// Package convert turns a markdown source byte-buffer into DOCX, HTML,
// or PDF via two stock upstream container images: pandoc (default
// `docker.io/pandoc/latex:latest`) handles MD↔DOCX and MD→HTML, and
// a headless-chromium image (default `docker.io/zenika/alpine-chrome:latest`)
// handles HTML→PDF. No custom image build is required — the operator
// just needs `podman` or `docker` on PATH and the runner pulls each
// image on first use via `--pull=missing`.
//
// Public surface:
//
//	ToDocx(ctx, source, meta)  → []byte (DOCX bytes)
//	ToHTML(ctx, source, meta)  → []byte (standalone HTML)
//	ToPDF (ctx, source, meta)  → []byte (PDF, via HTML + chromium)
//
//	Probe(ctx, override)        → Capabilities (call once at startup)
//	Available()                 → (Capabilities, bool)
//	SetImages(pandoc, chromium) — install image refs from config
//
// All three converters are safe for concurrent use; each call gets a
// fresh container. The pandoc image's entrypoint is `pandoc`, so the
// argv we pass after the image flows straight into pandoc. The
// alpine-chrome image's entrypoint is `chromium-browser`, so the argv
// flows into chromium-browser. No `sh -c` wrappers, no shell quoting.
//
// Metadata maps to the placeholders consumed by viewer-template.html.
// title/tracking_number/revision/status/is_draft typically come from
// the source filename (zddc.ParseFilename); client/project/contractor/
// project_number from the .zddc cascade `convert:` block.
package convert

import (
	"context"
	"fmt"
	"os"
	"path/filepath"
	"strings"
	"sync/atomic"
	"time"
)

// Metadata is the variable bag passed to pandoc as `--variable k=v`
// pairs. Fields with zero values are omitted. The viewer-template.html
// uses `$if(field)$ … $endif$` blocks so absent fields render cleanly.
type Metadata struct {
	Title          string
	TrackingNumber string
	Revision       string
	Status         string
	Client         string
	Project        string
	Contractor     string
	ProjectNumber  string
	GenerationTime time.Time
	IsDraft        bool
	NoTOC          bool
}

// Default images. Operator overrides via --convert-pandoc-image /
// --convert-chromium-image (see cmd/zddc-server). pandoc/latex carries
// TeX Live for native PDF too, so it's a superset of pandoc/core;
// operators wanting a slimmer footprint can switch to pandoc/core.
const (
	DefaultPandocImage   = "docker.io/pandoc/latex:latest"
	DefaultChromiumImage = "docker.io/zenika/alpine-chrome:latest"
)

var (
	pandocImage   atomic.Pointer[string]
	chromiumImage atomic.Pointer[string]
)

// SetImages installs the image refs used for subsequent ToDocx/ToHTML/
// ToPDF calls. Empty values keep the previous setting (or the
// DefaultPandocImage / DefaultChromiumImage constants on first call).
// Called from cmd/zddc-server/main.go after flag parsing.
func SetImages(pandoc, chromium string) {
	if pandoc != "" {
		s := pandoc
		pandocImage.Store(&s)
	}
	if chromium != "" {
		s := chromium
		chromiumImage.Store(&s)
	}
}

func currentPandocImage() string {
	if p := pandocImage.Load(); p != nil && *p != "" {
		return *p
	}
	return DefaultPandocImage
}

func currentChromiumImage() string {
	if p := chromiumImage.Load(); p != nil && *p != "" {
		return *p
	}
	return DefaultChromiumImage
}

// ToDocx renders source markdown to DOCX bytes. One container run via
// the pandoc image. Caller passes the full file content (envelope +
// body); pandoc handles `markdown+yaml_metadata_block` natively.
func ToDocx(ctx context.Context, source []byte, m Metadata) ([]byte, error) {
	r := currentRunner()
	if r == nil {
		return nil, ErrUnavailable
	}
	cmd := []string{
		"--from=markdown+yaml_metadata_block",
		"--to=docx",
		"--output=-",
	}
	cmd = append(cmd, metadataArgs(m)...)
	cmd = append(cmd, "-")
	return r.Run(ctx, currentPandocImage(), source, nil, cmd)
}

// ToHTML renders source markdown to standalone HTML using
// viewer-template.html. Embeds CSS + images via --embed-resources.
// Template + custom.css are bind-mounted into the container at /tpl
// from a per-call scratch dir.
func ToHTML(ctx context.Context, source []byte, m Metadata) ([]byte, error) {
	r := currentRunner()
	if r == nil {
		return nil, ErrUnavailable
	}
	scratch, err := writeAssetsToScratch()
	if err != nil {
		return nil, fmt.Errorf("scratch: %w", err)
	}
	defer os.RemoveAll(scratch)

	cmd := []string{
		"--from=markdown+yaml_metadata_block",
		"--to=html5",
		"--standalone",
		"--embed-resources",
		"--section-divs",
		"--id-prefix=",
		"--html-q-tags",
		"--template=/tpl/viewer-template.html",
	}
	if !m.NoTOC {
		cmd = append(cmd, "--toc", "--toc-depth=6")
	}
	cmd = append(cmd, metadataArgs(m)...)
	cmd = append(cmd, "--output=-", "-")

	mounts := []string{scratch + ":/tpl:ro"}
	return r.Run(ctx, currentPandocImage(), source, mounts, cmd)
}

// ToPDF renders source markdown to PDF in two stages: pandoc produces
// HTML using viewer-template.html (stage 1, pandoc image), then headless
// Chromium prints that HTML to PDF (stage 2, chromium image). The
// two-stage choice preserves the print-media CSS already authored in
// viewer-template.html — pandoc's native --pdf-engine path uses LaTeX
// which would bypass it entirely.
//
// Chromium runs from the alpine-chrome image whose entrypoint is
// `chromium-browser`; our cmd is the flag list passed straight to that
// binary. The host scratch dir is bind-mounted read-write at /pdf so
// chromium can write out.pdf and we read it back afterward.
func ToPDF(ctx context.Context, source []byte, m Metadata) ([]byte, error) {
	html, err := ToHTML(ctx, source, m)
	if err != nil {
		return nil, err
	}
	r := currentRunner()
	if r == nil {
		return nil, ErrUnavailable
	}

	scratch, err := os.MkdirTemp("", "zddc-pdf-")
	if err != nil {
		return nil, fmt.Errorf("scratch: %w", err)
	}
	defer os.RemoveAll(scratch)
	htmlPath := filepath.Join(scratch, "in.html")
	pdfPath := filepath.Join(scratch, "out.pdf")
	if err := os.WriteFile(htmlPath, html, 0o644); err != nil {
		return nil, fmt.Errorf("write html: %w", err)
	}
	if err := chmodTree(scratch, 0o755, 0o644); err != nil {
		return nil, err
	}

	mounts := []string{scratch + ":/pdf:rw"}
	// alpine-chrome's entrypoint is `chromium-browser`. --no-sandbox is
	// required because the container drops CAP_SYS_ADMIN; the threat
	// model is "malicious markdown drives chromium RCE", contained by
	// --network=none + --cap-drop=ALL + --read-only + tmpfs.
	cmd := []string{
		"--headless",
		"--disable-gpu",
		"--no-sandbox",
		"--user-data-dir=/tmp/chrome",
		"--no-pdf-header-footer",
		"--virtual-time-budget=10000",
		"--print-to-pdf=/pdf/out.pdf",
		"file:///pdf/in.html",
	}
	if _, err := r.Run(ctx, currentChromiumImage(), nil, mounts, cmd); err != nil {
		return nil, err
	}

	out, err := os.ReadFile(pdfPath)
	if err != nil {
		return nil, fmt.Errorf("read pdf: %w", err)
	}
	if len(out) < 4 || string(out[:4]) != "%PDF" {
		return nil, &ConvertError{
			Tool:     "chromium",
			ExitCode: 0,
			Stderr:   "chromium did not produce a valid PDF",
			Cause:    fmt.Errorf("invalid PDF magic in output (got %d bytes)", len(out)),
		}
	}
	return out, nil
}

// metadataArgs renders Metadata into pandoc -V flags. Order is stable
// so test fixtures don't churn. Empty values are omitted (the template
// uses $if(...)$ blocks).
func metadataArgs(m Metadata) []string {
	var out []string
	add := func(k, v string) {
		v = strings.TrimSpace(v)
		if v == "" {
			return
		}
		out = append(out, "-V", k+"="+v)
	}
	add("title", m.Title)
	add("tracking_number", m.TrackingNumber)
	add("revision", m.Revision)
	add("status", m.Status)
	add("client", m.Client)
	add("project", m.Project)
	add("contractor", m.Contractor)
	add("project_number", m.ProjectNumber)
	if !m.GenerationTime.IsZero() {
		add("generation_time", m.GenerationTime.Format("January 02, 2006 at 3:04:05 PM MST"))
	}
	if m.IsDraft {
		add("is_draft", "true")
	}
	if m.NoTOC {
		add("no-toc", "true")
	}
	return out
}