// Package pld reads the X1208 power-loss-detect line.
//
// The HAT drives GPIO 6 high while mains power is present and low while the
// Pi runs on cells.
package pld

import (
	"fmt"

	"github.com/warthog618/go-gpiocdev"
)

const (
	// DefaultChip is addressed by name, not by /dev path, so this does not
	// depend on any distribution-specific symlink.
	DefaultChip = "gpiochip0"
	// DefaultLine is the power-loss-detect line on the X1208.
	DefaultLine = 6

	consumer = "x1208-exporter"
)

// Line holds the GPIO request open across reads. The kernel grants the request
// exclusively, so reacquiring it per read risks EBUSY against our own
// not-yet-released handle.
type Line struct {
	chip   string
	offset int
	line   *gpiocdev.Line
}

// New returns a Line that has not yet been requested. The first Read requests it.
func New(chip string, offset int) *Line {
	if chip == "" {
		chip = DefaultChip
	}
	return &Line{chip: chip, offset: offset}
}

func (l *Line) ensureOpen() error {
	if l.line != nil {
		return nil
	}
	line, err := gpiocdev.RequestLine(l.chip, l.offset,
		gpiocdev.AsInput,
		gpiocdev.WithConsumer(consumer))
	if err != nil {
		return fmt.Errorf("request %s line %d: %w", l.chip, l.offset, err)
	}
	l.line = line
	return nil
}

// Close releases the line. It is safe to call more than once.
func (l *Line) Close() error {
	if l.line == nil {
		return nil
	}
	line := l.line
	l.line = nil
	return line.Close()
}

// Read reports whether mains power is present.
//
// On error the line is released so the next Read requests a fresh one.
//
// A read that succeeds is not proof that the value is meaningful: a floating
// or misconfigured line returns 0 with no error. Callers that act on "no AC"
// must corroborate it, not trust this alone.
func (l *Line) Read() (acPresent bool, err error) {
	if err := l.ensureOpen(); err != nil {
		return false, err
	}
	defer func() {
		if err != nil {
			_ = l.Close()
		}
	}()

	v, err := l.line.Value()
	if err != nil {
		return false, fmt.Errorf("read %s line %d: %w", l.chip, l.offset, err)
	}
	return v != 0, nil
}