Seed from go-template-repo, renamed to simplexcalc

The template's files at a77fd30, without its history or LICENSE, after
script/rename simplexcalc.

Model: opus-5-5

internal/middleware/responsewriter.go

@@ -0,0 +1,63 @@
+package middleware
+
+import (
+	"net/http"
+)
+
+// responseRecorder remembers the status code and byte count for the
+// logger and the metrics middleware. net/http gives no way to read
+// them back off an http.ResponseWriter, so the only way to know what
+// was answered is to be the thing that answered it.
+type responseRecorder struct {
+	http.ResponseWriter
+
+	status  int
+	written int64
+	wrote   bool
+}
+
+func newResponseRecorder(w http.ResponseWriter) *responseRecorder {
+	// A handler that writes a body without calling WriteHeader has
+	// sent 200; recording that up front means Status() is right for
+	// the common case without waiting for a call that never comes.
+	return &responseRecorder{ResponseWriter: w, status: http.StatusOK}
+}
+
+func (r *responseRecorder) WriteHeader(status int) {
+	if r.wrote {
+		return
+	}
+
+	r.status = status
+	r.wrote = true
+
+	r.ResponseWriter.WriteHeader(status)
+}
+
+func (r *responseRecorder) Write(b []byte) (int, error) {
+	r.wrote = true
+
+	n, err := r.ResponseWriter.Write(b)
+	r.written += int64(n)
+
+	//nolint:wrapcheck // pass-through writer: wrapping would obscure the underlying error.
+	return n, err
+}
+
+// Status returns the status code that was sent.
+func (r *responseRecorder) Status() int {
+	return r.status
+}
+
+// Written reports whether anything has been sent yet. The panic
+// recoverer needs this: it can only substitute a 500 for a response
+// that has not started.
+func (r *responseRecorder) Written() bool {
+	return r.wrote
+}
+
+// Unwrap lets http.ResponseController reach the underlying writer, so
+// wrapping does not cost the handler flushing or deadline control.
+func (r *responseRecorder) Unwrap() http.ResponseWriter {
+	return r.ResponseWriter
+}