Avoid reading all csv data into memory

cmd/md-fmt/main.go

@@ -6,6 +6,7 @@ import (
 	"encoding/csv"
 	"flag"
 	"fmt"
+	"io"
 	"log"
 	"os"
 	"strings"
@@ -31,52 +32,76 @@ func main() {
 	read.TrimLeadingSpace = true
 	read.LazyQuotes = *lazyQuotes
 
-	records, err := read.ReadAll()
+	rec, err := read.Read()
 	if err != nil {
-		log.Fatalf("failed to load records in memory: %s\n", err)
+		log.Fatalf("error reading from csv file: %s", err)
 	}
 
-	widths := make([]int, len(records[0]))
-	for _, row := range records {
-		for i, col := range row {
+	widths := make([]int, len(rec))
+	for i, col := range rec {
 		widths[i] = max(widths[i], len(col))
 	}
+	for {
+		rec, err := read.Read()
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			log.Fatalf("error reading from csv file: %s", err)
 		}
 
-	c := make([]string, len(records[0]))
-	log.Println(records[0], len(c))
+		for i, col := range rec {
+			widths[i] = max(widths[i], len(col))
+		}
+	}
+
+	c := make([]string, len(widths))
 	for i := 0; i < len(c); i++ {
 		c[i] = " %-*s "
 	}
 	pattern := fmt.Sprintf("|%s|\n", strings.Join(c, "|"))
-	log.Println(pattern)
 
-	sb := strings.Builder{}
+	// Reset file descriptor cursor and take new CSV reader from it
+	fd.Seek(0, 0)
+	read = csv.NewReader(fd)
+	read.Comma = sep
+	read.TrimLeadingSpace = true
+	read.LazyQuotes = *lazyQuotes
 
 	// Format header row
+	rec, err = read.Read()
+	if err != nil {
+		log.Fatalf("failed to read next csv record: %s", err)
+	}
+
 	curr := make([]any, 0, 2*len(widths))
 	for i := range widths {
-		curr = append(curr, widths[i], records[0][i])
+		curr = append(curr, widths[i], rec[i])
 	}
-	sb.WriteString(fmt.Sprintf(pattern, curr...))
+	fmt.Printf(pattern, curr...)
 
 	// Format header separator row
 	curr = curr[:0] // empty slice but preserve capacity
 	for i := range widths {
 		curr = append(curr, widths[i], strings.Repeat("-", widths[i]))
 	}
-	sb.WriteString(fmt.Sprintf(pattern, curr...))
+	fmt.Printf(pattern, curr...)
 
-	// Format rest of records
-	for i := 1; i < len(records); i++ {
-		curr = curr[:0]
-		for j := range widths {
-			curr = append(curr, widths[j], records[i][j])
+	for {
+		rec, err := read.Read()
+		if err == io.EOF {
+			break
 		}
-		sb.WriteString(fmt.Sprintf(pattern, curr...))
+		if err != nil {
+			log.Fatalf("error reading from csv file: %s", err)
 		}
 
-	fmt.Print(sb.String())
+		curr = curr[:0]
+		for i := range widths {
+			curr = append(curr, widths[i], rec[i])
+		}
+		fmt.Printf(pattern, curr...)
+	}
 }
 
 func max(a, b int) int {