Clarify and fix State's EOF behavior

state_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"cmp"
 	"io"
+	"math"
 	"testing"
 
 	"git.codemonkeysoftware.net/b/gigaparsec"
@@ -15,6 +16,30 @@ import (
 	"pgregory.net/rapid"
 )
 
+type customEOFReaderAt struct {
+	r             *bytes.Reader
+	eofAtExactFit bool
+}
+
+func newCustomEOFReaderAt(b []byte, eofAtExactFit bool) customEOFReaderAt {
+	return customEOFReaderAt{
+		r:             bytes.NewReader(b),
+		eofAtExactFit: eofAtExactFit,
+	}
+}
+
+func (r customEOFReaderAt) ReadAt(p []byte, off int64) (n int, err error) {
+	n, err = r.r.ReadAt(p, off)
+	if int64(len(p))+off >= r.r.Size() {
+		if r.eofAtExactFit {
+			err = io.EOF
+		} else {
+			err = nil
+		}
+	}
+	return n, err
+}
+
 func TestState(t *testing.T) {
 	t.Run("state reads the same position every time", rapid.MakeCheck(func(t *rapid.T) {
 		data := rapid.SliceOfN(rapid.Byte(), 1, 100).Draw(t, "data")
@@ -31,19 +56,59 @@ func TestState(t *testing.T) {
 		must.NoError(t, err)
 		must.SliceEqOp(t, expected, dst)
 	}))
-	t.Run("Read returns io.EOF iff it overruns source", rapid.MakeCheck(func(t *rapid.T) {
-		data := rapid.SliceOfN(rapid.Byte(), 0, 100).Draw(t, "data")
-		dst := pgen.SliceOfNZero[byte](0, 200).Draw(t, "dst")
-		st := gigaparsec.MakeState(bytes.NewReader(data))
+	t.Run("Read ends before end of source", rapid.MakeCheck(func(t *rapid.T) {
+		src := rapid.SliceOfN(rapid.Byte(), 1, 100).Draw(t, "src")
+		endReadAt := rapid.IntRange(0, len(src)-1).Draw(t, "endReadAt")
+		pos := rapid.Uint64Range(0, uint64(endReadAt)).Draw(t, "pos")
+		dst := pgen.SliceOfNZero[byte](0, endReadAt-int(pos)).Draw(t, "dst")
+		st := gigaparsec.MakeState(bytes.NewReader(src)).At(pos)
 
-		n, _, err := st.Read(dst)
-		t.Logf("n=%d", n)
-		must.EqOp(t, min(len(data), len(dst)), int(n))
-		if len(dst) > len(data) || st.Pos() == uint64(len(data)) {
-			must.ErrorIs(t, err, io.EOF)
-		} else {
-			must.NoError(t, err)
-		}
+		n, next, err := st.At(pos).Read(dst)
+
+		test.EqOp(t, uint64(len(dst)), n)
+		ptest.StateIsAt(t, next, pos+n)
+		test.NoError(t, err)
+		test.SliceEqOp(t, src[pos:pos+n], dst)
+	}))
+	t.Run("Non-empty Read ends at end of source", rapid.MakeCheck(func(t *rapid.T) {
+		readerReturnsEOF := rapid.Bool().Draw(t, "readerReturnsEOF")
+		src := rapid.SliceOfN(rapid.Byte(), 1, 100).Draw(t, "src")
+		dst := pgen.SliceOfNZero[byte](1, len(src)).Draw(t, "dst")
+		pos := uint64(len(src) - len(dst))
+		st := gigaparsec.MakeState(newCustomEOFReaderAt(src, readerReturnsEOF))
+
+		n, next, err := st.At(pos).Read(dst)
+
+		test.EqOp(t, uint64(len(dst)), n)
+		ptest.StateIsAt(t, next, pos+n)
+		test.NoError(t, err)
+		test.SliceEqOp(t, src[pos:pos+n], dst)
+	}))
+	t.Run("Read overruns source", rapid.MakeCheck(func(t *rapid.T) {
+		src := rapid.SliceOfN(rapid.Byte(), 1, 100).Draw(t, "src")
+		pos := rapid.Uint64Range(0, uint64(len(src))-1).Draw(t, "pos")
+		minDstLen := len(src) - int(pos) + 1
+		dst := pgen.SliceOfNZero[byte](minDstLen, minDstLen+10).Draw(t, "dst")
+		st := gigaparsec.MakeState(bytes.NewReader(src)).At(pos)
+
+		n, next, err := st.Read(dst)
+
+		test.EqOp(t, uint64(len(src)), n+pos)
+		ptest.StateIsAt(t, next, pos+n)
+		test.ErrorIs(t, err, io.EOF)
+		test.SliceEqOp(t, src[pos:pos+n], dst[:n])
+	}))
+	t.Run("Read starts after end of source", rapid.MakeCheck(func(t *rapid.T) {
+		src := rapid.SliceOfN(rapid.Byte(), 0, 100).Draw(t, "src")
+		dst := pgen.SliceOfNZero[byte](0, 100).Draw(t, "dst")
+		pos := rapid.Uint64Min(uint64(len(src))).Draw(t, "pos")
+		st := gigaparsec.MakeState(bytes.NewReader(src)).At(pos)
+
+		n, next, err := st.Read(dst)
+
+		test.EqOp(t, 0, n)
+		ptest.StateIsAt(t, next, pos)
+		test.ErrorIs(t, err, io.EOF)
 	}))
 	t.Run("next state reads next input", rapid.MakeCheck(func(t *rapid.T) {
 		const maxLen = 100
@@ -58,14 +123,6 @@ func TestState(t *testing.T) {
 		n, _, _ := next.Read(dst)
 		must.SliceEqOp(t, data[skip:skip+int(n)], dst[:n])
 	}))
-	t.Run("Read returns io.EOF if n is less than requested", rapid.MakeCheck(func(t *rapid.T) {
-		data := rapid.SliceOfN(rapid.Byte(), 0, 100).Draw(t, "data")
-		st := gigaparsec.MakeState(bytes.NewReader(data))
-
-		n, _, err := st.Read(make([]byte, len(data)+1))
-		test.ErrorIs(t, err, io.EOF)
-		test.EqOp(t, len(data), int(n))
-	}))
 	t.Run("At sets state position", rapid.MakeCheck(func(t *rapid.T) {
 		data := rapid.SliceOfN(rapid.Byte(), 1, 100).Draw(t, "data")
 		pos := rapid.Uint64Range(0, uint64(len(data)-1)).Draw(t, "pos")
@@ -95,7 +152,7 @@ func TestState(t *testing.T) {
 	}))
 	t.Run("Read returns an error if the ReaderAt fails", rapid.MakeCheck(func(t *rapid.T) {
 		expectedErr := pgen.Error().Draw(t, "expectedErr")
-		startPos := rapid.Uint64().Draw(t, "startPos")
+		startPos := rapid.Uint64Max(math.MaxInt64).Draw(t, "startPos")
 		dst := pgen.SliceOfNZero[byte](0, 100).Draw(t, "dst")
 		st := gigaparsec.MakeState(ptest.ErrReaderAt(expectedErr)).At(startPos)
 		n, next, err := st.Read(dst)