builtin: deprecate string.split_by_whitespace in favor of fields()

vlib/builtin/string.v

@@ -1616,8 +1616,32 @@ pub fn (s string) repeat(count int) string {
 // fields returns a string array of the string split by `\t` and ` `
 // Example: assert '\t\tv = v'.fields() == ['', '', 'v', '=', 'v']
 pub fn (s string) fields() []string {
-	// TODO do this in a better way
-	return s.replace('\t', ' ').split(' ')
+	mut res := []string{}
+	mut word_start := 0
+	mut word_end := 0
+	mut is_in_word := false
+	mut is_space := false
+	for i, c in s {
+		is_space = c in [` `, `\t`, `\n`]
+		if !is_in_word && !is_space {
+			word_start = i
+			is_in_word = true
+			continue
+		}
+		if is_space && is_in_word {
+			word_end = i
+			res << s[word_start..word_end]
+			is_in_word = false
+			word_end = 0
+			word_start = 0
+			continue
+		}
+	}
+	if is_in_word && word_start > 0 {
+		// collect the remainder word at the end
+		res << s[word_start..s.len]
+	}
+	return res
 }
 
 // strip_margin allows multi-line strings to be formatted in a way that removes white-space
@@ -1684,31 +1708,8 @@ pub fn (s string) strip_margin_custom(del byte) string {
 
 // split_by_whitespace - extract only the non whitespace tokens/words from the given string `s`.
 // example: '  sss   ssss'.split_by_whitespace() => ['sss', 'ssss']
+
+[deprecated: 'use string.fields() instead']
 pub fn (s string) split_by_whitespace() []string {
-	mut res := []string{}
-	mut word_start := 0
-	mut word_end := 0
-	mut is_in_word := false
-	mut is_space := false
-	for i, c in s {
-		is_space = c in [` `, `\t`, `\n`]
-		if !is_in_word && !is_space {
-			word_start = i
-			is_in_word = true
-			continue
-		}
-		if is_space && is_in_word {
-			word_end = i
-			res << s[word_start..word_end]
-			is_in_word = false
-			word_end = 0
-			word_start = 0
-			continue
-		}
-	}
-	if is_in_word && word_start > 0 {
-		// collect the remainder word at the end
-		res << s[word_start..s.len]
-	}
-	return res
+	return s.fields()
 }