Add levensthein suggestion on backend

Check if backend is mis-spelled (distance less than 3)
and if so suggest what backend might match.
Get the first match, don't iterate until the
end in this case, it'll be overkill.

Test plan:
* Have a slightly mis-spelled
  backend name (e g wgte). Verify
  error message suggests wget instead.
* Have an outrageously wrongly spelled
  backend (eg hateternal) and verify that
  no backend is suggested.
* Have a correctly spelled backend and
  verify no regression.

Author: jonaslu

internal/pkg/call/call.go

@@ -47,11 +47,13 @@ type backend interface {
 }
 
 func getBackend(callData *data.Call) (backend, error) {
-	if backendConstructor, exists := ValidBackends[callData.Backend]; exists {
+	requestedBackend := callData.Backend
+
+	if backendConstructor, exists := ValidBackends[requestedBackend]; exists {
 		return backendConstructor.constructor(callData, backendConstructor.BinaryName), nil
 	}
 
-	return nil, errors.Errorf("Unknown backend: %s", callData.Backend)
+	return nil, errors.Errorf("Unknown backend: %s", requestedBackend)
 }
 
 func ValidBackend(backendName string) bool {

internal/pkg/parse/backend.go

@@ -6,6 +6,7 @@ import (
 
 	"github.com/jonaslu/ain/internal/pkg/call"
 	"github.com/jonaslu/ain/internal/pkg/data"
+	"github.com/jonaslu/ain/internal/pkg/utils"
 )
 
 func parseBackendSection(template []sourceMarker, callData *data.Parse) *fatalMarker {
@@ -26,13 +27,19 @@ func parseBackendSection(template []sourceMarker, callData *data.Parse) *fatalMa
 		}
 	}
 
-	backendName := strings.ToLower(backendLines[0].lineContents)
+	requestedBackendName := strings.ToLower(backendLines[0].lineContents)
 
-	if !call.ValidBackend(backendName) {
-		return newFatalMarker(fmt.Sprintf("Unknown backend %s", backendName), backendLines[0])
+	if !call.ValidBackend(requestedBackendName) {
+		for backendName, _ := range call.ValidBackends {
+			if utils.LevenshteinDistance(requestedBackendName, backendName) < 3 {
+				return newFatalMarker(fmt.Sprintf("Unknown backend: %s. Did you mean %s", requestedBackendName, backendName), backendLines[0])
+			}
+		}
+
+		return newFatalMarker(fmt.Sprintf("Unknown backend %s", requestedBackendName), backendLines[0])
 	}
 
-	callData.Backend = backendName
+	callData.Backend = requestedBackendName
 
 	return nil
 }