From b92720b2402428dea94d2e41030b19cf6dc12ed5 Mon Sep 17 00:00:00 2001
From: Kazuma Watanabe <watassbass@gmail.com>
Date: Sun, 14 May 2023 10:20:10 +0000
Subject: [PATCH] Fix incorrect byte check in NewRangeScannerFragment

---
 pos_scanner.go      | 18 +++++++++++-------
 pos_scanner_test.go | 45 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 56 insertions(+), 7 deletions(-)

diff --git a/pos_scanner.go b/pos_scanner.go
index cff55392..4eb2ce06 100644
--- a/pos_scanner.go
+++ b/pos_scanner.go
@@ -26,10 +26,11 @@ type RangeScanner struct {
 	b        []byte
 	cb       bufio.SplitFunc
 
-	pos Pos    // position of next byte to process in b
-	cur Range  // latest range
-	tok []byte // slice of b that is covered by cur
-	err error  // error from last scan, if any
+	start Pos    // position of first byte in b
+	pos   Pos    // position of next byte to process in b
+	cur   Range  // latest range
+	tok   []byte // slice of b that is covered by cur
+	err   error  // error from last scan, if any
 }
 
 // NewRangeScanner creates a new RangeScanner for the given buffer, producing
@@ -55,11 +56,14 @@ func NewRangeScannerFragment(b []byte, filename string, start Pos, cb bufio.Spli
 		b:        b,
 		cb:       cb,
 		pos:      start,
+		start:    start,
 	}
 }
 
 func (sc *RangeScanner) Scan() bool {
-	if sc.pos.Byte >= len(sc.b) || sc.err != nil {
+	currentByte := sc.pos.Byte - sc.start.Byte
+
+	if currentByte >= len(sc.b) || sc.err != nil {
 		// All done
 		return false
 	}
@@ -67,7 +71,7 @@ func (sc *RangeScanner) Scan() bool {
 	// Since we're operating on an in-memory buffer, we always pass the whole
 	// remainder of the buffer to our SplitFunc and set isEOF to let it know
 	// that it has the whole thing.
-	advance, token, err := sc.cb(sc.b[sc.pos.Byte:], true)
+	advance, token, err := sc.cb(sc.b[currentByte:], true)
 
 	// Since we are setting isEOF to true this should never happen, but
 	// if it does we will just abort and assume the SplitFunc is misbehaving.
@@ -95,7 +99,7 @@ func (sc *RangeScanner) Scan() bool {
 	// we're being asked to skip over by the SplitFunc.
 	// adv is a slice covering any additional bytes we are skipping over, based
 	// on what the SplitFunc told us to do with advance.
-	adv := sc.b[sc.pos.Byte : sc.pos.Byte+advance]
+	adv := sc.b[currentByte : currentByte+advance]
 
 	// We now need to scan over our token to count the grapheme clusters
 	// so we can correctly advance Column, and count the newlines so we
diff --git a/pos_scanner_test.go b/pos_scanner_test.go
index bb6a89a5..57ff0cc1 100644
--- a/pos_scanner_test.go
+++ b/pos_scanner_test.go
@@ -194,3 +194,48 @@ func TestPosScanner(t *testing.T) {
 		})
 	}
 }
+
+func TestPosScannerFragment(t *testing.T) {
+	tests := map[string]struct {
+		Input    string
+		Start    Pos
+		Want     []Range
+		WantToks [][]byte
+	}{
+		"single line": {
+			"hello",
+			Pos{Byte: 10, Line: 2, Column: 1},
+			[]Range{
+				{
+					Start: Pos{Byte: 10, Line: 2, Column: 1},
+					End:   Pos{Byte: 15, Line: 2, Column: 6},
+				},
+			},
+			[][]byte{
+				[]byte("hello"),
+			},
+		},
+	}
+
+	for name, test := range tests {
+		t.Run(name, func(t *testing.T) {
+			src := []byte(test.Input)
+			sc := NewRangeScannerFragment(src, "", test.Start, bufio.ScanLines)
+			got := make([]Range, 0)
+			gotToks := make([][]byte, 0)
+			for sc.Scan() {
+				got = append(got, sc.Range())
+				gotToks = append(gotToks, sc.Bytes())
+			}
+			if sc.Err() != nil {
+				t.Fatalf("unexpected error: %s", sc.Err())
+			}
+			if !reflect.DeepEqual(got, test.Want) {
+				t.Errorf("incorrect ranges\ngot: %swant: %s", spew.Sdump(got), spew.Sdump(test.Want))
+			}
+			if !reflect.DeepEqual(gotToks, test.WantToks) {
+				t.Errorf("incorrect tokens\ngot: %swant: %s", spew.Sdump(gotToks), spew.Sdump(test.WantToks))
+			}
+		})
+	}
+}