package agentguard

import (
	"context"
	"crypto/hmac"
	"crypto/rand"
	"crypto/sha256"
	"encoding/json"
	"os"
	"sync"
)

var (
	permitSealOnce sync.Once
	permitSealKey  [32]byte
	permitSealErr  error
)

type permitPayload struct {
	Workspace CanonicalWorkspace
	Profile   ProviderProfile
}

// Permit is an opaque, process-local proof of a successful admission. Its
// fields intentionally cannot be constructed or modified by callers.
type Permit struct {
	payload permitPayload
	seal    []byte
	pins    []canonicalPath
}

// Workspace returns a defensive copy of the canonical task view pinned by the
// permit.
func (p *Permit) Workspace() (CanonicalWorkspace, bool) {
	if p == nil || len(p.seal) == 0 {
		return CanonicalWorkspace{}, false
	}
	return cloneWorkspace(p.payload.Workspace), true
}

// Admit validates and seals the current request.
func Admit(req AdmissionRequest) AdmissionResult {
	evaluated, blocked := evaluateAdmission(req)
	if blocked.Blocker != nil {
		return blocked
	}
	seal, err := sealPayload(evaluated.payload)
	if err != nil {
		return blockedResult(
			req, BlockerCodeInvalidAdmissionRequest,
			"admission permit could not be sealed",
		)
	}
	permit := &Permit{
		payload: evaluated.payload,
		seal:    seal,
		pins:    append([]canonicalPath(nil), evaluated.pins...),
	}
	workspace := cloneWorkspace(evaluated.workspace)
	return AdmissionResult{
		Status:    AdmissionStatusPermitted,
		Workspace: &workspace,
		Permit:    permit,
	}
}

// ValidatePermit re-evaluates all current revisions and filesystem identities
// before invocation.
func ValidatePermit(permit *Permit, req AdmissionRequest) AdmissionResult {
	if permit == nil || len(permit.seal) == 0 {
		return blockedResult(
			req, BlockerCodePermitInvalid,
			"provider invocation requires a valid admission permit",
		)
	}
	evaluated, blocked := evaluateAdmission(req)
	if blocked.Blocker != nil {
		return blocked
	}
	currentSeal, err := sealPayload(evaluated.payload)
	if err != nil {
		return blockedResult(
			req, BlockerCodePermitInvalid,
			"admission permit could not be validated",
		)
	}
	if !hmac.Equal(permit.seal, currentSeal) {
		return blockedResult(
			req, BlockerCodePermitStale,
			"admission permit does not match the current immutable revisions",
		)
	}
	for _, pin := range permit.pins {
		current, err := os.Stat(pin.path)
		if err != nil || !os.SameFile(pin.info, current) {
			return blockedResult(
				req, BlockerCodeWorkspaceIdentityMismatch,
				"workspace identity changed after admission",
			)
		}
	}
	workspace := cloneWorkspace(evaluated.workspace)
	return AdmissionResult{
		Status:    AdmissionStatusPermitted,
		Workspace: &workspace,
		Permit:    permit,
	}
}

// Invoke is the mandatory zero-side-effect boundary for unattended provider
// execution. The callback is never called when permit validation blocks.
func Invoke(
	ctx context.Context,
	permit *Permit,
	req AdmissionRequest,
	invoke func(context.Context, CanonicalWorkspace) error,
) (AdmissionResult, error) {
	result := ValidatePermit(permit, req)
	if !result.Allowed() {
		return result, nil
	}
	if invoke == nil {
		return blockedResult(
			req, BlockerCodeInvalidAdmissionRequest,
			"provider invocation callback is missing",
		), nil
	}
	return result, invoke(ctx, cloneWorkspace(*result.Workspace))
}

func sealPayload(payload permitPayload) ([]byte, error) {
	permitSealOnce.Do(func() {
		_, permitSealErr = rand.Read(permitSealKey[:])
	})
	if permitSealErr != nil {
		return nil, permitSealErr
	}
	encoded, err := json.Marshal(payload)
	if err != nil {
		return nil, err
	}
	mac := hmac.New(sha256.New, permitSealKey[:])
	_, _ = mac.Write(encoded)
	return mac.Sum(nil), nil
}

func cloneWorkspace(workspace CanonicalWorkspace) CanonicalWorkspace {
	workspace.WritableRoots = append([]string(nil), workspace.WritableRoots...)
	workspace.VCSMetadataRoots = append([]string(nil), workspace.VCSMetadataRoots...)
	return workspace
}