04-log-topn: implement TopN with bufio.Scanner and slices.SortFunc

Count paths in a map, copy into []KV, sort by count descending with
cmp.Compare, truncate to n. Scanner buffer raised to 1MB for long lines;
malformed lines (<3 fields) are skipped.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012MpK9niZm4jcs3mAJbxMfi

04-log-topn/main.go

@@ -4,7 +4,7 @@
 // ----------
 // Log lines look like:
 //
-//     2026-08-28T10:00:01Z GET /api/users 200 123ms
+//	2026-08-28T10:00:01Z GET /api/users 200 123ms
 //
 // Implement TopN(r, n) so that it:
 //
@@ -20,8 +20,11 @@
 package main
 
 import (
+	"bufio"
+	"cmp"
 	"fmt"
 	"io"
+	"slices"
 	"strings"
 )
 
@@ -38,7 +41,38 @@ func TopN(r io.Reader, n int) ([]KV, error) {
 	//   - map[string]int for counting
 	//   - copy map into []KV and sort by count desc (maps can't be sorted)
 	//   - truncate to n
-	return nil, nil
+
+	m := make(map[string]int)
+	sc := bufio.NewScanner(r)
+	sc.Buffer(make([]byte, 0, 64*1024), 1024*1024)
+	for sc.Scan() {
+		//fmt.Println(sc.Text())
+		fields := strings.Fields(sc.Text())
+		//fmt.Printf("len: %v, fields: %v\n", len(fields), fields)
+		if len(fields) < 3 {
+			continue
+		}
+		m[fields[2]]++
+	}
+
+	// copy over map into list of KV
+	l := make([]KV, len(m))
+	i := 0
+	for k, v := range m {
+		// println(i, k, v, len(l), len(m))
+		l[i] = KV{k, v}
+		i++
+	}
+
+	// sort by count desc
+	slices.SortFunc(l, func(a, b KV) int {
+		return cmp.Compare(b.Count, a.Count)
+	})
+	if len(l) > n {
+		l = l[:n]
+	}
+
+	return l, nil
 }
 
 func main() {