1 files changed, 241 insertions, 0 deletions
diff --git a/decode_scan.go b/decode_scan.go
new file mode 100644
index 0000000..e75f1c5
--- /dev/null
+++ b/decode_scan.go
@@ -0,0 +1,241 @@
+// Copyright (C) 2022  Luke Shumaker <lukeshu@lukeshu.com>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+package lowmemjson
+
+import (
+	"errors"
+	"fmt"
+	"io"
+)
+
+type ReadError struct {
+	Err    error
+	Offset int64
+}
+
+func (e *ReadError) Error() string {
+	return fmt.Sprintf("json: I/O error at input byte %v: %v", e.Offset, e.Err)
+}
+func (e *ReadError) Unwrap() error { return e.Err }
+
+type SyntaxError struct {
+	Err    string
+	Offset int64
+}
+
+func (e *SyntaxError) Error() string {
+	return fmt.Sprintf("json: syntax error at input byte %v: %v", e.Offset, e.Err)
+}
+
+type runeTypeScanner interface {
+	// The returned error is a *ReadError, a *SyntaxError, or nil.
+	// An EOF condition is represented either as
+	//
+	//   (char, size, RuneTypeEOF, nil)
+	//
+	// or
+	//
+	//   (char, size, RuneTypeError, &SyntaxError{Offset: offset: Err: io.ErrUnexepctedEOF})
+	ReadRuneType() (rune, int, RuneType, error)
+	// The returned error is a *ReadError, a *SyntaxError, io.EOF, or nil.
+	ReadRune() (rune, int, error)
+	UnreadRune() error
+	Reset()
+	InputOffset() int64
+}
+
+// runeTypeScannerImpl /////////////////////////////////////////////////////////////////////////////
+
+type runeTypeScannerImpl struct {
+	inner io.RuneReader
+
+	parser Parser
+	offset int64
+
+	repeat bool
+	stuck  bool
+	rRune  rune
+	rSize  int
+	rType  RuneType
+	rErr   error
+}
+
+var _ runeTypeScanner = (*runeTypeScannerImpl)(nil)
+
+func (sc *runeTypeScannerImpl) Reset() {
+	sc.parser.Reset()
+	sc.stuck = false
+	sc.repeat = false
+}
+
+func (sc *runeTypeScannerImpl) ReadRuneType() (rune, int, RuneType, error) {
+	switch {
+	case sc.stuck:
+		// do nothing
+	case sc.repeat:
+		if _, ok := sc.inner.(io.RuneScanner); ok {
+			sc.inner.ReadRune()
+		}
+	default:
+		var err error
+		sc.rRune, sc.rSize, err = sc.inner.ReadRune()
+		sc.offset += int64(sc.rSize)
+		switch err {
+		case nil:
+			sc.rType, err = sc.parser.HandleRune(sc.rRune)
+			if err != nil {
+				sc.rErr = &SyntaxError{
+					Offset: sc.offset,
+					Err:    err.Error(),
+				}
+			} else {
+				sc.rErr = nil
+			}
+		case io.EOF:
+			sc.rType, err = sc.parser.HandleEOF()
+			if err != nil {
+				sc.rErr = &SyntaxError{
+					Offset: sc.offset,
+					Err:    err.Error(),
+				}
+			} else {
+				sc.rErr = nil
+			}
+		default:
+			sc.rType = 0
+			sc.rErr = &ReadError{
+				Offset: sc.offset,
+				Err:    err,
+			}
+		}
+	}
+	sc.repeat = false
+	sc.stuck = sc.rType == RuneTypeEOF || sc.rType == RuneTypeError
+	return sc.rRune, sc.rSize, sc.rType, sc.rErr
+}
+
+func (sc *runeTypeScannerImpl) ReadRune() (rune, int, error) {
+	r, s, t, e := sc.ReadRuneType()
+	switch t {
+	case RuneTypeEOF:
+		return 0, 0, io.EOF
+	case RuneTypeError:
+		return 0, 0, e
+	default:
+		return r, s, nil
+	}
+}
+
+var ErrInvalidUnreadRune = errors.New("lowmemjson: invalid use of UnreadRune")
+
+// UnreadRune undoes a call to .ReadRune() or .ReadRuneType().  If the
+// last call to .ReadRune() or .ReadRuneType() has already been
+// unread, or if that call returned an error or RuneTypeEOF, then
+// ErrInvalidRune is returned.  Otherwise, nil is returned.
+func (sc *runeTypeScannerImpl) UnreadRune() error {
+	if sc.stuck || sc.repeat {
+		return ErrInvalidUnreadRune
+	}
+	sc.repeat = true
+	if rs, ok := sc.inner.(io.RuneScanner); ok {
+		_ = rs.UnreadRune()
+	}
+	return nil
+}
+
+func (sc *runeTypeScannerImpl) InputOffset() int64 {
+	ret := sc.offset
+	if sc.repeat {
+		ret -= int64(sc.rSize)
+	}
+	return ret
+}
+
+// noWSRuneTypeScanner /////////////////////////////////////////////////////////////////////////////
+
+type noWSRuneTypeScanner struct {
+	inner runeTypeScanner
+}
+
+var _ runeTypeScanner = (*noWSRuneTypeScanner)(nil)
+
+func (sc *noWSRuneTypeScanner) ReadRuneType() (rune, int, RuneType, error) {
+again:
+	r, s, t, e := sc.inner.ReadRuneType()
+	if t == RuneTypeSpace {
+		goto again
+	}
+	return r, s, t, e
+}
+
+func (sc *noWSRuneTypeScanner) ReadRune() (rune, int, error) {
+	r, s, t, e := sc.ReadRuneType()
+	switch t {
+	case RuneTypeEOF:
+		return 0, 0, io.EOF
+	case RuneTypeError:
+		return 0, 0, e
+	default:
+		return r, s, nil
+	}
+}
+
+func (sc *noWSRuneTypeScanner) UnreadRune() error  { return sc.inner.UnreadRune() }
+func (sc *noWSRuneTypeScanner) Reset()             { sc.inner.Reset() }
+func (sc *noWSRuneTypeScanner) InputOffset() int64 { return sc.inner.InputOffset() }
+
+// elemRuneTypeScanner /////////////////////////////////////////////////////////////////////////////
+
+type elemRuneTypeScanner struct {
+	inner runeTypeScanner
+
+	parser Parser
+	repeat bool
+	rType  RuneType
+}
+
+var _ runeTypeScanner = (*elemRuneTypeScanner)(nil)
+
+func (sc *elemRuneTypeScanner) ReadRuneType() (rune, int, RuneType, error) {
+	r, s, t, e := sc.inner.ReadRuneType()
+
+	// Check if we need to insert a premature EOF
+	if t != RuneTypeError && t != RuneTypeEOF {
+		if sc.repeat {
+			sc.repeat = false
+		} else {
+			sc.rType, _ = sc.parser.HandleRune(r)
+		}
+		if sc.rType == RuneTypeEOF {
+			_ = sc.inner.UnreadRune()
+		}
+		t = sc.rType
+	}
+	if t == RuneTypeEOF {
+		return 0, 0, RuneTypeEOF, nil
+	}
+
+	return r, s, t, e
+}
+
+func (sc *elemRuneTypeScanner) ReadRune() (rune, int, error) {
+	r, s, t, e := sc.ReadRuneType()
+	switch t {
+	case RuneTypeEOF:
+		return 0, 0, io.EOF
+	case RuneTypeError:
+		return 0, 0, e
+	default:
+		return r, s, nil
+	}
+}
+
+func (sc *elemRuneTypeScanner) UnreadRune() error {
+	sc.repeat = true
+	return sc.inner.UnreadRune()
+}
+
+func (sc *elemRuneTypeScanner) InputOffset() int64 { return sc.inner.InputOffset() }
+func (sc *elemRuneTypeScanner) Reset()             {}