package host

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

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

// DiskCollector checks filesystem block + inode pressure across mounts.
//
// It reads /proc/mounts to discover mounts and then uses statfs to compute usage.
// Pseudo filesystems are filtered out.
//
// Thresholds (PLAN.md):
//   - P1 if blocks OR inodes >= 92%
//   - P0 if blocks OR inodes >= 98%
//
// Issues are emitted per mount (one issue that includes both block+inode usage).
//
// NOTE: This collector is Linux-specific.
type DiskCollector struct {
	interval time.Duration

	readFile func(string) ([]byte, error)
	statfs   func(path string, st *syscall.Statfs_t) error
}

func NewDiskCollector() *DiskCollector {
	return &DiskCollector{
		interval: 10 * time.Second,
		readFile: os.ReadFile,
		statfs:   syscall.Statfs,
	}
}

func (c *DiskCollector) Name() string { return "host:disk" }

func (c *DiskCollector) Interval() time.Duration {
	if c.interval <= 0 {
		return 10 * time.Second
	}
	return c.interval
}

func (c *DiskCollector) 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
	}

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

	mounts := parseProcMounts(string(b))
	if len(mounts) == 0 {
		// Unusual but treat as degraded rather than hard error.
		return nil, collectors.Status{Health: collectors.HealthDegraded, Message: "no mounts found"}, nil
	}

	issues := make([]model.Issue, 0, 8)
	seenMount := map[string]struct{}{}

	partialErrs := 0
	for _, m := range mounts {
		if err := ctx.Err(); err != nil {
			return issues, collectors.Status{Health: collectors.HealthError, Message: "canceled"}, err
		}
		if shouldSkipMount(m) {
			continue
		}
		if _, ok := seenMount[m.MountPoint]; ok {
			continue
		}
		seenMount[m.MountPoint] = struct{}{}

		var st syscall.Statfs_t
		if err := c.statfs(m.MountPoint, &st); err != nil {
			partialErrs++
			continue
		}

		blockPct, blockFreeBytes := statfsBlockUsedPct(st)
		inodePct := statfsInodeUsedPct(st)

		pri, ok := diskPriority(blockPct, inodePct)
		if !ok {
			continue
		}

		evidence := map[string]string{
			"mount":            m.MountPoint,
			"fstype":           m.FSType,
			"block_used_pct":   fmt.Sprintf("%.1f", blockPct),
			"block_free_bytes": strconv.FormatUint(blockFreeBytes, 10),
		}
		if inodePct >= 0 {
			evidence["inode_used_pct"] = fmt.Sprintf("%.1f", inodePct)
		}

		issues = append(issues, model.Issue{
			ID:       fmt.Sprintf("host:disk:%s:usage", m.MountPoint),
			Category: model.CategoryStorage,
			Priority: pri,
			Title:    fmt.Sprintf("Disk usage high on %s", m.MountPoint),
			Details:  "Filesystem space and/or inodes are nearly exhausted.",
			Evidence: evidence,
			SuggestedFix: fmt.Sprintf(
				"Inspect usage:\n  df -h %s\n  df -i %s\nFind large directories:\n  sudo du -xh --max-depth=2 %s | sort -h | tail",
				m.MountPoint, m.MountPoint, m.MountPoint,
			),
		})
	}

	st := collectors.OKStatus()
	if partialErrs > 0 {
		st.Health = collectors.HealthDegraded
		st.Message = fmt.Sprintf("partial failures: %d mounts", partialErrs)
	}
	return issues, st, nil
}

type procMount struct {
	Device     string
	MountPoint string
	FSType     string
	Options    string
}

func parseProcMounts(content string) []procMount {
	s := bufio.NewScanner(strings.NewReader(content))
	out := make([]procMount, 0, 32)
	for s.Scan() {
		line := strings.TrimSpace(s.Text())
		if line == "" {
			continue
		}
		fields := strings.Fields(line)
		if len(fields) < 3 {
			continue
		}
		m := procMount{
			Device:     unescapeProcMountsField(fields[0]),
			MountPoint: unescapeProcMountsField(fields[1]),
			FSType:     fields[2],
		}
		if len(fields) >= 4 {
			m.Options = fields[3]
		}
		out = append(out, m)
	}
	return out
}

// /proc/mounts escapes special characters as octal sequences.
// The most common one is a space as \040.
func unescapeProcMountsField(s string) string {
	replacer := strings.NewReplacer(
		"\\040", " ",
		"\\011", "\t",
		"\\012", "\n",
		"\\134", "\\",
	)
	return replacer.Replace(s)
}

var pseudoFSTypes = map[string]struct{}{
	"proc":        {},
	"sysfs":       {},
	"tmpfs":       {},
	"devtmpfs":    {},
	"devpts":      {},
	"cgroup":      {},
	"cgroup2":     {},
	"pstore":      {},
	"securityfs":  {},
	"debugfs":     {},
	"tracefs":     {},
	"configfs":    {},
	"hugetlbfs":   {},
	"mqueue":      {},
	"rpc_pipefs":  {},
	"fusectl":     {},
	"binfmt_misc": {},
	"autofs":      {},
	"bpf":         {},
	"ramfs":       {},
	"nsfs":        {},
	"efivarfs":    {},
	"overlay":     {}, // common container overlay mounts

	"squashfs":  {}, // typically read-only images
	"selinuxfs": {},
	"systemd-1": {},
	"overlayfs": {}, // (non-standard) conservative skip

	"cgroupfs":        {},
	"procfs":          {},
	"fuse.lxcfs":      {},
	"fuse.gvfsd-fuse": {},
}

func shouldSkipMount(m procMount) bool {
	if m.MountPoint == "" {
		return true
	}
	// Filter by fstype.
	if _, ok := pseudoFSTypes[m.FSType]; ok {
		return true
	}
	// Filter common pseudo mountpoints.
	if strings.HasPrefix(m.MountPoint, "/proc") || strings.HasPrefix(m.MountPoint, "/sys") {
		return true
	}
	if strings.HasPrefix(m.MountPoint, "/dev") {
		// /dev itself can be a real mount in some cases, but usually isn't useful for disk pressure.
		return true
	}
	return false
}

func statfsBlockUsedPct(st syscall.Statfs_t) (usedPct float64, freeBytes uint64) {
	// Mirror df(1) semantics closely:
	// total = f_blocks
	// used  = f_blocks - f_bfree
	// avail = f_bavail (space available to unprivileged user)
	// use%  = used / (used + avail)
	if st.Blocks == 0 {
		return 0, 0
	}

	bsize := uint64(st.Bsize)
	blocks := uint64(st.Blocks)
	bfree := uint64(st.Bfree)
	bavail := uint64(st.Bavail)

	usedBlocks := blocks - bfree
	denom := usedBlocks + bavail
	if denom == 0 {
		return 0, 0
	}

	freeBytes = bavail * bsize
	usedPct = (float64(usedBlocks) / float64(denom)) * 100.0
	return usedPct, freeBytes
}

// statfsInodeUsedPct returns inode used percent. If inodes are unavailable (f_files==0), returns -1.
func statfsInodeUsedPct(st syscall.Statfs_t) float64 {
	if st.Files == 0 {
		return -1
	}
	total := float64(st.Files)
	free := float64(st.Ffree)
	used := total - free
	return (used / total) * 100.0
}

func diskPriority(blockPct, inodePct float64) (model.Priority, bool) {
	maxPct := blockPct
	if inodePct > maxPct {
		maxPct = inodePct
	}
	// inodePct may be -1 if not supported; ignore in that case.
	if inodePct < 0 {
		maxPct = blockPct
	}

	switch {
	case maxPct >= 98.0:
		return model.PriorityP0, true
	case maxPct >= 92.0:
		return model.PriorityP1, true
	default:
		return "", false
	}
}

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