// Package edgeconfig is the seam for publish-time generated nginx config.
//
// Producers (auth gating, grain routes, redirects, …) implement Fragment;
// a Writer renders fragments atomically to the shared nginx include
// directory. A Reloader fans the change out to every running nginx
// instance, whose reload handler validates (nginx -t) before reloading.
//
// Risk model: only files inside the include directory ever change — the
// main nginx config is static and never touched at runtime. A broken
// fragment fails `nginx -t` and the previous config keeps serving.
//
// Dependency rule: imports only stdlib and other platform packages.
package edgeconfig

import (
	"context"
	"errors"
	"fmt"
	"hash/adler32"
	"os"
	"path/filepath"
	"sort"
	"strconv"
	"strings"
)

// Fragment is one named piece of generated nginx config owned by exactly
// one producer. Name determines include order: it must start with a number
// prefix ("10-auth", "20-routes"); the writer sorts by name.
type Fragment interface {
	Name() string
	Render() ([]byte, error)
}

// Reloader fans a config change out to all nginx instances. Providers are
// selected per environment: kube in-cluster, http for compose/dev, null for
// tests. Implementations must be safe for concurrent use.
type Reloader interface {
	Reload(ctx context.Context) Result
}

// TargetResult is the outcome for one nginx instance.
type TargetResult struct {
	Target string // host:port or pod identifier
	Err    error  // nil on success
}

// Result aggregates per-target outcomes of one Reload call.
type Result struct {
	Targets []TargetResult
}

// Errs returns the non-nil errors, or nil if every target succeeded.
func (r Result) Errs() []error {
	var errs []error
	for _, t := range r.Targets {
		if t.Err != nil {
			errs = append(errs, fmt.Errorf("%s: %w", t.Target, t.Err))
		}
	}
	return errs
}

// OK reports whether every target reloaded successfully.
func (r Result) OK() bool { return len(r.Errs()) == 0 }

// Writer renders fragments into dir (the nginx include directory mounted
// from the RWX volume). Every file write is atomic: tmp file + rename, so
// a reader (nginx reload handler) never sees a partial fragment.
type Writer struct {
	dir string
}

// NewWriter returns a Writer that renders fragments into dir.
func NewWriter(dir string) *Writer { return &Writer{dir: dir} }

// Dir is the include directory fragments are rendered into.
func (w *Writer) Dir() string { return w.dir }

// Write renders each fragment atomically and reports which names changed
// since the last known content. Fragments whose Render returns an error
// abort the whole flush: nothing is written, so nginx keeps serving the
// previous complete set. The returned names are sorted.
func (w *Writer) Write(fragments ...Fragment) (changed []string, err error) {
	type rendered struct {
		frag Fragment
		data []byte
	}
	rs := make([]rendered, 0, len(fragments))
	for _, f := range fragments {
		data, err := f.Render()
		if err != nil {
			return nil, fmt.Errorf("edgeconfig: render %s: %w", f.Name(), err)
		}
		rs = append(rs, rendered{f, data})
	}
	sort.Slice(rs, func(i, j int) bool { return rs[i].frag.Name() < rs[j].frag.Name() })

	for _, r := range rs {
		if err := w.writeAtomic(r.frag.Name(), r.data); err != nil {
			return nil, fmt.Errorf("edgeconfig: write %s: %w", r.frag.Name(), err)
		}
	}
	for _, r := range rs {
		changed = append(changed, r.frag.Name())
	}
	return changed, nil
}

// writeAtomic writes data to name via a tmp file + rename in dir.
func (w *Writer) writeAtomic(name string, data []byte) error {
	if err := validName(name); err != nil {
		return err
	}
	final := filepath.Join(w.dir, name)
	tmp, err := os.CreateTemp(w.dir, ".edgeconfig-*")
	if err != nil {
		return err
	}
	tmpName := tmp.Name()
	defer os.Remove(tmpName) // no-op after successful rename
	if _, err := tmp.Write(data); err != nil {
		tmp.Close()
		return err
	}
	if err := tmp.Sync(); err != nil {
		tmp.Close()
		return err
	}
	if err := tmp.Close(); err != nil {
		return err
	}
	return os.Rename(tmpName, final)
}

func validName(name string) error {
	if name == "" || strings.ContainsAny(name, "/\\") || strings.Contains(name, "..") ||
		filepath.Base(name) != name {
		return errors.New("invalid fragment name " + strconv.Quote(name))
	}
	return nil
}

// Checksum is the content fingerprint the reload handler compares against
// (adler32 — cheap, stable across processes; matches nothing security-
// sensitive, this is change detection only).
func Checksum(data []byte) uint32 { return adler32.Checksum(data) }