Re-added SliceReaderAt

TODO.txt

@@ -1,3 +1,4 @@
+Test State against both possible ReaderAt EOF behaviors
 Write Repeat tests
 Think about not requiring so much Pos() when making messages
 Rename Seq2 to Seq

gigaparsec.go

@@ -116,6 +116,22 @@ type ReaderAt[T any] interface {
 	ReadAt(p []T, off int64) (n int, err error)
 }
 
+type SliceReaderAt[T any] []T
+
+func (s SliceReaderAt[T]) ReadAt(dst []T, off int64) (n int, err error) {
+	if off < 0 {
+		return 0, errors.New("SliceReaderAt.ReadAt: negative offset")
+	}
+	if off >= int64(len(s)) {
+		return 0, io.EOF
+	}
+	n = copy(dst, s[off:])
+	if n < len(dst) {
+		err = io.EOF
+	}
+	return n, err
+}
+
 func MakeState[In any](r ReaderAt[In]) State[In] {
 	return State[In]{r: r}
 }

state_test.go

@@ -2,11 +2,13 @@ package gigaparsec_test
 
 import (
 	"bytes"
+	"cmp"
 	"io"
 	"testing"
 
 	"git.codemonkeysoftware.net/b/gigaparsec"
 	ptest "git.codemonkeysoftware.net/b/gigaparsec/test"
+	"git.codemonkeysoftware.net/b/gigaparsec/test/generator"
 	pgen "git.codemonkeysoftware.net/b/gigaparsec/test/generator"
 	"github.com/shoenig/test"
 	"github.com/shoenig/test/must"
@@ -102,3 +104,42 @@ func TestState(t *testing.T) {
 		test.Zero(t, n)
 	}))
 }
+
+func TestSliceReaderAt(t *testing.T) {
+	const maxLen = 100
+	t.Run("offset ≥ 0", rapid.MakeCheck(func(t *rapid.T) {
+		src := rapid.SliceOfN(rapid.Byte(), 0, maxLen).Draw(t, "src")
+		dst := generator.SliceOfNZero[byte](0, maxLen).Draw(t, "dst")
+		offset := rapid.Int64Range(0, int64(len(src))+10).Draw(t, "offset")
+
+		n, err := gigaparsec.SliceReaderAt[byte](src).ReadAt(dst, offset)
+
+		switch cmp.Compare(len(src), int(offset)+len(dst)) {
+		case -1:
+			// Read overruns src.
+			test.ErrorIs(t, err, io.EOF)
+			test.EqOp(t, max(0, len(src)-int(offset)), n)
+		case 0:
+			// Read exactly reaches end of source.
+			// io.ReaderAt spec allows error to be either io.EOF or nil.
+			test.EqOp(t, len(dst), n)
+		case 1:
+			// Read ends before end of source.
+			test.NoError(t, err)
+			test.EqOp(t, len(dst), n)
+		}
+
+		if offset < int64(len(src)) {
+			test.SliceEqOp(t, src[offset:offset+int64(n)], dst[:n])
+		}
+	}))
+	t.Run("offset < 0", rapid.MakeCheck(func(t *rapid.T) {
+		src := rapid.SliceOfN(rapid.Byte(), 0, maxLen).Draw(t, "src")
+		dst := generator.SliceOfNZero[byte](0, maxLen).Draw(t, "dst")
+		offset := rapid.Int64Max(-1).Draw(t, "offset")
+
+		n, err := gigaparsec.SliceReaderAt[byte](src).ReadAt(dst, offset)
+		test.Error(t, err)
+		test.EqOp(t, 0, n)
+	}))
+}