// Package fingerprint identifies a chat-completions conversation without a
// session id: the system prompt and the first user message never change from
// turn to turn, so hashing them pins the conversation.
package fingerprint

import (
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"strings"
)

// maxPart is the number of bytes of each input string that contributes to the
// key: 4 KiB keeps a huge first message from slowing every turn.
const maxPart = 4096

// Of returns the lowercase hex SHA-256 of the system prompt and the first user
// message of a chat-completions body (first 4 KiB of each, joined with "\n"),
// or "" when the body is not a JSON object with a "messages" array containing a
// user message.
func Of(body []byte) string {
	var doc struct {
		Messages []json.RawMessage `json:"messages"`
	}
	if err := json.Unmarshal(body, &doc); err != nil || doc.Messages == nil {
		return ""
	}

	var system, user string
	seenSystem, seenUser := false, false
	for _, raw := range doc.Messages {
		var msg struct {
			Role    string          `json:"role"`
			Content json.RawMessage `json:"content"`
		}
		if err := json.Unmarshal(raw, &msg); err != nil {
			return ""
		}
		switch msg.Role {
		case "system":
			if !seenSystem {
				seenSystem = true
				system = contentText(msg.Content)
			}
		case "user":
			if !seenUser {
				seenUser = true
				user = contentText(msg.Content)
			}
		}
	}
	if !seenUser {
		return ""
	}

	if len(system) > maxPart {
		system = system[:maxPart]
	}
	if len(user) > maxPart {
		user = user[:maxPart]
	}
	sum := sha256.Sum256([]byte(system + "\n" + user))
	return hex.EncodeToString(sum[:])
}

// contentText renders a message content value: a JSON string is returned as-is,
// an array of parts is the concatenation of its text parts (other types
// ignored), and anything else is "".
func contentText(raw json.RawMessage) string {
	var text string
	if err := json.Unmarshal(raw, &text); err == nil {
		return text
	}
	var parts []struct {
		Type string `json:"type"`
		Text string `json:"text"`
	}
	if err := json.Unmarshal(raw, &parts); err != nil {
		return ""
	}
	var b strings.Builder
	for _, p := range parts {
		if p.Type == "text" {
			b.WriteString(p.Text)
		}
	}
	return b.String()
}