/*   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"
    "io"
    "fmt"
    "bytes"
    "os/exec"
    "context"
    "syscall"
    "strconv"
    "os/signal"
    "golang.org/x/sys/unix"
    "gitlab.com/whom/shs/ast"
    "gitlab.com/whom/shs/log"
)

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, pid int

/* cancel func for current running process
 */
var CurCancel *context.CancelFunc


/* 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 {
        <- sigChan
        log.Log(log.DEBUG,
            "caught SIGINT",
            "jobctl")

        if CurCancel != nil {
            (*CurCancel)()
        }
    }
}

// for some reason not implemented in stdlib
func tcsetpgrp(fd int, pgrp int) error {
    return unix.IoctlSetPointerInt(fd, unix.TIOCSPGRP, pgrp)
}

// wrapper for convenience
func setSigState() {
    signal.Ignore(syscall.SIGTTOU, syscall.SIGTTIN, syscall.SIGTSTP)
    signal.Notify(sigChan, syscall.SIGINT, syscall.SIGQUIT)
}

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

    go signalHandler()

    setSigState()
    pid = os.Getpid()
    var errr error
    pgid, errr = syscall.Getpgid(pid)
    if errr != nil {
        log.Log(log.ERR,
            "Failure to get pgid: " + errr.Error(),
            "jobctl")
        return false
    }

    termPgrp, err := unix.IoctlGetInt(0, unix.TIOCGPGRP)
    if err != nil {
        log.Log(log.ERR,
            "Failure to get pgrp: " + err.Error(),
            "jobctl")
        return false
    }

    if pgid != termPgrp {
        syscall.Kill(-termPgrp, unix.SIGTTIN)
    }

    syscall.Setpgid(0, 0)
    tcsetpgrp(0, pid)

    return true
}

/* Tears down session
 */
func TeardownShell() {
    close(sigChan)
    close(waitChan)
    for k := range JobMap {
        JobMap[k].Cancel()
    }
}

/* Check background processes states
 * If any have finished, reap them
 */
func CheckBGProcs() {
    for key, proc := range JobMap {
        maybeFilePath := fmt.Sprintf("/proc/%d", key)
        _, err := os.Stat(maybeFilePath)

        if os.IsNotExist(err) {
            if err := proc.Ctl.Process.Release(); err != nil {
                log.Log(log.ERR,
                    fmt.Sprintf("Failed to release exited process %d: %s",
                        key, err.Error()),
                    "jobctl")
            } else {
                // not actually an error, just abusing default visibility
                log.Log(log.ERR,
                    fmt.Sprintf("Child [%d] exited", key),
                    "jobctl")
                delete(JobMap, key)
            }
        } else if err != nil {
            log.Log(log.DEBUG,
                "Error checking if process exists in proc: " + err.Error(),
                "jobctl")
        }
    }
}

/* 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,
    stdin  io.Reader,
    stdout io.Writer,
    stderr io.Writer ) {
    c, cancel := context.WithCancel(context.Background())
    cmd := exec.CommandContext(c, path, args...)
    cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}

    if !background {
        cmd.SysProcAttr.Foreground = true
    }

    if stdin == nil {
        stdin = os.Stdin
    }

    if stdout == nil {
        stdout = os.Stdout
    }

    if stderr == nil {
        stderr = os.Stderr
    }

    cmd.Stdin = stdin
    cmd.Stdout = stdout
    cmd.Stderr = stderr

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

    // TODO: check for clobber?
    // unlikely, but PIDs do reset after a while
    JobMap[cpid] = Proc{ Ctl: cmd, Cancel: cancel }
    CurCancel = &cancel

    if !background {
        cmd.Wait()
        tcsetpgrp(0, pgid)
        delete(JobMap, cpid)
        CurCancel = nil
    }
}

/* 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 {
    if in == nil {
        log.Log(log.ERR, "no arguments given", "call")
        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, nil, nil, nil)
    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.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, nil, nil, nil)
    return nil
}

/* takes one argument: pid of process to foreground
 * returns nil
 *
 * Example:
 *    (bg vim file.txt)
 *    ( <call to ps or jobs> )
 *    (fg <pid>)
 */
func Fg(in *ast.Token, vt ast.VarTable, ft ast.FuncTable) *ast.Token {
    if len(JobMap) < 1 {
        return nil
    }

    pid, err := strconv.ParseFloat(in.Value(), 64)
    if err != nil {
        log.Log(log.ERR,
            "value supplied to fg could not be cast to float",
            "fg")
        return nil
    }

    ipid := int(pid)
    proc, ok := JobMap[ipid]
    if !ok {
        log.Log(log.ERR,
            "Process not found, was it started by this shell?",
            "fg")
        return nil
    }

    if err := tcsetpgrp(0, ipid); err != nil {
        log.Log(log.ERR,
            "Error foregrounding process: " + err.Error(),
            "fg")
        return nil
    }

    proc.Ctl.Wait()
    return nil
}

/* Takes 0 args
 * returns a list of PIDs
 *
 * 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,
    }

    var _inner *ast.Token
    iter := &_inner
    for k := range JobMap {
        (*iter) = &ast.Token{
            Tag: ast.STRING,
        }

        (*iter).Set(fmt.Sprintf("%d", k))
        iter = &(*iter).Next
    }

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

    out := new(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())
    }

    LaunchProcess(path, args, false, nil, out, nil)
    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 {
    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
    }

    proc, ok := JobMap[int(pid)]
    if !ok {
        log.Log(log.ERR,
            "Couldnt find process " + in.Value() + ", was it started by this shell?",
            "kill")
        return nil
    }

    // if this doesnt work do proc.Ctl.Kill()
    proc.Cancel()
    delete(JobMap, int(pid))
    return nil
}