package hostcollect

import (
	"context"
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"sort"
	"strconv"
	"strings"
	"time"

	"github.com/itworx/pulse/internal/process"
)

// maxProcessCPUPercent matches the bound internal/process enforces. A busy multi-core
// task legitimately exceeds 100%.
const maxProcessCPUPercent = 10000

// processKey identifies a task across scans. The PID alone is not enough: PIDs are
// reused, and charging a new process with the CPU time of the dead one it replaced
// would show a fresh task at an absurd utilisation. The kernel's start time makes the
// identity stable.
type processKey struct {
	pid       int
	startTime uint64
}

type processCPUSample struct {
	at    time.Time
	ticks uint64
}

// processStat is the subset of /proc/<pid>/stat the inventory needs.
type processStat struct {
	pid       int
	comm      string
	state     string
	ticks     uint64
	startTime uint64
	rssPages  uint64
}

func (s processStat) key() processKey { return processKey{pid: s.pid, startTime: s.startTime} }

// parseProcessStat reads /proc/<pid>/stat.
//
// The second field is the executable name in parentheses and it is neither escaped nor
// quoted: a process can legitimately be called ") (" or "my app (2)". Splitting the
// line on whitespace therefore corrupts every field after it. The parser instead cuts
// at the LAST closing parenthesis, which is the only reliable delimiter.
func parseProcessStat(data []byte, pid int) (processStat, error) {
	text := string(data)
	open := strings.IndexByte(text, '(')
	closing := strings.LastIndexByte(text, ')')
	if open < 0 || closing < open {
		return processStat{}, errors.New("hostcollect: process stat has no comm field")
	}
	comm := text[open+1 : closing]
	fields := strings.Fields(text[closing+1:])
	// Field 3 (state) becomes index 0 here; the inventory needs up to field 24 (rss).
	const (
		stateIndex     = 0
		utimeIndex     = 11
		stimeIndex     = 12
		startTimeIndex = 19
		rssIndex       = 21
	)
	if len(fields) <= rssIndex {
		return processStat{}, errors.New("hostcollect: process stat is truncated")
	}
	utime, utimeOK := parseUint(fields[utimeIndex])
	stime, stimeOK := parseUint(fields[stimeIndex])
	startTime, startOK := parseUint(fields[startTimeIndex])
	if !utimeOK || !stimeOK || !startOK {
		return processStat{}, errors.New("hostcollect: process stat has malformed counters")
	}
	// rss is signed in the kernel's own printf but never negative in practice; a
	// negative or malformed value degrades to zero rather than failing the process.
	rss, _ := parseUint(fields[rssIndex])
	return processStat{
		pid:       pid,
		comm:      sanitizeName(comm),
		state:     processState(fields[stateIndex]),
		ticks:     utime + stime,
		startTime: startTime,
		rssPages:  rss,
	}, nil
}

// processState expands the kernel's single-letter state so the UI does not have to.
func processState(value string) string {
	if value == "" {
		return "unknown"
	}
	switch value[0] {
	case 'R':
		return "running"
	case 'S':
		return "sleeping"
	case 'D':
		return "uninterruptible"
	case 'Z':
		return "zombie"
	case 'T':
		return "stopped"
	case 't':
		return "tracing-stop"
	case 'X', 'x':
		return "dead"
	case 'I':
		return "idle"
	case 'K':
		return "wakekill"
	case 'W':
		return "waking"
	case 'P':
		return "parked"
	default:
		return "unknown"
	}
}

// Processes reads the process inventory.
//
// A process that exits between readdir and open is the normal case, not an error: the
// scan skips it and keeps going. Only a failure that invalidates the whole scan (no
// uptime, unreadable procfs root) returns an error, because a half-empty inventory
// published as complete would be worse than no inventory at all.
func (c *Collector) Processes(ctx context.Context) (process.RawSnapshot, error) {
	if c == nil {
		return process.RawSnapshot{}, errors.New("hostcollect: collector is nil")
	}
	if err := ctx.Err(); err != nil {
		return process.RawSnapshot{}, err
	}
	now := c.options.Now().UTC()
	uptimeSeconds, err := c.uptimeSeconds()
	if err != nil {
		return process.RawSnapshot{}, err
	}
	entries, err := os.ReadDir(c.options.ProcRoot)
	if err != nil {
		return process.RawSnapshot{}, fmt.Errorf("hostcollect: list processes: %w", err)
	}

	stats := make([]processStat, 0, len(entries))
	for _, entry := range entries {
		if err := ctx.Err(); err != nil {
			return process.RawSnapshot{}, err
		}
		pid, ok := processID(entry.Name())
		if !ok {
			continue
		}
		data, readErr := readLimited(c.procPath(entry.Name(), "stat"), maxProcessFileBytes)
		if readErr != nil {
			continue
		}
		stat, parseErr := parseProcessStat(data, pid)
		if parseErr != nil {
			continue
		}
		stats = append(stats, stat)
	}

	percentages := c.cpuPercentages(now, stats)
	rows := c.rank(stats, percentages)

	processes := make([]process.RawProcess, 0, len(rows))
	for _, stat := range rows {
		if err := ctx.Err(); err != nil {
			return process.RawSnapshot{}, err
		}
		row, ok := c.describe(stat, uptimeSeconds, percentages[stat.key()])
		if !ok {
			continue
		}
		processes = append(processes, row)
	}
	sort.Slice(processes, func(i, j int) bool { return processes[i].PID < processes[j].PID })

	return process.RawSnapshot{
		Source: process.Source{
			ID:                "process",
			Type:              "agent",
			CapabilityVersion: process.ContractVersion,
			ObservedAt:        now,
		},
		Processes:  processes,
		ObservedAt: now,
	}, nil
}

// cpuPercentages turns accumulated CPU ticks into a utilisation percentage relative to
// the previous scan and replaces the retained state with the current one. The map is
// rebuilt from the live process list every scan, so it cannot grow without bound as
// processes come and go.
func (c *Collector) cpuPercentages(now time.Time, stats []processStat) map[processKey]float64 {
	percentages := make(map[processKey]float64, len(stats))
	current := make(map[processKey]processCPUSample, len(stats))

	c.mu.Lock()
	previous := c.processes
	for _, stat := range stats {
		key := stat.key()
		current[key] = processCPUSample{at: now, ticks: stat.ticks}
		sample, seen := previous[key]
		if !seen {
			continue
		}
		elapsed := now.Sub(sample.at).Seconds()
		if elapsed <= 0 || stat.ticks < sample.ticks {
			continue
		}
		percent := float64(stat.ticks-sample.ticks) / c.options.ClockTicks / elapsed * 100
		if percent < 0 {
			percent = 0
		}
		if percent > maxProcessCPUPercent {
			percent = maxProcessCPUPercent
		}
		percentages[key] = percent
	}
	c.processes = current
	c.mu.Unlock()

	return percentages
}

// rank keeps the busiest processes when the inventory exceeds MaxRows. Truncating by
// cost rather than by PID order keeps the rows an operator actually needs.
func (c *Collector) rank(stats []processStat, percentages map[processKey]float64) []processStat {
	ordered := append([]processStat(nil), stats...)
	sort.SliceStable(ordered, func(i, j int) bool {
		left, right := percentages[ordered[i].key()], percentages[ordered[j].key()]
		if left != right {
			return left > right
		}
		if ordered[i].rssPages != ordered[j].rssPages {
			return ordered[i].rssPages > ordered[j].rssPages
		}
		return ordered[i].pid < ordered[j].pid
	})
	if len(ordered) > c.options.ProcessLimits.MaxRows {
		ordered = ordered[:c.options.ProcessLimits.MaxRows]
	}
	return ordered
}

// describe fills in the fields that need a second read, for the retained rows only.
//
// Only the program name is taken from the command line, never the arguments: argv
// routinely carries tokens, passwords and connection strings, and the snapshot is
// stored and rendered. The read itself is bounded, so an enormous argv costs one page,
// not a copy of the whole command line.
func (c *Collector) describe(stat processStat, uptimeSeconds, cpuPercent float64) (process.RawProcess, bool) {
	directory := strconv.Itoa(stat.pid)
	name := stat.comm
	if data, err := readLimited(c.procPath(directory, "cmdline"), maxCmdlineBytes); err == nil {
		if argv0 := commandName(data); argv0 != "" {
			name = argv0
		}
	}
	if name == "" {
		// Neither comm nor cmdline is usable — the task is gone or unreadable.
		return process.RawProcess{}, false
	}
	memoryBytes := stat.rssPages * uint64(c.options.PageSize) //nolint:gosec // PageSize is validated positive
	if rss, ok := c.residentBytes(directory); ok {
		memoryBytes = rss
	}
	runtimeSeconds := uptimeSeconds - float64(stat.startTime)/c.options.ClockTicks
	if runtimeSeconds < 0 || runtimeSeconds > maxUptimeSeconds {
		runtimeSeconds = 0
	}
	return process.RawProcess{
		PID:            stat.pid,
		Name:           truncate(name, maxNameBytes),
		State:          stat.state,
		RuntimeSeconds: runtimeSeconds,
		CPUPercent:     cpuPercent,
		MemoryBytes:    memoryBytes,
	}, true
}

// residentBytes prefers VmRSS from /proc/<pid>/status, which the kernel already
// reports in kB and keeps consistent, over the page count in stat. Kernel threads have
// no VmRSS at all, which is why the stat value remains the fallback.
func (c *Collector) residentBytes(directory string) (uint64, bool) {
	data, err := readLimited(c.procPath(directory, "status"), maxProcessFileBytes)
	if err != nil {
		return 0, false
	}
	for _, line := range lines(data) {
		key, rest, found := strings.Cut(line, ":")
		if !found || key != "VmRSS" {
			continue
		}
		fields := strings.Fields(rest)
		if len(fields) == 0 {
			return 0, false
		}
		value, ok := parseUint(fields[0])
		if !ok {
			return 0, false
		}
		if len(fields) > 1 && strings.EqualFold(fields[1], "kB") {
			const kib = 1024
			if value > (1<<64-1)/kib {
				return 0, false
			}
			value *= kib
		}
		return value, true
	}
	return 0, false
}

// commandName extracts the program name from a NUL separated command line.
func commandName(data []byte) string {
	text := string(data)
	if index := strings.IndexByte(text, 0); index >= 0 {
		text = text[:index]
	}
	text = strings.TrimSpace(text)
	if text == "" {
		return ""
	}
	return sanitizeName(filepath.Base(text))
}

// sanitizeName drops control characters so a hostile process name cannot inject
// terminal escapes or NUL bytes into logs and stored snapshots.
func sanitizeName(value string) string {
	cleaned := strings.Map(func(r rune) rune {
		if r < 0x20 || r == 0x7f {
			return -1
		}
		return r
	}, value)
	return truncate(strings.TrimSpace(cleaned), maxNameBytes)
}

func processID(name string) (int, bool) {
	pid, err := strconv.Atoi(name)
	if err != nil || pid < 1 {
		return 0, false
	}
	return pid, true
}