// Copyright 2016 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// This file implements source, a buffered rune reader
// which is specialized for the needs of the Go scanner:
// Contiguous sequences of runes (literals) are extracted
// directly as []byte without the need to re-encode the
// runes in UTF-8 (as would be necessary with bufio.Reader).
//
// This file is self-contained (go tool compile source.go
// compiles) and thus could be made into its own package.

package syntax

import (
	"io"
	"unicode/utf8"
)

// starting points for line and column numbers
const linebase = 1
const colbase = 1

// max. number of bytes to unread
const maxunread = 10

// buf [...read...|...|...unread...|s|...free...]
//         ^      ^   ^            ^
//         |      |   |            |
//        suf     r0  r            w

type source struct {
	src  io.Reader
	errh func(line, pos uint, msg string)

	// source buffer
	buf         [4 << 10]byte
	r0, r, w    int   // previous/current read and write buf positions, excluding sentinel
	line0, line uint  // previous/current line
	col0, col   uint  // previous/current column (byte offsets from line start)
	ioerr       error // pending io error

	// literal buffer
	lit []byte // literal prefix
	suf int    // literal suffix; suf >= 0 means we are scanning a literal
}

// init initializes source to read from src and to report errors via errh.
// errh must not be nil.
func (s *source) init(src io.Reader, errh func(line, pos uint, msg string)) {
	s.src = src
	s.errh = errh

	s.buf[0] = utf8.RuneSelf // terminate with sentinel
	s.r0, s.r, s.w = 0, 0, 0
	s.line0, s.line = 0, linebase
	s.col0, s.col = 0, colbase
	s.ioerr = nil

	s.lit = s.lit[:0]
	s.suf = -1
}

// ungetr sets the reading position to a previous reading
// position, usually the one of the most recently read
// rune, but possibly earlier (see unread below).
func (s *source) ungetr() {
	s.r, s.line, s.col = s.r0, s.line0, s.col0
}

// unread moves the previous reading position to a position
// that is n bytes earlier in the source. The next ungetr
// call will set the reading position to that moved position.
// The "unread" runes must be single byte and not contain any
// newlines; and 0 <= n <= maxunread must hold.
func (s *source) unread(n int) {
	s.r0 -= n
	s.col0 -= uint(n)
}

func (s *source) error(msg string) {
	s.errh(s.line0, s.col0, msg)
}

// getr reads and returns the next rune.
//
// If a read or source encoding error occurs, getr
// calls the error handler installed with init.
// The handler must exist.
//
// The (line, col) position passed to the error handler
// is always at the current source reading position.
func (s *source) getr() rune {
redo:
	s.r0, s.line0, s.col0 = s.r, s.line, s.col

	// We could avoid at least one test that is always taken in the
	// for loop below by duplicating the common case code (ASCII)
	// here since we always have at least the sentinel (utf8.RuneSelf)
	// in the buffer. Measure and optimize if necessary.

	// make sure we have at least one rune in buffer, or we are at EOF
	for s.r+utf8.UTFMax > s.w && !utf8.FullRune(s.buf[s.r:s.w]) && s.ioerr == nil && s.w-s.r < len(s.buf) {
		s.fill() // s.w-s.r < len(s.buf) => buffer is not full
	}

	// common case: ASCII and enough bytes
	// (invariant: s.buf[s.w] == utf8.RuneSelf)
	if b := s.buf[s.r]; b < utf8.RuneSelf {
		s.r++
		// TODO(gri) Optimization: Instead of adjusting s.col for each character,
		// remember the line offset instead and then compute the offset as needed
		// (which is less often).
		s.col++
		if b == 0 {
			s.error("invalid NUL character")
			goto redo
		}
		if b == '\n' {
			s.line++
			s.col = colbase
		}
		return rune(b)
	}

	// EOF
	if s.r == s.w {
		if s.ioerr != io.EOF {
			// ensure we never start with a '/' (e.g., rooted path) in the error message
			s.error("I/O error: " + s.ioerr.Error())
		}
		return -1
	}

	// uncommon case: not ASCII
	r, w := utf8.DecodeRune(s.buf[s.r:s.w])
	s.r += w
	s.col += uint(w)

	if r == utf8.RuneError && w == 1 {
		s.error("invalid UTF-8 encoding")
		goto redo
	}

	// BOM's are only allowed as the first character in a file
	const BOM = 0xfeff
	if r == BOM {
		if s.r0 > 0 { // s.r0 is always > 0 after 1st character (fill will set it to maxunread)
			s.error("invalid BOM in the middle of the file")
		}
		goto redo
	}

	return r
}

func (s *source) fill() {
	// Slide unread bytes to beginning but preserve last read char
	// (for one ungetr call) plus maxunread extra bytes (for one
	// unread call).
	if s.r0 > maxunread {
		n := s.r0 - maxunread // number of bytes to slide down
		// save literal prefix, if any
		// (make sure we keep maxunread bytes and the last
		// read char in the buffer)
		if s.suf >= 0 {
			// we have a literal
			if s.suf < n {
				// save literal prefix
				s.lit = append(s.lit, s.buf[s.suf:n]...)
				s.suf = 0
			} else {
				s.suf -= n
			}
		}
		copy(s.buf[:], s.buf[n:s.w])
		s.r0 = maxunread // eqv: s.r0 -= n
		s.r -= n
		s.w -= n
	}

	// read more data: try a limited number of times
	for i := 100; i > 0; i-- {
		n, err := s.src.Read(s.buf[s.w : len(s.buf)-1]) // -1 to leave space for sentinel
		if n < 0 {
			panic("negative read") // incorrect underlying io.Reader implementation
		}
		s.w += n
		if n > 0 || err != nil {
			s.buf[s.w] = utf8.RuneSelf // sentinel
			if err != nil {
				s.ioerr = err
			}
			return
		}
	}

	s.buf[s.w] = utf8.RuneSelf // sentinel
	s.ioerr = io.ErrNoProgress
}

func (s *source) startLit() {
	s.suf = s.r0
	s.lit = s.lit[:0] // reuse lit
}

func (s *source) stopLit() []byte {
	lit := s.buf[s.suf:s.r]
	if len(s.lit) > 0 {
		lit = append(s.lit, lit...)
	}
	s.killLit()
	return lit
}

func (s *source) killLit() {
	s.suf = -1 // no pending literal
}