package attach

import (
	"io"
	"net/http"
	"strings"

	"gocloud.dev/blob"
)

const defaultStaticContentType = "application/octet-stream"

// StaticHandler serves GET prefix/<partition>/<filename> from bucket.
// filename is the original disk_name or a thumb_* sibling stored in the
// original's partition. The blob key is the validated 4-segment path;
// unvalidated request segments never reach NewReader (T-05-13).
func StaticHandler(bucket *blob.Bucket, prefix string) http.Handler {
	prefix = strings.TrimSuffix(prefix, "/")
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if r.Method != http.MethodGet && r.Method != http.MethodHead {
			http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
			return
		}
		if bucket == nil {
			http.NotFound(w, r)
			return
		}
		rel, ok := stripStaticPrefix(r.URL.Path, prefix)
		if !ok {
			http.NotFound(w, r)
			return
		}
		key, ok := parsePublicBlobPath(rel)
		if !ok {
			http.NotFound(w, r)
			return
		}
		reader, err := bucket.NewReader(r.Context(), key, nil)
		if err != nil {
			http.NotFound(w, r)
			return
		}
		defer reader.Close()
		ct := reader.ContentType()
		if ct == "" {
			ct = defaultStaticContentType
		}
		w.Header().Set("Content-Type", ct)
		if r.Method == http.MethodHead {
			return
		}
		_, _ = io.Copy(w, reader)
	})
}

func stripStaticPrefix(path, prefix string) (string, bool) {
	if prefix == "" {
		return strings.TrimPrefix(path, "/"), true
	}
	if path == prefix {
		return "", false
	}
	if strings.HasPrefix(path, prefix+"/") {
		return path[len(prefix)+1:], true
	}
	return "", false
}

// parsePublicBlobPath accepts exactly 3 partition groups plus a filename
// and returns that path as the blob key. Originals must live in
// PartitionDirectory(filename); thumb_* names are stored beside the
// original, so their partition is not derived from the thumb filename.
// Rejects "..", empty segments, extra slashes, and mismatched original
// partitions.
func parsePublicBlobPath(p string) (key string, ok bool) {
	if p == "" || strings.Contains(p, "\\") || strings.Contains(p, "..") || strings.Contains(p, "//") {
		return "", false
	}
	parts := strings.Split(p, "/")
	if len(parts) != 4 {
		return "", false
	}
	for _, part := range parts {
		if part == "" || part == "." || part == ".." {
			return "", false
		}
	}
	filename := parts[3]
	got := strings.Join(parts[:3], "/")
	if !strings.HasPrefix(filename, "thumb_") {
		want := strings.TrimSuffix(PartitionDirectory(filename), "/")
		if got != want {
			return "", false
		}
	}
	return got + "/" + filename, true
}