// Package greenled provides the canonical TERA Core-X48 PRO green-LED
// mapping and the kernel gpio-leds controller interface.
package greenled

import (
	"context"
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"strconv"
	"strings"
	"sync"
	"time"
)

const (
	ChannelCount     = 48
	DefaultSysfsRoot = "/sys/class/leds"
	MinPulseDuration = 100 * time.Millisecond
	MaxPulseDuration = 5 * time.Second
)

// State exposes the serial-channel identity and its deterministic board LED
// mapping. A forced value of true means the green LED is held on. false
// releases the PCF8574 line back to RX activity.
type State struct {
	Channel    string `json:"channel"`
	LEDName    string `json:"led_name"`
	I2CBus     int    `json:"i2c_bus"`
	I2CAddress string `json:"i2c_address"`
	GPIOOffset int    `json:"gpio_offset"`
	Available  bool   `json:"available"`
	Forced     bool   `json:"forced"`
	Message    string `json:"message,omitempty"`
}

// Controller is the service-facing hardware boundary. Implementations must
// not use raw I2C while the kernel owns the PCF8574 gpio-leds clients.
type Controller interface {
	States(ctx context.Context) ([]State, error)
	SetForce(ctx context.Context, channel string, force bool) (State, error)
	Pulse(ctx context.Context, channel string, dwell time.Duration) (State, error)
}

// UnavailableError means the board control path has not been deployed or is
// temporarily unavailable. It deliberately contains no raw host path.
type UnavailableError struct {
	Reason string
}

func (e *UnavailableError) Error() string {
	if e == nil || e.Reason == "" {
		return "green LED controller unavailable"
	}
	return "green LED controller unavailable: " + e.Reason
}

func IsUnavailable(err error) bool {
	var unavailable *UnavailableError
	return errors.As(err, &unavailable)
}

// UnavailableController keeps API reads deterministic before the DTB and
// privileged hardware helper are deployed.
type UnavailableController struct {
	Reason string
}

func (c UnavailableController) States(context.Context) ([]State, error) {
	states := make([]State, 0, ChannelCount)
	for channel := 1; channel <= ChannelCount; channel++ {
		state, _ := stateForChannel(channel)
		state.Message = c.reason()
		states = append(states, state)
	}
	return states, nil
}

func (c UnavailableController) SetForce(context.Context, string, bool) (State, error) {
	return State{}, &UnavailableError{Reason: c.reason()}
}

func (c UnavailableController) Pulse(context.Context, string, time.Duration) (State, error) {
	return State{}, &UnavailableError{Reason: c.reason()}
}

func (c UnavailableController) reason() string {
	if strings.TrimSpace(c.Reason) != "" {
		return strings.TrimSpace(c.Reason)
	}
	return "tera-hwd is not connected"
}

// SysfsController controls only /sys/class/leds/chNN-green-led-force/
// brightness entries created by the active gpio-leds device tree fragment.
type SysfsController struct {
	root string
	mu   sync.Mutex
}

func NewSysfsController(root string) *SysfsController {
	if strings.TrimSpace(root) == "" {
		root = DefaultSysfsRoot
	}
	return &SysfsController{root: filepath.Clean(root)}
}

func (c *SysfsController) States(ctx context.Context) ([]State, error) {
	if err := ctx.Err(); err != nil {
		return nil, err
	}
	c.mu.Lock()
	defer c.mu.Unlock()

	states := make([]State, 0, ChannelCount)
	for channel := 1; channel <= ChannelCount; channel++ {
		state, _ := stateForChannel(channel)
		states = append(states, c.readStateLocked(state))
	}
	return states, nil
}

func (c *SysfsController) SetForce(ctx context.Context, channel string, force bool) (State, error) {
	if err := ctx.Err(); err != nil {
		return State{}, err
	}
	state, err := stateForChannelName(channel)
	if err != nil {
		return State{}, err
	}
	c.mu.Lock()
	defer c.mu.Unlock()
	if err := c.writeForceLocked(state, force); err != nil {
		return State{}, err
	}
	return c.readStateLocked(state), nil
}

func (c *SysfsController) Pulse(ctx context.Context, channel string, dwell time.Duration) (State, error) {
	if dwell < MinPulseDuration || dwell > MaxPulseDuration {
		return State{}, fmt.Errorf("dwell_ms must be between %d and %d", MinPulseDuration.Milliseconds(), MaxPulseDuration.Milliseconds())
	}
	if err := ctx.Err(); err != nil {
		return State{}, err
	}
	state, err := stateForChannelName(channel)
	if err != nil {
		return State{}, err
	}
	c.mu.Lock()
	defer c.mu.Unlock()
	if err := c.writeForceLocked(state, true); err != nil {
		return State{}, err
	}

	timer := time.NewTimer(dwell)
	select {
	case <-ctx.Done():
		if releaseErr := c.writeForceLocked(state, false); releaseErr != nil {
			return State{}, releaseErr
		}
		return State{}, ctx.Err()
	case <-timer.C:
	}
	if !timer.Stop() {
		select {
		case <-timer.C:
		default:
		}
	}
	if err := c.writeForceLocked(state, false); err != nil {
		return State{}, err
	}
	return c.readStateLocked(state), nil
}

func stateForChannelName(channel string) (State, error) {
	normalized := strings.ToUpper(strings.TrimSpace(channel))
	if len(normalized) != 4 || !strings.HasPrefix(normalized, "CH") {
		return State{}, fmt.Errorf("channel must be CH01 through CH48")
	}
	number, err := strconv.Atoi(normalized[2:])
	if err != nil {
		return State{}, fmt.Errorf("channel must be CH01 through CH48")
	}
	return stateForChannel(number)
}

func stateForChannel(channel int) (State, error) {
	if channel < 1 || channel > ChannelCount {
		return State{}, fmt.Errorf("channel must be CH01 through CH48")
	}
	return State{
		Channel:    fmt.Sprintf("CH%02d", channel),
		LEDName:    fmt.Sprintf("ch%02d-green-led-force", channel),
		I2CBus:     3,
		I2CAddress: fmt.Sprintf("0x%02x", 0x20+(channel-1)/8),
		GPIOOffset: (channel - 1) % 8,
	}, nil
}

func (c *SysfsController) readStateLocked(state State) State {
	value, err := os.ReadFile(c.brightnessPath(state))
	if err != nil {
		if os.IsNotExist(err) {
			state.Message = "kernel gpio-leds entry is unavailable"
			return state
		}
		state.Message = "kernel gpio-leds state cannot be read"
		return state
	}

	state.Available = true
	switch strings.TrimSpace(string(value)) {
	case "0":
		state.Forced = false
	case "1":
		state.Forced = true
	default:
		state.Message = "kernel gpio-leds brightness value is invalid"
	}
	return state
}

func (c *SysfsController) writeForceLocked(state State, force bool) error {
	value := []byte("0\n")
	if force {
		value = []byte("1\n")
	}
	if err := os.WriteFile(c.brightnessPath(state), value, 0); err != nil {
		if os.IsNotExist(err) {
			return &UnavailableError{Reason: "kernel gpio-leds entry is unavailable"}
		}
		if os.IsPermission(err) {
			return &UnavailableError{Reason: "green LED hardware helper lacks gpio-leds access"}
		}
		return fmt.Errorf("green LED gpio-leds write failed")
	}
	return nil
}

func (c *SysfsController) brightnessPath(state State) string {
	return filepath.Join(c.root, state.LEDName, "brightness")
}