Add line number to JSON parse error

Author: James Haggerty

jp.go

@@ -3,6 +3,7 @@ package main
 import (
 	"encoding/json"
 	"fmt"
+	"io"
 	"io/ioutil"
 	"os"
 
@@ -83,19 +84,27 @@ func runMain(c *cli.Context) int {
 		return 0
 	}
 	var input interface{}
-	var jsonParser *json.Decoder
+	var inputStream io.Reader
 	if c.String("filename") != "" {
 		f, err := os.Open(c.String("filename"))
 		if err != nil {
 			return errMsg("Error opening input file: %s", err)
 		}
-		jsonParser = json.NewDecoder(f)
+		inputStream = f
 
 	} else {
-		jsonParser = json.NewDecoder(os.Stdin)
+		inputStream = os.Stdin
 	}
+	newlineNumberReader := NewLineNumberReader(inputStream)
+	jsonParser := json.NewDecoder(newlineNumberReader)
 	if err := jsonParser.Decode(&input); err != nil {
-		errMsg("Error parsing input json: %s\n", err)
+		if syntaxError, ok := err.(*json.SyntaxError); ok {
+			line, char := newlineNumberReader.ConvertOffset(syntaxError.Offset)
+			errMsg("Error parsing input json: %s (line: %d, char: %d)\n",
+				syntaxError, line, char)
+		} else {
+			errMsg("Error parsing input json: %s", err)
+		}
 		return 2
 	}
 	result, err := jmespath.Search(expression, input)
@@ -121,3 +130,51 @@ func runMain(c *cli.Context) int {
 	os.Stdout.WriteString("\n")
 	return 0
 }
+
+type LineNumberReader struct {
+	actualReader     io.Reader
+	newlinePositions []int64
+	bytesRead        int64
+}
+
+func NewLineNumberReader(actualReader io.Reader) *LineNumberReader {
+	return &LineNumberReader{
+		actualReader: actualReader,
+	}
+}
+
+func (lnr *LineNumberReader) Read(p []byte) (n int, err error) {
+	n, err = lnr.actualReader.Read(p)
+
+	if err != nil || n == 0 {
+		return
+	}
+
+	for i, v := range p {
+		if i >= n {
+			return
+		}
+
+		if v == '\n' {
+			lnr.newlinePositions = append(lnr.newlinePositions, lnr.bytesRead+int64(i)+1)
+		}
+	}
+
+	lnr.bytesRead = lnr.bytesRead + int64(n)
+	return
+}
+
+func (lnr *LineNumberReader) ConvertOffset(offset int64) (linePos int, charPos int64) {
+	lastNewlinePos := int64(0)
+	lastLineNumber := 0
+	for lineNumber, newlinePos := range lnr.newlinePositions {
+		if newlinePos >= offset {
+			break
+		}
+
+		lastNewlinePos = newlinePos
+		lastLineNumber = lineNumber + 1
+	}
+
+	return lastLineNumber + 1, offset - lastNewlinePos
+}