersSHA256,
		Generation:        generation,
		CanonicalRevision: "sha256:" + generation,
	}, nil
}

func decodeSelector(data []byte) (Selector, error) {
	if len(data) == 0 || len(data) > maximumSelectorBytes || hasDuplicateObjectKeys(data) {
		return Selector{}, ErrIntegrity
	}
	var value Selector
	if err := decodeExact(data, &value); err != nil || !validSelector(value) {
		return Selector{}, ErrIntegrity
	}
	canonical, err := encodeSelector(value)
	if err != nil || !bytes.Equal(data, canonical) {
		return Selector{}, ErrIntegrity
	}
	return value, nil
}

func encodeSelector(value Selector) ([]byte, error) {
	if !validSelector(value) {
		return nil, ErrIntegrity
	}
	encoded, err := json.Marshal(value)
	if err != nil {
		return nil, fmt.Errorf("%w: selector encoding", ErrIntegrity)
	}
	return append(encoded, '\n'), nil
}

func validSelector(value Selector) bool {
	if value.Version != SchemaVersion || !validHex(value.Transaction, 32) {
		return false
	}
	switch value.State {
	case "blocked":
		return value.Generation == "" && len(value.PreviousGenerations) == 0
	case "ready":
		if !validHex(value.Generation, 64) || len(value.PreviousGenerations) > retainedPredecessors {
			return false
		}
		seen := map[string]struct{}{value.Generation: {}}
		for _, generation := range value.PreviousGenerations {
			if !validHex(generation, 64) {
				return false
			}
			if _, duplicate := seen[generation]; duplicate {
				return false
			}
			seen[generation] = struct{}{}
		}
		return true
	default:
		return false
	}
}

func newManifest(snapshot Snapshot) (manifest, error) {
	if err := validateSnapshot(snapshot); err != nil {
		return manifest{}, err
	}
	files := []manifestFile{{Name: "auth.yaml", Size: len(snapshot.Auth), SHA256: snapshot.AuthSHA256}}
	if snapshot.Mode == "local" {
		files = append(files, manifestFile{Name: "users.yaml", Size: len(snapshot.Users), SHA256: snapshot.UsersSHA256})
	}
	return manifest{Version: SchemaVersion, Generation: snapshot.Generation, Mode: snapshot.Mode, CanonicalRevision: snapshot.CanonicalRevision, Files: files}, nil
}

func decodeManifest(data []byte) (manifest, error) {
	if len(data) == 0 || len(data) > maximumManifestBytes || hasDuplicateObjectKeys(data) {
		return manifest{}, ErrIntegrity
	}
	var value manifest
	if err := decodeExact(data, &value); err != nil || !validManifest(value) {
		return manifest{}, ErrIntegrity
	}
	canonical, err := encodeManifest(value)
	if err != nil || !bytes.Equal(data, canonical) {
		return manifest{}, ErrIntegrity
	}
	return value, nil
}

func encodeManifest(value manifest) ([]byte, error) {
	if !validManifest(value) {
		return nil, ErrIntegrity
	}
	encoded, err := json.Marshal(value)
	if err != nil {
		return nil, fmt.Errorf("%w: manifest encoding", ErrIntegrity)
	}
	return append(encoded, '\n'), nil
}

func validateManifest(value manifest, snapshot Snapshot) error {
	expected, err := newManifest(snapshot)
	if err != nil || !manifestEqual(value, expected) {
		return ErrIntegrity
	}
	return nil
}

func manifestEqual(left, right manifest) bool {
	if left.Version != right.Version || left.Generation != right.Generation || left.Mode != right.Mode || left.CanonicalRevision != right.CanonicalRevision || len(left.Files) != len(right.Files) {
		return false
	}
	for index := range left.Files {
		if left.Files[index] != right.Files[index] {
			return false
		}
	}
	return true
}

func validManifest(value manifest) bool {
	if value.Version != SchemaVersion || !validHex(value.Generation, 64) || (value.Mode != "local" && value.Mode != "oidc") || value.CanonicalRevision != "sha256:"+value.Generation {
		return false
	}
	want := []string{"auth.yaml"}
	if value.Mode == "local" {
		want = append(want, "users.yaml")
	}
	if len(value.Files) != len(want) {
		return false
	}
	for index, file := range value.Files {
		maximum := maximumAuthBytes
		if file.Name == "users.yaml" {
			maximum = maximumUsersBytes
		}
		if file.Name != want[index] || file.Size < 0 || file.Size > maximum || !validHex(file.SHA256, 64) {
			return false
		}
	}
	return true
}

func validateSnapshot(snapshot Snapshot) error {
	expected, err := NewSnapshot(snapshot.Mode, snapshot.Auth, snapshot.Users)
	if err != nil || expected.AuthSHA256 != snapshot.AuthSHA256 || expected.UsersSHA256 != snapshot.UsersSHA256 || expected.Generation != snapshot.Generation || expected.CanonicalRevision != snapshot.CanonicalRevision {
		return ErrIntegrity
	}
	return nil
}

func decodeExact(data []byte, destination any) error {
	decoder := json.NewDecoder(bytes.NewReader(data))
	decoder.DisallowUnknownFields()
	if err := decoder.Decode(destination); err != nil {
		return err
	}
	if err := decoder.Decode(&struct{}{}); err != io.EOF {
		return fmt.Errorf("multiple JSON values")
	}
	return nil
}

func hasDuplicateObjectKeys(data []byte) bool {
	decoder := json.NewDecoder(bytes.NewReader(data))
	return scanJSONValue(decoder, nil) != nil || decoder.Decode(&struct{}{}) != io.EOF
}

func scanJSONValue(decoder *json.Decoder, seen map[string]struct{}) error {
	token, err := decoder.Token()
	if err != nil {
		return err
	}
	delimiter, ok := token.(json.Delim)
	if !ok {
		return nil
	}
	switch delimiter {
	case '{':
		keys := make(map[string]struct{})
		for decoder.More() {
			keyToken, err := decoder.Token()
			if err != nil {
				return err
			}
			key, ok := keyToken.(string)
			if !ok {
				return fmt.Errorf("object key")
			}
			if _, exists := keys[key]; exists {
				return fmt.Errorf("duplicate object key")
			}
			keys[key] = struct{}{}
			if err := scanJSONValue(decoder, keys); err != nil {
				return err
			}
		}
		_, err := decoder.Token()
		return err
	case '[':
		for decoder.More() {
			if err := scanJSONValue(decoder, nil); err != nil {
				return err
			}
		}
		_, err := decoder.Token()
		return err
	default:
		return fmt.Errorf("unexpected delimiter")
	}
}

func validHex(value string, length int) bool {
	if len(value) != length || strings.ToLower(value) != value {
		return false
	}
	_, err := hex.DecodeString(value)
	return err == nil
}