package protocol

import (
	"crypto/sha256"
	"encoding/binary"
	"encoding/hex"
)

// RequestFingerprint 计算第 7.3 节发送请求指纹（十六进制小写 SHA-256）。
// 字段：to.kind、to.id、body.enc、content_type、解码后正文、meta（键排序）、
// send_at_ms、delay_ms、offline.keep、offline.ttl_seconds、receipt。
// 不含 talk_password 与 rid。meta 键顺序不影响结果。
// receipt / offline 使用文档默认值后的有效值；缺省的 send_at_ms、delay_ms 不写入对应槽位。
func RequestFingerprint(s *Send) (string, error) {
	raw, err := DecodeBody(s.Body)
	if err != nil {
		return "", err
	}
	metaJSON, err := MetaCanonicalJSON(s.Meta)
	if err != nil {
		return "", badRequest("invalid meta")
	}

	h := sha256.New()
	writeStr(h, s.To.Kind)
	writeStr(h, s.To.ID)
	writeStr(h, s.Body.Enc)
	writeStr(h, EffectiveContentType(s.Body))
	writeBytes(h, raw)
	writeBytes(h, metaJSON)

	writeOptInt64(h, s.SendAtMs)
	writeOptInt64(h, s.DelayMs)

	keep := EffectiveOfflineKeep(s)
	writeBool(h, keep)
	ttl := EffectiveOfflineTTL(s)
	writeInt64(h, ttl)
	writeBool(h, EffectiveReceipt(s))

	sum := h.Sum(nil)
	return hex.EncodeToString(sum), nil
}

type hashWriter interface {
	Write(p []byte) (int, error)
}

func writeStr(h hashWriter, s string) {
	writeBytes(h, []byte(s))
}

func writeBytes(h hashWriter, b []byte) {
	var lenBuf [8]byte
	binary.BigEndian.PutUint64(lenBuf[:], uint64(len(b)))
	_, _ = h.Write(lenBuf[:])
	_, _ = h.Write(b)
}

func writeBool(h hashWriter, v bool) {
	if v {
		_, _ = h.Write([]byte{1})
	} else {
		_, _ = h.Write([]byte{0})
	}
}

func writeInt64(h hashWriter, v int64) {
	var buf [8]byte
	binary.BigEndian.PutUint64(buf[:], uint64(v))
	_, _ = h.Write(buf[:])
}

func writeOptInt64(h hashWriter, v *int64) {
	if v == nil {
		_, _ = h.Write([]byte{0})
		return
	}
	_, _ = h.Write([]byte{1})
	writeInt64(h, *v)
}