package testsuite

import (
	"bytes"
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"strconv"
	"strings"
	"syscall"
	"unsafe"

	"hakurei.app/fhs"
)

// Stat represents status information read from /proc/pid/stat.
type Stat struct {
	// The process ID.
	PID int
	// The filename of the executable, with parenthesis stripped.
	Comm string
	// One of the following characters, indicating process state:
	//
	//	R      Running
	//
	//	S      Sleeping in an interruptible wait
	//
	//	D      Waiting in uninterruptible disk sleep
	//
	//	Z      Zombie
	//
	//	T      Stopped (on a signal) or (before Linux
	//	       2.6.33) trace stopped
	//
	//	t      Tracing stop (Linux 2.6.33 onward)
	//
	//	W      Paging (only before Linux 2.6.0)
	//
	//	X      Dead (from Linux 2.6.0 onward)
	//
	//	x      Dead (Linux 2.6.33 to 3.13 only)
	//
	//	K      Wakekill (Linux 2.6.33 to 3.13 only)
	//
	//	W      Waking (Linux 2.6.33 to 3.13 only)
	//
	//	P      Parked (Linux 3.9 to 3.13 only)
	//
	//	I      Idle (Linux 4.14 onward)
	State byte
	// The process ID of the parent of this process.
	PPID int
	// The process group ID of the process.
	PGRP int
	// The session ID of the process.
	Session int
	// The controlling terminal of the process.
	TTYNR int
	// The ID of the foreground process group of the controlling terminal of the
	// process.
	TPGID int
	// The kernel flags word of the process. For bit meanings, see the PF_*
	// defines in the Linux kernel source file include/linux/sched.h.
	Flags uint
	// The number of minor faults the process has made which have not required
	// loading a memory page from disk.
	MinFlt uint
	// The number of minor faults that the process's waited-for children have
	// made.
	CMinFlt uint
	// The number of major faults the process has made which have required
	// loading a memory page from disk.
	MajFlt uint
	// The number of major faults that the process's waited-for children have
	// made.
	CMajFlt uint
	// Amount of time that this process has been scheduled in user mode,
	// measured in clock ticks.
	UTime uint
	// Amount of time that this process has been scheduled in kernel mode,
	// measured in clock ticks.
	STime uint
	// Amount of time that this process's waited-for children have been
	// scheduled in user mode, measured in clock ticks.
	CUTime int
	// Amount of time that this process's waited-for children have been
	// scheduled in kernel mode, measured in clock ticks.
	CSTime int
	// For processes running a real-time scheduling policy, this is the negated
	// scheduling priority, minus one.
	Priority int
	// The nice value, a value in the range 19 (low priority) to -20 (high
	// priority).
	Nice int
	// Number of threads in this process.
	NumThreads int

	// unmaintained field: itrealvalue

	// The time the process started after system boot. Since Linux 2.6, the
	// value is expressed in clock ticks.
	StartTime uint64
	// Virtual memory size in bytes.
	VSize uint
	// Resident set size in pages.
	RSS int
	// Soft limit in bytes on the rss of the process.
	RSSLim uint64
	// The address above which program text can run.
	StartCode uint64
	// The address below which program text can run.
	EndCode uint64
	// The address of the start (i.e., bottom) of the stack.
	StartStack uint64
	// The current value of ESP (stack pointer), as found in the kernel stack
	// page for the process.
	KSTKESP uint64
	// The current EIP (instruction pointer).
	KSTKEIP uint64

	// obsolete fields: signal, blocked, sigignore, sigcatch

	// This is the "channel" in which the process is waiting. It is the address
	// of a location in the kernel where the process is sleeping.
	WChan uint64

	// unmaintained fields: nswap, cnswap

	// Signal to be sent to parent when we die.
	ExitSignal int
	// CPU number last executed on.
	Processor int
	// Real-time scheduling priority, a number in the range 1 to 99 for processes
	// scheduled under a real-time policy, or 0, for non-real-time processes.
	RTPriority uint
	// Scheduling policy (see sched_setscheduler(2)). Decode using the SCHED_*
	// constants in linux/sched.h.
	Policy uint
	// Aggregated block I/O delays, measured in clock ticks (centiseconds).
	DelayAcctBlkIOTicks uint64
	// Guest time of the process (time spent running a virtual CPU for a guest
	// operating system), measured in clock ticks.
	GuestTime int
	// Guest time of the process's children, measured in clock ticks.
	CGuestTime int
}

// Executable is like [os.Executable], but for the process referred to by s.
func (s *Stat) Executable() (string, error) {
	path, err := os.Readlink(filepath.Join(fhs.Proc, strconv.Itoa(s.PID), "exe"))

	// When the executable has been deleted then Readlink returns a
	// path appended with " (deleted)".
	return strings.TrimSuffix(path, " (deleted)"), err
}

// Stat populates stat with the proc filesystem entry referred to by s.
func (s *Stat) Stat(stat *syscall.Stat_t) (err error) {
	err = syscall.Stat(filepath.Join(fhs.Proc, strconv.Itoa(s.PID)), stat)
	if err != nil {
		err = os.NewSyscallError("stat", err)
	}
	return
}

// Args reads arguments of the process referred to by s.
func (s *Stat) Args() ([]string, error) {
	p, err := os.ReadFile(filepath.Join(fhs.Proc, strconv.Itoa(s.PID), "cmdline"))
	if err != nil {
		return nil, err
	}
	a := bytes.Split(p, []byte{0})
	if len(a) > 0 && len(a[len(a)-1]) == 0 {
		a = a[:len(a)-1]
	}

	args := make([]string, len(a))
	for i, arg := range a {
		args[i] = unsafe.String(unsafe.SliceData(arg), len(arg))
	}
	return args, nil
}

// ErrBadDelimiters is returned by [Stat.UnmarshalText] if one or both bytes of
// the comm delimiter pair were missing or misplaced.
var ErrBadDelimiters = errors.New("missing comm delimiters")

// UnmarshalText populates the structure pointed to by s from text.
func (s *Stat) UnmarshalText(text []byte) (err error) {
	var (
		discard uint64
		_uint64 = &discard
		_int64  = (*int64)(unsafe.Pointer(&discard))

		ld = bytes.Index(text, []byte("("))
		rd = bytes.LastIndex(text, []byte(")"))
	)

	if ld <= 0 || rd < 0 {
		return ErrBadDelimiters
	}

	if s.PID, err = strconv.Atoi(
		unsafe.String(unsafe.SliceData(text), ld-1),
	); err != nil {
		return
	}

	s.Comm = string(text[ld+1 : rd])

	var (
		n int

		state string
	)
	n, err = fmt.Fscan(
		bytes.NewBuffer(text[rd+2:]),
		&state,
		&s.PPID,
		&s.PGRP,
		&s.Session,
		&s.TTYNR,
		&s.TPGID,
		&s.Flags,
		&s.MinFlt,
		&s.CMinFlt,
		&s.MajFlt,
		&s.CMajFlt,
		&s.UTime,
		&s.STime,
		&s.CUTime,
		&s.CSTime,
		&s.Priority,
		&s.Nice,
		&s.NumThreads,
		_int64,
		&s.StartTime,
		&s.VSize,
		&s.RSS,
		&s.RSSLim,
		&s.StartCode,
		&s.EndCode,
		&s.StartStack,
		&s.KSTKESP,
		&s.KSTKEIP,
		_uint64,
		_uint64,
		_uint64,
		_uint64,
		&s.WChan,
		_uint64,
		_uint64,
		&s.ExitSignal,
		&s.Processor,
		&s.RTPriority,
		&s.Policy,
		&s.DelayAcctBlkIOTicks,
		&s.GuestTime,
		&s.CGuestTime,
	)
	if err != nil {
		err = fmt.Errorf("field %d: %w", n, err)
	} else if len(state) != 1 {
		err = fmt.Errorf("invalid state %q", state)
	} else {
		s.State = state[0]
	}
	return
}

// A StatScanner continuously scans the proc filesystem for process status
// information in /proc/pid/stat.
type StatScanner struct {
	// Current entry.
	stat Stat
	// Cached top-level /proc entries.
	dents []os.DirEntry
	// Current progress through dents.
	i int
	// Whether the previous call to Scan had repopulated dents.
	wrapped bool
	// First stored error: a non-nil err disables the scanner.
	err error
}

// IsNotExist returns whether an error is [os.ErrNotExist] or ESRCH.
func IsNotExist(err error) bool {
	return errors.Is(err, os.ErrNotExist) || errors.Is(err, syscall.ESRCH)
}

// Scan reads a process status information entry. It returns false if an
// unrecoverable error is encountered, after which Scan no longer scans new
// entries.
func (s *StatScanner) Scan() bool {
	if s.err != nil {
		return false
	}

	if s.wrapped = s.i == len(s.dents); s.wrapped {
		if s.dents, s.err = os.ReadDir(fhs.Proc); s.err != nil {
			return false
		}
		s.i = 0
		if len(s.dents) == 0 {
			s.err = syscall.ENOTRECOVERABLE
			return false
		}
	}

	for s.i < len(s.dents) {
		dent := s.dents[s.i]
		s.i++
		if !dent.IsDir() {
			continue
		}

		pid, err := strconv.Atoi(dent.Name())
		if err != nil {
			continue
		}

		var p []byte
		p, err = os.ReadFile(filepath.Join(fhs.Proc, dent.Name(), "stat"))
		if err != nil {
			if IsNotExist(err) {
				continue
			}
			s.err = err
			return false
		}

		s.err = s.stat.UnmarshalText(p)
		if s.err == nil && pid != s.stat.PID {
			s.err = fmt.Errorf(
				"bad status information: dent=%d, stat=%d",
				pid, s.stat.PID,
			)
		}
		return s.err == nil
	}
	return s.Scan()
}

// Stat returns the address of the [Stat] structure populated by the last call
// to Scan.
func (s *StatScanner) Stat() *Stat { return &s.stat }

// Err returns the stored error value.
func (s *StatScanner) Err() error { return s.err }

// Repopulated returns whether the last Scan call had re-read the proc filesystem.
func (s *StatScanner) Repopulated() bool { return s.wrapped }