Determine relative paths using absolute paths

In some cases it was impossible to determine the relative paths between
two directories if those directories were not absolute paths.

Author: jstemmer

main.go

@@ -6,7 +6,6 @@ import (
 	"fmt"
 	"io"
 	"os"
-	"path"
 	"path/filepath"
 	"sort"
 	"strings"
@@ -146,9 +145,20 @@ func main() {
 		os.Exit(1)
 	}
 
+	var basedir string
+	if relative {
+		basedir, err = filepath.Abs(filepath.Dir(outputFile))
+		if err != nil {
+			if !silent {
+				fmt.Fprintf(os.Stderr, "could not determine absolute path: %s\n", err)
+			}
+			os.Exit(1)
+		}
+	}
+
 	tags := []Tag{}
 	for _, file := range files {
-		ts, err := Parse(file, relative, path.Dir(outputFile))
+		ts, err := Parse(file, relative, basedir)
 		if err != nil {
 			if !silent {
 				fmt.Fprintf(os.Stderr, "parse error: %s\n\n", err)

parser.go

@@ -208,8 +208,9 @@ func (p *tagParser) parseInterfaceMethods(name string, s *ast.InterfaceType) {
 func (p *tagParser) createTag(name string, pos token.Pos, tagType TagType) Tag {
 	f := p.fset.File(pos).Name()
 	if p.relative {
-		if rel, err := filepath.Rel(p.basepath, f); err != nil {
-			// log error, but continue
+		if abs, err := filepath.Abs(f); err != nil {
+			fmt.Fprintf(os.Stderr, "could not determine absolute path: %s\n", err)
+		} else if rel, err := filepath.Rel(p.basepath, abs); err != nil {
 			fmt.Fprintf(os.Stderr, "could not determine relative path: %s\n", err)
 		} else {
 			f = rel

parser_test.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"path/filepath"
 	"strconv"
 	"testing"
 )
@@ -91,7 +92,13 @@ var testCases = []struct {
 
 func TestParse(t *testing.T) {
 	for _, testCase := range testCases {
-		tags, err := Parse(testCase.filename, testCase.relative, testCase.basepath)
+		basepath, err := filepath.Abs(testCase.basepath)
+		if err != nil {
+			t.Errorf("[%s] could not determine base path: %s\n", testCase.filename, err)
+			continue
+		}
+
+		tags, err := Parse(testCase.filename, testCase.relative, basepath)
 		if err != nil {
 			t.Errorf("[%s] Parse error: %s", testCase.filename, err)
 			continue