Improvements in performance and memory usage

I profiled fzf and it turned out that it was spending significant amount
of time repeatedly converting character arrays into Unicode codepoints.
This commit greatly improves search performance after the initial scan
by memoizing the converted results.

This commit also addresses the problem of unbounded memory usage of fzf.
fzf is a short-lived process that usually processes small input, so it
was implemented to cache the intermediate results very aggressively with
no notion of cache expiration/eviction. I still think a proper
implementation of caching scheme is definitely an overkill. Instead this
commit introduces limits to the maximum size (or minimum selectivity) of
the intermediate results that can be cached.

src/tokenizer.go

@@ -16,15 +16,9 @@ type Range struct {
 	end   int
 }
 
-// Transformed holds the result of tokenization and transformation
-type Transformed struct {
-	whole *string
-	parts []Token
-}
-
 // Token contains the tokenized part of the strings and its prefix length
 type Token struct {
-	text         *string
+	text         *[]rune
 	prefixLength int
 }
 
@@ -81,8 +75,8 @@ func withPrefixLengths(tokens []string, begin int) []Token {
 	for idx, token := range tokens {
 		// Need to define a new local variable instead of the reused token to take
 		// the pointer to it
-		str := token
-		ret[idx] = Token{text: &str, prefixLength: prefixLength}
+		runes := []rune(token)
+		ret[idx] = Token{text: &runes, prefixLength: prefixLength}
 		prefixLength += len([]rune(token))
 	}
 	return ret
@@ -142,33 +136,40 @@ func Tokenize(str *string, delimiter *regexp.Regexp) []Token {
 	return withPrefixLengths(tokens, 0)
 }
 
-func joinTokens(tokens []Token) string {
+func joinTokens(tokens *[]Token) *string {
 	ret := ""
-	for _, token := range tokens {
-		ret += *token.text
+	for _, token := range *tokens {
+		ret += string(*token.text)
 	}
-	return ret
+	return &ret
+}
+
+func joinTokensAsRunes(tokens *[]Token) *[]rune {
+	ret := []rune{}
+	for _, token := range *tokens {
+		ret = append(ret, *token.text...)
+	}
+	return &ret
 }
 
 // Transform is used to transform the input when --with-nth option is given
-func Transform(tokens []Token, withNth []Range) *Transformed {
+func Transform(tokens []Token, withNth []Range) *[]Token {
 	transTokens := make([]Token, len(withNth))
 	numTokens := len(tokens)
-	whole := ""
 	for idx, r := range withNth {
-		part := ""
+		part := []rune{}
 		minIdx := 0
 		if r.begin == r.end {
 			idx := r.begin
 			if idx == rangeEllipsis {
-				part += joinTokens(tokens)
+				part = append(part, *joinTokensAsRunes(&tokens)...)
 			} else {
 				if idx < 0 {
 					idx += numTokens + 1
 				}
 				if idx >= 1 && idx <= numTokens {
 					minIdx = idx - 1
-					part += *tokens[idx-1].text
+					part = append(part, *tokens[idx-1].text...)
 				}
 			}
 		} else {
@@ -195,11 +196,10 @@ func Transform(tokens []Token, withNth []Range) *Transformed {
 			minIdx = util.Max(0, begin-1)
 			for idx := begin; idx <= end; idx++ {
 				if idx >= 1 && idx <= numTokens {
-					part += *tokens[idx-1].text
+					part = append(part, *tokens[idx-1].text...)
 				}
 			}
 		}
-		whole += part
 		var prefixLength int
 		if minIdx < numTokens {
 			prefixLength = tokens[minIdx].prefixLength
@@ -208,7 +208,5 @@ func Transform(tokens []Token, withNth []Range) *Transformed {
 		}
 		transTokens[idx] = Token{&part, prefixLength}
 	}
-	return &Transformed{
-		whole: &whole,
-		parts: transTokens}
+	return &transTokens
 }