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[pf3gnuchains/gcc-fork.git] / libgo / go / tabwriter / tabwriter.go

// Copyright 2009 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.

// The tabwriter package implements a write filter (tabwriter.Writer)
// that translates tabbed columns in input into properly aligned text.
//
// The package is using the Elastic Tabstops algorithm described at
// http://nickgravgaard.com/elastictabstops/index.html.
//
package tabwriter

import (
        "bytes"
        "io"
        "os"
        "utf8"
)


// ----------------------------------------------------------------------------
// Filter implementation

// A cell represents a segment of text terminated by tabs or line breaks.
// The text itself is stored in a separate buffer; cell only describes the
// segment's size in bytes, its width in runes, and whether it's an htab
// ('\t') terminated cell.
//
type cell struct {
        size  int  // cell size in bytes
        width int  // cell width in runes
        htab  bool // true if the cell is terminated by an htab ('\t')
}


// A Writer is a filter that inserts padding around tab-delimited
// columns in its input to align them in the output.
//
// The Writer treats incoming bytes as UTF-8 encoded text consisting
// of cells terminated by (horizontal or vertical) tabs or line
// breaks (newline or formfeed characters). Cells in adjacent lines
// constitute a column. The Writer inserts padding as needed to
// make all cells in a column have the same width, effectively
// aligning the columns. It assumes that all characters have the
// same width except for tabs for which a tabwidth must be specified.
// Note that cells are tab-terminated, not tab-separated: trailing
// non-tab text at the end of a line does not form a column cell.
//
// The Writer assumes that all Unicode code points have the same width;
// this may not be true in some fonts.
//
// If DiscardEmptyColumns is set, empty columns that are terminated
// entirely by vertical (or "soft") tabs are discarded. Columns
// terminated by horizontal (or "hard") tabs are not affected by
// this flag.
//
// If a Writer is configured to filter HTML, HTML tags and entities
// are simply passed through. The widths of tags and entities are
// assumed to be zero (tags) and one (entities) for formatting purposes.
//
// A segment of text may be escaped by bracketing it with Escape
// characters. The tabwriter passes escaped text segments through
// unchanged. In particular, it does not interpret any tabs or line
// breaks within the segment. If the StripEscape flag is set, the
// Escape characters are stripped from the output; otherwise they
// are passed through as well. For the purpose of formatting, the
// width of the escaped text is always computed excluding the Escape
// characters.
//
// The formfeed character ('\f') acts like a newline but it also
// terminates all columns in the current line (effectively calling
// Flush). Cells in the next line start new columns. Unless found
// inside an HTML tag or inside an escaped text segment, formfeed
// characters appear as newlines in the output.
//
// The Writer must buffer input internally, because proper spacing
// of one line may depend on the cells in future lines. Clients must
// call Flush when done calling Write.
//
type Writer struct {
        // configuration
        output   io.Writer
        minwidth int
        tabwidth int
        padding  int
        padbytes [8]byte
        flags    uint

        // current state
        buf     bytes.Buffer // collected text excluding tabs or line breaks
        pos     int          // buffer position up to which cell.width of incomplete cell has been computed
        cell    cell         // current incomplete cell; cell.width is up to buf[pos] excluding ignored sections
        endChar byte         // terminating char of escaped sequence (Escape for escapes, '>', ';' for HTML tags/entities, or 0)
        lines   [][]cell     // list of lines; each line is a list of cells
        widths  []int        // list of column widths in runes - re-used during formatting
}


func (b *Writer) addLine() { b.lines = append(b.lines, []cell{}) }


// Reset the current state.
func (b *Writer) reset() {
        b.buf.Reset()
        b.pos = 0
        b.cell = cell{}
        b.endChar = 0
        b.lines = b.lines[0:0]
        b.widths = b.widths[0:0]
        b.addLine()
}


// Internal representation (current state):
//
// - all text written is appended to buf; tabs and line breaks are stripped away
// - at any given time there is a (possibly empty) incomplete cell at the end
//   (the cell starts after a tab or line break)
// - cell.size is the number of bytes belonging to the cell so far
// - cell.width is text width in runes of that cell from the start of the cell to
//   position pos; html tags and entities are excluded from this width if html
//   filtering is enabled
// - the sizes and widths of processed text are kept in the lines list
//   which contains a list of cells for each line
// - the widths list is a temporary list with current widths used during
//   formatting; it is kept in Writer because it's re-used
//
//                    |<---------- size ---------->|
//                    |                            |
//                    |<- width ->|<- ignored ->|  |
//                    |           |             |  |
// [---processed---tab------------<tag>...</tag>...]
// ^                  ^                         ^
// |                  |                         |
// buf                start of incomplete cell  pos


// Formatting can be controlled with these flags.
const (
        // Ignore html tags and treat entities (starting with '&'
        // and ending in ';') as single characters (width = 1).
        FilterHTML uint = 1 << iota

        // Strip Escape characters bracketing escaped text segments
        // instead of passing them through unchanged with the text.
        StripEscape

        // Force right-alignment of cell content.
        // Default is left-alignment.
        AlignRight

        // Handle empty columns as if they were not present in
        // the input in the first place.
        DiscardEmptyColumns

        // Always use tabs for indentation columns (i.e., padding of
        // leading empty cells on the left) independent of padchar.
        TabIndent

        // Print a vertical bar ('|') between columns (after formatting).
        // Discarded colums appear as zero-width columns ("||").
        Debug
)


// A Writer must be initialized with a call to Init. The first parameter (output)
// specifies the filter output. The remaining parameters control the formatting:
//
//      minwidth        minimal cell width including any padding
//      tabwidth        width of tab characters (equivalent number of spaces)
//      padding         padding added to a cell before computing its width
//      padchar         ASCII char used for padding
//                      if padchar == '\t', the Writer will assume that the
//                      width of a '\t' in the formatted output is tabwidth,
//                      and cells are left-aligned independent of align_left
//                      (for correct-looking results, tabwidth must correspond
//                      to the tab width in the viewer displaying the result)
//      flags           formatting control
//
// To format in tab-separated columns with a tab stop of 8:
//      b.Init(w, 8, 1, 8, '\t', 0);
//
// To format in space-separated columns with at least 4 spaces between columns:
//      b.Init(w, 0, 4, 8, ' ', 0);
//
func (b *Writer) Init(output io.Writer, minwidth, tabwidth, padding int, padchar byte, flags uint) *Writer {
        if minwidth < 0 || tabwidth < 0 || padding < 0 {
                panic("negative minwidth, tabwidth, or padding")
        }
        b.output = output
        b.minwidth = minwidth
        b.tabwidth = tabwidth
        b.padding = padding
        for i := range b.padbytes {
                b.padbytes[i] = padchar
        }
        if padchar == '\t' {
                // tab padding enforces left-alignment
                flags &^= AlignRight
        }
        b.flags = flags

        b.reset()

        return b
}


// debugging support (keep code around)
func (b *Writer) dump() {
        pos := 0
        for i, line := range b.lines {
                print("(", i, ") ")
                for _, c := range line {
                        print("[", string(b.buf.Bytes()[pos:pos+c.size]), "]")
                        pos += c.size
                }
                print("\n")
        }
        print("\n")
}


// local error wrapper so we can distinguish os.Errors we want to return
// as errors from genuine panics (which we don't want to return as errors)
type osError struct {
        err os.Error
}


func (b *Writer) write0(buf []byte) {
        n, err := b.output.Write(buf)
        if n != len(buf) && err == nil {
                err = os.EIO
        }
        if err != nil {
                panic(osError{err})
        }
}


func (b *Writer) writeN(src []byte, n int) {
        for n > len(src) {
                b.write0(src)
                n -= len(src)
        }
        b.write0(src[0:n])
}


var (
        newline = []byte{'\n'}
        tabs    = []byte("\t\t\t\t\t\t\t\t")
)


func (b *Writer) writePadding(textw, cellw int, useTabs bool) {
        if b.padbytes[0] == '\t' || useTabs {
                // padding is done with tabs
                if b.tabwidth == 0 {
                        return // tabs have no width - can't do any padding
                }
                // make cellw the smallest multiple of b.tabwidth
                cellw = (cellw + b.tabwidth - 1) / b.tabwidth * b.tabwidth
                n := cellw - textw // amount of padding
                if n < 0 {
                        panic("internal error")
                }
                b.writeN(tabs, (n+b.tabwidth-1)/b.tabwidth)
                return
        }

        // padding is done with non-tab characters
        b.writeN(b.padbytes[0:], cellw-textw)
}


var vbar = []byte{'|'}

func (b *Writer) writeLines(pos0 int, line0, line1 int) (pos int) {
        pos = pos0
        for i := line0; i < line1; i++ {
                line := b.lines[i]

                // if TabIndent is set, use tabs to pad leading empty cells
                useTabs := b.flags&TabIndent != 0

                for j, c := range line {
                        if j > 0 && b.flags&Debug != 0 {
                                // indicate column break
                                b.write0(vbar)
                        }

                        if c.size == 0 {
                                // empty cell
                                if j < len(b.widths) {
                                        b.writePadding(c.width, b.widths[j], useTabs)
                                }
                        } else {
                                // non-empty cell
                                useTabs = false
                                if b.flags&AlignRight == 0 { // align left
                                        b.write0(b.buf.Bytes()[pos : pos+c.size])
                                        pos += c.size
                                        if j < len(b.widths) {
                                                b.writePadding(c.width, b.widths[j], false)
                                        }
                                } else { // align right
                                        if j < len(b.widths) {
                                                b.writePadding(c.width, b.widths[j], false)
                                        }
                                        b.write0(b.buf.Bytes()[pos : pos+c.size])
                                        pos += c.size
                                }
                        }
                }

                if i+1 == len(b.lines) {
                        // last buffered line - we don't have a newline, so just write
                        // any outstanding buffered data
                        b.write0(b.buf.Bytes()[pos : pos+b.cell.size])
                        pos += b.cell.size
                } else {
                        // not the last line - write newline
                        b.write0(newline)
                }
        }
        return
}


// Format the text between line0 and line1 (excluding line1); pos
// is the buffer position corresponding to the beginning of line0.
// Returns the buffer position corresponding to the beginning of
// line1 and an error, if any.
//
func (b *Writer) format(pos0 int, line0, line1 int) (pos int) {
        pos = pos0
        column := len(b.widths)
        for this := line0; this < line1; this++ {
                line := b.lines[this]

                if column < len(line)-1 {
                        // cell exists in this column => this line
                        // has more cells than the previous line
                        // (the last cell per line is ignored because cells are
                        // tab-terminated; the last cell per line describes the
                        // text before the newline/formfeed and does not belong
                        // to a column)

                        // print unprinted lines until beginning of block
                        pos = b.writeLines(pos, line0, this)
                        line0 = this

                        // column block begin
                        width := b.minwidth // minimal column width
                        discardable := true // true if all cells in this column are empty and "soft"
                        for ; this < line1; this++ {
                                line = b.lines[this]
                                if column < len(line)-1 {
                                        // cell exists in this column
                                        c := line[column]
                                        // update width
                                        if w := c.width + b.padding; w > width {
                                                width = w
                                        }
                                        // update discardable
                                        if c.width > 0 || c.htab {
                                                discardable = false
                                        }
                                } else {
                                        break
                                }
                        }
                        // column block end

                        // discard empty columns if necessary
                        if discardable && b.flags&DiscardEmptyColumns != 0 {
                                width = 0
                        }

                        // format and print all columns to the right of this column
                        // (we know the widths of this column and all columns to the left)
                        b.widths = append(b.widths, width) // push width
                        pos = b.format(pos, line0, this)
                        b.widths = b.widths[0 : len(b.widths)-1] // pop width
                        line0 = this
                }
        }

        // print unprinted lines until end
        return b.writeLines(pos, line0, line1)
}


// Append text to current cell.
func (b *Writer) append(text []byte) {
        b.buf.Write(text)
        b.cell.size += len(text)
}


// Update the cell width.
func (b *Writer) updateWidth() {
        b.cell.width += utf8.RuneCount(b.buf.Bytes()[b.pos:b.buf.Len()])
        b.pos = b.buf.Len()
}


// To escape a text segment, bracket it with Escape characters.
// For instance, the tab in this string "Ignore this tab: \xff\t\xff"
// does not terminate a cell and constitutes a single character of
// width one for formatting purposes.
//
// The value 0xff was chosen because it cannot appear in a valid UTF-8 sequence.
//
const Escape = '\xff'


// Start escaped mode.
func (b *Writer) startEscape(ch byte) {
        switch ch {
        case Escape:
                b.endChar = Escape
        case '<':
                b.endChar = '>'
        case '&':
                b.endChar = ';'
        }
}


// Terminate escaped mode. If the escaped text was an HTML tag, its width
// is assumed to be zero for formatting purposes; if it was an HTML entity,
// its width is assumed to be one. In all other cases, the width is the
// unicode width of the text.
//
func (b *Writer) endEscape() {
        switch b.endChar {
        case Escape:
                b.updateWidth()
                if b.flags&StripEscape == 0 {
                        b.cell.width -= 2 // don't count the Escape chars
                }
        case '>': // tag of zero width
        case ';':
                b.cell.width++ // entity, count as one rune
        }
        b.pos = b.buf.Len()
        b.endChar = 0
}


// Terminate the current cell by adding it to the list of cells of the
// current line. Returns the number of cells in that line.
//
func (b *Writer) terminateCell(htab bool) int {
        b.cell.htab = htab
        line := &b.lines[len(b.lines)-1]
        *line = append(*line, b.cell)
        b.cell = cell{}
        return len(*line)
}


func handlePanic(err *os.Error) {
        if e := recover(); e != nil {
                *err = e.(osError).err // re-panics if it's not a local osError
        }
}


// Flush should be called after the last call to Write to ensure
// that any data buffered in the Writer is written to output. Any
// incomplete escape sequence at the end is simply considered
// complete for formatting purposes.
//
func (b *Writer) Flush() (err os.Error) {
        defer b.reset() // even in the presence of errors
        defer handlePanic(&err)

        // add current cell if not empty
        if b.cell.size > 0 {
                if b.endChar != 0 {
                        // inside escape - terminate it even if incomplete
                        b.endEscape()
                }
                b.terminateCell(false)
        }

        // format contents of buffer
        b.format(0, 0, len(b.lines))

        return
}


var hbar = []byte("---\n")

// Write writes buf to the writer b.
// The only errors returned are ones encountered
// while writing to the underlying output stream.
//
func (b *Writer) Write(buf []byte) (n int, err os.Error) {
        defer handlePanic(&err)

        // split text into cells
        n = 0
        for i, ch := range buf {
                if b.endChar == 0 {
                        // outside escape
                        switch ch {
                        case '\t', '\v', '\n', '\f':
                                // end of cell
                                b.append(buf[n:i])
                                b.updateWidth()
                                n = i + 1 // ch consumed
                                ncells := b.terminateCell(ch == '\t')
                                if ch == '\n' || ch == '\f' {
                                        // terminate line
                                        b.addLine()
                                        if ch == '\f' || ncells == 1 {
                                                // A '\f' always forces a flush. Otherwise, if the previous
                                                // line has only one cell which does not have an impact on
                                                // the formatting of the following lines (the last cell per
                                                // line is ignored by format()), thus we can flush the
                                                // Writer contents.
                                                if err = b.Flush(); err != nil {
                                                        return
                                                }
                                                if ch == '\f' && b.flags&Debug != 0 {
                                                        // indicate section break
                                                        b.write0(hbar)
                                                }
                                        }
                                }

                        case Escape:
                                // start of escaped sequence
                                b.append(buf[n:i])
                                b.updateWidth()
                                n = i
                                if b.flags&StripEscape != 0 {
                                        n++ // strip Escape
                                }
                                b.startEscape(Escape)

                        case '<', '&':
                                // possibly an html tag/entity
                                if b.flags&FilterHTML != 0 {
                                        // begin of tag/entity
                                        b.append(buf[n:i])
                                        b.updateWidth()
                                        n = i
                                        b.startEscape(ch)
                                }
                        }

                } else {
                        // inside escape
                        if ch == b.endChar {
                                // end of tag/entity
                                j := i + 1
                                if ch == Escape && b.flags&StripEscape != 0 {
                                        j = i // strip Escape
                                }
                                b.append(buf[n:j])
                                n = i + 1 // ch consumed
                                b.endEscape()
                        }
                }
        }

        // append leftover text
        b.append(buf[n:])
        n = len(buf)
        return
}


// NewWriter allocates and initializes a new tabwriter.Writer.
// The parameters are the same as for the the Init function.
//
func NewWriter(output io.Writer, minwidth, tabwidth, padding int, padchar byte, flags uint) *Writer {
        return new(Writer).Init(output, minwidth, tabwidth, padding, padchar, flags)
}