// Package config loads, validates, and merges a TOML override file over the
// default synth.ClassFreqConfigs map. The single public entry point is Load.
package config

import (
	"errors"
	"fmt"
	"hash/fnv"
	"io/fs"
	"os"
	"path/filepath"
	"sort"
	"strings"
	"time"

	"github.com/BurntSushi/toml"

	"github.com/netsynth/netsynth/classify"
	"github.com/netsynth/netsynth/synth"
)

// SoundOverride holds optional per-class sound parameters decoded from TOML.
// Pointer fields: nil = not set by user (keep default), non-nil = user override.
type SoundOverride struct {
	Frequency *float64 `toml:"frequency"`
	Waveform  *string  `toml:"waveform"`
}

// RawRule holds a user-defined classification rule as decoded from TOML.
// Port is a pointer so we can distinguish "not set" (nil, matches any port) from port=0.
type RawRule struct {
	Port     *uint16 `toml:"port"`
	Protocol string  `toml:"protocol"`
	Class    string  `toml:"class"`
}

// rawConfig is the top-level TOML decode target.
type rawConfig struct {
	Sounds map[string]SoundOverride `toml:"sounds"`
	Rules  []RawRule                `toml:"rules"`
	Groups map[string]string        `toml:"groups"`
}

// LoadResult is the return type from Load(). It carries the merged FreqConfig map,
// the user-defined classification rules (to be prepended before DefaultRules by the caller),
// the resolved config file path (empty string if no config was found),
// and the set of classes that were auto-assigned frequencies.
type LoadResult struct {
	FreqCfgs    map[classify.TrafficClass]synth.FreqConfig
	UserRules   []classify.Rule
	ConfigPath  string
	AutoClasses map[classify.TrafficClass]bool // classes with auto-assigned frequencies
}

// validWaveforms maps TOML waveform strings to WaveformType constants.
var validWaveforms = map[string]synth.WaveformType{
	"sine":     synth.WaveformSine,
	"square":   synth.WaveformSquare,
	"sawtooth": synth.WaveformSawtooth,
	"triangle": synth.WaveformTriangle,
}

// Load finds, parses, validates, and merges a TOML config file.
//
// configPath is the --config flag value; empty string triggers auto-discovery.
// Returns a LoadResult with the merged FreqConfig map, user-defined rules, and resolved path.
// Returns an error on: explicit file not found, parse errors, unknown keys,
// type mismatches, invalid waveform values, or invalid rule definitions.
// Returns no error (uses defaults) when no config is found during auto-discovery.
func Load(configPath string) (LoadResult, error) {
	path, explicit, err := resolvePath(configPath)
	if err != nil {
		return LoadResult{}, err
	}
	if path == "" {
		// No config found during auto-discovery — use defaults silently (CFG-02)
		return LoadResult{
			FreqCfgs:    copyDefaults(),
			UserRules:   []classify.Rule{},
			ConfigPath:  "",
			AutoClasses: map[classify.TrafficClass]bool{},
		}, nil
	}

	raw, err := parseFile(path)
	if err != nil {
		if explicit && errors.Is(err, fs.ErrNotExist) {
			return LoadResult{}, fmt.Errorf("config file not found: %s", configPath)
		}
		return LoadResult{}, err
	}

	if err := validate(raw); err != nil {
		return LoadResult{}, err
	}

	userRules := convertRules(raw.Rules)
	freqCfgs := copyDefaults()
	autoClasses := map[classify.TrafficClass]bool{}
	// Add auto-freq entries BEFORE merge so that [sounds.X] overrides for user classes apply.
	addAutoFreqEntries(freqCfgs, userRules, autoClasses)
	merge(freqCfgs, raw.Sounds)
	applyGroupOverrides(freqCfgs, raw.Groups)

	return LoadResult{
		FreqCfgs:    freqCfgs,
		UserRules:   userRules,
		ConfigPath:  path,
		AutoClasses: autoClasses,
	}, nil
}

// resolvePath resolves the config path from an explicit flag value or auto-discovery.
// Returns (path, explicit, error) where explicit=true means the user specified a path.
func resolvePath(configPath string) (string, bool, error) {
	if configPath != "" {
		return configPath, true, nil
	}
	return discoverPath(), false, nil
}

// discoverPath probes the standard discovery locations in precedence order.
// Returns the first existing config path, or "" if none found.
// Discovery order: ./netsynth.toml > ~/.config/netsynth/config.toml
func discoverPath() string {
	// 1. Working directory
	if _, err := os.Stat("netsynth.toml"); err == nil {
		return "netsynth.toml"
	}
	// 2. XDG config dir (~/.config/netsynth/config.toml or $XDG_CONFIG_HOME/netsynth/config.toml)
	dir, err := os.UserConfigDir()
	if err != nil {
		return ""
	}
	p := filepath.Join(dir, "netsynth", "config.toml")
	if _, err := os.Stat(p); err == nil {
		return p
	}
	return ""
}

// parseFile decodes the TOML file at path and checks for unknown keys via Undecoded().
// Returns fs.ErrNotExist-wrapped error when the file does not exist.
func parseFile(path string) (rawConfig, error) {
	var raw rawConfig
	md, err := toml.DecodeFile(path, &raw)
	if err != nil {
		// Preserve the fs.ErrNotExist sentinel so Load can distinguish explicit vs discovered.
		if errors.Is(err, fs.ErrNotExist) {
			return raw, err
		}
		return raw, fmt.Errorf("config parse error: %w", err)
	}
	// Detect field-level typos within [sounds.<class>] blocks (CFG-05).
	// Note: class-name typos in [sounds.<name>] are NOT caught here because all
	// map keys are valid decode targets. Class validation happens in merge (D-09).
	if undecoded := md.Undecoded(); len(undecoded) > 0 {
		keyPath := strings.Join(undecoded[0], ".")
		return raw, fmt.Errorf("config: unknown key %q — check spelling", keyPath)
	}
	return raw, nil
}

// validate checks waveform strings and rules before merge so we fail fast at startup (D-11).
func validate(raw rawConfig) error {
	for _, override := range raw.Sounds {
		if override.Waveform != nil {
			if _, err := parseWaveform(*override.Waveform); err != nil {
				return err
			}
		}
	}
	return validateRules(raw.Rules)
}

// validateRules checks that each rule has a valid protocol and a non-empty class.
func validateRules(rules []RawRule) error {
	validProtocols := map[string]bool{"tcp": true, "udp": true, "icmp": true}
	for i, r := range rules {
		if r.Protocol == "" {
			return fmt.Errorf("config: rules[%d]: protocol is required", i)
		}
		if !validProtocols[r.Protocol] {
			return fmt.Errorf("config: rules[%d]: invalid protocol %q -- valid: tcp, udp, icmp", i, r.Protocol)
		}
		if r.Class == "" {
			return fmt.Errorf("config: rules[%d]: class is required", i)
		}
	}
	return nil
}

// convertRules converts a slice of RawRule (from TOML) into classify.Rule slice.
func convertRules(raw []RawRule) []classify.Rule {
	result := make([]classify.Rule, len(raw))
	for i, r := range raw {
		var port uint16
		if r.Port != nil {
			port = *r.Port
		}
		result[i] = classify.Rule{
			Protocol: r.Protocol,
			DstPort:  port,
			Class:    classify.TrafficClass(r.Class),
		}
	}
	return result
}

// autoAssignFreq computes a deterministic frequency in [2500, 4000] Hz for a class name
// using FNV-32a hashing. Same input always produces the same output.
func autoAssignFreq(className string) float64 {
	h := fnv.New32a()
	h.Write([]byte(className))
	const (
		baseHz   = 2500.0
		stepHz   = 50.0
		numSteps = uint32(31) // [2500, 4000] Hz in 50 Hz steps
	)
	return baseHz + float64(h.Sum32()%numSteps)*stepHz
}

// addAutoFreqEntries adds a FreqConfig entry for each user-defined class that doesn't
// already have one in the map. Built-in classes that appear in user rules are skipped.
// Must be called AFTER merge() so that [sounds.X] overrides are already applied.
// autoClasses is populated with the class names that were auto-assigned.
func addAutoFreqEntries(cfgs map[classify.TrafficClass]synth.FreqConfig, userRules []classify.Rule, autoClasses map[classify.TrafficClass]bool) {
	for _, rule := range userRules {
		if _, exists := cfgs[rule.Class]; !exists {
			baseHz := autoAssignFreq(string(rule.Class))
			cfgs[rule.Class] = synth.FreqConfig{
				BaseHz:       baseHz,
				WaveformType: synth.WaveformSine,
				Harmonics:    synth.WaveformPresetHarmonics(synth.WaveformSine, baseHz, synth.SampleRate),
				Pan:          0.0,
			}
			autoClasses[rule.Class] = true
		}
	}
}

// parseWaveform converts a TOML waveform string to a WaveformType.
func parseWaveform(s string) (synth.WaveformType, error) {
	if wt, ok := validWaveforms[s]; ok {
		return wt, nil
	}
	valid := []string{"sine", "square", "sawtooth", "triangle"}
	return 0, fmt.Errorf("config: invalid waveform %q — valid values: %s", s, strings.Join(valid, ", "))
}

// waveformString converts a WaveformType back to its TOML string representation.
func waveformString(wt synth.WaveformType) string {
	switch wt {
	case synth.WaveformSine:
		return "sine"
	case synth.WaveformSquare:
		return "square"
	case synth.WaveformSawtooth:
		return "sawtooth"
	case synth.WaveformTriangle:
		return "triangle"
	default:
		return "custom"
	}
}

// applyGroupOverrides overlays [groups] reassignments onto freqCfgs.Group in-place.
// Unknown class names produce a warning to stderr (D-09).
// Unknown group names are silently accepted -- users can invent custom groups (D-09).
func applyGroupOverrides(cfgs map[classify.TrafficClass]synth.FreqConfig, groups map[string]string) {
	for className, groupName := range groups {
		cls := classify.TrafficClass(className)
		cfg, known := cfgs[cls]
		if !known {
			fmt.Fprintf(os.Stderr, "Warning: config: [groups]: unknown class %q (ignored)\n", className)
			continue
		}
		cfg.Group = groupName
		cfgs[cls] = cfg
	}
}

// groupOrder defines the canonical display order for --print-config section headers (D-04).
var groupOrder = []string{
	"Infrastructure", "Web", "Mail", "Remote Access",
	"File Transfer", "Database", "Discovery", "VoIP", "Unknown",
}

// PrintConfig returns the effective configuration as commented TOML output.
// The output includes a header with source path and generation date, an optional
// [[rules]] section for user-defined rules, and a [sounds.*] section for all
// traffic classes in deterministic order (built-ins first, then user-defined sorted).
// Each sound entry is annotated as (default), (override), or (auto-assigned).
func PrintConfig(result LoadResult) string {
	var sb strings.Builder

	// Header
	fmt.Fprintf(&sb, "# NetSynth effective configuration\n")
	if result.ConfigPath != "" {
		fmt.Fprintf(&sb, "# Config source: %s\n", result.ConfigPath)
	} else {
		fmt.Fprintf(&sb, "# Config source: none (using defaults)\n")
	}
	fmt.Fprintf(&sb, "# Generated: %s\n", time.Now().UTC().Format("2006-01-02T15:04:05Z"))
	fmt.Fprintf(&sb, "\n")

	// [[rules]] section (if any user rules exist)
	if len(result.UserRules) > 0 {
		fmt.Fprintf(&sb, "# Classification rules (user-defined, prepended before built-in rules)\n")
		for _, rule := range result.UserRules {
			fmt.Fprintf(&sb, "[[rules]]\n")
			if rule.DstPort != 0 {
				fmt.Fprintf(&sb, "port = %d\n", rule.DstPort)
			}
			fmt.Fprintf(&sb, "protocol = %q\n", rule.Protocol)
			fmt.Fprintf(&sb, "class = %q\n", string(rule.Class))
			fmt.Fprintf(&sb, "\n")
		}
	}

	// [sounds.*] section — built-in classes grouped by family, then user-defined

	// Build group -> []TrafficClass index from AllClasses()
	builtinByGroup := map[string][]classify.TrafficClass{}
	builtinSet := map[classify.TrafficClass]bool{}
	for _, cls := range classify.AllClasses() {
		builtinSet[cls] = true
		cfg := result.FreqCfgs[cls]
		grp := cfg.Group
		builtinByGroup[grp] = append(builtinByGroup[grp], cls)
	}

	// Sort each group by ascending BaseHz (D-05) using result.FreqCfgs (effective Hz, not defaults)
	for grp := range builtinByGroup {
		classes := builtinByGroup[grp]
		sort.Slice(classes, func(i, j int) bool {
			return result.FreqCfgs[classes[i]].BaseHz < result.FreqCfgs[classes[j]].BaseHz
		})
	}

	// Emit built-in classes grouped with headers (D-04)
	for _, grp := range groupOrder {
		classes, ok := builtinByGroup[grp]
		if !ok || len(classes) == 0 {
			continue
		}
		fmt.Fprintf(&sb, "# %s\n\n", grp)
		for _, cls := range classes {
			cfg := result.FreqCfgs[cls]
			annotation := classAnnotation(cls, cfg, result.AutoClasses)
			fmt.Fprintf(&sb, "# %s -- %.1f Hz (%s)\n", string(cls), cfg.BaseHz, annotation)
			fmt.Fprintf(&sb, "[sounds.%s]\n", string(cls))
			fmt.Fprintf(&sb, "frequency = %.1f\n", cfg.BaseHz)
			fmt.Fprintf(&sb, "waveform = %q\n", waveformString(cfg.WaveformType))
			fmt.Fprintf(&sb, "\n")
		}
	}

	// Check for custom groups (from [groups] reassignment) that are not in groupOrder.
	// For non-canonical group names (user-invented), collect and emit separately.
	customGroups := map[string][]classify.TrafficClass{}
	for _, cls := range classify.AllClasses() {
		cfg := result.FreqCfgs[cls]
		grp := cfg.Group
		isCanonical := false
		for _, cg := range groupOrder {
			if grp == cg {
				isCanonical = true
				break
			}
		}
		if !isCanonical {
			customGroups[grp] = append(customGroups[grp], cls)
		}
	}
	// Sort and emit custom group sections
	var customGroupNames []string
	for grp := range customGroups {
		customGroupNames = append(customGroupNames, grp)
	}
	sort.Strings(customGroupNames)
	for _, grp := range customGroupNames {
		classes := customGroups[grp]
		sort.Slice(classes, func(i, j int) bool {
			return result.FreqCfgs[classes[i]].BaseHz < result.FreqCfgs[classes[j]].BaseHz
		})
		fmt.Fprintf(&sb, "# %s\n\n", grp)
		for _, cls := range classes {
			cfg := result.FreqCfgs[cls]
			annotation := classAnnotation(cls, cfg, result.AutoClasses)
			fmt.Fprintf(&sb, "# %s -- %.1f Hz (%s)\n", string(cls), cfg.BaseHz, annotation)
			fmt.Fprintf(&sb, "[sounds.%s]\n", string(cls))
			fmt.Fprintf(&sb, "frequency = %.1f\n", cfg.BaseHz)
			fmt.Fprintf(&sb, "waveform = %q\n", waveformString(cfg.WaveformType))
			fmt.Fprintf(&sb, "\n")
		}
	}

	// Emit user-defined classes (in FreqCfgs but not in AllClasses) under "# User-defined" (D-06)
	var userClasses []string
	for cls := range result.FreqCfgs {
		if !builtinSet[cls] {
			userClasses = append(userClasses, string(cls))
		}
	}
	sort.Strings(userClasses)
	if len(userClasses) > 0 {
		fmt.Fprintf(&sb, "# User-defined\n\n")
		for _, clsStr := range userClasses {
			cls := classify.TrafficClass(clsStr)
			cfg := result.FreqCfgs[cls]
			annotation := classAnnotation(cls, cfg, result.AutoClasses)
			fmt.Fprintf(&sb, "# %s -- %.1f Hz (%s)\n", clsStr, cfg.BaseHz, annotation)
			fmt.Fprintf(&sb, "[sounds.%s]\n", clsStr)
			fmt.Fprintf(&sb, "frequency = %.1f\n", cfg.BaseHz)
			fmt.Fprintf(&sb, "waveform = %q\n", waveformString(cfg.WaveformType))
			fmt.Fprintf(&sb, "\n")
		}
	}

	return sb.String()
}

// classAnnotation returns the annotation string for a traffic class entry.
// Returns "default", "override", or "auto-assigned".
func classAnnotation(cls classify.TrafficClass, cfg synth.FreqConfig, autoClasses map[classify.TrafficClass]bool) string {
	if autoClasses[cls] {
		return "auto-assigned"
	}
	defaultCfg, isBuiltin := synth.ClassFreqConfigs[cls]
	if !isBuiltin {
		// User-defined class that was manually specified in [sounds.*] (not auto-assigned)
		return "override"
	}
	if cfg.BaseHz == defaultCfg.BaseHz && cfg.WaveformType == defaultCfg.WaveformType {
		return "default"
	}
	return "override"
}

// copyDefaults returns a shallow copy of synth.ClassFreqConfigs.
// Shallow copy is safe because merge assigns fresh Harmonics slices from
// WaveformPresetHarmonics, never mutating the original default slice.
func copyDefaults() map[classify.TrafficClass]synth.FreqConfig {
	result := make(map[classify.TrafficClass]synth.FreqConfig, len(synth.ClassFreqConfigs))
	for k, v := range synth.ClassFreqConfigs {
		result[k] = v
	}
	return result
}

// merge overlays per-class overrides onto the defaults map in-place.
// Only non-nil pointer fields in each SoundOverride are applied.
func merge(
	defaults map[classify.TrafficClass]synth.FreqConfig,
	overrides map[string]SoundOverride,
) map[classify.TrafficClass]synth.FreqConfig {
	for className, override := range overrides {
		class := classify.TrafficClass(className)
		cfg, known := defaults[class]
		if !known {
			// D-09: unknown class name = warning (not error), in case Phase 7 defines it
			fmt.Fprintf(os.Stderr, "Warning: config: unknown class %q (ignored)\n", className)
			continue
		}
		if override.Frequency != nil {
			cfg.BaseHz = *override.Frequency
			// Regenerate harmonics when a waveform preset is active (Pitfall 3)
			if cfg.WaveformType != synth.WaveformCustom {
				cfg.Harmonics = synth.WaveformPresetHarmonics(cfg.WaveformType, cfg.BaseHz, synth.SampleRate)
			}
		}
		if override.Waveform != nil {
			wt, _ := parseWaveform(*override.Waveform) // already validated above
			cfg.WaveformType = wt
			cfg.Harmonics = synth.WaveformPresetHarmonics(wt, cfg.BaseHz, synth.SampleRate)
		}
		defaults[class] = cfg
	}
	return defaults
}