/*
Volt Cluster — Workload Scheduler.

Implements scheduling strategies for assigning workloads to cluster nodes.
The scheduler considers:
  - Resource availability (CPU, memory, disk)
  - Label selectors and affinity rules
  - Node health status
  - Current workload distribution (spread/pack strategies)

Strategies:
  - BinPack: Pack workloads onto fewest nodes (maximize density)
  - Spread:  Distribute evenly across nodes (maximize availability)
  - Manual:  Explicit node selection by name/label

Copyright (c) Armored Gates LLC. All rights reserved.
AGPSL v5 — Source-available. Anti-competition clauses apply.
*/
package cluster

import (
	"fmt"
	"sort"
)

// ── Strategy ─────────────────────────────────────────────────────────────────

// ScheduleStrategy defines how workloads are assigned to nodes.
type ScheduleStrategy string

const (
	StrategyBinPack ScheduleStrategy = "binpack"
	StrategySpread  ScheduleStrategy = "spread"
	StrategyManual  ScheduleStrategy = "manual"
)

// ── Scheduler ────────────────────────────────────────────────────────────────

// Scheduler assigns workloads to nodes based on a configurable strategy.
type Scheduler struct {
	strategy ScheduleStrategy
}

// NewScheduler creates a scheduler with the given strategy.
func NewScheduler(strategy ScheduleStrategy) *Scheduler {
	if strategy == "" {
		strategy = StrategyBinPack
	}
	return &Scheduler{strategy: strategy}
}

// SelectNode chooses the best node for a workload based on the current strategy.
// Returns the selected NodeInfo or an error if no suitable node exists.
func (s *Scheduler) SelectNode(
	nodes []*NodeInfo,
	required WorkloadResources,
	selector map[string]string,
	existingSchedule []*ScheduledWorkload,
) (*NodeInfo, error) {

	// Filter to eligible nodes
	eligible := s.filterEligible(nodes, required, selector)
	if len(eligible) == 0 {
		return nil, fmt.Errorf("no eligible nodes: checked %d nodes, none meet resource/label requirements", len(nodes))
	}

	switch s.strategy {
	case StrategySpread:
		return s.selectSpread(eligible, existingSchedule), nil
	case StrategyBinPack:
		return s.selectBinPack(eligible), nil
	case StrategyManual:
		// Manual strategy returns the first eligible node matching the selector
		return eligible[0], nil
	default:
		return s.selectBinPack(eligible), nil
	}
}

// filterEligible returns nodes that are healthy, match labels, and have sufficient resources.
func (s *Scheduler) filterEligible(nodes []*NodeInfo, required WorkloadResources, selector map[string]string) []*NodeInfo {
	var eligible []*NodeInfo

	for _, node := range nodes {
		// Must be ready
		if node.Status != NodeStatusReady {
			continue
		}

		// Must match label selector
		if !matchLabels(node.Labels, selector) {
			continue
		}

		// Must have sufficient resources
		availMem := node.Resources.MemoryTotalMB - node.Resources.MemoryUsedMB
		if required.MemoryMB > 0 && availMem < required.MemoryMB {
			continue
		}

		// CPU check (basic — just core count)
		if required.CPUCores > 0 && node.Resources.CPUCores < required.CPUCores {
			continue
		}

		// Disk check
		availDisk := (node.Resources.DiskTotalGB - node.Resources.DiskUsedGB) * 1024 // convert to MB
		if required.DiskMB > 0 && availDisk < required.DiskMB {
			continue
		}

		eligible = append(eligible, node)
	}

	return eligible
}

// selectBinPack picks the node with the LEAST available memory (pack tight).
func (s *Scheduler) selectBinPack(nodes []*NodeInfo) *NodeInfo {
	sort.Slice(nodes, func(i, j int) bool {
		availI := nodes[i].Resources.MemoryTotalMB - nodes[i].Resources.MemoryUsedMB
		availJ := nodes[j].Resources.MemoryTotalMB - nodes[j].Resources.MemoryUsedMB
		return availI < availJ // least available first
	})
	return nodes[0]
}

// selectSpread picks the node with the fewest currently scheduled workloads.
func (s *Scheduler) selectSpread(nodes []*NodeInfo, schedule []*ScheduledWorkload) *NodeInfo {
	// Count workloads per node
	counts := make(map[string]int)
	for _, sw := range schedule {
		if sw.Status == "running" || sw.Status == "pending" {
			counts[sw.NodeID]++
		}
	}

	// Sort by workload count (ascending)
	sort.Slice(nodes, func(i, j int) bool {
		return counts[nodes[i].NodeID] < counts[nodes[j].NodeID]
	})

	return nodes[0]
}

// ── Scoring (for future extensibility) ───────────────────────────────────────

// NodeScore represents a scored node for scheduling decisions.
type NodeScore struct {
	Node  *NodeInfo
	Score float64
}

// ScoreNodes evaluates and ranks all eligible nodes for a workload.
// Higher scores are better.
func ScoreNodes(nodes []*NodeInfo, required WorkloadResources) []NodeScore {
	var scores []NodeScore

	for _, node := range nodes {
		if node.Status != NodeStatusReady {
			continue
		}

		score := 0.0

		// Resource availability score (0-50 points)
		if node.Resources.MemoryTotalMB > 0 {
			memPct := float64(node.Resources.MemoryTotalMB-node.Resources.MemoryUsedMB) / float64(node.Resources.MemoryTotalMB)
			score += memPct * 50
		}

		// CPU headroom score (0-25 points)
		if node.Resources.CPUCores > required.CPUCores {
			score += 25
		}

		// Health score (0-25 points)
		if node.MissedBeats == 0 {
			score += 25
		} else {
			score += float64(25-node.MissedBeats*5)
			if score < 0 {
				score = 0
			}
		}

		scores = append(scores, NodeScore{Node: node, Score: score})
	}

	sort.Slice(scores, func(i, j int) bool {
		return scores[i].Score > scores[j].Score
	})

	return scores
}