reverse headers + ci

Author: pascal-blokur

.github/workflows/go.yaml

@@ -0,0 +1,44 @@
+name: Go
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches:
+      - *
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repo
+        uses: actions/checkout@v5
+        with:
+          fetch-depth: 0 # needed because of the new-from-rev in golangci
+
+      - name: Set up Go
+        uses: actions/setup-go@v6
+        with:
+          go-version: "1.25.x"
+
+      - name: golangci-lint
+        uses: golangci/golangci-lint-action@v8
+        with:
+          args: --timeout 5m0s
+
+      - id: govulncheck
+        uses: golang/govulncheck-action@v1
+
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repo
+        uses: actions/checkout@v5
+
+      - name: Set up Go
+        uses: actions/setup-go@v6
+        with:
+          go-version: "1.25.x"
+
+      - name: Running Unit Tests
+        run: go test ./...

errors.go

@@ -0,0 +1,12 @@
+package csvheaders
+
+// ReplacerError is an error that can be returned when using the Replacer.
+type ReplacerError string
+
+// Error implements the error interface.
+func (e ReplacerError) Error() string {
+	return string(e)
+}
+
+// ErrDuplicateHeader is returned when the header map registers the same header twice.
+const ErrDuplicateHeader = ReplacerError("duplicate header")

replacer.go

@@ -21,6 +21,22 @@ func NewReplacer(reader *csv.Reader, inToOut map[string]string) *Replacer {
 	return &Replacer{Reader: reader, inToOut: inToOut}
 }
 
+// NewReverseReplacer returns a replacer using mappings from struct definition to file contents.
+// If there is a duplicate value in the headers mapping, this returns an error.
+func NewReverseReplacer(reader *csv.Reader, outToIn map[string]string) (*Replacer, error) {
+	r := &Replacer{Reader: reader, inToOut: make(map[string]string)}
+
+	for out, in := range outToIn {
+		if h, alreadyRegistered := r.inToOut[in]; alreadyRegistered {
+			return nil, fmt.Errorf("struct tag %s registered with CSV headers %s and %s: %w", in, out, h, ErrDuplicateHeader)
+		}
+
+		r.inToOut[in] = out
+	}
+
+	return r, nil
+}
+
 // Read a record from the reader.
 func (r *Replacer) Read() ([]string, error) {
 	record, err := r.Reader.Read()