/*   SHS: Syntactically Homogeneous Shell
 *   Copyright (C) 2019  Aidan Hahn
 *
 *   This program is free software: you can redistribute it and/or modify
 *   it under the terms of the GNU General Public License as published by
 *   the Free Software Foundation, either version 3 of the License, or
 *   (at your option) any later version.
 *
 *   This program is distributed in the hope that it will be useful,
 *   but WITHOUT ANY WARRANTY; without even the implied warranty of
 *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *   GNU General Public License for more details.
 *
 *   You should have received a copy of the GNU General Public License
 *   along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */

package stdlib

import (
    "os"
    "errors"
    "os/exec"
    "context"
    "syscall"
    "golang.org/x/sys/unix"
    "gitlab.com/whom/shs/log"
)

func tcsetpgrp(fd int, pgrp int) error {
    return unix.IoctlSetPointerInt(fd, unix.TIOCSPGRP, pgrp)
}

var sigChan chan os.Signal
var waitChan chan error

/* mapping of pid to Proc
 */
var JobMap = map[int]Proc{}

/* id of shell process group
 */
var pgid int

/* Holds an os/exec Cmd object and its context cancel function
 */
type Proc struct {
    Ctl     *exec.Cmd
    Cancel  context.CancelFunc
}

var catchMe = []os.Signal{
    syscall.SIGINT,
}

/* Exit code of last run process
 */
var LastExitCode int

// Job control handlers
func signalHandler() {
    for {
        sig := <- sigChan
        switch sig {
            case syscall.SIGINT:
                log.Log(log.DEBUG,
                    "caught SIGINT!",
                    "jobctl")
        }
    }
}

func waitHandler() {
    for {
        w := <- waitChan
        exit := 0
        if w != nil {
            log.Log(log.ERR,
                "Child returned error: " + w.Error(),
                "jobctl")

            // something outrageous
            exit = -1024
            var e *exec.ExitError
            if errors.As(w, &e) {
                exit = e.Pid()
            }

            LastExitCode = exit
        }
    }
}

/* Sets pgid
 * Installs a sigchld handler
 * returns true if success, false on error
 */
func InitShellFeatures() bool {
    // TODO: adjust, make configurable, i dunno
    sigChan = make(chan os.Signal, 5)
    waitChan = make(chan error, 5)

    go signalHandler()
    go waitHandler()

    pid := os.Getpid()
    pgid = syscall.Getpgrp()
    if pid != pgid {
        syscall.Setpgid(0, 0)
    }
    tcsetpgrp(0, pid)

    return true
}

/* Tears down session
 */
func TeardownShell() {
    close(sigChan)
    close(waitChan)
    // TODO: Exit all processes in the JobMap
}

/* Makes and stores a new process in the job control
 * uses os/exec Cmd object. sets SysProcAttr.Foreground
 * calls Cmd.Start()
 */
func LaunchProcess(path string, args []string, background bool) {
    c, cancel := context.WithCancel(context.Background())
    cmd := exec.CommandContext(c, path, args...)
    cmd.SysProcAttr = &syscall.SysProcAttr{Foreground: !background, Pgid: pgid}

    cmd.Stdin = os.Stdin
    cmd.Stdout = os.Stdout
    cmd.Stderr = os.Stderr

    cmd.Start()
    pid := cmd.Process.Pid

    // TODO: check for clobber?
    JobMap[pid] = Proc{ Ctl: cmd, Cancel: cancel }
    go func(){
        waitChan <- cmd.Wait()
    }()

    if background {
        cmd.Process.Signal(syscall.SIGTSTP)
    }
}

/* Takes n arguments (list of tokens generated by lexing a shell command)
 * Evaluates arguments, but does not err on undefined symbols (note the last arg to Eval(...))
 * Executes shell command and returns nil
 *
 * Example (l vim file.txt)
 */
func Call(in *ast.Token, vt ast.VarTable, ft ast.FuncTable) *ast.Token {
    in = in.Eval(ft, vt, true)
    if in == nil {
        return nil
    }

    if in.Tag == ast.LIST {
        log.Log(log.ERR, "couldnt exec, target bin is a list", "call")
        return nil
    }

    path, err := exec.LookPath(in.Value())
    if err != nil {
        log.Log(log.ERR, "Couldnt exec " + in.Value() + ", file not found", "call")
        return nil
    }

    args := []string{}
    for i := in.Next; i != nil; i = i.Next {
        if i.Tag == ast.LIST {
            log.Log(log.ERR, "Couldnt exec " + path + ", element in arguments is a list", "call")
            return nil
        }

        args = append(args, i.Value())
    }

    LaunchProcess(path, args, false)
    return nil
}

/* Starts a call in the background
 * Takes n args (a shell command not delimited by string delimiters)
 *
 * Example: (bg vim file.txt)
 */
func Bgcall(in *ast.Token, vt ast.VarTable, ft ast.FuncTable) *ast.Token {
    if in == nil {
        return nil
    }

    if in.Tag == ast.LIST {
        log.Log(log.ERR, "couldnt exec, target bin is a list", "call")
        return nil
    }

    path, err := exec.LookPath(in.Value())
    if err != nil {
       log.Log(log.ERR, "Couldnt exec " + in.Value() + ", file not found", "call")
        return nil
    }

    args := []string{}
    for i := in.Next; i != nil; i = i.Next {
        if i.Tag == ast.LIST {
            log.Log(log.ERR, "Couldnt exec " + path + ", element in arguments is a list", "call")
            return nil
        }

        args = append(args, i.Value())
    }

    LaunchProcess(path, args, true)
    return nil
}

/* brings last BG'ed process into the foreground
 * returns nil
 *
 * Example:
 *    (bg vim file.txt)
 *    (fg)
 */
func Fg(in *ast.Token, vt ast.VarTable, ft ast.FuncTable) *ast.Token {
    if len(bgProcs) < 1 {
        return nil
    }

    cmd := bgProcs[0]
    bgProcs = bgProcs[1:]

    signalChan := make(chan os.Signal, 2)
    signal.Notify(signalChan, sigs...)
    go func() {
        sig := <-signalChan
        cmd.Process.Signal(sig)
    }()

    cmd.Process.Signal(syscall.SIGCONT)
    err := cmd.Wait()
    close(signalChan)
    signal.Reset(sigs...)
    if err != nil {
        if exitError, ok := err.(*exec.ExitError); ok {
            LastExitCode = exitError.ExitCode()
        } else {
            log.Log(log.ERR, "Execution step returned error: " + err.Error(), "call")
        }
    }

    return nil
}

/* Takes 0 args
 * returns a string containing info about current jobs
 * returns total jobs as well as their PIDs and place in the bg queue
 *
 * Example:
 *    (bg ping google.com)
 *    (bg .........)
 *    (jobs)
 */
func Jobs(in *ast.Token, vt ast.VarTable, fg ast.FuncTable) *ast.Token {
    ret := &ast.Token{
        Tag: ast.LIST,
    }

    _inner := &ast.Token{
        Tag: ast.STRING,
    }

    ret.Direct(_inner)
    _inner.Set(fmt.Sprintf("Total: %d", len(bgProcs)))

    iter := &_inner
    for i := 0; i < len(bgProcs); i += 1 {
        (*iter).Next = &ast.Token{
            Tag: ast.STRING,
        }
        (*iter).Next.Set(fmt.Sprintf("[%d]: %d", i, bgProcs[i].Process.Pid))
        iter = &(*iter).Next
    }

    return ret
}

/* calls a command (blocks until completion)
 * captures stdout and returns it as a string
 *
 * Example: ($ echo hello world)
 */
func ReadCmd(in *ast.Token, vt ast.VarTable, ft ast.FuncTable) *ast.Token {
    in = in.Eval(ft, vt, true)

    if in == nil {
        return nil
    }

    if in.Tag == ast.LIST {
        log.Log(log.ERR, "couldnt exec, target bin is a list", "call")
        return nil
    }

    var out bytes.Buffer
    path, err := exec.LookPath(in.Value())
    if err != nil {
        log.Log(log.ERR, "Couldnt exec " + in.Value() + ", file not found", "call")
        return nil
    }

    args := []string{}
    for i := in.Next; i != nil; i = i.Next {
        if i.Tag == ast.LIST {
            log.Log(log.ERR, "Couldnt exec " + path + ", element in arguments is a list", "call")
            return nil
        }

        args = append(args, i.Value())
    }

    var cmd *exec.Cmd
    if len(args) > 0 {
        cmd = exec.Command(path, args...)
    } else {
        cmd = exec.Command(path)
    }
    cmd.Stdout = &out
    cmd.Stderr = os.Stderr
    cmd.Stdin  = os.Stdin

    signalChan := make(chan os.Signal, 2)
    signal.Notify(signalChan, sigs...)
    go func() {
        sig := <-signalChan
        cmd.Process.Signal(sig)
    }()

    err = cmd.Run()
    close(signalChan)
    signal.Reset(sigs...)
    if err != nil {
         if exitError, ok := err.(*exec.ExitError); ok {
            LastExitCode = exitError.ExitCode()
        } else {
            log.Log(log.ERR, "Execution step returned error: " + err.Error(), "$")
        }
    }

    output := out.String()
    olen := len(output)
    if olen > 0 && output[olen - 1] == '\n' {
        output = output[:olen - 1]
    }

    ret := &ast.Token{Tag: ast.STRING}
    ret.Set(output)
    return ret

}

/* Takes 0 arguments
 * returns the exit code of the last executed program
 *
 * Example:
 *     (sudo apt update)
 *     (?) <- gets exit code
 */
func GetExit(in *ast.Token, vt ast.VarTable, ft ast.FuncTable) *ast.Token {
    ret := &ast.Token{Tag: ast.NUMBER}
    ret.Set(fmt.Sprintf("%d", LastExitCode))
    return ret
}

/* takes an argument (pid of process to be killed)
 * calls Process.Kill() on it
 * do not use this if you already have a native implementation
 * (this function not added to functable in stdlib.go)
 */
func Kill(in *ast.Token, vt ast.VarTable, ft ast.FuncTable) *ast.Token {
    in = in.Eval(ft, vt, true)

    if in.Tag == ast.LIST {
        log.Log(log.ERR, "non-number argument to kill function", "kill")
        return nil
    }

    pid, err := strconv.ParseInt(in.Value(), 10, 64)
    if err != nil {
        log.Log(log.ERR, "error parsing arg to kill: " + err.Error(), "kill")
        return nil
    }

    found := false
    newBgProcs := []*exec.Cmd{}
    for _, i := range bgProcs {
        if i.Process.Pid != int(pid) {
            newBgProcs = append(newBgProcs, i)
        } else {
            found = true
            err = i.Process.Kill()
            if err != nil {
                log.Log(log.ERR, fmt.Sprintf("error killing process %d: %s",
                    int(pid), err.Error()), "kill")
                newBgProcs = append(newBgProcs, i)
            }
        }
    }

    bgProcs = newBgProcs

    if !found {
        // docs say no error on unix systems 
        proc, _ := os.FindProcess(int(pid))
        err = proc.Kill()
        if err != nil {
            log.Log(log.ERR, fmt.Sprintf("error killing process %d: %s",
                int(pid), err.Error()), "kill")
        }
    }

    return nil
}