Merge branch 'windows_expt' into devel. Error message about file creation on windows.

.gitignore

 test_tmp*
-ackley
 ackley_mc
+ackley_mc.exe
 *.csv
 /ackley_mc.exe

Makefile

@@ -34,6 +34,7 @@ clean:
 	$(GO) clean
 	rm -rf bin/*
 	rm -rf examples/*.csv
+	rm -rf *.csv
 	rm -rf */*_delme.*
 	rm -rf */test_tmp*
 	rm -rf /tmp/go-build[0-9]*

README.md

@@ -18,11 +18,11 @@ I do not have a mac for testing, so this is limited to linux and windows. It is
     cd ackley_mc
     go build .
 
-and you should end up with an executable called `ackley_mc` or `ackley_mc.exe` under windows. To build without graphics:
+and you should end up with an executable called `ackley_mc` or `ackley_mc.exe` under windows. To build without graphics,
 
     git clone git@gitlab.rrz.uni-hamburg.de:Bae5157/ackley_mc.git
     cd ackley_mc
-    go build -tags=no_gfx .
+    go build .
  
 ### Graphics yes / no ?
 

mc_work/dorun.go

@@ -4,8 +4,10 @@ package mcwork
 import (
 	"bufio"
 	"bytes"
+	"errors"
 	"fmt"
 	"io"
+	"io/fs"
 	"math"
 	"math/rand"
 	"os"
@@ -62,6 +64,43 @@ func isNotSane(mcPrm *McPrm) error {
 	return nil
 }
 
+// fExists returns true if the file exists.
+// It is part of the exercise, working around file permissions on windows
+func fExists (name string) bool {
+	var err error
+	if _, err := os.Stat (name); err == nil {
+		return true
+	}
+	if errors.Is (err, fs.ErrNotExist) {
+		return false
+	}
+	return true
+}
+
+
+// createFix is a wrapper around os.Create() which I seem to need for windows.
+// On windows, I cannot overwrite an existing file. To be very exact, the
+// O_CREATE fails if a file exists. O_TRUNC also fails.
+func createFix (name string) (*os.File, error) {
+	var rErr = `Remove the file %s and try again. This only happens on windows: %w`
+	var f *os.File
+	var err error
+	if f, err = os.Create(name); err == nil {
+		return f, nil // Most common path (no problem)
+	}
+	
+	if !fExists(name) { // does not exist, but we have a real error.
+		return nil, err // Return original error
+	} // Next step try to remove original file
+    if e := os.Remove (name); e != nil {
+		return nil, fmt.Errorf (rErr, name, e)
+	}
+	if f, err = os.Create(name); err == nil { // second attempt, after removing name
+		return f, nil
+	}
+	return nil, err
+}
+
 // setupRun does things like get the cooling rate, seed the random numbers.
 func setupRun(mcPrm *McPrm, cprm *cprm) error {
 	var err error
@@ -83,8 +122,8 @@ func setupRun(mcPrm *McPrm, cprm *cprm) error {
 		if mcPrm.fOutName, err = setSuffix(mcPrm.fOutName, ".csv"); err != nil {
 			return err
 		}
-		if cprm.fOutRaw, err = os.Create(mcPrm.fOutName); err != nil {
-			return fmt.Errorf ("Trying to create file: %w", err)
+		if cprm.fOutRaw, err = createFix(mcPrm.fOutName); err != nil {
+			return err
 		}
 		cprm.fOut = bufio.NewWriter(cprm.fOutRaw)
 		cprm.doTxtFval = true // other parts of code can see that we are writing to file