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PhutilUTF8StringTruncator.php
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Sun, Nov 24, 06:08

PhutilUTF8StringTruncator.php
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<?php
/**
* Truncate a UTF-8 string to a some maximum number of bytes, codepoints, or
* glyphs.
*
* This class takes some UTF-8 string as input, and emits a valid UTF-8 string
* which is no longer than given byte, codepoint, or glyph limits.
*
* $short = id(new PhutilUTF8StringTruncator())
* ->setMaximumGlyphs(80)
* ->truncateString($long);
*
* Byte limits restrict the number of bytes the result may contain. They are
* appropriate when you care about how much storage a string requires.
*
* Codepoint limits restrict the number of codepoints the result may contain.
* Since codepoints may have up to 4 bytes, the resulting strings may require
* have more than this many bytes. This kind of limit is appropriate when you
* are using UTF-8 storage.
*
* Glyph limits restrict the display size of the string. Because a single glyph
* may have an arbitrary number of combining characters, this does not impose
* a storage size limit on the string: a string with only one glyph may require
* an arbitrarily large number of bytes.
*
* You can set more than one limit; the smallest limit will be used.
*
* NOTE: This function makes a best effort to apply some reasonable rules but
* will not work well for the full range of unicode languages.
*/
final class PhutilUTF8StringTruncator extends Phobject {
private $maximumBytes;
private $maximumCodepoints;
private $maximumGlyphs;
private $minimumLimit;
private $terminator = "\xE2\x80\xA6";
private $terminatorBytes = 3;
private $terminatorCodepoints = 1;
private $terminatorGlyphs = 1;
public function setMaximumBytes($maximum_bytes) {
$this->maximumBytes = $maximum_bytes;
$this->didUpdateMaxima();
return $this;
}
public function setMaximumCodepoints($maximum_codepoints) {
$this->maximumCodepoints = $maximum_codepoints;
$this->didUpdateMaxima();
return $this;
}
public function setMaximumGlyphs($maximum_glyphs) {
$this->maximumGlyphs = $maximum_glyphs;
$this->didUpdateMaxima();
return $this;
}
private function didUpdateMaxima() {
$this->minimumLimit = INF;
if ($this->maximumBytes) {
$this->minimumLimit = min($this->minimumLimit, $this->maximumBytes);
}
if ($this->maximumCodepoints) {
$this->minimumLimit = min($this->minimumLimit, $this->maximumCodepoints);
}
if ($this->maximumGlyphs) {
$this->minimumLimit = min($this->minimumLimit, $this->maximumGlyphs);
}
}
public function setTerminator($terminator) {
$this->terminator = $terminator;
$this->terminatorBytes = strlen($terminator);
$this->terminatorCodepoints = count(phutil_utf8v($terminator));
$this->terminatorGlyphs = count(phutil_utf8v_combined($terminator));
return $this;
}
public function truncateString($string) {
// First, check if the string has fewer bytes than the most restrictive
// limit. Codepoints and glyphs always take up at least one byte, so we can
// just return the string unmodified if we're under all of the limits.
$byte_len = strlen($string);
if ($byte_len <= $this->minimumLimit) {
return $string;
}
// We're going to vectorize the string so we can deal with it in terms
// of unicode characters. If the string is huge (like 10MB) and we are
// only extracting a tiny piece of it (like the first 1024 bytes), we
// want to avoid vectorizing the entire string in cases where there is
// no possibility that we'll need all of it. Try to compute a "hard limit":
// an upper bound on the number of bytes we can ever need to process.
$hard_limits = array();
if ($this->maximumBytes) {
$hard_limits[] = $this->maximumBytes;
}
if ($this->maximumCodepoints) {
// No UTF8 character is longer than 6 bytes, so we can impose a ceiling
// if we have a codepoint limit.
$hard_limits[] = ($this->maximumCodepoints * 6);
}
if ($hard_limits) {
// Add a few more bytes onto the end so that we have a little more of
// the string than we actually need and can get the right terminator
// behavior.
$hard_limit = max($hard_limits) + 32;
} else {
$hard_limit = null;
}
// Build a vector of characters first.
$string_pv = phutil_utf8v($string, $hard_limit);
$point_len = count($string_pv);
// We always need the combined vector, even if we're only doing byte or
// codepoint truncation, because we don't want to truncate to half of a
// combining character.
$string_gv = phutil_utf8v_combine_characters($string_pv);
$glyph_len = count($string_gv);
// Now, check if we're still over the limits. For example, a string may
// be over the raw byte limit but under the glyph limit if it contains
// several multibyte characters.
$too_long = false;
if ($this->maximumBytes && ($byte_len > $this->maximumBytes)) {
$too_long = true;
}
if ($this->maximumCodepoints && ($point_len > $this->maximumCodepoints)) {
$too_long = true;
}
if ($this->maximumGlyphs && ($glyph_len > $this->maximumGlyphs)) {
$too_long = true;
}
if (!$too_long) {
return $string;
}
// This string is legitimately longer than at least one of the limits, so
// we need to truncate it. Find the minimum cutoff point: this is the last
// glyph we can possibly return while satisfying the limits and having space
// for the terminator.
$cutoff = $glyph_len;
if ($this->maximumBytes) {
if ($byte_len <= $this->maximumBytes) {
$cutoff = $glyph_len;
} else {
$bytes = $this->terminatorBytes;
for ($ii = 0; $ii < $glyph_len; $ii++) {
$bytes += strlen($string_gv[$ii]);
if ($bytes > $this->maximumBytes) {
$cutoff = $ii;
break;
}
}
}
}
if ($this->maximumCodepoints) {
if ($point_len <= $this->maximumCodepoints) {
$cutoff = min($cutoff, $glyph_len);
} else {
$points = 0;
for ($ii = 0; $ii < $glyph_len; $ii++) {
$glyph_bytes = strlen($string_gv[$ii]);
while ($points < $point_len) {
$glyph_bytes -= strlen($string_pv[$points]);
$points++;
if ($glyph_bytes <= 0) {
break;
}
}
$points_total = $points + $this->terminatorCodepoints;
if ($points_total > $this->maximumCodepoints) {
$cutoff = min($cutoff, $ii);
break;
}
}
}
}
if ($this->maximumGlyphs) {
if ($glyph_len <= $this->maximumGlyphs) {
$cutoff = min($cutoff, $glyph_len);
} else {
$cutoff = min($cutoff, $this->maximumGlyphs - $this->terminatorGlyphs);
}
}
// If we don't have enough characters for anything, just return the
// terminator.
if ($cutoff <= 0) {
return $this->terminator;
}
// Otherwise, we're going to try to cut the string off somewhere reasonable
// rather than somewhere arbitrary.
// NOTE: This is not complete, and there are many other word boundary
// characters and reasonable places to break words in the UTF-8 character
// space. For now, this gives us reasonable behavior for latin languages. We
// don't necessarily have access to PCRE+Unicode so there isn't a great way
// for us to look up character attributes.
// If we encounter these, prefer to break on them instead of cutting the
// string off in the middle of a word.
static $break_characters = array(
' ' => true,
"\n" => true,
';' => true,
':' => true,
'[' => true,
'(' => true,
',' => true,
'-' => true,
);
// If we encounter these, shorten to this character exactly without
// appending the terminal.
static $stop_characters = array(
'.' => true,
'!' => true,
'?' => true,
);
// Search backward in the string, looking for reasonable places to break it.
$word_boundary = null;
$stop_boundary = null;
$any_nonboundary = false;
// If we do a word break with a terminal, we have to look beyond at least
// the number of characters in the terminal. If the terminal is longer than
// the required length, we'll skip this whole block and return it on its
// own.
// Only search backward for a while. At some point we don't get a better
// result by looking through the whole string, and if this is "MMM..." or
// a non-latin language without word break characters we're just wasting
// time.
// See PHI654. We also only look for a break near the end of the text,
// relative to the length of the text. If the text is something like
// "O123: MMMMMM..." or "See path/to/long/thing", we want to cut the very
// long word in half, not just render "O123..." or "See...".
$search = max(0, $cutoff - 256, $cutoff / 2);
for ($ii = min($cutoff, $glyph_len - 1); $ii >= $search; $ii--) {
$c = $string_gv[$ii];
if (isset($break_characters[$c])) {
$word_boundary = $ii;
} else if (isset($stop_characters[$c])) {
$stop_boundary = $ii + 1;
break;
} else {
$any_nonboundary = true;
if ($word_boundary !== null) {
break;
}
}
}
if ($stop_boundary !== null) {
// We found a character like ".". Cut the string there, without appending
// the terminal.
$string_part = array_slice($string_gv, 0, $stop_boundary);
return implode('', $string_part);
}
// If we didn't find any boundary characters or we found ONLY boundary
// characters, just break at the maximum character length.
if ($word_boundary === null || !$any_nonboundary) {
$word_boundary = $cutoff;
}
$string_part = array_slice($string_gv, 0, $word_boundary);
$string_part = implode('', $string_part);
return $string_part.$this->terminator;
}
}

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