// Package backend owns validated, immutable OpenAI-compatible backend
// definitions.
package backend

import (
	"errors"
	"fmt"
	"net/url"
	"regexp"
	"sort"
	"strings"

	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
	"gitea.maximumdirect.net/eric/promptkit/internal/jsonvalue"
	"gitea.maximumdirect.net/eric/promptkit/internal/llm"
)

const (
	// OpenRouterID is the reserved ID of Promptkit's built-in OpenRouter
	// backend.
	OpenRouterID = "openrouter"

	openRouterEndpoint  = "https://openrouter.ai/api/v1"
	openRouterAPIKeyEnv = "OPENROUTER_API_KEY"

	openRouterConcurrencyLimit = 16
	defaultQueueCapacity       = 1024
)

// ErrBackendNotFound identifies a registry lookup for an unknown backend ID.
var ErrBackendNotFound = errors.New("backend not found")

var environmentVariableName = regexp.MustCompile(`^[A-Za-z_][A-Za-z0-9_]*$`)

// Registry is an immutable collection of validated backend definitions.
type Registry struct {
	backends map[string]domain.Backend
}

// NewRegistry constructs a registry containing the built-in OpenRouter
// definition followed by the supplied additions. Every ID must be unique.
func NewRegistry(additions []domain.Backend) (*Registry, error) {
	registry := &Registry{
		backends: make(map[string]domain.Backend, len(additions)+1),
	}

	definitions := make([]domain.Backend, 0, len(additions)+1)
	definitions = append(definitions, domain.Backend{
		ID:               OpenRouterID,
		Endpoint:         openRouterEndpoint,
		APIKeyEnv:        openRouterAPIKeyEnv,
		ConcurrencyLimit: openRouterConcurrencyLimit,
	})
	definitions = append(definitions, additions...)

	for _, definition := range definitions {
		definition.ID = strings.TrimSpace(definition.ID)
		if definition.ID == "" {
			return nil, errors.New("backend ID must not be blank")
		}
		if _, exists := registry.backends[definition.ID]; exists {
			return nil, fmt.Errorf("backend ID %q is already registered", definition.ID)
		}

		normalized, err := normalizeBackend(definition)
		if err != nil {
			return nil, err
		}
		registry.backends[normalized.ID] = normalized
	}

	return registry, nil
}

// GetBackend returns a defensive copy of the backend registered with id.
func (r *Registry) GetBackend(id string) (domain.Backend, error) {
	if r == nil {
		return domain.Backend{}, fmt.Errorf("%w: %q", ErrBackendNotFound, id)
	}
	definition, ok := r.backends[id]
	if !ok {
		return domain.Backend{}, fmt.Errorf("%w: %q", ErrBackendNotFound, id)
	}
	extraParams, err := jsonvalue.CopyMap(definition.ExtraParams)
	if err != nil {
		return domain.Backend{}, fmt.Errorf("copy backend %q: %w", id, err)
	}
	definition.ExtraParams = extraParams
	return definition, nil
}

// CapacityPolicies returns a copy of the normalized policies for limited
// backends.
func (r *Registry) CapacityPolicies() map[string]domain.BackendCapacityPolicy {
	policies := make(map[string]domain.BackendCapacityPolicy)
	if r == nil {
		return policies
	}
	for id, definition := range r.backends {
		if definition.ConcurrencyLimit == 0 {
			continue
		}
		policies[id] = domain.BackendCapacityPolicy{
			ConcurrencyLimit: definition.ConcurrencyLimit,
			QueueCapacity:    definition.QueueCapacity,
		}
	}
	return policies
}

func normalizeBackend(definition domain.Backend) (domain.Backend, error) {
	definition.Endpoint = strings.TrimSpace(definition.Endpoint)
	if err := validateEndpoint(definition.Endpoint); err != nil {
		return domain.Backend{}, fmt.Errorf("backend %q endpoint: %w", definition.ID, err)
	}

	definition.APIKeyEnv = strings.TrimSpace(definition.APIKeyEnv)
	if definition.APIKeyEnv != "" && !environmentVariableName.MatchString(definition.APIKeyEnv) {
		return domain.Backend{}, fmt.Errorf(
			"backend %q api key environment variable %q is invalid",
			definition.ID,
			definition.APIKeyEnv,
		)
	}

	if definition.ConcurrencyLimit < 0 {
		return domain.Backend{}, fmt.Errorf(
			"backend %q concurrency limit must not be negative",
			definition.ID,
		)
	}
	if definition.QueueCapacity < 0 {
		return domain.Backend{}, fmt.Errorf(
			"backend %q queue capacity must not be negative",
			definition.ID,
		)
	}
	if definition.ConcurrencyLimit == 0 {
		if definition.QueueCapacitySet {
			return domain.Backend{}, fmt.Errorf(
				"backend %q queue capacity requires a positive concurrency limit",
				definition.ID,
			)
		}
		definition.QueueCapacity = 0
	} else {
		if !definition.QueueCapacitySet {
			definition.QueueCapacity = defaultQueueCapacity
			definition.QueueCapacitySet = true
		}
		maxInt := int(^uint(0) >> 1)
		if definition.QueueCapacity > maxInt-definition.ConcurrencyLimit {
			return domain.Backend{}, fmt.Errorf(
				"backend %q total capacity overflows int",
				definition.ID,
			)
		}
	}

	keys := make([]string, 0, len(definition.ExtraParams))
	for key := range definition.ExtraParams {
		keys = append(keys, key)
	}
	sort.Strings(keys)
	for _, key := range keys {
		if key == "" {
			return domain.Backend{}, fmt.Errorf("backend %q extra parameter key must not be empty", definition.ID)
		}
		if llm.IsReservedOpenAIChatRequestField(key) {
			return domain.Backend{}, fmt.Errorf(
				"backend %q extra parameter %q collides with a reserved request field",
				definition.ID,
				key,
			)
		}
	}

	extraParams, err := jsonvalue.CopyMap(definition.ExtraParams)
	if err != nil {
		return domain.Backend{}, fmt.Errorf("backend %q extra parameters: %w", definition.ID, err)
	}
	definition.ExtraParams = extraParams
	return definition, nil
}

func validateEndpoint(endpoint string) error {
	if endpoint == "" {
		return errors.New("must not be blank")
	}
	if strings.Contains(endpoint, "#") {
		return errors.New("must not contain a fragment")
	}

	parsed, err := url.Parse(endpoint)
	if err != nil {
		return fmt.Errorf("must be a valid URL: %w", err)
	}
	scheme := strings.ToLower(parsed.Scheme)
	if scheme != "http" && scheme != "https" {
		return errors.New("must use http or https")
	}
	if !parsed.IsAbs() || parsed.Hostname() == "" {
		return errors.New("must be absolute and include a host")
	}
	if parsed.User != nil {
		return errors.New("must not contain user information")
	}
	if parsed.RawQuery != "" || parsed.ForceQuery {
		return errors.New("must not contain a query string")
	}
	return nil
}