package convert

import (
	"context"
	"fmt"
	"log/slog"
	"os/exec"
	"strings"
	"sync"
	"sync/atomic"
	"time"
)

// Capabilities is the snapshot of "can we convert right now?". The
// only hard requirement is a container runtime on PATH — image presence
// is left to `--pull=missing` at conversion time, so a missing image
// surfaces as a normal ConvertError (not a probe failure).
type Capabilities struct {
	Engine        string    // "podman" | "docker" | ""
	EngineVer     string    // first line of "<engine> --version"
	PandocImage   string    // resolved pandoc image ref
	ChromiumImage string    // resolved chromium image ref
	ProbedAt      time.Time
	Err           error
}

// Ready reports whether conversions can be attempted. The first
// conversion may still fail if the configured image isn't reachable
// from the host's registry (the runner will surface a clear error
// from podman/docker stderr).
func (c Capabilities) Ready() bool {
	return c.Engine != "" && c.Err == nil
}

// Reason returns a short human-friendly explanation when Ready() is
// false. Used as the body of a 503.
func (c Capabilities) Reason() string {
	if c.Engine == "" {
		return "no container runtime (podman or docker) found on PATH"
	}
	if c.Err != nil {
		return c.Err.Error()
	}
	return "unavailable"
}

var (
	caps      atomic.Pointer[Capabilities]
	probeCool sync.Mutex
)

// Available returns the current Capabilities snapshot and whether
// conversions can proceed.
func Available() (Capabilities, bool) {
	p := caps.Load()
	if p == nil {
		return Capabilities{}, false
	}
	return *p, p.Ready()
}

// Probe locates the container engine and installs a containerRunner
// as the package default. Call once at server startup. Returns the
// captured Capabilities for logging.
//
// Engine order: engineOverride (if non-empty) → podman → docker. First
// hit wins. Image presence is NOT probed: the runner uses
// `--pull=missing` so the first conversion request will pull whichever
// image it needs.
//
// Any failure here is non-fatal: the server still starts, conversion
// endpoints just return 503. This matches the user's locked-in
// requirement that no-container-runtime ⇒ "can't do conversions".
func Probe(ctx context.Context, engineOverride string) Capabilities {
	probeCool.Lock()
	defer probeCool.Unlock()

	now := time.Now()
	c := Capabilities{
		PandocImage:   currentPandocImage(),
		ChromiumImage: currentChromiumImage(),
		ProbedAt:      now,
	}

	engine := resolveEngine(engineOverride)
	if engine == "" {
		c.Err = fmt.Errorf("no container runtime found (tried: %s)", strings.Join(enginesTried(engineOverride), ", "))
		caps.Store(&c)
		slog.Warn("convert: probe failed", "reason", c.Err.Error())
		return c
	}
	c.Engine = engine

	if v, err := probeVersion(ctx, engine); err == nil {
		c.EngineVer = v
	}

	InstallRunner(newContainerRunner(engine))
	caps.Store(&c)
	slog.Info("convert: ready",
		"engine", engine,
		"engine_version", c.EngineVer,
		"pandoc_image", c.PandocImage,
		"chromium_image", c.ChromiumImage)
	return c
}

// Reprobe re-runs Probe with the existing configuration. Used by the
// handler when a request hits a not-Ready state — gives the operator
// a way to recover (e.g. installed podman after the server started)
// without a server restart. Cooldown of 60 s between probes to keep
// error-path requests cheap.
func Reprobe(ctx context.Context, engineOverride string) Capabilities {
	if p := caps.Load(); p != nil {
		if time.Since(p.ProbedAt) < 60*time.Second {
			return *p
		}
	}
	return Probe(ctx, engineOverride)
}

func resolveEngine(override string) string {
	if override != "" {
		if p, err := exec.LookPath(override); err == nil {
			return p
		}
		return ""
	}
	for _, name := range []string{"podman", "docker"} {
		if p, err := exec.LookPath(name); err == nil {
			return p
		}
	}
	return ""
}

func enginesTried(override string) []string {
	if override != "" {
		return []string{override}
	}
	return []string{"podman", "docker"}
}

func probeVersion(ctx context.Context, engine string) (string, error) {
	c := exec.CommandContext(ctx, engine, "--version")
	out, err := c.CombinedOutput()
	if err != nil {
		return "", err
	}
	line := strings.SplitN(strings.TrimSpace(string(out)), "\n", 2)[0]
	return line, nil
}