32 lines
1.1 KiB
Go
32 lines
1.1 KiB
Go
// Copyright (c) Tailscale Inc & AUTHORS
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// SPDX-License-Identifier: BSD-3-Clause
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// Package truncate provides a utility function for safely truncating UTF-8
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// strings to a fixed length, respecting multi-byte codepoints.
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package truncate
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// String returns a prefix of a UTF-8 string s, having length no greater than n
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// bytes. If s exceeds this length, it is truncated at a point ≤ n so that the
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// result does not end in a partial UTF-8 encoding. If s is less than or equal
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// to this length, it is returned unmodified.
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func String[String ~string | ~[]byte](s String, n int) String {
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if n >= len(s) {
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return s
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}
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// Back up until we find the beginning of a UTF-8 encoding.
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for n > 0 && s[n-1]&0xc0 == 0x80 { // 0x10... is a continuation byte
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n--
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}
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// If we're at the beginning of a multi-byte encoding, back up one more to
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// skip it. It's possible the value was already complete, but it's simpler
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// if we only have to check in one direction.
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//
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// Otherwise, we have a single-byte code (0x00... or 0x01...).
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if n > 0 && s[n-1]&0xc0 == 0xc0 { // 0x11... starts a multibyte encoding
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n--
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}
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return s[:n]
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}
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