internal/astutil/cursor: FindPos: don't assume Files are in Pos order

+ test

Change-Id: I75c4c3b7789feda874da7644bda8ba93f87f5efb
Reviewed-on: https://go-review.googlesource.com/c/tools/+/650645
Reviewed-by: Robert Findley <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: adonovan

internal/astutil/cursor/cursor.go

@@ -428,15 +428,21 @@ func (c Cursor) FindPos(start, end token.Pos) (Cursor, bool) {
 		ev := events[i]
 		if ev.index > i { // push?
 			n := ev.node
-			var nodeStart, nodeEnd token.Pos
+			var nodeEnd token.Pos
 			if file, ok := n.(*ast.File); ok {
-				nodeStart, nodeEnd = file.FileStart, file.FileEnd
+				nodeEnd = file.FileEnd
+				// Note: files may be out of Pos order.
+				if file.FileStart > start {
+					i = ev.index // disjoint, after; skip to next file
+					continue
+				}
 			} else {
-				nodeStart, nodeEnd = n.Pos(), n.End()
-			}
-			if nodeStart > start {
-				break // disjoint, after; stop
+				nodeEnd = n.End()
+				if n.Pos() > start {
+					break // disjoint, after; stop
+				}
 			}
+			// Inv: node.{Pos,FileStart} <= start
 			if end <= nodeEnd {
 				// node fully contains target range
 				best = i

internal/astutil/cursor/cursor_test.go

@@ -14,6 +14,7 @@ import (
 	"go/token"
 	"iter"
 	"log"
+	"math/rand"
 	"path/filepath"
 	"reflect"
 	"slices"
@@ -332,8 +333,31 @@ func TestCursor_FindNode(t *testing.T) {
 			}
 		}
 	}
+}
+
+// TestCursor_FindPos_order ensures that FindPos does not assume files are in Pos order.
+func TestCursor_FindPos_order(t *testing.T) {
+	// Pick an arbitrary decl.
+	target := netFiles[7].Decls[0]
+
+	// Find the target decl by its position.
+	cur, ok := cursor.Root(netInspect).FindPos(target.Pos(), target.End())
+	if !ok || cur.Node() != target {
+		t.Fatalf("unshuffled: FindPos(%T) = (%v, %t)", target, cur, ok)
+	}
 
-	// TODO(adonovan): FindPos needs a test (not just a benchmark).
+	// Shuffle the files out of Pos order.
+	files := slices.Clone(netFiles)
+	rand.Shuffle(len(files), func(i, j int) {
+		files[i], files[j] = files[j], files[i]
+	})
+
+	// Find it again.
+	inspect := inspector.New(files)
+	cur, ok = cursor.Root(inspect).FindPos(target.Pos(), target.End())
+	if !ok || cur.Node() != target {
+		t.Fatalf("shuffled: FindPos(%T) = (%v, %t)", target, cur, ok)
+	}
 }
 
 func TestCursor_Edge(t *testing.T) {