// Package config parses the per-channel binding files at
// <profile>/channels/<name>.json. Plexum's channel.Registry already
// parses the {agent_id, plugin} core; this package additionally pulls
// out the plugin-specific `fabric` extension block describing which
// Fabric (guild, channelId) the Plexum channel name maps to.
package config

import (
	"encoding/json"
	"errors"
	"fmt"
	"io/fs"
	"os"
	"path/filepath"
	"strings"
)

// PluginName is what manifest.json declares as the plugin name; we
// only consume channels/*.json entries with this value in `plugin`.
const PluginName = "plexum-fabric-channel"

// FabricBinding is one Plexum-channel ↔ Fabric-channel mapping. Built
// by Load from channels/<name>.json files.
type FabricBinding struct {
	// PlexumChannelName: the basename of channels/<name>.json (no .json).
	// Also matches the ChannelContract.Name the plugin manifest advertises.
	PlexumChannelName string
	// AgentID this channel routes to (also recorded in Plexum's channel
	// registry; we re-read here for plugin-internal convenience).
	AgentID string
	// FabricGuildNodeID — which Fabric guild owns the channel.
	FabricGuildNodeID string
	// FabricChannelID — the channel id within that guild.
	FabricChannelID string
}

// On-disk shape; we ignore fields outside the `fabric` block.
type wireConfig struct {
	AgentID string `json:"agent_id"`
	Plugin  string `json:"plugin"`
	Fabric  struct {
		GuildNodeID string `json:"guild_node_id"`
		ChannelID   string `json:"channel_id"`
	} `json:"fabric"`
}

// Load returns all FabricBindings discovered under channelsDir. Files
// whose `plugin` field doesn't match PluginName are silently skipped
// (other channel plugins coexist in the same dir). Files with our
// plugin name but missing fabric.{guild_node_id, channel_id} are
// errors — we won't silently route nowhere.
func Load(channelsDir string) ([]FabricBinding, error) {
	entries, err := os.ReadDir(channelsDir)
	if err != nil {
		if errors.Is(err, fs.ErrNotExist) {
			return nil, nil
		}
		return nil, fmt.Errorf("fabric/config: read %s: %w", channelsDir, err)
	}
	var out []FabricBinding
	for _, e := range entries {
		if e.IsDir() || !strings.HasSuffix(e.Name(), ".json") {
			continue
		}
		path := filepath.Join(channelsDir, e.Name())
		raw, rerr := os.ReadFile(path)
		if rerr != nil {
			return nil, fmt.Errorf("fabric/config: read %s: %w", path, rerr)
		}
		var w wireConfig
		if jerr := json.Unmarshal(raw, &w); jerr != nil {
			return nil, fmt.Errorf("fabric/config: parse %s: %w", path, jerr)
		}
		if w.Plugin != PluginName {
			continue
		}
		name := strings.TrimSuffix(e.Name(), ".json")
		if w.AgentID == "" {
			return nil, fmt.Errorf("fabric/config: %s missing agent_id", path)
		}
		if w.Fabric.GuildNodeID == "" || w.Fabric.ChannelID == "" {
			return nil, fmt.Errorf("fabric/config: %s missing fabric.{guild_node_id, channel_id}", path)
		}
		out = append(out, FabricBinding{
			PlexumChannelName: name,
			AgentID:           w.AgentID,
			FabricGuildNodeID: w.Fabric.GuildNodeID,
			FabricChannelID:   w.Fabric.ChannelID,
		})
	}
	return out, nil
}

// ByFabricChannel indexes bindings by (guild_node_id, channel_id) for
// fast inbound lookup. Plugin builds this map once at startup.
type ByFabricChannel map[string]*FabricBinding

// Key composes the index key.
func Key(guildNodeID, channelID string) string {
	return guildNodeID + "/" + channelID
}

// Index returns a ready-to-query ByFabricChannel.
func Index(bindings []FabricBinding) ByFabricChannel {
	out := make(ByFabricChannel, len(bindings))
	for i := range bindings {
		b := bindings[i]
		out[Key(b.FabricGuildNodeID, b.FabricChannelID)] = &b
	}
	return out
}