Fix error message code snippet

json.go

@@ -3,6 +3,8 @@ package main
 import (
 	"fmt"
 	"strconv"
+	"strings"
+	"unicode/utf8"
 )
 
 type jsonParser struct {
@@ -49,6 +51,9 @@ func (p *jsonParser) next() {
 	} else {
 		p.lastChar = 0
 	}
+	if p.lastChar == '\n' {
+		p.lineNumber++
+	}
 	p.sourceTail.writeByte(p.lastChar)
 }
 
@@ -357,5 +362,20 @@ func (p *jsonParser) skipComment() {
 }
 
 func (p *jsonParser) errorSnippet(message string) error {
-	return fmt.Errorf("%s on node %d.\n%s\n", message, p.lineNumber, p.sourceTail.string())
+	lines := strings.Split(p.sourceTail.string(), "\n")
+	lastLineLen := utf8.RuneCountInString(lines[len(lines)-1])
+	pointer := strings.Repeat(".", lastLineLen-1) + "^"
+	lines = append(lines, pointer)
+
+	paddedLines := make([]string, len(lines))
+	for i, line := range lines {
+		paddedLines[i] = "   " + line
+	}
+
+	return fmt.Errorf(
+		"%s on line %d.\n\n%s\n\n",
+		message,
+		p.lineNumber,
+		strings.Join(paddedLines, "\n"),
+	)
 }

ring.go

@@ -1,32 +1,49 @@
 package main
 
+import (
+	"strings"
+)
+
 type ring struct {
-	buf        [100]byte
+	buf        [70]byte
 	start, end int
 }
 
 func (r *ring) writeByte(b byte) {
-	if b == '\n' {
-		r.end = 0
-		r.start = r.end
-		return
-	}
 	r.buf[r.end] = b
-	r.end++
-	if r.end >= len(r.buf) {
-		r.end = 0
-	}
+	r.end = (r.end + 1) % len(r.buf)
+
 	if r.end == r.start {
-		r.start++
-		if r.start >= len(r.buf) {
-			r.start = 0
-		}
+		r.start = (r.start + 1) % len(r.buf)
 	}
 }
 
 func (r *ring) string() string {
-	if r.start < r.end {
-		return string(r.buf[r.start:r.end])
+	var lines []byte
+	newlineCount := 0
+
+	for i := r.end - 1; ; i-- {
+		if i < 0 {
+			i = len(r.buf) - 1
+		}
+
+		if r.buf[i] == '\n' {
+			newlineCount++
+			if newlineCount == 2 {
+				break
+			}
+		}
+
+		lines = append(lines, r.buf[i])
+
+		if i == r.start {
+			break
+		}
 	}
-	return string(r.buf[r.start:]) + string(r.buf[:r.end])
+
+	for i, j := 0, len(lines)-1; i < j; i, j = i+1, j-1 {
+		lines[i], lines[j] = lines[j], lines[i]
+	}
+
+	return strings.TrimRight(string(lines), "\t \n")
 }