github.com/primecitizens/pcz/std@v0.2.1/text/unicode/utf8/decode.go

// SPDX-License-Identifier: Apache-2.0
// Copyright 2023 The Prime Citizens
//
// 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.

// Package utf8 implements functions and constants to support text encoded in
// UTF-8. It includes functions to translate between runes and UTF-8 byte sequences.
// See https://en.wikipedia.org/wiki/UTF-8
package utf8

import (
	. "github.com/primecitizens/pcz/std/text/unicode/common"
)

// Valid reports whether s consists entirely of valid UTF-8-encoded runes.
func Valid(s string) bool {
	// Fast path. Check for and skip 8 bytes of ASCII characters per iteration.
	for len(s) >= 8 {
		// Combining two 32 bit loads allows the same code to be used
		// for 32 and 64 bit platforms.
		// The compiler can generate a 32bit load for first32 and second32
		// on many platforms. See test/codegen/memcombine.go.
		first32 := uint32(s[0]) | uint32(s[1])<<8 | uint32(s[2])<<16 | uint32(s[3])<<24
		second32 := uint32(s[4]) | uint32(s[5])<<8 | uint32(s[6])<<16 | uint32(s[7])<<24
		if (first32|second32)&0x80808080 != 0 {
			// Found a non ASCII byte (>= RuneSelf).
			break
		}
		s = s[8:]
	}

	for i, n := 0, len(s); i < n; {
		si := s[i]
		if si < RuneSelf {
			i++
			continue
		}
		x := first[si]
		if x == xx {
			return false // Illegal starter byte.
		}
		size := int(x & 7)
		if i+size > n {
			return false // Short or invalid.
		}
		accept := acceptRanges[x>>4]
		if c := s[i+1]; c < accept.lo || accept.hi < c {
			return false
		} else if size == 2 {
		} else if c := s[i+2]; c < locb || hicb < c {
			return false
		} else if size == 3 {
		} else if c := s[i+3]; c < locb || hicb < c {
			return false
		}
		i += size
	}
	return true
}

// Count returns the number of runes in s. Erroneous and short
// encodings are treated as single runes of width 1 byte.
func Count(s string) (n int) {
	for i := 0; i < len(s); n++ {
		c := s[i]
		if c < RuneSelf {
			// ASCII fast path
			i++
			continue
		}
		x := first[c]
		if x == xx {
			i++ // invalid.
			continue
		}
		size := int(x & 7)
		if i+size > len(s) {
			i++ // Short or invalid.
			continue
		}
		accept := acceptRanges[x>>4]
		if c := s[i+1]; c < accept.lo || accept.hi < c {
			size = 1
		} else if size == 2 {
		} else if c := s[i+2]; c < locb || hicb < c {
			size = 1
		} else if size == 3 {
		} else if c := s[i+3]; c < locb || hicb < c {
			size = 1
		}
		i += size
	}
	return n
}

// First unpacks the first UTF-8 encoding in s and returns the rune and
// its width in bytes. If p is empty it returns (RuneError, 0).
//
// Otherwise, if the encoding is invalid, it returns
// (RuneError, 1). Both are impossible results for correct,
// non-empty UTF-8.
//
// An encoding is invalid if it is incorrect UTF-8, encodes a rune that is
// out of range, or is not the shortest possible UTF-8 encoding for the
// value. No other validation is performed.
func First(s string) (r rune, size int) {
	if len(s) == 0 {
		return RuneError, 0
	}
	s0 := s[0]
	x := first[s0]
	if x >= as {
		// The following code simulates an additional check for x == xx and
		// handling the ASCII and invalid cases accordingly. This mask-and-or
		// approach prevents an additional branch.
		mask := rune(x) << 31 >> 31 // Create 0x0000 or 0xFFFF.
		return rune(s[0])&^mask | RuneError&mask, 1
	}
	sz := int(x & 7)
	accept := acceptRanges[x>>4]
	if len(s) < sz {
		return RuneError, 1
	}
	s1 := s[1]
	if s1 < accept.lo || accept.hi < s1 {
		return RuneError, 1
	}
	if sz <= 2 { // <= instead of == to help the compiler eliminate some bounds checks
		return rune(s0&mask2)<<6 | rune(s1&maskx), 2
	}
	s2 := s[2]
	if s2 < locb || hicb < s2 {
		return RuneError, 1
	}
	if sz <= 3 {
		return rune(s0&mask3)<<12 | rune(s1&maskx)<<6 | rune(s2&maskx), 3
	}
	s3 := s[3]
	if s3 < locb || hicb < s3 {
		return RuneError, 1
	}
	return rune(s0&mask4)<<18 | rune(s1&maskx)<<12 | rune(s2&maskx)<<6 | rune(s3&maskx), 4
}

// Last unpacks the last UTF-8 encoding in p and returns the rune and
// its width in bytes. If p is empty it returns (RuneError, 0). Otherwise, if
// the encoding is invalid, it returns (RuneError, 1). Both are impossible
// results for correct, non-empty UTF-8.
//
// An encoding is invalid if it is incorrect UTF-8, encodes a rune that is
// out of range, or is not the shortest possible UTF-8 encoding for the
// value. No other validation is performed.
func Last(s string) (r rune, size int) {
	end := len(s)
	if end == 0 {
		return RuneError, 0
	}
	start := end - 1
	r = rune(s[start])
	if r < RuneSelf {
		return r, 1
	}
	// guard against O(n^2) behavior when traversing
	// backwards through strings with long sequences of
	// invalid UTF-8.
	lim := end - MaxRuneLen
	if lim < 0 {
		lim = 0
	}
	for start--; start >= lim; start-- {
		if RuneStart(s[start]) {
			break
		}
	}
	if start < 0 {
		start = 0
	}
	r, size = First(s[start:end])
	if start+size != end {
		return RuneError, 1
	}
	return r, size
}

// FullRune reports whether the bytes in s begin with a full UTF-8 encoding of a rune.
//
// An invalid encoding is considered a full Rune since it will convert as a width-1 error rune.
func FullRune(s string) bool {
	if len(s) == 0 {
		return false
	}
	x := first[s[0]]
	if len(s) >= int(x&7) {
		return true // ASCII, invalid, or valid.
	}
	// Must be short or invalid.
	accept := acceptRanges[x>>4]
	if len(s) > 1 && (s[1] < accept.lo || accept.hi < s[1]) {
		return true
	} else if len(s) > 2 && (s[2] < locb || hicb < s[2]) {
		return true
	}
	return false
}

// RuneStart reports whether the byte could be the first byte of an encoded,
// possibly invalid rune. Second and subsequent bytes always have the top two
// bits set to 10.
func RuneStart(b byte) bool {
	return b&0xC0 != 0x80
}

// RuneValid reports whether r can be legally encoded as UTF-8.
// Code points that are out of range or a surrogate half are illegal.
func RuneValid(r rune) bool {
	switch {
	case 0 <= r && r < SurrogateMin,
		SurrogateMax < r && r <= MaxRune:
		return true
	}
	return false
}