package ect

import (
	"errors"
	"fmt"
)

// DefaultClockSkewTolerance is the recommended clock skew between agents (Section 6.2).
const DefaultClockSkewTolerance = 30 // seconds

// DefaultMaxAncestorLimit is the recommended max ancestor traversal for cycle detection (Section 6.3).
const DefaultMaxAncestorLimit = 10000

// ECTStore provides lookup of ECTs by task ID (and optionally workflow ID) for DAG validation.
// Implemented by ledger or in-memory cache of verified parent ECTs.
type ECTStore interface {
	// GetByTid returns the payload for the given task ID, or nil if not found.
	GetByTid(tid string) *Payload
	// Contains returns true if (tid, wid) already exists. wid may be empty for global scope.
	Contains(tid, wid string) bool
}

// DAGConfig holds parameters for DAG validation.
type DAGConfig struct {
	ClockSkewTolerance int // seconds; recommended 30
	MaxAncestorLimit   int // recommended 10000
}

// DefaultDAGConfig returns recommended defaults.
func DefaultDAGConfig() DAGConfig {
	return DAGConfig{
		ClockSkewTolerance: DefaultClockSkewTolerance,
		MaxAncestorLimit:   DefaultMaxAncestorLimit,
	}
}

// ValidateDAG runs Section 6.2 validation rules: uniqueness, parent existence,
// temporal ordering, acyclicity, parent policy decision.
func ValidateDAG(ect *Payload, store ECTStore, cfg DAGConfig) error {
	if store == nil {
		return errors.New("ect: ECTStore required for DAG validation")
	}
	if cfg.ClockSkewTolerance <= 0 {
		cfg.ClockSkewTolerance = DefaultClockSkewTolerance
	}
	if cfg.MaxAncestorLimit <= 0 {
		cfg.MaxAncestorLimit = DefaultMaxAncestorLimit
	}

	// 1. Task ID Uniqueness
	if store.Contains(ect.Tid, ect.Wid) {
		return fmt.Errorf("ect: task ID already exists: %s", ect.Tid)
	}

	// 2. Parent Existence and 3. Temporal Ordering
	for _, parentID := range ect.Par {
		parent := store.GetByTid(parentID)
		if parent == nil {
			return fmt.Errorf("ect: parent task not found: %s", parentID)
		}
		// parent.iat < child.iat + clock_skew_tolerance  =>  parent.iat - ect.iat <= clock_skew_tolerance
		if parent.Iat >= ect.Iat+int64(cfg.ClockSkewTolerance) {
			return fmt.Errorf("ect: parent task not earlier than current: %s", parentID)
		}
	}

	// 4. Acyclicity (and depth limit)
	visited := make(map[string]struct{})
	if hasCycle(ect.Tid, ect.Par, store, visited, cfg.MaxAncestorLimit) {
		return errors.New("ect: circular dependency or depth limit exceeded")
	}

	// 5. Parent Policy Decision
	for _, parentID := range ect.Par {
		parent := store.GetByTid(parentID)
		if parent.PolDecision == PolDecisionRejected || parent.PolDecision == PolDecisionPendingHumanReview {
			if !ect.CompensationRequired {
				return errors.New("ect: parent has non-approved pol_decision; current ECT must be compensation/remediation or have compensation_required true")
			}
		}
	}

	return nil
}

// hasCycle returns true if following par from the given parent IDs leads back to targetTid
// or if traversal exceeds maxDepth. visited is mutated.
func hasCycle(targetTid string, parentIDs []string, store ECTStore, visited map[string]struct{}, maxDepth int) bool {
	if len(visited) >= maxDepth {
		return true
	}
	for _, parentID := range parentIDs {
		if parentID == targetTid {
			return true
		}
		if _, ok := visited[parentID]; ok {
			continue
		}
		visited[parentID] = struct{}{}
		parent := store.GetByTid(parentID)
		if parent != nil {
			if hasCycle(targetTid, parent.Par, store, visited, maxDepth) {
				return true
			}
		}
	}
	return false
}