package host

import (
	"bufio"
	"context"
	"fmt"
	"os"
	"strconv"
	"strings"
	"sync"
	"time"

	"tower/internal/collectors"
	"tower/internal/model"
)

// MemCollector checks MemAvailable and swap pressure from /proc/meminfo.
//
// Thresholds (PLAN.md):
// Memory (MemAvailable as % of MemTotal):
//   - P2 if <= 15% sustained 60s
//   - P1 if <= 10% sustained 60s
//   - P0 if <=  5% sustained 30s
//
// Swap pressure (only if RAM is also tight):
//   - P1 if swap used >= 50% AND MemAvailable <= 10% sustained 60s
//   - P0 if swap used >= 80% AND MemAvailable <=  5% sustained 30s
//
// Emits up to two issues:
//   - host:mem:available
//   - host:mem:swap
//
// NOTE: Linux-specific.
// Thread-safe: Collect() can be called concurrently.
type MemCollector struct {
	interval time.Duration

	now      func() time.Time
	readFile func(string) ([]byte, error)

	mu sync.Mutex

	memPri   model.Priority
	memSince time.Time

	swapPri   model.Priority
	swapSince time.Time
}

func NewMemCollector() *MemCollector {
	return &MemCollector{
		interval: 5 * time.Second,
		now:      time.Now,
		readFile: os.ReadFile,
	}
}

func (c *MemCollector) Name() string { return "host:mem" }

func (c *MemCollector) Interval() time.Duration {
	if c.interval <= 0 {
		return 5 * time.Second
	}
	return c.interval
}

func (c *MemCollector) Collect(ctx context.Context) ([]model.Issue, collectors.Status, error) {
	if err := ctx.Err(); err != nil {
		return nil, collectors.Status{Health: collectors.HealthError, Message: "canceled"}, err
	}

	now := c.now()
	b, err := c.readFile("/proc/meminfo")
	if err != nil {
		return nil, collectors.Status{Health: collectors.HealthError, Message: "failed reading /proc/meminfo"}, err
	}

	mi := parseProcMeminfo(string(b))
	memTotalKB, okT := mi["MemTotal"]
	memAvailKB, okA := mi["MemAvailable"]
	if !okT || !okA || memTotalKB <= 0 {
		return nil, collectors.Status{Health: collectors.HealthDegraded, Message: "missing MemTotal/MemAvailable"}, nil
	}

	memAvailPct := (float64(memAvailKB) / float64(memTotalKB)) * 100.0

	desiredMemPri, memWindow := desiredMemPriority(memAvailPct)
	c.mu.Lock()
	c.memPri, c.memSince = updateSustained(now, c.memPri, c.memSince, desiredMemPri)
	memPri, memSince := c.memPri, c.memSince
	c.mu.Unlock()

	issues := make([]model.Issue, 0, 2)
	if memPri != "" && !memSince.IsZero() && now.Sub(memSince) >= memWindow {
		issues = append(issues, model.Issue{
			ID:       "host:mem:available",
			Category: model.CategoryMemory,
			Priority: memPri,
			Title:    "Low available memory",
			Details:  "MemAvailable is low and has remained low for a sustained period.",
			Evidence: map[string]string{
				"mem_available_kb":  strconv.FormatInt(memAvailKB, 10),
				"mem_total_kb":      strconv.FormatInt(memTotalKB, 10),
				"mem_available_pct": fmt.Sprintf("%.1f", memAvailPct),
			},
			SuggestedFix: "Identify memory hogs:\n  free -h\n  ps aux --sort=-rss | head\nConsider restarting runaway processes or adding RAM.",
		})
	}

	swapTotalKB, okST := mi["SwapTotal"]
	swapFreeKB, okSF := mi["SwapFree"]
	swapUsedPct := 0.0
	if okST && okSF && swapTotalKB > 0 {
		swapUsedKB := swapTotalKB - swapFreeKB
		swapUsedPct = (float64(swapUsedKB) / float64(swapTotalKB)) * 100.0
	}

	desiredSwapPri, swapWindow := desiredSwapPriority(memAvailPct, swapTotalKB, swapUsedPct)
	c.mu.Lock()
	c.swapPri, c.swapSince = updateSustained(now, c.swapPri, c.swapSince, desiredSwapPri)
	swapPri, swapSince := c.swapPri, c.swapSince
	c.mu.Unlock()
	if swapPri != "" && !swapSince.IsZero() && now.Sub(swapSince) >= swapWindow {
		issues = append(issues, model.Issue{
			ID:       "host:mem:swap",
			Category: model.CategoryMemory,
			Priority: swapPri,
			Title:    "High swap usage with low RAM",
			Details:  "Swap usage is high while available RAM is also low, indicating memory pressure.",
			Evidence: map[string]string{
				"swap_used_pct":     fmt.Sprintf("%.1f", swapUsedPct),
				"swap_total_kb":     strconv.FormatInt(swapTotalKB, 10),
				"mem_available_pct": fmt.Sprintf("%.1f", memAvailPct),
			},
			SuggestedFix: "Find swapping processes:\n  vmstat 1\n  smem -r 2>/dev/null || true\nConsider reducing memory usage or increasing RAM/swap.",
		})
	}

	return issues, collectors.OKStatus(), nil
}

func parseProcMeminfo(content string) map[string]int64 {
	out := map[string]int64{}
	s := bufio.NewScanner(strings.NewReader(content))
	for s.Scan() {
		line := strings.TrimSpace(s.Text())
		if line == "" {
			continue
		}
		// Example: "MemAvailable:  12345 kB"
		fields := strings.Fields(line)
		if len(fields) < 2 {
			continue
		}
		key := strings.TrimSuffix(fields[0], ":")
		v, err := strconv.ParseInt(fields[1], 10, 64)
		if err != nil {
			continue
		}
		out[key] = v
	}
	return out
}

func desiredMemPriority(memAvailPct float64) (model.Priority, time.Duration) {
	switch {
	case memAvailPct <= 5.0:
		return model.PriorityP0, 30 * time.Second
	case memAvailPct <= 10.0:
		return model.PriorityP1, 60 * time.Second
	case memAvailPct <= 15.0:
		return model.PriorityP2, 60 * time.Second
	default:
		return "", 0
	}
}

func desiredSwapPriority(memAvailPct float64, swapTotalKB int64, swapUsedPct float64) (model.Priority, time.Duration) {
	if swapTotalKB <= 0 {
		return "", 0
	}
	// Only alert on swap when RAM is also tight.
	switch {
	case swapUsedPct >= 80.0 && memAvailPct <= 5.0:
		return model.PriorityP0, 30 * time.Second
	case swapUsedPct >= 50.0 && memAvailPct <= 10.0:
		return model.PriorityP1, 60 * time.Second
	default:
		return "", 0
	}
}

// updateSustained updates current severity and its since timestamp.
// If desired is empty, it clears the state.
func updateSustained(now time.Time, current model.Priority, since time.Time, desired model.Priority) (model.Priority, time.Time) {
	if desired == "" {
		return "", time.Time{}
	}
	if current != desired || since.IsZero() {
		return desired, now
	}
	return current, since
}

var _ collectors.Collector = (*MemCollector)(nil)