func unescapeEntity()

in html/escape.go [57:163]
87 lines of code
41 McCabe index (conditional complexity)

func unescapeEntity(b []byte, dst, src int, attribute bool) (dst1, src1 int) {
	// https://html.spec.whatwg.org/multipage/syntax.html#consume-a-character-reference

	// i starts at 1 because we already know that s[0] == '&'.
	i, s := 1, b[src:]

	if len(s) <= 1 {
		b[dst] = b[src]
		return dst + 1, src + 1
	}

	if s[i] == '#' {
		if len(s) <= 3 { // We need to have at least "&#.".
			b[dst] = b[src]
			return dst + 1, src + 1
		}
		i++
		c := s[i]
		hex := false
		if c == 'x' || c == 'X' {
			hex = true
			i++
		}

		x := '\x00'
		for i < len(s) {
			c = s[i]
			i++
			if hex {
				if '0' <= c && c <= '9' {
					x = 16*x + rune(c) - '0'
					continue
				} else if 'a' <= c && c <= 'f' {
					x = 16*x + rune(c) - 'a' + 10
					continue
				} else if 'A' <= c && c <= 'F' {
					x = 16*x + rune(c) - 'A' + 10
					continue
				}
			} else if '0' <= c && c <= '9' {
				x = 10*x + rune(c) - '0'
				continue
			}
			if c != ';' {
				i--
			}
			break
		}

		if i <= 3 { // No characters matched.
			b[dst] = b[src]
			return dst + 1, src + 1
		}

		if 0x80 <= x && x <= 0x9F {
			// Replace characters from Windows-1252 with UTF-8 equivalents.
			x = replacementTable[x-0x80]
		} else if x == 0 || (0xD800 <= x && x <= 0xDFFF) || x > 0x10FFFF {
			// Replace invalid characters with the replacement character.
			x = '\uFFFD'
		}

		return dst + utf8.EncodeRune(b[dst:], x), src + i
	}

	// Consume the maximum number of characters possible, with the
	// consumed characters matching one of the named references.

	for i < len(s) {
		c := s[i]
		i++
		// Lower-cased characters are more common in entities, so we check for them first.
		if 'a' <= c && c <= 'z' || 'A' <= c && c <= 'Z' || '0' <= c && c <= '9' {
			continue
		}
		if c != ';' {
			i--
		}
		break
	}

	entityName := string(s[1:i])
	if entityName == "" {
		// No-op.
	} else if attribute && entityName[len(entityName)-1] != ';' && len(s) > i && s[i] == '=' {
		// No-op.
	} else if x := entity[entityName]; x != 0 {
		return dst + utf8.EncodeRune(b[dst:], x), src + i
	} else if x := entity2[entityName]; x[0] != 0 {
		dst1 := dst + utf8.EncodeRune(b[dst:], x[0])
		return dst1 + utf8.EncodeRune(b[dst1:], x[1]), src + i
	} else if !attribute {
		maxLen := len(entityName) - 1
		if maxLen > longestEntityWithoutSemicolon {
			maxLen = longestEntityWithoutSemicolon
		}
		for j := maxLen; j > 1; j-- {
			if x := entity[entityName[:j]]; x != 0 {
				return dst + utf8.EncodeRune(b[dst:], x), src + j + 1
			}
		}
	}

	dst1, src1 = dst+i, src+i
	copy(b[dst:dst1], b[src:src1])
	return dst1, src1
}