builtin: add a non allocating rune.length_in_bytes() method and tests for it

vlib/builtin/rune.v

@@ -53,3 +53,21 @@ pub fn (c rune) repeat(count int) string {
 	res := unsafe { utf32_to_str_no_malloc(u32(c), &buffer[0]) }
 	return res.repeat(count)
 }
+
+pub fn (c rune) length_in_bytes() int {
+	code := u32(c)
+	if code <= 0x7F {
+		return 1
+	} else if code <= 0x7FF {
+		return 2
+	} else if 0xD800 <= code && code <= 0xDFFF {
+		// between min and max for surrogates
+		return -1
+	} else if code <= 0xFFFF {
+		return 3
+	} else if code <= 0x10FFFF {
+		// 0x10FFFF is the maximum valid unicode code point
+		return 4
+	}
+	return -1
+}

vlib/builtin/rune_test.v

@@ -11,3 +11,27 @@ fn test_repeat() {
 	assert r1.repeat(0) == ''
 	assert r2.repeat(0) == ''
 }
+
+fn test_length_in_bytes() {
+	assert rune(0x0).length_in_bytes() == 1
+	assert `A`.length_in_bytes() == 1 // latin letter
+	assert rune(0x7F).length_in_bytes() == 1
+	//
+	assert rune(0x80).length_in_bytes() == 2
+	assert `Д`.length_in_bytes() == 2 // cyrillic letter
+	assert rune(0x7FF).length_in_bytes() == 2
+	//
+	assert rune(0x800).length_in_bytes() == 3
+	assert `喂`.length_in_bytes() == 3 // hey
+	assert rune(0xFFFF).length_in_bytes() == 3
+	//
+	assert rune(0xD800).length_in_bytes() == -1 // min for surrogates
+	assert rune(0xD866).length_in_bytes() == -1 // invalid
+	assert rune(0xDFFF).length_in_bytes() == -1 // max for surrogates
+	//
+	assert rune(0x100000).length_in_bytes() == 4
+	assert rune(0x10FFD7).length_in_bytes() == 4 // "Supplementary Private Use Area-B" ¯\_(ツ)_/¯
+	assert rune(0x10FFFF).length_in_bytes() == 4
+	//
+	assert rune(0x110000).length_in_bytes() == -1
+}