Revert "proper utf16 support"

This reverts commit 66ab48772f.

this change broke utf-8 scanning of multi-byte runes

parser/parser.go

@@ -3,15 +3,12 @@ package parser
 import (
 	"bufio"
 	"bytes"
-	"encoding/binary"
 	"errors"
 	"fmt"
 	"io"
-	"log/slog"
 	"strconv"
 	"strings"
-	"unicode/utf16"
-	"unicode/utf8"
+	"unicode"
 )
 
 type File struct {
@@ -72,31 +69,33 @@ func ParseFile(r io.Reader) (*File, error) {
 	var b bytes.Buffer
 	var role string
 
+	var lineCount int
+	var linePos int
+
+	var utf16 bool
+
 	var f File
 
 	br := bufio.NewReader(r)
-
-	var sc scannerDecoder = utf8ScannerDecoder{}
-	if bom, err := br.Peek(2); err != nil {
-		slog.Warn("error reading byte-order mark", "error", err)
-	} else if bytes.Equal(bom, []byte{0xFE, 0xFF}) {
-		sc = utf16ScannerDecoder{binary.LittleEndian}
-		//nolint:errcheck
-		br.Discard(2)
-	} else if bytes.Equal(bom, []byte{0xFF, 0xFE}) {
-		sc = utf16ScannerDecoder{binary.BigEndian}
-		//nolint:errcheck
-		br.Discard(2)
-	}
-
-	scanner := bufio.NewScanner(br)
-	scanner.Split(sc.ScanBytes)
-	for scanner.Scan() {
-		r, err := sc.DecodeRune(scanner.Bytes())
-		if err != nil {
+	for {
+		r, _, err := br.ReadRune()
+		if errors.Is(err, io.EOF) {
+			break
+		} else if err != nil {
 			return nil, err
 		}
 
+		// the utf16 byte order mark will be read as "unreadable" by ReadRune()
+		if isUnreadable(r) && lineCount == 0 && linePos == 0 {
+			utf16 = true
+			continue
+		}
+
+		// skip the second byte if we're reading utf16
+		if utf16 && r == 0 {
+			continue
+		}
+
 		next, r, err := parseRuneForState(r, curr)
 		if errors.Is(err, io.ErrUnexpectedEOF) {
 			return nil, fmt.Errorf("%w: %s", err, b.String())
@@ -104,6 +103,13 @@ func ParseFile(r io.Reader) (*File, error) {
 			return nil, err
 		}
 
+		if isNewline(r) {
+			lineCount++
+			linePos = 0
+		} else {
+			linePos++
+		}
+
 		// process the state transition, some transitions need to be intercepted and redirected
 		if next != curr {
 			switch curr {
@@ -303,6 +309,10 @@ func isNewline(r rune) bool {
 	return r == '\r' || r == '\n'
 }
 
+func isUnreadable(r rune) bool {
+	return r == unicode.ReplacementChar
+}
+
 func isValidMessageRole(role string) bool {
 	return role == "system" || role == "user" || role == "assistant"
 }
@@ -315,39 +325,3 @@ func isValidCommand(cmd string) bool {
 		return false
 	}
 }
-
-type scannerDecoder interface {
-	ScanBytes(data []byte, atEOF bool) (advance int, token []byte, err error)
-	DecodeRune([]byte) (rune, error)
-}
-
-type utf8ScannerDecoder struct{}
-
-func (utf8ScannerDecoder) ScanBytes(data []byte, atEOF bool) (advance int, token []byte, err error) {
-	return scanBytesN(data, 1, atEOF)
-}
-
-func (utf8ScannerDecoder) DecodeRune(data []byte) (rune, error) {
-	r, _ := utf8.DecodeRune(data)
-	return r, nil
-}
-
-type utf16ScannerDecoder struct {
-	binary.ByteOrder
-}
-
-func (utf16ScannerDecoder) ScanBytes(data []byte, atEOF bool) (advance int, token []byte, err error) {
-	return scanBytesN(data, 2, atEOF)
-}
-
-func (e utf16ScannerDecoder) DecodeRune(data []byte) (rune, error) {
-	return utf16.Decode([]uint16{e.ByteOrder.Uint16(data)})[0], nil
-}
-
-func scanBytesN(data []byte, n int, atEOF bool) (int, []byte, error) {
-	if atEOF && len(data) == 0 {
-		return 0, nil, nil
-	}
-
-	return n, data[:n], nil
-}