task 005: use strings.Builder for string concatenation in loops

Replace per-rune += string(r) heap allocations with strings.Builder in
SanitizeFilename (internal/utils.go) and splitLines (internal/batch/processor.go).
Both now call Grow/Reset appropriately to pre-allocate capacity.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

2 files changed, 16 insertions, 12 deletions

diff --git a/internal/batch/processor.go b/internal/batch/processor.go
index 6867c3c..314d67f 100644
--- a/internal/batch/processor.go
+++ b/internal/batch/processor.go
@@ -103,20 +103,21 @@ func parseBatchLine(line string) *WordEntry {
 	}
 }
 
-// splitLines splits a string by newlines
+// splitLines splits a string by newlines, handling both \n and \r\n line endings.
+// Uses strings.Builder to avoid per-character heap allocations from += concatenation.
 func splitLines(s string) []string {
 	var lines []string
-	current := ""
+	var current strings.Builder
 	for _, r := range s {
 		if r == '\n' {
-			lines = append(lines, current)
-			current = ""
+			lines = append(lines, current.String())
+			current.Reset()
 		} else if r != '\r' {
-			current += string(r)
+			current.WriteRune(r)
 		}
 	}
-	if current != "" {
-		lines = append(lines, current)
+	if current.Len() > 0 {
+		lines = append(lines, current.String())
 	}
 	return lines
 }
diff --git a/internal/utils.go b/internal/utils.go
index 779b95a..c135f9b 100644
--- a/internal/utils.go
+++ b/internal/utils.go
@@ -4,6 +4,7 @@ import (
 	"crypto/md5"
 	"encoding/hex"
 	"fmt"
+	"strings"
 	"time"
 )
 
@@ -22,17 +23,19 @@ func GenerateCardID(bulgarianWord string) string {
 	return fmt.Sprintf("%d_%s", epochMillis, hashStr)
 }
 
-// SanitizeFilename creates a safe filename from a string
+// SanitizeFilename creates a safe filename from a string.
+// Uses strings.Builder to avoid per-rune heap allocations.
 func SanitizeFilename(s string) string {
-	result := ""
+	var b strings.Builder
+	b.Grow(len(s))
 	for _, r := range s {
 		if isAlphaNumeric(r) || r == '-' || r == '_' {
-			result += string(r)
+			b.WriteRune(r)
 		} else {
-			result += "_"
+			b.WriteByte('_')
 		}
 	}
-	return result
+	return b.String()
 }
 
 // isAlphaNumeric checks if a rune is alphanumeric