From 7230a74f494582cbee55f7f8fa81888ccd593d9a Mon Sep 17 00:00:00 2001
From: Alejandro Mery <amery@jpi.io>
Date: Tue, 29 Aug 2023 01:56:28 +0000
Subject: [PATCH 1/3] lexer: runes.Reader renamed to lexer.Reader

Signed-off-by: Alejandro Mery <amery@jpi.io>
---
 lexer/lexer.go             | 2 ++
 {runes => lexer}/reader.go | 2 +-
 runes/docs.go              | 2 --
 3 files changed, 3 insertions(+), 3 deletions(-)
 create mode 100644 lexer/lexer.go
 rename {runes => lexer}/reader.go (99%)
 delete mode 100644 runes/docs.go

diff --git a/lexer/lexer.go b/lexer/lexer.go
new file mode 100644
index 0000000..33b17b6
--- /dev/null
+++ b/lexer/lexer.go
@@ -0,0 +1,2 @@
+// Package lexer provides basic helpers to implement parsers
+package lexer
diff --git a/runes/reader.go b/lexer/reader.go
similarity index 99%
rename from runes/reader.go
rename to lexer/reader.go
index 1e73048..a36b5a9 100644
--- a/runes/reader.go
+++ b/lexer/reader.go
@@ -1,4 +1,4 @@
-package runes
+package lexer
 
 import (
 	"bytes"
diff --git a/runes/docs.go b/runes/docs.go
deleted file mode 100644
index ba5c527..0000000
--- a/runes/docs.go
+++ /dev/null
@@ -1,2 +0,0 @@
-// Package runes helps us work with runes
-package runes

From edcba80baa85dc90f0e0dd4e93a90ce32f2c859f Mon Sep 17 00:00:00 2001
From: Alejandro Mery <amery@jpi.io>
Date: Tue, 29 Aug 2023 00:45:52 +0000
Subject: [PATCH 2/3] lexer: fix ReadRune() to actually move the cursor

Signed-off-by: Alejandro Mery <amery@jpi.io>
---
 lexer/reader.go | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/lexer/reader.go b/lexer/reader.go
index a36b5a9..75ea32a 100644
--- a/lexer/reader.go
+++ b/lexer/reader.go
@@ -153,6 +153,9 @@ func (b *Reader) ReadRune() (rune, int, error) {
 
 	// decode rune
 	r, l := utf8.DecodeRune(b.buf[b.cursor:])
+	// step over
+	b.cursor += l
+
 	return r, l, nil
 }
 

From 6cca2996caeab7f15e95774625d5116a35f44859 Mon Sep 17 00:00:00 2001
From: Alejandro Mery <amery@jpi.io>
Date: Mon, 28 Aug 2023 23:16:01 +0000
Subject: [PATCH 3/3] lexer: Implement Reader.UnreadRune() and
 Reader.PeekRune()

Signed-off-by: Alejandro Mery <amery@jpi.io>
---
 lexer/reader.go | 41 ++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 40 insertions(+), 1 deletion(-)

diff --git a/lexer/reader.go b/lexer/reader.go
index 75ea32a..7c0aaa3 100644
--- a/lexer/reader.go
+++ b/lexer/reader.go
@@ -2,6 +2,7 @@ package lexer
 
 import (
 	"bytes"
+	"errors"
 	"io"
 	"strings"
 	"unicode/utf8"
@@ -18,7 +19,14 @@ const (
 
 // implemented interfaces
 var (
-	_ io.RuneReader = (*Reader)(nil)
+	_ io.RuneReader  = (*Reader)(nil)
+	_ io.RuneScanner = (*Reader)(nil)
+)
+
+var (
+	// ErrInvalidUnreadRune indicates UnreadRune() was calls after an
+	// action other than a successful ReadRune()
+	ErrInvalidUnreadRune = errors.New("invalid UnreadRune() call")
 )
 
 // Reader is a RuneReader aimed at implementing text parsers
@@ -28,6 +36,8 @@ type Reader struct {
 	buf    []byte
 	off    int
 	cursor int
+
+	lastRuneSize int
 }
 
 // String returns what's already Read but not yet emitted or discarded
@@ -54,6 +64,9 @@ func (b *Reader) Discard() {
 		// step
 		b.off = b.cursor
 	}
+
+	// and prevent UnreadRune()
+	b.lastRuneSize = -1
 }
 
 // ready tells how many bytes are ready to decode
@@ -139,6 +152,8 @@ func (b *Reader) ReadRune() (rune, int, error) {
 	for {
 		err := b.needsBytes(count)
 		if err != nil {
+			b.lastRuneSize = -1
+
 			return 0, 0, err
 		}
 
@@ -155,10 +170,34 @@ func (b *Reader) ReadRune() (rune, int, error) {
 	r, l := utf8.DecodeRune(b.buf[b.cursor:])
 	// step over
 	b.cursor += l
+	// and remember for UnreadRune()
+	b.lastRuneSize = l
 
 	return r, l, nil
 }
 
+// UnreadRune moves the cursor where it was before the last call to ReadRune
+func (b *Reader) UnreadRune() error {
+	if b.lastRuneSize > 0 {
+		b.cursor -= b.lastRuneSize
+		b.lastRuneSize = -1
+		return nil
+	}
+
+	return ErrInvalidUnreadRune
+}
+
+// PeekRune returns information about the next rune without moving the
+// cursor
+func (b *Reader) PeekRune() (rune, int, error) {
+	r, l, err := b.ReadRune()
+	if err != nil {
+		return r, l, err
+	}
+	err = b.UnreadRune()
+	return r, l, err
+}
+
 // NewReader creates a new runes [Reader] using the given [io.Reader]
 func NewReader(r io.Reader) *Reader {
 	if r == nil {