init
This commit is contained in:
160
internal/utf8/decode.go
Executable file
160
internal/utf8/decode.go
Executable file
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// Package utf8 implements encoding and decoding of UTF-8 coded numbers.
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package utf8
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import (
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"errors"
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"fmt"
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"io"
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"github.com/mewkiz/flac/internal/ioutilx"
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)
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const (
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tx = 0x80 // 1000 0000
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t2 = 0xC0 // 1100 0000
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t3 = 0xE0 // 1110 0000
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t4 = 0xF0 // 1111 0000
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t5 = 0xF8 // 1111 1000
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t6 = 0xFC // 1111 1100
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t7 = 0xFE // 1111 1110
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t8 = 0xFF // 1111 1111
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maskx = 0x3F // 0011 1111
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mask2 = 0x1F // 0001 1111
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mask3 = 0x0F // 0000 1111
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mask4 = 0x07 // 0000 0111
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mask5 = 0x03 // 0000 0011
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mask6 = 0x01 // 0000 0001
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rune1Max = 1<<7 - 1
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rune2Max = 1<<11 - 1
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rune3Max = 1<<16 - 1
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rune4Max = 1<<21 - 1
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rune5Max = 1<<26 - 1
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rune6Max = 1<<31 - 1
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rune7Max = 1<<36 - 1
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)
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// Decode decodes a "UTF-8" coded number and returns it.
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//
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// ref: http://permalink.gmane.org/gmane.comp.audio.compression.flac.devel/3033
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//
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// Algorithm description:
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// - read one byte B0 from the stream
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// - if B0 = 0xxxxxxx then the read value is B0 -> end
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// - if B0 = 10xxxxxx, the encoding is invalid
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// - if B0 = 11xxxxxx, set L to the number of leading binary 1s minus 1:
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// B0 = 110xxxxx -> L = 1
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// B0 = 1110xxxx -> L = 2
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// B0 = 11110xxx -> L = 3
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// B0 = 111110xx -> L = 4
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// B0 = 1111110x -> L = 5
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// B0 = 11111110 -> L = 6
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// - assign the bits following the encoding (the x bits in the examples) to
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// a variable R with a magnitude of at least 36 bits
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// - loop from 1 to L
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// - left shift R 6 bits
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// - read B from the stream
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// - if B does not match 10xxxxxx, the encoding is invalid
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// - set R = R or <the lower 6 bits from B>
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// - the read value is R
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func Decode(r io.Reader) (x uint64, err error) {
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c0, err := ioutilx.ReadByte(r)
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if err != nil {
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return 0, err
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}
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// 1-byte, 7-bit sequence?
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if c0 < tx {
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// if c0 == 0xxxxxxx
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// total: 7 bits (7)
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return uint64(c0), nil
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}
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// unexpected continuation byte?
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if c0 < t2 {
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// if c0 == 10xxxxxx
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return 0, errors.New("frame.decodeUTF8Int: unexpected continuation byte")
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}
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// get number of continuation bytes and store bits from c0.
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var l int
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switch {
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case c0 < t3:
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// if c0 == 110xxxxx
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// total: 11 bits (5 + 6)
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l = 1
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x = uint64(c0 & mask2)
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case c0 < t4:
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// if c0 == 1110xxxx
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// total: 16 bits (4 + 6 + 6)
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l = 2
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x = uint64(c0 & mask3)
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case c0 < t5:
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// if c0 == 11110xxx
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// total: 21 bits (3 + 6 + 6 + 6)
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l = 3
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x = uint64(c0 & mask4)
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case c0 < t6:
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// if c0 == 111110xx
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// total: 26 bits (2 + 6 + 6 + 6 + 6)
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l = 4
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x = uint64(c0 & mask5)
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case c0 < t7:
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// if c0 == 1111110x
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// total: 31 bits (1 + 6 + 6 + 6 + 6 + 6)
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l = 5
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x = uint64(c0 & mask6)
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case c0 < t8:
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// if c0 == 11111110
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// total: 36 bits (0 + 6 + 6 + 6 + 6 + 6 + 6)
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l = 6
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x = 0
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}
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// store bits from continuation bytes.
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for i := 0; i < l; i++ {
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x <<= 6
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c, err := ioutilx.ReadByte(r)
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if err != nil {
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if err == io.EOF {
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return 0, io.ErrUnexpectedEOF
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}
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return 0, err
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}
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if c < tx || t2 <= c {
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// if c != 10xxxxxx
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return 0, errors.New("frame.decodeUTF8Int: expected continuation byte")
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}
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x |= uint64(c & maskx)
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}
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// check if number representation is larger than necessary.
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switch l {
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case 1:
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if x <= rune1Max {
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return 0, fmt.Errorf("frame.decodeUTF8Int: larger number representation than necessary; x (%d) stored in %d bytes, could be stored in %d bytes", x, l+1, l)
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}
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case 2:
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if x <= rune2Max {
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return 0, fmt.Errorf("frame.decodeUTF8Int: larger number representation than necessary; x (%d) stored in %d bytes, could be stored in %d bytes", x, l+1, l)
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}
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case 3:
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if x <= rune3Max {
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return 0, fmt.Errorf("frame.decodeUTF8Int: larger number representation than necessary; x (%d) stored in %d bytes, could be stored in %d bytes", x, l+1, l)
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}
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case 4:
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if x <= rune4Max {
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return 0, fmt.Errorf("frame.decodeUTF8Int: larger number representation than necessary; x (%d) stored in %d bytes, could be stored in %d bytes", x, l+1, l)
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}
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case 5:
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if x <= rune5Max {
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return 0, fmt.Errorf("frame.decodeUTF8Int: larger number representation than necessary; x (%d) stored in %d bytes, could be stored in %d bytes", x, l+1, l)
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}
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case 6:
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if x <= rune6Max {
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return 0, fmt.Errorf("frame.decodeUTF8Int: larger number representation than necessary; x (%d) stored in %d bytes, could be stored in %d bytes", x, l+1, l)
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}
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}
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return x, nil
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}
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72
internal/utf8/encode.go
Executable file
72
internal/utf8/encode.go
Executable file
@@ -0,0 +1,72 @@
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package utf8
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import (
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"io"
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"github.com/mewkiz/flac/internal/ioutilx"
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"github.com/mewkiz/pkg/errutil"
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)
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// Encode encodes x as a "UTF-8" coded number.
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func Encode(w io.Writer, x uint64) error {
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// 1-byte, 7-bit sequence?
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if x <= rune1Max {
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if err := ioutilx.WriteByte(w, byte(x)); err != nil {
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return errutil.Err(err)
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}
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return nil
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}
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// get number of continuation bytes and store bits of c0.
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var (
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// number of continuation bytes.,
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l int
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// bits of c0.
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bits uint64
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)
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switch {
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case x <= rune2Max:
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// if c0 == 110xxxxx
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// total: 11 bits (5 + 6)
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l = 1
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bits = t2 | (x>>6)&mask2
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case x <= rune3Max:
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// if c0 == 1110xxxx
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// total: 16 bits (4 + 6 + 6)
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l = 2
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bits = t3 | (x>>(6*2))&mask3
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case x <= rune4Max:
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// if c0 == 11110xxx
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// total: 21 bits (3 + 6 + 6 + 6)
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l = 3
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bits = t4 | (x>>(6*3))&mask4
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case x <= rune5Max:
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// if c0 == 111110xx
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// total: 26 bits (2 + 6 + 6 + 6 + 6)
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l = 4
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bits = t5 | (x>>(6*4))&mask5
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case x <= rune6Max:
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// if c0 == 1111110x
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// total: 31 bits (1 + 6 + 6 + 6 + 6 + 6)
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l = 5
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bits = t6 | (x>>(6*5))&mask6
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case x <= rune7Max:
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// if c0 == 11111110
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// total: 36 bits (0 + 6 + 6 + 6 + 6 + 6 + 6)
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l = 6
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bits = 0
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}
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// Store bits of c0.
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if err := ioutilx.WriteByte(w, byte(bits)); err != nil {
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return errutil.Err(err)
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}
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// Store continuation bytes.
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for i := l - 1; i >= 0; i-- {
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bits := tx | (x>>uint(6*i))&maskx
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if err := ioutilx.WriteByte(w, byte(bits)); err != nil {
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return errutil.Err(err)
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}
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}
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return nil
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}
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