package page

import (
	"bytes"
	"encoding/base64"
	"encoding/json"
	"fmt"
	"io"
	"strconv"
	"time"
	"unicode/utf8"
	"uuid"
)

const maxCursorBytes = 16384

// CursorError identifies malformed, version, sort, value, or size cursor failures via
// Kind. It intentionally omits cursor payloads from its message.
type CursorError struct{ Kind string }

// Error implements error without including cursor contents.
func (e *CursorError) Error() string { return "page: cursor " + e.Kind }

// ValueError identifies an unsupported concrete type or invalid supported value
// at Position in the row key, without exposing the value itself.
type ValueError struct {
	Position int
	Type     string
}

// Error implements error without including key values.
func (e *ValueError) Error() string {
	return fmt.Sprintf("page: invalid key value at %d (%s)", e.Position, e.Type)
}

type envelope struct {
	Version int         `json:"v"`
	Sort    string      `json:"sort"`
	Values  [][2]string `json:"values"`
}

func encodeCursor(sortID string, values []any) (string, error) {
	e := envelope{Version: 1, Sort: sortID, Values: make([][2]string, len(values))}
	for i, v := range values {
		pair, ok := encodeValue(v)
		if !ok {
			return "", &ValueError{Position: i, Type: fmt.Sprintf("%T", v)}
		}
		e.Values[i] = pair
	}
	data, err := json.Marshal(e)
	if err != nil {
		return "", &CursorError{Kind: "malformed"}
	}
	if base64.RawURLEncoding.EncodedLen(len(data)) > maxCursorBytes {
		return "", &CursorError{Kind: "size"}
	}
	return base64.RawURLEncoding.EncodeToString(data), nil
}

func encodeValue(value any) ([2]string, bool) {
	var tag, payload string
	switch v := value.(type) {
	case int:
		tag, payload = "int", strconv.FormatInt(int64(v), 10)
	case int8:
		tag, payload = "int8", strconv.FormatInt(int64(v), 10)
	case int16:
		tag, payload = "int16", strconv.FormatInt(int64(v), 10)
	case int32:
		tag, payload = "int32", strconv.FormatInt(int64(v), 10)
	case int64:
		tag, payload = "int64", strconv.FormatInt(v, 10)
	case string:
		if !utf8.ValidString(v) {
			return [2]string{}, false
		}
		tag, payload = "string", v
	case time.Time:
		v = v.UTC()
		if v.Year() < 0 || v.Year() > 9999 {
			return [2]string{}, false
		}
		tag, payload = "time", v.Format(time.RFC3339Nano)
	case uuid.UUID:
		tag, payload = "uuid", v.String()
	default:
		return [2]string{}, false
	}
	return [2]string{tag, payload}, true
}

func decodeCursor(token string, sort Sort) ([]any, error) {
	failure := func(kind string) ([]any, error) { return nil, &CursorError{Kind: kind} }
	if len(token) > maxCursorBytes {
		return failure("size")
	}
	data, err := base64.RawURLEncoding.Strict().DecodeString(token)
	if err != nil || !utf8.Valid(data) || base64.RawURLEncoding.EncodeToString(data) != token {
		return failure("malformed")
	}
	// invariant: exactly three distinct envelope fields; struct decoding alone permits duplicates.
	decoder := json.NewDecoder(bytes.NewReader(data))
	start, err := decoder.Token()
	if err != nil || start != json.Delim('{') {
		return failure("malformed")
	}
	fields := make(map[string]json.RawMessage, 3)
	for decoder.More() {
		name, err := decoder.Token()
		if err != nil {
			return failure("malformed")
		}
		key, ok := name.(string)
		if !ok || (key != "v" && key != "sort" && key != "values") {
			return failure("malformed")
		}
		if _, exists := fields[key]; exists {
			return failure("malformed")
		}
		var raw json.RawMessage
		if decoder.Decode(&raw) != nil {
			return failure("malformed")
		}
		fields[key] = raw
	}
	if _, err := decoder.Token(); err != nil || len(fields) != 3 {
		return failure("malformed")
	}
	if _, err := decoder.Token(); err != io.EOF {
		return failure("malformed")
	}
	var version int
	if bytes.Equal(fields["v"], []byte("null")) || json.Unmarshal(fields["v"], &version) != nil {
		return failure("malformed")
	}
	if version != 1 {
		return failure("version")
	}
	var id string
	if !jsonString(fields["sort"], &id) {
		return failure("malformed")
	}
	if id != sort.ID {
		return failure("sort")
	}
	var entries []json.RawMessage
	if len(fields["values"]) == 0 || fields["values"][0] != '[' || json.Unmarshal(fields["values"], &entries) != nil || len(entries) != len(sort.Columns) {
		return failure("malformed")
	}
	values := make([]any, len(entries))
	for i, entry := range entries {
		var pair []json.RawMessage
		if len(entry) == 0 || entry[0] != '[' || json.Unmarshal(entry, &pair) != nil || len(pair) != 2 {
			return failure("malformed")
		}
		var tag, payload string
		if !jsonString(pair[0], &tag) || !jsonString(pair[1], &payload) {
			return failure("malformed")
		}
		value, ok := decodeValue(tag, payload)
		if !ok {
			return failure("value")
		}
		values[i] = value
	}
	return values, nil
}

func jsonString(raw json.RawMessage, value *string) bool {
	return len(raw) > 0 && raw[0] == '"' && json.Unmarshal(raw, value) == nil && utf8.ValidString(*value)
}

func decodeValue(tag, payload string) (any, bool) {
	switch tag {
	case "int", "int8", "int16", "int32", "int64":
		bits := strconv.IntSize
		switch tag {
		case "int8":
			bits = 8
		case "int16":
			bits = 16
		case "int32":
			bits = 32
		case "int64":
			bits = 64
		}
		n, err := strconv.ParseInt(payload, 10, bits)
		if err != nil || strconv.FormatInt(n, 10) != payload {
			return nil, false
		}
		switch tag {
		case "int":
			return int(n), true
		case "int8":
			return int8(n), true
		case "int16":
			return int16(n), true
		case "int32":
			return int32(n), true
		default:
			return n, true
		}
	case "string":
		return payload, utf8.ValidString(payload)
	case "time":
		t, err := time.Parse(time.RFC3339Nano, payload)
		t = t.UTC()
		return t, err == nil && t.Year() >= 0 && t.Year() <= 9999
	case "uuid":
		id, err := uuid.Parse(payload)
		return id, err == nil && id.String() == payload
	default:
		return nil, false
	}
}