font/sfnt: new package for parsing SFNT fonts.
Updates golang/go#16904
Change-Id: Ic4c55865741b712f7d82448276fc357f2edf9b45
Reviewed-on: https://go-review.googlesource.com/33417
Reviewed-by: Dave Day <djd@golang.org>
diff --git a/font/sfnt/sfnt.go b/font/sfnt/sfnt.go
new file mode 100644
index 0000000..e0b977f
--- /dev/null
+++ b/font/sfnt/sfnt.go
@@ -0,0 +1,304 @@
+// 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.
+
+// Package sfnt implements a decoder for SFNT font file formats, including
+// TrueType and OpenType.
+package sfnt // import "golang.org/x/image/font/sfnt"
+
+// This implementation was written primarily to the
+// https://www.microsoft.com/en-us/Typography/OpenTypeSpecification.aspx
+// specification. Additional documentation is at
+// http://developer.apple.com/fonts/TTRefMan/
+
+import (
+ "errors"
+ "io"
+)
+
+// These constants are not part of the specifications, but are limitations used
+// by this implementation.
+const (
+ maxNumTables = 256
+ // (maxTableOffset + maxTableLength) will not overflow an int32.
+ maxTableLength = 1 << 29
+ maxTableOffset = 1 << 29
+)
+
+var (
+ errInvalidBounds = errors.New("sfnt: invalid bounds")
+ errInvalidHeadTable = errors.New("sfnt: invalid head table")
+ errInvalidMaxpTable = errors.New("sfnt: invalid maxp table")
+ errInvalidSourceData = errors.New("sfnt: invalid source data")
+ errInvalidTableOffset = errors.New("sfnt: invalid table offset")
+ errInvalidTableTagOrder = errors.New("sfnt: invalid table tag order")
+ errInvalidVersion = errors.New("sfnt: invalid version")
+
+ errUnsupportedNumberOfTables = errors.New("sfnt: unsupported number of tables")
+ errUnsupportedTableOffsetLength = errors.New("sfnt: unsupported table offset or length")
+ errUnsupportedVersion = errors.New("sfnt: unsupported version")
+)
+
+// Units are an integral number of abstract, scalable "font units". The em
+// square is typically 1000 or 2048 "font units". This would map to a certain
+// number (e.g. 30 pixels) of physical pixels, depending on things like the
+// display resolution (DPI) and font size (e.g. a 12 point font).
+type Units int32
+
+func u16(b []byte) uint16 {
+ _ = b[1] // Bounds check hint to compiler.
+ return uint16(b[0])<<8 | uint16(b[1])<<0
+}
+
+func u32(b []byte) uint32 {
+ _ = b[3] // Bounds check hint to compiler.
+ return uint32(b[0])<<24 | uint32(b[1])<<16 | uint32(b[2])<<8 | uint32(b[3])<<0
+}
+
+// source is a source of byte data. Conceptually, it is like an io.ReaderAt,
+// except that a common source of SFNT font data is in-memory instead of
+// on-disk: a []byte containing the entire data, either as a global variable
+// (e.g. "goregular.TTF") or the result of an ioutil.ReadFile call. In such
+// cases, as an optimization, we skip the io.Reader / io.ReaderAt model of
+// copying from the source to a caller-supplied buffer, and instead provide
+// direct access to the underlying []byte data.
+type source struct {
+ b []byte
+ r io.ReaderAt
+
+ // TODO: add a caching layer, if we're using the io.ReaderAt? Note that
+ // this might make a source no longer safe to use concurrently.
+}
+
+// valid returns whether exactly one of s.b and s.r is nil.
+func (s *source) valid() bool {
+ return (s.b == nil) != (s.r == nil)
+}
+
+// view returns the length bytes at the given offset. buf is an optional
+// scratch buffer to reduce allocations when calling view multiple times. A nil
+// buf is valid. The []byte returned may be a sub-slice of buf[:cap(buf)], or
+// it may be an unrelated slice. In any case, the caller should not modify the
+// contents of the returned []byte, other than passing that []byte back to this
+// method on the same source s.
+func (s *source) view(buf []byte, offset, length int) ([]byte, error) {
+ if 0 > offset || offset > offset+length {
+ return nil, errInvalidBounds
+ }
+
+ // Try reading from the []byte.
+ if s.b != nil {
+ if offset+length > len(s.b) {
+ return nil, errInvalidBounds
+ }
+ return s.b[offset : offset+length], nil
+ }
+
+ // Read from the io.ReaderAt.
+ if length <= cap(buf) {
+ buf = buf[:length]
+ } else {
+ // Round length up to the nearest KiB. The slack can lead to fewer
+ // allocations if the buffer is re-used for multiple source.view calls.
+ n := length
+ n += 1023
+ n &^= 1023
+ buf = make([]byte, length, n)
+ }
+ if n, err := s.r.ReadAt(buf, int64(offset)); n != length {
+ return nil, err
+ }
+ return buf, nil
+}
+
+// u16 returns the uint16 in the table t at the relative offset i.
+//
+// buf is an optional scratch buffer as per the source.view method.
+func (s *source) u16(buf []byte, t table, i int) (uint16, error) {
+ if i < 0 || uint(t.length) < uint(i+2) {
+ return 0, errInvalidBounds
+ }
+ buf, err := s.view(buf, int(t.offset)+i, 2)
+ if err != nil {
+ return 0, err
+ }
+ return u16(buf), nil
+}
+
+// table is a section of the font data.
+type table struct {
+ offset, length uint32
+}
+
+// Parse parses an SFNT font from a []byte data source.
+func Parse(src []byte) (*Font, error) {
+ f := &Font{src: source{b: src}}
+ if err := f.initialize(); err != nil {
+ return nil, err
+ }
+ return f, nil
+}
+
+// ParseReaderAt parses an SFNT font from an io.ReaderAt data source.
+func ParseReaderAt(src io.ReaderAt) (*Font, error) {
+ f := &Font{src: source{r: src}}
+ if err := f.initialize(); err != nil {
+ return nil, err
+ }
+ return f, nil
+}
+
+// Font is an SFNT font.
+type Font struct {
+ src source
+
+ // https://www.microsoft.com/typography/otspec/otff.htm#otttables
+ // "Required Tables".
+ cmap table
+ head table
+ hhea table
+ hmtx table
+ maxp table
+ name table
+ os2 table
+ post table
+
+ // https://www.microsoft.com/typography/otspec/otff.htm#otttables
+ // "Tables Related to TrueType Outlines".
+ //
+ // This implementation does not support hinting, so it does not read the
+ // cvt, fpgm gasp or prep tables.
+ glyf table
+ loca table
+
+ // https://www.microsoft.com/typography/otspec/otff.htm#otttables
+ // "Tables Related to PostScript Outlines".
+ //
+ // TODO: cff, cff2, vorg?
+
+ // https://www.microsoft.com/typography/otspec/otff.htm#otttables
+ // "Advanced Typographic Tables".
+ //
+ // TODO: base, gdef, gpos, gsub, jstf, math?
+
+ // https://www.microsoft.com/typography/otspec/otff.htm#otttables
+ // "Other OpenType Tables".
+ //
+ // TODO: hdmx, kern, vmtx? Others?
+
+ cached struct {
+ numGlyphs int
+ unitsPerEm Units
+ }
+}
+
+// NumGlyphs returns the number of glyphs in f.
+func (f *Font) NumGlyphs() int { return f.cached.numGlyphs }
+
+// UnitsPerEm returns the number of units per em for f.
+func (f *Font) UnitsPerEm() Units { return f.cached.unitsPerEm }
+
+func (f *Font) initialize() error {
+ if !f.src.valid() {
+ return errInvalidSourceData
+ }
+ var buf []byte
+
+ // https://www.microsoft.com/typography/otspec/otff.htm "Organization of an
+ // OpenType Font" says that "The OpenType font starts with the Offset
+ // Table", which is 12 bytes.
+ buf, err := f.src.view(buf, 0, 12)
+ if err != nil {
+ return err
+ }
+ switch u32(buf) {
+ default:
+ return errInvalidVersion
+ case 0x00010000:
+ // No-op.
+ case 0x4f54544f: // "OTTO".
+ // TODO: support CFF fonts.
+ return errUnsupportedVersion
+ }
+ numTables := int(u16(buf[4:]))
+ if numTables > maxNumTables {
+ return errUnsupportedNumberOfTables
+ }
+
+ // "The Offset Table is followed immediately by the Table Record entries...
+ // sorted in ascending order by tag", 16 bytes each.
+ buf, err = f.src.view(buf, 12, 16*numTables)
+ if err != nil {
+ return err
+ }
+ for b, first, prevTag := buf, true, uint32(0); len(b) > 0; b = b[16:] {
+ tag := u32(b)
+ if first {
+ first = false
+ } else if tag <= prevTag {
+ return errInvalidTableTagOrder
+ }
+ prevTag = tag
+
+ o, n := u32(b[8:12]), u32(b[12:16])
+ if o > maxTableOffset || n > maxTableLength {
+ return errUnsupportedTableOffsetLength
+ }
+ // We ignore the checksums, but "all tables must begin on four byte
+ // boundries [sic]".
+ if o&3 != 0 {
+ return errInvalidTableOffset
+ }
+
+ // Match the 4-byte tag as a uint32. For example, "OS/2" is 0x4f532f32.
+ switch tag {
+ case 0x4f532f32:
+ f.os2 = table{o, n}
+ case 0x636d6170:
+ f.cmap = table{o, n}
+ case 0x676c7966:
+ f.glyf = table{o, n}
+ case 0x68656164:
+ f.head = table{o, n}
+ case 0x68686561:
+ f.hhea = table{o, n}
+ case 0x686d7478:
+ f.hmtx = table{o, n}
+ case 0x6c6f6361:
+ f.loca = table{o, n}
+ case 0x6d617870:
+ f.maxp = table{o, n}
+ case 0x6e616d65:
+ f.name = table{o, n}
+ case 0x706f7374:
+ f.post = table{o, n}
+ }
+ }
+
+ var u uint16
+
+ // https://www.microsoft.com/typography/otspec/head.htm
+ if f.head.length != 54 {
+ return errInvalidHeadTable
+ }
+ u, err = f.src.u16(buf, f.head, 18)
+ if err != nil {
+ return err
+ }
+ if u == 0 {
+ return errInvalidHeadTable
+ }
+ f.cached.unitsPerEm = Units(u)
+
+ // https://www.microsoft.com/typography/otspec/maxp.htm
+ if f.maxp.length != 32 {
+ return errInvalidMaxpTable
+ }
+ u, err = f.src.u16(buf, f.maxp, 4)
+ if err != nil {
+ return err
+ }
+ f.cached.numGlyphs = int(u)
+
+ return nil
+}
diff --git a/font/sfnt/sfnt_test.go b/font/sfnt/sfnt_test.go
new file mode 100644
index 0000000..b80e410
--- /dev/null
+++ b/font/sfnt/sfnt_test.go
@@ -0,0 +1,40 @@
+// 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.
+
+package sfnt
+
+import (
+ "bytes"
+ "testing"
+
+ "golang.org/x/image/font/gofont/goregular"
+)
+
+func TestParse(t *testing.T) {
+ f, err := Parse(goregular.TTF)
+ if err != nil {
+ t.Fatalf("Parse: %v", err)
+ }
+ testFont(t, f)
+}
+
+func TestParseReaderAt(t *testing.T) {
+ f, err := ParseReaderAt(bytes.NewReader(goregular.TTF))
+ if err != nil {
+ t.Fatalf("ParseReaderAt: %v", err)
+ }
+ testFont(t, f)
+}
+
+func testFont(t *testing.T, f *Font) {
+ if got, want := f.UnitsPerEm(), Units(2048); got != want {
+ t.Errorf("UnitsPerEm: got %d, want %d", got, want)
+ }
+ // The exact number of glyphs in goregular.TTF can vary, and future
+ // versions may add more glyphs, but https://blog.golang.org/go-fonts says
+ // that "The WGL4 character set... [has] more than 650 characters in all.
+ if got, want := f.NumGlyphs(), 650; got <= want {
+ t.Errorf("NumGlyphs: got %d, want > %d", got, want)
+ }
+}
