better syntax errors for reencoder

3 files changed, 31 insertions, 22 deletions

diff --git a/lib/lowmemjson/adapter_test.go b/lib/lowmemjson/adapter_test.go
index 0d82f78..630c7ca 100644
--- a/lib/lowmemjson/adapter_test.go
+++ b/lib/lowmemjson/adapter_test.go
@@ -112,13 +112,6 @@ type (
 	UnmarshalTypeError    = json.UnmarshalTypeError
 )
 
-type SyntaxError struct {
-	msg    string
-	Offset int64
-}
-
-func (e *SyntaxError) Error() string { return e.msg }
-
 const (
 	startDetectingCyclesAfter = 1000
 )
diff --git a/lib/lowmemjson/borrowed_misc.go b/lib/lowmemjson/borrowed_misc.go
index 343c924..5c6bbb6 100644
--- a/lib/lowmemjson/borrowed_misc.go
+++ b/lib/lowmemjson/borrowed_misc.go
@@ -5,9 +5,23 @@
 package lowmemjson
 
 import (
+	"fmt"
 	"reflect"
 )
 
+// A SyntaxError is a description of a JSON syntax error.
+//
+// from scanner.go
+type SyntaxError struct {
+	msg    string // description of error
+	Offset int64  // error occurred after reading Offset bytes
+}
+
+func (e *SyntaxError) Error() string {
+	return fmt.Sprintf("JSON syntax error at input byte %v: %v",
+		e.Offset, e.msg)
+}
+
 // from encode.go
 func isEmptyValue(v reflect.Value) bool {
 	switch v.Kind() {
diff --git a/lib/lowmemjson/reencode.go b/lib/lowmemjson/reencode.go
index 76aedc9..e0783b0 100644
--- a/lib/lowmemjson/reencode.go
+++ b/lib/lowmemjson/reencode.go
@@ -39,6 +39,7 @@ type ReEncoder struct {
 
 	// state: .WriteRune
 	err            error
+	inputPos       int64
 	written        int
 	stack          []reencodeState
 	stack0IsNumber bool
@@ -77,7 +78,7 @@ func (enc *ReEncoder) Write(p []byte) (int, error) {
 
 func (enc *ReEncoder) Flush() error {
 	if enc.bufLen > 0 {
-		return fmt.Errorf("unflushed unicode garbage: %q", enc.buf[:enc.bufLen])
+		return &SyntaxError{fmt.Sprintf("EOF: unflushed unicode garbage: %q", enc.buf[:enc.bufLen]), enc.inputPos}
 	}
 	switch len(enc.stack) {
 	case 0:
@@ -89,7 +90,7 @@ func (enc *ReEncoder) Flush() error {
 		}
 		fallthrough
 	default:
-		return fmt.Errorf("in the middle of a value")
+		return &SyntaxError{fmt.Sprintf("EOF: in the middle of a value"), enc.inputPos}
 	}
 }
 
@@ -103,6 +104,7 @@ func (enc *ReEncoder) WriteRune(c rune) (n int, err error) {
 	}
 	enc.written = 0
 	enc.err = enc.state(c)
+	enc.inputPos += int64(utf8.RuneLen(c))
 	return enc.written, enc.err
 }
 
@@ -203,7 +205,7 @@ func (enc *ReEncoder) stateAny(c rune) error {
 		enc.replaceState(enc.stateInNull, false)
 		enc.stateBuf = append(enc.stateBuf[:0], 'n')
 	default:
-		return fmt.Errorf("decode value: unexpected character: %c", c)
+		return &SyntaxError{fmt.Sprintf("any: unexpected character: %c", c), enc.inputPos}
 	}
 	return enc.emitByte(byte(c))
 }
@@ -233,7 +235,7 @@ func (enc *ReEncoder) _stateInObject(c rune, nonempty bool) error {
 			}
 		}
 	default:
-		return fmt.Errorf("decode object: unexpected character: %c", c)
+		return &SyntaxError{fmt.Sprintf("object: unexpected character: %c", c), enc.inputPos}
 	}
 	return enc.emitByte(byte(c))
 }
@@ -255,7 +257,7 @@ func (enc *ReEncoder) stateInKV(c rune) error {
 		}
 		return nil
 	default:
-		return fmt.Errorf("decode object member: unexpected character: %c", c)
+		return &SyntaxError{fmt.Sprintf("object member: unexpected character: %c", c), enc.inputPos}
 	}
 }
 func (enc *ReEncoder) stateAfterV(c rune) error {
@@ -273,7 +275,7 @@ func (enc *ReEncoder) stateAfterV(c rune) error {
 			return err
 		}
 	default:
-		return fmt.Errorf("decode object member: unexpected character: %c", c)
+		return &SyntaxError{fmt.Sprintf("object member: unexpected character: %c", c), enc.inputPos}
 	}
 	return enc.emitByte(byte(c))
 }
@@ -321,7 +323,7 @@ func (enc *ReEncoder) stateAfterItem(c rune) error {
 			return err
 		}
 	default:
-		return fmt.Errorf("decode array: unexpected character: %c", c)
+		return &SyntaxError{fmt.Sprintf("array: unexpected character: %c", c), enc.inputPos}
 	}
 	return enc.emitByte(byte(c))
 }
@@ -339,7 +341,7 @@ func (enc *ReEncoder) stateInString(c rune) error {
 	case 0x0020 <= c && c <= 0x10FFFF:
 		return enc.emit(writeStringChar(enc.Out, c, false, enc.UnicodeEscape))
 	default:
-		return fmt.Errorf("decode string: unexpected character: %c", c)
+		return &SyntaxError{fmt.Sprintf("string: unexpected character: %c", c), enc.inputPos}
 	}
 }
 func (enc *ReEncoder) stateInBackslash(c rune) error {
@@ -369,7 +371,7 @@ func (enc *ReEncoder) stateInBackslash(c rune) error {
 		enc.replaceState(enc.stateInUnicode, false)
 		return nil
 	default:
-		return fmt.Errorf("decode backslash sequence: unexpected character: %c", c)
+		return &SyntaxError{fmt.Sprintf("string backslash sequence: unexpected character: %c", c), enc.inputPos}
 	}
 }
 func (enc *ReEncoder) stateInUnicode(c rune) error {
@@ -381,7 +383,7 @@ func (enc *ReEncoder) stateInUnicode(c rune) error {
 	case 'A' <= c && c <= 'F':
 		enc.stateBuf = append(enc.stateBuf, byte(c)-'A'+10)
 	default:
-		return fmt.Errorf("decode backslash sequence: unexpected character: %c", c)
+		return &SyntaxError{fmt.Sprintf("string unicode sequence: unexpected character: %c", c), enc.inputPos}
 	}
 	if len(enc.stateBuf) == 4 {
 		enc.replaceState(enc.stateInString, false)
@@ -447,7 +449,7 @@ func (enc *ReEncoder) stateNumberA(c rune) error { // start
 	case '1', '2', '3', '4', '5', '6', '7', '8', '9':
 		enc.replaceState(enc.stateNumberD, true)
 	default:
-		return fmt.Errorf("decode number: unexpected character: %c", c)
+		return &SyntaxError{fmt.Sprintf("number: unexpected character: %c", c), enc.inputPos}
 	}
 	return enc.emitByte(byte(c))
 }
@@ -458,7 +460,7 @@ func (enc *ReEncoder) stateNumberB(c rune) error { // got a leading "-"
 	case '1', '2', '3', '4', '5', '6', '7', '8', '9':
 		enc.replaceState(enc.stateNumberD, true)
 	default:
-		return fmt.Errorf("decode number: unexpected character: %c", c)
+		return &SyntaxError{fmt.Sprintf("number: unexpected character: %c", c), enc.inputPos}
 	}
 	return enc.emitByte(byte(c))
 }
@@ -500,7 +502,7 @@ func (enc *ReEncoder) stateNumberE(c rune) error { // got a ".", ready to read a
 		enc.replaceState(enc.stateNumberF, true)
 		return enc.emitByte(byte(c))
 	default:
-		return fmt.Errorf("decode number: unexpected character: %c", c)
+		return &SyntaxError{fmt.Sprintf("number: unexpected character: %c", c), enc.inputPos}
 	}
 }
 func (enc *ReEncoder) stateNumberF(c rune) error { // in the fraction part
@@ -550,7 +552,7 @@ func (enc *ReEncoder) stateNumberG(c rune) error { // got a leading "e"
 		return enc.emitByte(byte(c))
 	default:
 		enc.stateBuf = enc.stateBuf[:0]
-		return fmt.Errorf("decode number: unexpected character: %c", c)
+		return &SyntaxError{fmt.Sprintf("number: unexpected character: %c", c), enc.inputPos}
 	}
 }
 func (enc *ReEncoder) stateNumberH(c rune) error { // in the exponent's number part
@@ -582,7 +584,7 @@ func (enc *ReEncoder) stateInFalse(c rune) error { return enc._stateInLiteral(c,
 func (enc *ReEncoder) stateInNull(c rune) error  { return enc._stateInLiteral(c, "null") }
 func (enc *ReEncoder) _stateInLiteral(c rune, full string) error {
 	if c != rune(full[len(enc.stateBuf)]) {
-		return fmt.Errorf("decode %s: unexpected character: %c", full, c)
+		return &SyntaxError{fmt.Sprintf("%s: unexpected character: %c", full, c), enc.inputPos}
 	}
 	enc.stateBuf = append(enc.stateBuf, byte(c))
 	if len(enc.stateBuf) == len(full) {