whitespace: switch to cheaper check

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

git/commits.go

@@ -57,17 +57,21 @@ var FieldNames = map[string]string{
 	"%G?": "verification_flag",
 }
 
+// Check warns if changes introduce whitespace errors.
+// Returns non-zero if any issues are found.
+func Check(commit string) ([]byte, error) {
+	cmd := exec.Command("git", "show", "--check", commit)
+	cmd.Stderr = os.Stderr
+	return cmd.Output()
+}
+
 // Show returns the diff of a commit.
 //
 // NOTE: This could be expensive for very large commits.
 func Show(commit string) ([]byte, error) {
 	cmd := exec.Command("git", "show", commit)
 	cmd.Stderr = os.Stderr
-	out, err := cmd.Output()
+	return cmd.Output()
-	if err != nil {
-		return nil, err
-	}
-	return out, nil
 }
 
 // CommitEntry represents a single commit's information from `git`.

rules/danglingwhitespace/rule.go

@@ -1,9 +1,6 @@
 package danglingwhitespace
 
 import (
-	"bytes"
-	"fmt"
-
 	"github.com/vbatts/git-validation/git"
 	"github.com/vbatts/git-validation/validate"
 )
@@ -23,27 +20,18 @@ func init() {
 }
 
 func ValidateDanglingWhitespace(c git.CommitEntry) (vr validate.Result) {
-	diff, err := git.Show(c["commit"])
-	if err != nil {
-		return validate.Result{Pass: false, Msg: err.Error(), CommitEntry: c}
-	}
-
 	vr.CommitEntry = c
+	vr.Msg = "commit does not have any whitespace errors"
 	vr.Pass = true
-	for _, line := range bytes.Split(diff, newLine) {
+
-		if !bytes.HasPrefix(line, diffAddLine) || bytes.Equal(line, diffAddLine) {
+	_, err := git.Check(c["commit"])
-			continue
+	if err != nil {
-		}
-		if len(bytes.TrimSpace(line)) != len(line) {
 		vr.Pass = false
-			vr.Msg = fmt.Sprintf("line %q has trailiing spaces", string(line))
+		if err.Error() == "exit status 2" {
+			vr.Msg = "has whitespace errors. See `git show --check " + c["commit"] + "`."
+		} else {
+			vr.Msg = "errored with: " + err.Error()
 		}
 	}
-	vr.Msg = "all added diff lines do not have trailing spaces"
 	return
 }
-
-var (
-	newLine     = []byte("\n")
-	diffAddLine = []byte("+ ")
-)