// Copyright 2010 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.

// TODO(gri) consider making this a separate package outside the go directory.

package token

import (
	"fmt"
	"sort"
	"sync"
)

// -----------------------------------------------------------------------------
// Positions

// Position describes an arbitrary source position
// including the file, line, and column location.
// A Position is valid if the line number is > 0.
//
type Position struct {
	Filename string // filename, if any
	Offset   int    // offset, starting at 0
	Line     int    // line number, starting at 1
	Column   int    // column number, starting at 1 (character count)
}

// IsValid returns true if the position is valid.
func (pos *Position) IsValid() bool { return pos.Line > 0 }

// String returns a string in one of several forms:
//
//	file:line:column    valid position with file name
//	line:column         valid position without file name
//	file                invalid position with file name
//	-                   invalid position without file name
//
func (pos Position) String() string {
	s := pos.Filename
	if pos.IsValid() {
		if s != "" {
			s += ":"
		}
		s += fmt.Sprintf("%d:%d", pos.Line, pos.Column)
	}
	if s == "" {
		s = "-"
	}
	return s
}

// Pos is a compact encoding of a source position within a file set.
// It can be converted into a Position for a more convenient, but much
// larger, representation.
//
// The Pos value for a given file is a number in the range [base, base+size],
// where base and size are specified when adding the file to the file set via
// AddFile.
//
// To create the Pos value for a specific source offset, first add
// the respective file to the current file set (via FileSet.AddFile)
// and then call File.Pos(offset) for that file. Given a Pos value p
// for a specific file set fset, the corresponding Position value is
// obtained by calling fset.Position(p).
//
// Pos values can be compared directly with the usual comparison operators:
// If two Pos values p and q are in the same file, comparing p and q is
// equivalent to comparing the respective source file offsets. If p and q
// are in different files, p < q is true if the file implied by p was added
// to the respective file set before the file implied by q.
//
type Pos int

// The zero value for Pos is NoPos; there is no file and line information
// associated with it, and NoPos().IsValid() is false. NoPos is always
// smaller than any other Pos value. The corresponding Position value
// for NoPos is the zero value for Position.
// 
const NoPos Pos = 0

// IsValid returns true if the position is valid.
func (p Pos) IsValid() bool {
	return p != NoPos
}

// -----------------------------------------------------------------------------
// File

// A File is a handle for a file belonging to a FileSet.
// A File has a name, size, and line offset table.
//
type File struct {
	set  *FileSet
	name string // file name as provided to AddFile
	base int    // Pos value range for this file is [base...base+size]
	size int    // file size as provided to AddFile

	// lines and infos are protected by set.mutex
	lines []int
	infos []lineInfo
}

// Name returns the file name of file f as registered with AddFile.
func (f *File) Name() string {
	return f.name
}

// Base returns the base offset of file f as registered with AddFile.
func (f *File) Base() int {
	return f.base
}

// Size returns the size of file f as registered with AddFile.
func (f *File) Size() int {
	return f.size
}

// LineCount returns the number of lines in file f.
func (f *File) LineCount() int {
	f.set.mutex.RLock()
	n := len(f.lines)
	f.set.mutex.RUnlock()
	return n
}

// AddLine adds the line offset for a new line.
// The line offset must be larger than the offset for the previous line
// and smaller than the file size; otherwise the line offset is ignored.
//
func (f *File) AddLine(offset int) {
	f.set.mutex.Lock()
	if i := len(f.lines); (i == 0 || f.lines[i-1] < offset) && offset < f.size {
		f.lines = append(f.lines, offset)
	}
	f.set.mutex.Unlock()
}

// SetLines sets the line offsets for a file and returns true if successful.
// The line offsets are the offsets of the first character of each line;
// for instance for the content "ab\nc\n" the line offsets are {0, 3}.
// An empty file has an empty line offset table.
// Each line offset must be larger than the offset for the previous line
// and smaller than the file size; otherwise SetLines fails and returns
// false.
//
func (f *File) SetLines(lines []int) bool {
	// verify validity of lines table
	size := f.size
	for i, offset := range lines {
		if i > 0 && offset <= lines[i-1] || size <= offset {
			return false
		}
	}

	// set lines table
	f.set.mutex.Lock()
	f.lines = lines
	f.set.mutex.Unlock()
	return true
}

// SetLinesForContent sets the line offsets for the given file content.
func (f *File) SetLinesForContent(content []byte) {
	var lines []int
	line := 0
	for offset, b := range content {
		if line >= 0 {
			lines = append(lines, line)
		}
		line = -1
		if b == '\n' {
			line = offset + 1
		}
	}

	// set lines table
	f.set.mutex.Lock()
	f.lines = lines
	f.set.mutex.Unlock()
}

// A lineInfo object describes alternative file and line number
// information (such as provided via a //line comment in a .go
// file) for a given file offset.
type lineInfo struct {
	// fields are exported to make them accessible to gob
	Offset   int
	Filename string
	Line     int
}

// AddLineInfo adds alternative file and line number information for
// a given file offset. The offset must be larger than the offset for
// the previously added alternative line info and smaller than the
// file size; otherwise the information is ignored.
//
// AddLineInfo is typically used to register alternative position
// information for //line filename:line comments in source files.
//
func (f *File) AddLineInfo(offset int, filename string, line int) {
	f.set.mutex.Lock()
	if i := len(f.infos); i == 0 || f.infos[i-1].Offset < offset && offset < f.size {
		f.infos = append(f.infos, lineInfo{offset, filename, line})
	}
	f.set.mutex.Unlock()
}

// Pos returns the Pos value for the given file offset;
// the offset must be <= f.Size().
// f.Pos(f.Offset(p)) == p.
//
func (f *File) Pos(offset int) Pos {
	if offset > f.size {
		panic("illegal file offset")
	}
	return Pos(f.base + offset)
}

// Offset returns the offset for the given file position p;
// p must be a valid Pos value in that file.
// f.Offset(f.Pos(offset)) == offset.
//
func (f *File) Offset(p Pos) int {
	if int(p) < f.base || int(p) > f.base+f.size {
		panic("illegal Pos value")
	}
	return int(p) - f.base
}

// Line returns the line number for the given file position p;
// p must be a Pos value in that file or NoPos.
//
func (f *File) Line(p Pos) int {
	// TODO(gri) this can be implemented much more efficiently
	return f.Position(p).Line
}

func searchLineInfos(a []lineInfo, x int) int {
	return sort.Search(len(a), func(i int) bool { return a[i].Offset > x }) - 1
}

// info returns the file name, line, and column number for a file offset.
func (f *File) info(offset int) (filename string, line, column int) {
	filename = f.name
	if i := searchInts(f.lines, offset); i >= 0 {
		line, column = i+1, offset-f.lines[i]+1
	}
	if len(f.infos) > 0 {
		// almost no files have extra line infos
		if i := searchLineInfos(f.infos, offset); i >= 0 {
			alt := &f.infos[i]
			filename = alt.Filename
			if i := searchInts(f.lines, alt.Offset); i >= 0 {
				line += alt.Line - i - 1
			}
		}
	}
	return
}

func (f *File) position(p Pos) (pos Position) {
	offset := int(p) - f.base
	pos.Offset = offset
	pos.Filename, pos.Line, pos.Column = f.info(offset)
	return
}

// Position returns the Position value for the given file position p;
// p must be a Pos value in that file or NoPos.
//
func (f *File) Position(p Pos) (pos Position) {
	if p != NoPos {
		if int(p) < f.base || int(p) > f.base+f.size {
			panic("illegal Pos value")
		}
		pos = f.position(p)
	}
	return
}

// -----------------------------------------------------------------------------
// FileSet

// A FileSet represents a set of source files.
// Methods of file sets are synchronized; multiple goroutines
// may invoke them concurrently.
//
type FileSet struct {
	mutex sync.RWMutex // protects the file set
	base  int          // base offset for the next file
	files []*File      // list of files in the order added to the set
	last  *File        // cache of last file looked up
}

// NewFileSet creates a new file set.
func NewFileSet() *FileSet {
	s := new(FileSet)
	s.base = 1 // 0 == NoPos
	return s
}

// Base returns the minimum base offset that must be provided to
// AddFile when adding the next file.
//
func (s *FileSet) Base() int {
	s.mutex.RLock()
	b := s.base
	s.mutex.RUnlock()
	return b

}

// AddFile adds a new file with a given filename, base offset, and file size
// to the file set s and returns the file. Multiple files may have the same
// name. The base offset must not be smaller than the FileSet's Base(), and
// size must not be negative.
//
// Adding the file will set the file set's Base() value to base + size + 1
// as the minimum base value for the next file. The following relationship
// exists between a Pos value p for a given file offset offs:
//
//	int(p) = base + offs
//
// with offs in the range [0, size] and thus p in the range [base, base+size].
// For convenience, File.Pos may be used to create file-specific position
// values from a file offset.
//
func (s *FileSet) AddFile(filename string, base, size int) *File {
	s.mutex.Lock()
	defer s.mutex.Unlock()
	if base < s.base || size < 0 {
		panic("illegal base or size")
	}
	// base >= s.base && size >= 0
	f := &File{s, filename, base, size, []int{0}, nil}
	base += size + 1 // +1 because EOF also has a position
	if base < 0 {
		panic("token.Pos offset overflow (> 2G of source code in file set)")
	}
	// add the file to the file set
	s.base = base
	s.files = append(s.files, f)
	s.last = f
	return f
}

// Iterate calls f for the files in the file set in the order they were added
// until f returns false.
// 
func (s *FileSet) Iterate(f func(*File) bool) {
	for i := 0; ; i++ {
		var file *File
		s.mutex.RLock()
		if i < len(s.files) {
			file = s.files[i]
		}
		s.mutex.RUnlock()
		if file == nil || !f(file) {
			break
		}
	}
}

func searchFiles(a []*File, x int) int {
	return sort.Search(len(a), func(i int) bool { return a[i].base > x }) - 1
}

func (s *FileSet) file(p Pos) *File {
	// common case: p is in last file
	if f := s.last; f != nil && f.base <= int(p) && int(p) <= f.base+f.size {
		return f
	}
	// p is not in last file - search all files
	if i := searchFiles(s.files, int(p)); i >= 0 {
		f := s.files[i]
		// f.base <= int(p) by definition of searchFiles
		if int(p) <= f.base+f.size {
			s.last = f
			return f
		}
	}
	return nil
}

// File returns the file that contains the position p.
// If no such file is found (for instance for p == NoPos),
// the result is nil.
//
func (s *FileSet) File(p Pos) (f *File) {
	if p != NoPos {
		s.mutex.RLock()
		f = s.file(p)
		s.mutex.RUnlock()
	}
	return
}

// Position converts a Pos in the fileset into a general Position.
func (s *FileSet) Position(p Pos) (pos Position) {
	if p != NoPos {
		s.mutex.RLock()
		if f := s.file(p); f != nil {
			pos = f.position(p)
		}
		s.mutex.RUnlock()
	}
	return
}

// -----------------------------------------------------------------------------
// Helper functions

func searchInts(a []int, x int) int {
	// This function body is a manually inlined version of:
	//
	//   return sort.Search(len(a), func(i int) bool { return a[i] > x }) - 1
	//
	// With better compiler optimizations, this may not be needed in the
	// future, but at the moment this change improves the go/printer
	// benchmark performance by ~30%. This has a direct impact on the
	// speed of gofmt and thus seems worthwhile (2011-04-29).
	// TODO(gri): Remove this when compilers have caught up.
	i, j := 0, len(a)
	for i < j {
		h := i + (j-i)/2 // avoid overflow when computing h
		// i ≤ h < j
		if a[h] <= x {
			i = h + 1
		} else {
			j = h
		}
	}
	return i - 1
}