// file_get fetches a previously-saved file by its opaque file_id and
// returns the metadata + base64-encoded bytes.
//
// Why scope is checked POST-fetch: file_id is the only key the caller
// knows; the scope (and therefore the authorisation envelope) is
// stored on the FileMeta row. We must read the row first to know which
// scope to validate. The trade-off is that file_id existence is
// observable (a foreign caller can probe IDs and learn that one
// exists), but the bytes themselves are still gated. file_id is a UUID,
// so the probe surface is impractical.
//
// Why base64 in the response: same reason as file_save — JSON can't
// carry arbitrary bytes natively. Callers that want a paste link or a
// direct download go through a separate path.
package tools

import (
	"context"
	"encoding/base64"
	"encoding/json"
	"errors"
	"fmt"
	"time"

	"gitea.stevedudenhoeffer.com/steve/executus/tool"
)

type fileGetArgs struct {
	FileID string `json:"file_id" description:"Opaque file ID returned by file_save or file_list."`
}

type fileGetResult struct {
	Name          string `json:"name"`
	ContentBase64 string `json:"content_base64"`
	Mime          string `json:"mime"`
	SizeBytes     int64  `json:"size_bytes"`
	CreatedAt     string `json:"created_at"` // RFC3339
}

// NewFileGet constructs the file_get tool. storage nil → "not
// configured" at execute time.
func NewFileGet(storage FileStorage) tool.Tool {
	return tool.NewGatedTool[fileGetArgs](
		"file_get",
		"Fetch a saved file by its file_id. Returns name, base64 content, MIME, size, and created_at. The caller must have access to the file's scope (skill / own user: / own run:).",
		tool.Permission{
			AuthoringRequirement: tool.RequirementAnyone,
			OperatesOn:           tool.ScopeCaller,
			SafeForShare:         true,
			Categories:           []string{"storage", "read"},
		},
		func(ctx context.Context, inv tool.Invocation, args fileGetArgs) (string, error) {
			if storage == nil {
				return "", fmt.Errorf("file_get: not configured")
			}
			if args.FileID == "" {
				return "", fmt.Errorf("file_get: file_id required")
			}

			meta, content, err := storage.FileGet(ctx, args.FileID)
			if err != nil {
				if errors.Is(err, ErrFileNotFound) {
					return "", fmt.Errorf("file_get: not found")
				}
				return "", fmt.Errorf("file_get: %w", err)
			}

			// Cross-skill access check: a file's SkillID must match the
			// current invocation's SkillID. Without this, a caller
			// could probe another skill's file_ids and read content.
			// One exception — the descendant grant (see
			// file_descendant_grant.go): workers this run dispatched.
			grantedViaDescendant := false
			if meta.SkillID != inv.SkillID {
				if !descendantFileGrant(ctx, storage, inv, meta.SkillID) {
					return "", fmt.Errorf("file_get: file does not belong to this skill")
				}
				grantedViaDescendant = true
			}

			// Scope check: even within the same skill, the scope on the
			// row gates access (e.g. user:bob's file is unreadable by
			// alice). The descendant grant stands in for it — the file's
			// scope is the WORKER's run, never the caller's.
			if err := ValidateScope(inv, meta.Scope, inv.CallerIsAdmin); err != nil && !grantedViaDescendant {
				return "", fmt.Errorf("file_get: %w", err)
			}

			res := fileGetResult{
				Name:          meta.Name,
				ContentBase64: base64.StdEncoding.EncodeToString(content),
				Mime:          meta.MimeType,
				SizeBytes:     meta.SizeBytes,
				CreatedAt:     meta.CreatedAt.UTC().Format(time.RFC3339),
			}
			b, err := json.Marshal(res)
			if err != nil {
				return "", fmt.Errorf("file_get: marshal: %w", err)
			}
			return string(b), nil
		},
	)
}