fix: hash a hunk past the scanner's line limit

internal/git/diff.go

@@ -1,7 +1,6 @@
 package git
 
 import (
-	"bufio"
 	"bytes"
 	"crypto/sha256"
 	"fmt"
@@ -229,29 +228,34 @@ func getGitDiff(data DiffContext, executor gitCommandExecutor) ([]*diff.FileDiff
 	return gitDiff, nil
 }
 
-func hunkHash(hunk *diff.Hunk) [32]byte {
-	// Generate a hash for a hunk based on its added and removed lines.
-	var lines []byte
-	data := hunk.Body
+const (
+	addedLine      = '+'
+	removedLine    = '-'
+	noNewlineAtEOF = '\\'
+)
 
-	if len(data) == 0 {
-		return sha256.Sum256(nil)
+func splitLine(data []byte) (line, rest []byte) {
+	i := bytes.IndexByte(data, '\n')
+	if i < 0 {
+		return data, nil
 	}
+	return bytes.TrimSuffix(data[:i], []byte("\r")), data[i+1:]
+}
 
-	scanner := bufio.NewScanner(bytes.NewReader(data))
+func hunkHash(hunk *diff.Hunk) [32]byte {
+	sum := sha256.New()
+	var line []byte
 
-	for scanner.Scan() {
-		line := scanner.Text()
+	for data := hunk.Body; len(data) > 0; {
+		line, data = splitLine(data)
 		if len(line) == 0 {
 			continue
 		}
 		switch line[0] {
-		case '+', '-':
-			// Include the line type and content
-			lines = append(lines, line...)
-		default:
-			// Skip context lines
+		case addedLine, removedLine, noNewlineAtEOF:
+			_, _ = sum.Write(line)
+			_, _ = sum.Write([]byte{'\n'})
 		}
 	}
-	return sha256.Sum256(lines)
+	return [32]byte(sum.Sum(nil))
 }

internal/git/diff_test.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"strings"
 	"testing"
 
 	"github.com/multimediallc/codeowners-plus/pkg/codeowners"
@@ -129,7 +130,7 @@ Binary files a/assets/img/offline.png and b/assets/img/offline.png differ`,
 			expectedErr:   false,
 			expectedFiles: 2,
 			expectedHunks: map[string]int{
-				"file1.go":                1,
+				"file1.go":               1,
 				"assets/img/offline.png": 0,
 			},
 		},
@@ -387,6 +388,48 @@ func TestHunkHash(t *testing.T) {
 			hunk2Body:    []byte(``),
 			expectedSame: true,
 		},
+		{
+			name:         "two lines vs their concatenation",
+			hunkBody:     []byte("+total = x\n+y\n"),
+			hunk2Body:    []byte("+total = x+y\n"),
+			expectedSame: false,
+		},
+		{
+			name:         "added then removed vs one joined line",
+			hunkBody:     []byte("+foo\n-bar\n"),
+			hunk2Body:    []byte("+foo-bar\n"),
+			expectedSame: false,
+		},
+		{
+			name:         "a trailing newline is a change",
+			hunkBody:     []byte("+value\n\\ No newline at end of file\n"),
+			hunk2Body:    []byte("+value\n"),
+			expectedSame: false,
+		},
+		{
+			name:         "differ only past the old 64KB scanner limit",
+			hunkBody:     []byte("+keep()\n+" + strings.Repeat("x", 70*1024) + "A"),
+			hunk2Body:    []byte("+keep()\n+" + strings.Repeat("x", 70*1024) + "B"),
+			expectedSame: false,
+		},
+		{
+			name:         "identical over-long hunks",
+			hunkBody:     []byte("+keep()\n+" + strings.Repeat("x", 70*1024) + "A"),
+			hunk2Body:    []byte("+keep()\n+" + strings.Repeat("x", 70*1024) + "A"),
+			expectedSame: true,
+		},
+		{
+			name:         "an unterminated CR is content",
+			hunkBody:     []byte("+keep()\n+value\r"),
+			hunk2Body:    []byte("+keep()\n+value"),
+			expectedSame: false,
+		},
+		{
+			name:         "a CRLF file hashes like its LF twin",
+			hunkBody:     []byte("+keep()\r\n+value\r\n"),
+			hunk2Body:    []byte("+keep()\n+value\n"),
+			expectedSame: true,
+		},
 	}
 
 	for _, tc := range tt {
@@ -806,3 +849,6 @@ func TestDiffOfDiffs(t *testing.T) {
 		}
 	}
 }
+
+// A hunk matching the approval-time diff is dropped as already reviewed, so a
+// collision past the old 64KB read limit retained an approval over unseen change.