// Package campaign parses campaign content (a campaign.md file with YAML
// frontmatter plus a Markdown body) and syncs it into listmonk as a
// campaign, mirroring the content-as-code pattern eec's internal/course
// package uses for course steps — but flattened, since a campaign has no
// step sequencing.
package campaign

import (
	"fmt"
	"os"
	"strings"

	"gopkg.in/yaml.v3"
)

// Frontmatter is the parsed YAML block at the top of a campaign.md file.
type Frontmatter struct {
	Subject string `yaml:"subject"`
	// Lists names Listmonk lists by name, not numeric ID (unlike eec's
	// course_steps.list_id) — resolved to IDs at sync time so a campaign
	// file stays legible without cross-referencing the Listmonk admin UI.
	Lists     []string `yaml:"lists"`
	FromEmail string   `yaml:"from_email"`
	Tags      []string `yaml:"tags"`
	// TemplateID is optional; 0 means "use listmonk's default template".
	TemplateID int `yaml:"template_id"`
	// Type defaults to "regular" when omitted; "optin" is rejected in v1
	// (see Validate) since this tool has no opt-in-confirmation workflow.
	Type string `yaml:"type"`
	// SegmentQuery is a raw SQL boolean expression run against subscribers,
	// the same segmentation mechanism listmonk's own admin UI search box
	// uses — see internal/campaign/sync.go for how this gets materialized
	// into list membership.
	SegmentQuery string `yaml:"segment_query"`
	// PreviewEmails overrides the sync-wide default preview address for
	// this campaign's automatic post-sync preview.
	PreviewEmails []string `yaml:"preview_emails"`
	// Attachments are paths relative to the campaign's own directory
	// (typically under assets/), uploaded to listmonk's media library and
	// attached to the campaign.
	Attachments []string `yaml:"attachments"`
}

// Campaign is one parsed campaign.md, identified by its directory name.
type Campaign struct {
	Slug string // campaigns/<slug>/ — also sent to listmonk as the campaign's Name
	Dir  string // full path to campaigns/<slug>/, for resolving Attachments
	Frontmatter
	Body string
}

// ParseFile reads a campaign.md file with a leading
// "---\n...\n---\n" frontmatter block and returns the parsed frontmatter and
// body, or a descriptive error — same shape as eec's course.parseFrontmatter.
func ParseFile(path string) (Frontmatter, string, error) {
	raw, err := os.ReadFile(path)
	if err != nil {
		return Frontmatter{}, "", err
	}
	text := string(raw)
	if !strings.HasPrefix(text, "---\n") {
		return Frontmatter{}, "", fmt.Errorf("missing frontmatter (expected file to start with '---')")
	}
	rest := text[4:]
	end := strings.Index(rest, "\n---\n")
	if end == -1 {
		return Frontmatter{}, "", fmt.Errorf("unterminated frontmatter (missing closing '---')")
	}
	rawFM := rest[:end]
	body := strings.TrimPrefix(rest[end+len("\n---\n"):], "\n")

	var fm Frontmatter
	if err := yaml.Unmarshal([]byte(rawFM), &fm); err != nil {
		return Frontmatter{}, "", fmt.Errorf("parsing frontmatter: %w", err)
	}
	if fm.Type == "" {
		fm.Type = "regular"
	}
	if err := fm.Validate(); err != nil {
		return Frontmatter{}, "", err
	}
	return fm, body, nil
}

// Validate checks the fields sync.go depends on before ever talking to
// listmonk, so a bad file is rejected with one clear message instead of a
// confusing API error partway through syncing.
func (fm Frontmatter) Validate() error {
	if fm.Subject == "" {
		return fmt.Errorf("frontmatter missing 'subject'")
	}
	if len(fm.Lists) == 0 && fm.SegmentQuery == "" {
		return fmt.Errorf("frontmatter must set 'lists' and/or 'segment_query' — a campaign needs a target audience")
	}
	if fm.FromEmail == "" {
		return fmt.Errorf("frontmatter missing 'from_email'")
	}
	if fm.Type != "regular" {
		return fmt.Errorf("type %q is not supported yet (only 'regular' campaigns) — optin campaigns need their own confirmation workflow this tool doesn't have", fm.Type)
	}
	return nil
}