refactor: improve token handling in getAvailableTokensForContent and truncateContentByTokens functions

tokens.go

@@ -6,17 +6,16 @@ import (
 	"text/template"
 
 	"github.com/tmc/langchaingo/llms"
-	"github.com/tmc/langchaingo/textsplitter"
 )
 
 // getAvailableTokensForContent calculates how many tokens are available for content
 // by rendering the template with empty content and counting tokens
-func getAvailableTokensForContent(template *template.Template, data map[string]interface{}) (int, error) {
+func getAvailableTokensForContent(tmpl *template.Template, data map[string]interface{}) (int, error) {
 	if tokenLimit <= 0 {
 		return 0, nil // No limit when disabled
 	}
 
-	// Create a copy of data and set Content to empty
+	// Create a copy of data and set "Content" to empty
 	templateData := make(map[string]interface{})
 	for k, v := range data {
 		templateData[k] = v
@@ -25,7 +24,7 @@ func getAvailableTokensForContent(template *template.Template, data map[string]i
 
 	// Execute template with empty content
 	var promptBuffer bytes.Buffer
-	if err := template.Execute(&promptBuffer, templateData); err != nil {
+	if err := tmpl.Execute(&promptBuffer, templateData); err != nil {
 		return 0, fmt.Errorf("error executing template: %v", err)
 	}
 
@@ -51,39 +50,50 @@ func getTokenCount(content string) (int, error) {
 	return llms.CountTokens(llmModel, content), nil
 }
 
-// truncateContentByTokens truncates the content to fit within the specified token limit
+// truncateContentByTokens truncates the content so that its token count does not exceed availableTokens.
+// This implementation uses a binary search on runes to find the longest prefix whose token count is within the limit.
+// If availableTokens is 0 or negative, the original content is returned.
 func truncateContentByTokens(content string, availableTokens int) (string, error) {
 	if availableTokens <= 0 || tokenLimit <= 0 {
 		return content, nil
 	}
-	tokenCount, err := getTokenCount(content)
+	totalTokens, err := getTokenCount(content)
 	if err != nil {
 		return "", fmt.Errorf("error counting tokens: %v", err)
 	}
-	if tokenCount <= availableTokens {
+	if totalTokens <= availableTokens {
 		return content, nil
 	}
 
-	splitter := textsplitter.NewTokenSplitter(
-		textsplitter.WithChunkSize(availableTokens),
-		textsplitter.WithChunkOverlap(0),
-		textsplitter.WithModelName(llmModel),
-	)
-	chunks, err := splitter.SplitText(content)
+	// Convert content to runes for safe slicing.
+	runes := []rune(content)
+	low := 0
+	high := len(runes)
+	validCut := 0
+
+	for low <= high {
+		mid := (low + high) / 2
+		substr := string(runes[:mid])
+		count, err := getTokenCount(substr)
 		if err != nil {
-		return "", fmt.Errorf("error splitting content: %v", err)
+			return "", fmt.Errorf("error counting tokens in substring: %v", err)
+		}
+		if count <= availableTokens {
+			validCut = mid
+			low = mid + 1
+		} else {
+			high = mid - 1
+		}
 	}
 
-	// Validate first chunk's token count
-	firstChunk := chunks[0]
-	chunkTokens, err := getTokenCount(firstChunk)
+	truncated := string(runes[:validCut])
+	// Final verification
+	finalTokens, err := getTokenCount(truncated)
 	if err != nil {
-		return "", fmt.Errorf("error counting tokens in chunk: %v", err)
+		return "", fmt.Errorf("error counting tokens in final truncated content: %v", err)
 	}
-	if chunkTokens > availableTokens {
-		return "", fmt.Errorf("first chunk uses %d tokens which exceeds the limit of %d tokens", chunkTokens, availableTokens)
+	if finalTokens > availableTokens {
+		return "", fmt.Errorf("truncated content still exceeds the available token limit")
 	}
-
-	// return the first chunk
-	return firstChunk, nil
+	return truncated, nil
 }