package main

// The read builtin: read [-r] [-p prompt] [name ...] reads a line and splits
// it into fields at the characters of $IFS (default: blank, tab, newline).

import (
	"errors"
	"fmt"
	"io"
	"os"
	"os/signal"
	"strings"
	"syscall"

	"github.com/chzyer/readline"
	"golang.org/x/sys/unix"
)

// builtinRead implements read. Each name gets one field, the last one the
// rest of the line; without names the whole line is stored in $REPLY. The
// status is 1 at the end of the input.
func builtinRead(args []string, stdin *os.File) error {
	raw := false
	prompt := ""
	args = args[1:]
	for len(args) > 0 && strings.HasPrefix(args[0], "-") && args[0] != "-" {
		opt := args[0]
		args = args[1:]
		if opt == "--" {
			break
		}
		for k := 1; k < len(opt); k++ {
			switch opt[k] {
			case 'r':
				raw = true
			case 'p':
				switch {
				case k+1 < len(opt):
					prompt = opt[k+1:]
				case len(args) > 0:
					prompt, args = args[0], args[1:]
				default:
					return errors.New("read: -p: option requires an argument")
				}
				k = len(opt)
			default:
				return fmt.Errorf("read: -%c: invalid option\nusage: read [-r] [-p prompt] [name ...]", opt[k])
			}
		}
	}
	for _, name := range args {
		if !isVarName(name) {
			return fmt.Errorf("read: `%s': not a valid identifier", sanitize(name))
		}
	}

	terminal := readline.IsTerminal(int(stdin.Fd()))
	if prompt != "" && terminal {
		fmt.Fprint(stdio(2), prompt)
	}
	line, literal, eof, err := readInputLine(stdin, raw, interactiveShell && terminal)
	if err != nil {
		return err
	}

	if len(args) == 0 {
		os.Setenv("REPLY", line)
	} else {
		ifs, ok := os.LookupEnv("IFS")
		if !ok {
			ifs = " \t\n"
		}
		fields := splitFields(line, literal, ifs, len(args))
		for i, name := range args {
			value := ""
			if i < len(fields) {
				value = fields[i]
			}
			os.Setenv(name, value)
		}
	}
	if eof {
		return statusError(1)
	}
	return nil
}

// readInputLine reads a line from f, byte by byte so that nothing after the
// line is consumed. Unless raw is set, a backslash escapes the next
// character (literal marks the escaped characters, which never split
// fields) and backslash-newline continues the line. eof reports that the
// input ended before a newline. If interruptible is set, Ctrl+C aborts the
// read with status 130.
func readInputLine(f *os.File, raw, interruptible bool) (line string, literal []bool, eof bool, err error) {
	readByte := plainByteReader(f)
	if interruptible {
		intr := make(chan os.Signal, 1)
		signal.Notify(intr, os.Interrupt)
		defer signal.Stop(intr)
		readByte = interruptibleByteReader(f, intr)
	}

	var b []byte
	for {
		c, err := readByte()
		if err == io.EOF {
			return string(b), literal, true, nil
		}
		if err != nil {
			return "", nil, false, err
		}
		if c == '\n' {
			return string(b), literal, false, nil
		}
		if c == '\\' && !raw {
			c, err = readByte()
			if err == io.EOF {
				return string(b), literal, true, nil
			}
			if err != nil {
				return "", nil, false, err
			}
			if c == '\n' {
				continue // the line goes on
			}
			b = append(b, c)
			literal = append(literal, true)
			continue
		}
		b = append(b, c)
		literal = append(literal, false)
	}
}

func plainByteReader(f *os.File) func() (byte, error) {
	buf := make([]byte, 1)
	return func() (byte, error) {
		for {
			n, err := f.Read(buf)
			if n == 1 {
				return buf[0], nil
			}
			if err != nil {
				return 0, err
			}
		}
	}
}

// interruptibleByteReader reads from the terminal f only when input is
// available, so that a SIGINT on intr can end the read in between.
func interruptibleByteReader(f *os.File, intr chan os.Signal) func() (byte, error) {
	next := plainByteReader(f)
	fds := []unix.PollFd{{Fd: int32(f.Fd()), Events: unix.POLLIN}}
	return func() (byte, error) {
		for {
			select {
			case <-intr:
				fmt.Fprintln(stdio(2))
				return 0, statusError(128 + int(syscall.SIGINT))
			default:
			}
			n, err := unix.Poll(fds, 100)
			if err != nil && !errors.Is(err, unix.EINTR) {
				return 0, err
			}
			if n > 0 {
				return next()
			}
		}
	}
}

// splitFields splits line into at most n fields like read does: IFS
// whitespace (blank, tab, newline in ifs) around fields is removed, other IFS
// characters separate fields (also empty ones). The last field is the rest of
// the line. Characters marked in literal are never separators.
func splitFields(line string, literal []bool, ifs string, n int) []string {
	isSep := func(i int) bool { return !literal[i] && strings.IndexByte(ifs, line[i]) >= 0 }
	isSpace := func(i int) bool { return isSep(i) && strings.IndexByte(" \t\n", line[i]) >= 0 }

	start, end := 0, len(line)
	for start < end && isSpace(start) {
		start++
	}
	for end > start && isSpace(end-1) {
		end--
	}

	var fields []string
	i := start
	for i < end && len(fields) < n-1 {
		j := i
		for j < end && !isSep(j) {
			j++
		}
		fields = append(fields, line[i:j])
		// The separator: whitespace, at most one other IFS character,
		// whitespace
		for j < end && isSpace(j) {
			j++
		}
		if j < end && isSep(j) && !isSpace(j) {
			j++
			for j < end && isSpace(j) {
				j++
			}
		}
		i = j
	}
	if i < end {
		fields = append(fields, line[i:end])
	}
	return fields
}