// Private messages: sent through the same POST /posts.json endpoint as
// topics, but with target_usernames / target_group_names / target_emails
// instead of a category — Discourse then creates a private-message topic
// (archetype private_message) visible only to sender and recipients.
package discourse

import (
	"context"
	"fmt"
	"strings"
)

// SendMessageRequest sends a private message. Exactly one of the three
// target lists is the common case; any non-empty one works. At least one
// target overall is required — Discourse 422s without one.
type SendMessageRequest struct {
	Title            string
	Raw              string
	TargetUsernames  []string
	TargetGroupNames []string
	TargetEmails     []string
}

// sendMessageWire is the POST /posts.json body: Discourse expects the
// target lists comma-joined strings, not JSON arrays.
type sendMessageWire struct {
	Title            string `json:"title"`
	Raw              string `json:"raw"`
	TargetUsernames  string `json:"target_usernames,omitempty"`
	TargetGroupNames string `json:"target_group_names,omitempty"`
	TargetEmails     string `json:"target_emails,omitempty"`
}

// SendMessageResult is the response: a fresh private-message topic,
// same shape as a topic creation (URL() renders its link).
type SendMessageResult = CreateTopicResult

// SendMessage posts a private message and returns the new PM topic's
// ids. Unknown recipients fail server-side as 403 (ErrForbidden) —
// that is the "not permitted to view the requested resource" case.
func (c *Client) SendMessage(ctx context.Context, req SendMessageRequest) (*SendMessageResult, error) {
	if strings.TrimSpace(req.Title) == "" {
		return nil, fmt.Errorf("%w: message title is required", ErrInvalidRequest)
	}
	if strings.TrimSpace(req.Raw) == "" {
		return nil, fmt.Errorf("%w: message raw body is required", ErrInvalidRequest)
	}
	if len(req.TargetUsernames) == 0 && len(req.TargetGroupNames) == 0 && len(req.TargetEmails) == 0 {
		return nil, fmt.Errorf("%w: at least one target (username, group or email) is required", ErrInvalidRequest)
	}
	wire := sendMessageWire{
		Title:            req.Title,
		Raw:              req.Raw,
		TargetUsernames:  strings.Join(cleanList(req.TargetUsernames), ","),
		TargetGroupNames: strings.Join(cleanList(req.TargetGroupNames), ","),
		TargetEmails:     strings.Join(cleanList(req.TargetEmails), ","),
	}
	var out SendMessageResult
	if err := c.Post(ctx, "/posts.json", wire, &out); err != nil {
		return nil, err
	}
	if out.TopicID == 0 {
		return nil, fmt.Errorf("%w: message sent but response carries no topic_id", ErrMalformedResponse)
	}
	return &out, nil
}

// cleanList trims whitespace and drops empties from a target list.
func cleanList(in []string) []string {
	out := make([]string, 0, len(in))
	for _, v := range in {
		if v = strings.TrimSpace(v); v != "" {
			out = append(out, v)
		}
	}
	return out
}