// Package yijing provides a deterministic hexagram seed from birth + local time.
// It is a cultural rhythm seed for lifestyle tips — not fortune-telling.
package yijing

import (
	"fmt"
	"time"
)

// Context is the seed passed to LLM prompts.
type Context struct {
	GuaIndex   int    // 1..64
	GuaName    string // short label
	Line       int    // 1..6 changing line hint
	DayPart    string // morning|noon|afternoon|evening|night
	NowLocal   string
	BirthDate  string
	BirthTime  string
}

var guaNames = []string{
	"乾", "坤", "屯", "蒙", "需", "讼", "师", "比",
	"小畜", "履", "泰", "否", "同人", "大有", "谦", "豫",
	"随", "蛊", "临", "观", "噬嗑", "贲", "剥", "复",
	"无妄", "大畜", "颐", "大过", "坎", "离", "咸", "恒",
	"遁", "大壮", "晋", "明夷", "家人", "睽", "蹇", "解",
	"损", "益", "夬", "姤", "萃", "升", "困", "井",
	"革", "鼎", "震", "艮", "渐", "归妹", "丰", "旅",
	"巽", "兑", "涣", "节", "中孚", "小过", "既济", "未济",
}

// Seed builds hexagram context from birth date/time and now (Asia/Shanghai preferred).
func Seed(birthDate string, birthTime *string, now time.Time) Context {
	loc, err := time.LoadLocation("Asia/Shanghai")
	if err != nil {
		loc = time.FixedZone("CST", 8*3600)
	}
	now = now.In(loc)

	bt := "12:00"
	if birthTime != nil && *birthTime != "" {
		bt = *birthTime
	}

	sum := hash(birthDate) + hash(bt) + hash(now.Format("2006-01-02")) + now.Hour()*17 + now.Minute()/10
	gua := (sum % 64) + 1
	line := (sum/7)%6 + 1
	name := guaNames[gua-1]

	return Context{
		GuaIndex:  gua,
		GuaName:   name,
		Line:      line,
		DayPart:   dayPart(now.Hour()),
		NowLocal:  now.Format("2006-01-02 15:04"),
		BirthDate: birthDate,
		BirthTime: bt,
	}
}

// PromptLine is a compact Chinese summary for the model.
func (c Context) PromptLine() string {
	return fmt.Sprintf(
		"出生 %s %s；此刻 %s（时段 %s）；卦象种子：%s（第%d卦）· 动爻%d",
		c.BirthDate, c.BirthTime, c.NowLocal, c.DayPart, c.GuaName, c.GuaIndex, c.Line,
	)
}

func dayPart(h int) string {
	switch {
	case h < 6:
		return "night"
	case h < 11:
		return "morning"
	case h < 14:
		return "noon"
	case h < 18:
		return "afternoon"
	default:
		return "evening"
	}
}

func hash(s string) int {
	n := 0
	for i := 0; i < len(s); i++ {
		n = n*131 + int(s[i])
	}
	if n < 0 {
		n = -n
	}
	return n
}