added LocateError

errors.go

@@ -1,6 +1,10 @@
 package goparsify
 
-import "fmt"
+import (
+	"bufio"
+	"fmt"
+	"strings"
+)
 
 // Error represents a parse error. These will often be set, the parser will back up a little and
 // find another viable path. In general when combining errors the longest error should be returned.
@@ -24,3 +28,50 @@ type UnparsedInputError struct {
 func (e UnparsedInputError) Error() string {
 	return "left unparsed: " + e.Remaining
 }
+
+// LocalError locates the error position in the input string s and returns the
+// error description along with a cursor to the input.
+func (e *Error) LocateError(s string) string {
+	if len(s) < e.Pos() {
+		return e.Error()
+	}
+
+	pos := e.Pos()
+
+	// find the line.
+	var (
+		lino, prev, off int
+		line, indent    []byte
+	)
+	byline := bufio.NewScanner(strings.NewReader(s))
+	for byline.Scan() {
+		lino++
+		line = byline.Bytes()
+		prev = off
+		off += len(line) + 1
+		// log.Printf("lino:%3d len:%4d prev:%4d  pos:%4d off:%4d rel:%4d line: %s",
+		// 	lino, len(line), prev, pos, off, (pos - prev), string(line))
+		if prev <= pos && pos <= off {
+			break
+		}
+	}
+	indent = make([]byte, len(line[:pos-prev]))
+	for i, c := range indent {
+		if c != '\t' {
+			indent[i] = ' '
+		}
+	}
+	off = pos - prev
+	if off > 40 {
+		indent = indent[off-30:]
+		line = line[off-30:]
+		line[0] = '.'
+		line[1] = '.'
+		line[2] = '.'
+	}
+	if len(line) > 70 {
+		line = line[:70]
+		line[69], line[68], line[67] = '.', '.', '.'
+	}
+	return fmt.Sprintf("Parsing error in line %d:\n%s\n%s^\n%v\n", lino, string(line), string(indent), e.Error())
+}