// Draft generation for Phase 5 (FR-8): turn a message into a short reply using
// a template and the OmniRoute gateway.
package reply

import (
	"context"
	"fmt"
	"log/slog"
	"strings"

	"gitea.lab.audasmedia.com.au/sam/email_agent/internal/store"
)

// maxPromptBodyChars bounds how much of the original message is sent to the
// model. Personal mail is usually short; this keeps the request cheap.
const maxPromptBodyChars = 4000

// maxDraftChars bounds the generated body before it is stored.
const maxDraftChars = 8000

// Request is one draft request.
type Request struct {
	// Message is the mail being replied to.
	Message store.Message
	// Category, Importance, and Action come from the latest classification.
	Category   string
	Importance string
	Action     string
	// TemplateID selects a built-in template. Empty picks one from Action.
	TemplateID string
	// Instruction is optional extra guidance Sam typed (for example "say the
	// invoice is paid"). It is appended to the template instruction.
	Instruction string
	// Signer is the name used for the sign-off (default "Sam").
	Signer string
}

// Generator produces reply drafts from an LLM client.
type Generator struct {
	client *Client
	logger *slog.Logger
}

// NewGenerator builds a Generator.
func NewGenerator(client *Client, logger *slog.Logger) *Generator {
	if logger == nil {
		logger = slog.New(slog.DiscardHandler)
	}
	return &Generator{client: client, logger: logger}
}

// Configured reports whether the LLM gateway is configured.
func (g *Generator) Configured() bool { return g.client.Configured() }

// Model returns the configured model alias.
func (g *Generator) Model() string { return g.client.Model() }

// Draft generates a reply body and returns it with the template that was used.
func (g *Generator) Draft(ctx context.Context, req Request) (body, templateID string, err error) {
	tmpl, ok := TemplateByID(req.TemplateID)
	if !ok {
		tmpl = TemplateForAction(req.Action)
		if strings.TrimSpace(req.TemplateID) != "" {
			g.logger.Warn("unknown reply template; using the action default",
				"template", req.TemplateID, "used", tmpl.ID)
		}
	}

	system, user := BuildPrompt(req, tmpl)
	body, err = g.client.Complete(ctx, system, user)
	if err != nil {
		return "", "", err
	}
	body = cleanDraft(body, maxDraftChars)
	if body == "" {
		return "", "", fmt.Errorf("the model returned an empty draft")
	}
	return body, tmpl.ID, nil
}

// BuildPrompt builds the system and user prompts for a draft. It is exported so
// the prompt shape can be tested without a live gateway.
func BuildPrompt(req Request, tmpl Template) (system, user string) {
	signer := strings.TrimSpace(req.Signer)
	if signer == "" {
		signer = "Sam"
	}

	system = strings.Join([]string{
		"You write short, plain-text email replies on behalf of " + signer + ".",
		"Return only the reply body: no subject line, no markdown, no placeholders, no quoted history.",
		"Match the language of the incoming message.",
		"Keep replies under 120 words and end with a sign-off of \"" + signer + "\".",
		"If the incoming message is unclear, write a brief acknowledgement and ask one short question.",
		"Never invent facts, prices, dates, or commitments. If a detail is missing, leave it out or ask about it.",
	}, " ")

	var b strings.Builder
	b.WriteString("TEMPLATE: ")
	b.WriteString(tmpl.Name)
	b.WriteString("\n")
	b.WriteString("INSTRUCTION: ")
	b.WriteString(tmpl.Instruction)
	b.WriteString("\n")
	if extra := strings.TrimSpace(req.Instruction); extra != "" {
		b.WriteString("SAM'S NOTE: ")
		b.WriteString(extra)
		b.WriteString("\n")
	}
	if cat := strings.TrimSpace(req.Category); cat != "" {
		b.WriteString("CLASSIFICATION: category=")
		b.WriteString(cat)
		if imp := strings.TrimSpace(req.Importance); imp != "" {
			b.WriteString(" importance=")
			b.WriteString(imp)
		}
		if act := strings.TrimSpace(req.Action); act != "" {
			b.WriteString(" suggested_action=")
			b.WriteString(act)
		}
		b.WriteString("\n")
	}
	b.WriteString("\n--- ORIGINAL MESSAGE ---\n")
	b.WriteString("From: ")
	b.WriteString(strings.TrimSpace(req.Message.FromAddr))
	b.WriteString("\n")
	b.WriteString("Arrived at: ")
	b.WriteString(strings.TrimSpace(req.Message.SourceAddress))
	b.WriteString("\n")
	b.WriteString("Subject: ")
	b.WriteString(strings.TrimSpace(req.Message.Subject))
	b.WriteString("\n\n")
	b.WriteString(truncate(req.Message.BodyText, maxPromptBodyChars))
	b.WriteString("\n--- END ---\n\nWrite the reply body now.")

	return system, b.String()
}

// cleanDraft trims a generated draft. Some models wrap output in a code fence or
// prefix it with a "Subject:" line even when told not to; strip those.
func cleanDraft(s string, max int) string {
	s = strings.TrimSpace(s)
	s = strings.TrimPrefix(s, "```")
	s = strings.TrimSuffix(s, "```")
	s = strings.TrimSpace(s)
	if strings.HasPrefix(strings.ToLower(s), "subject:") {
		if i := strings.IndexByte(s, '\n'); i >= 0 {
			s = strings.TrimSpace(s[i+1:])
		}
	}
	return truncate(s, max)
}

func truncate(s string, max int) string {
	s = strings.TrimSpace(s)
	if len(s) <= max {
		return s
	}
	return strings.TrimSpace(s[:max]) + "…"
}