package threadspost

import (
	"fmt"
	"strings"
)

// Threads Graph API text post hard limit (main post body).
const MaxPublishRunes = 500

// Threads reply/comment limit (outreach, not main post).
const MaxReplyRunes = 1000

// ViralSweetMin/Max: engagement sweet spot on Threads feed (Buffer / creator data, 2024–2026).
const ViralSweetMin = 80
const ViralSweetMax = 220

// ViralSoftWarn: engagement tends to drop beyond this length in the main visible post.
const ViralSoftWarn = 300

type LengthBand string

const (
	BandEmpty    LengthBand = "empty"
	BandTooShort LengthBand = "too_short"
	BandSweet    LengthBand = "sweet"
	BandLong     LengthBand = "long"
	BandOverSoft LengthBand = "over_soft"
	BandOverHard LengthBand = "over_hard"
)

func RuneLen(text string) int {
	return len([]rune(strings.TrimSpace(text)))
}

func ClampPublish(text string) string {
	text = strings.TrimSpace(text)
	runes := []rune(text)
	if len(runes) > MaxPublishRunes {
		return string(runes[:MaxPublishRunes])
	}
	return text
}

func PublishBand(text string) LengthBand {
	n := RuneLen(text)
	switch {
	case n == 0:
		return BandEmpty
	case n < ViralSweetMin:
		return BandTooShort
	case n <= ViralSweetMax:
		return BandSweet
	case n <= ViralSoftWarn:
		return BandLong
	case n <= MaxPublishRunes:
		return BandOverSoft
	default:
		return BandOverHard
	}
}

func ValidatePublish(text string) error {
	if strings.TrimSpace(text) == "" {
		return fmt.Errorf("貼文內文不可為空")
	}
	n := RuneLen(text)
	if n > MaxPublishRunes {
		return fmt.Errorf("貼文超過 Threads 上限 %d 字（目前 %d 字）", MaxPublishRunes, n)
	}
	return nil
}

func ValidateReply(text string) error {
	if strings.TrimSpace(text) == "" {
		return fmt.Errorf("留言內文不可為空")
	}
	n := RuneLen(text)
	if n > MaxReplyRunes {
		return fmt.Errorf("留言超過 Threads 上限 %d 字（目前 %d 字）", MaxReplyRunes, n)
	}
	return nil
}

func PublishHint(text string) string {
	switch PublishBand(text) {
	case BandTooShort:
		return fmt.Sprintf("偏短（爆款常見 %d～%d 字）", ViralSweetMin, ViralSweetMax)
	case BandSweet:
		return fmt.Sprintf("長度適中（爆款常見 %d～%d 字）", ViralSweetMin, ViralSweetMax)
	case BandLong:
		return fmt.Sprintf("略長（超過 %d 字互動可能下降）", ViralSoftWarn)
	case BandOverSoft:
		return fmt.Sprintf("接近 Threads 主文上限 %d 字", MaxPublishRunes)
	case BandOverHard:
		return fmt.Sprintf("超過 Threads 上限 %d 字，請縮短", MaxPublishRunes)
	default:
		return ""
	}
}