From c9ee9916ebdf986af1c38ce6d02421913c231038 Mon Sep 17 00:00:00 2001
From: Brandon Dyck <brandon@dyck.us>
Date: Fri, 13 Sep 2024 11:17:32 -0600
Subject: [PATCH] Test ReaderAtCursor with failing ReaderAt

---
 cursor/cursor_test.go | 13 +++++++++++++
 parser_test.go        |  4 ++--
 test/generator/gen.go | 11 +++++++++++
 3 files changed, 26 insertions(+), 2 deletions(-)
 create mode 100644 test/generator/gen.go

diff --git a/cursor/cursor_test.go b/cursor/cursor_test.go
index 3fa3e8e..6072438 100644
--- a/cursor/cursor_test.go
+++ b/cursor/cursor_test.go
@@ -6,6 +6,8 @@ import (
 	"testing"
 
 	"git.codemonkeysoftware.net/b/gigaparsec/cursor"
+	ptest "git.codemonkeysoftware.net/b/gigaparsec/test"
+	"git.codemonkeysoftware.net/b/gigaparsec/test/generator"
 	"github.com/shoenig/test"
 	"github.com/shoenig/test/must"
 	"pgregory.net/rapid"
@@ -15,6 +17,7 @@ func Todo(t *testing.T) {
 	t.Errorf("TODO")
 }
 
+// TODO move this to generator
 func SliceOfNZero[T any](minLen, maxLen int) *rapid.Generator[[]T] {
 	return rapid.Map(rapid.IntRange(minLen, maxLen), func(n int) []T {
 		return make([]T, n)
@@ -110,4 +113,14 @@ func TestReaderAtCursor(t *testing.T) {
 	testCursor(t, func(b []byte) cursor.ReaderAtCursor {
 		return cursor.NewReaderAt(bytes.NewReader(b))
 	})
+	t.Run("Read returns an error if the ReaderAt fails", rapid.MakeCheck(func(t *rapid.T) {
+		expectedErr := generator.Error().Draw(t, "expectedErr")
+		startPos := rapid.Uint64().Draw(t, "startPos")
+		dst := SliceOfNZero[byte](0, 100).Draw(t, "dst")
+		c := cursor.NewReaderAt(ptest.ErrReaderAt(expectedErr)).At(startPos)
+		n, next, err := c.Read(dst)
+		test.ErrorIs(t, err, expectedErr)
+		test.EqOp(t, startPos, next.Pos())
+		test.Zero(t, n)
+	}))
 }
diff --git a/parser_test.go b/parser_test.go
index ad6af6d..1096b8c 100644
--- a/parser_test.go
+++ b/parser_test.go
@@ -2,12 +2,12 @@ package gigaparsec_test
 
 import (
 	"bytes"
-	"errors"
 	"testing"
 
 	"git.codemonkeysoftware.net/b/gigaparsec"
 	"git.codemonkeysoftware.net/b/gigaparsec/cursor"
 	ptest "git.codemonkeysoftware.net/b/gigaparsec/test"
+	"git.codemonkeysoftware.net/b/gigaparsec/test/generator"
 	"github.com/shoenig/test"
 	"github.com/shoenig/test/must"
 	"pgregory.net/rapid"
@@ -52,7 +52,7 @@ func TestSlice(t *testing.T) {
 		assertParseFails(t, input, gigaparsec.Slice(s))
 	}))
 	t.Run("fails when read fails", rapid.MakeCheck(func(t *rapid.T) {
-		expectedErr := rapid.Map(rapid.StringN(0, 100, -1), errors.New).Draw(t, "err")
+		expectedErr := generator.Error().Draw(t, "expectedErr")
 		c := ptest.ErrCursor[byte](expectedErr)
 		s := rapid.SliceOfN(rapid.Byte(), 0, 100).Draw(t, "s")
 		result, err := gigaparsec.Slice(s)(gigaparsec.MakeState(c))
diff --git a/test/generator/gen.go b/test/generator/gen.go
new file mode 100644
index 0000000..6a63231
--- /dev/null
+++ b/test/generator/gen.go
@@ -0,0 +1,11 @@
+package generator
+
+import (
+	"errors"
+
+	"pgregory.net/rapid"
+)
+
+func Error() *rapid.Generator[error] {
+	return rapid.Map(rapid.String(), errors.New)
+}