Feat/reverse headers

.github/workflows/go.yaml

@@ -0,0 +1,43 @@
+name: Go
+
+on:
+  push:
+    branches: [ "*" ]
+  pull_request: 
+
+
+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()

replacer_test.go

@@ -3,6 +3,7 @@ package csvheaders_test
 import (
 	"bytes"
 	"encoding/csv"
+	"errors"
 	"slices"
 	"testing"
 
@@ -14,6 +15,7 @@ func TestReplacer(t *testing.T) {
 
 	t.Run("Read", testReplacerRead)
 	t.Run("ReadAll", testReplacerReadAll)
+	t.Run("Reverse", testReplacerReverse)
 }
 
 func testReplacerRead(t *testing.T) {
@@ -55,21 +57,20 @@ field 1,Charles,Monica
 func testReplacerReadAll(t *testing.T) {
 	t.Parallel()
 
-	var rawRows = []byte(`recording artist,title,isrc
-THE BEATLES,LET IT BE,US1239875
-recording artist,my band,FR789230987
+	var rawRows = []byte(`recording artist,title
+THE BEATLES,LET IT BE
+recording artist,my band
 `)
 
 	want := [][]string{
-		{"ARTIST", "TITLE", "ISRC"},
-		{"THE BEATLES", "LET IT BE", "US1239875"},
-		{"recording artist", "my band", "FR789230987"},
+		{"ARTIST", "TITLE"},
+		{"THE BEATLES", "LET IT BE"},
+		{"recording artist", "my band"},
 	}
 
 	headerReplacements := map[string]string{
 		"recording artist": "ARTIST",
 		"title":            "TITLE",
-		"isrc":             "ISRC",
 	}
 
 	r := csvheaders.NewReplacer(csv.NewReader(bytes.NewBuffer(rawRows)), headerReplacements)
@@ -79,6 +80,65 @@ recording artist,my band,FR789230987
 		t.Fatal(err)
 	}
 
+	compareResults(t, want, got)
+}
+
+func testReplacerReverse(t *testing.T) {
+	t.Parallel()
+
+	var rawRows = []byte(`field 1,field 2,field 10
+value 1,field 2,value 10
+`)
+
+	testCases := map[string]struct {
+		headerMapping map[string]string
+		expectedError error
+		want          [][]string
+	}{
+		"duplicated header": {
+			headerMapping: map[string]string{
+				"field 1": "FIELD",
+				"field 2": "FIELD",
+			},
+			expectedError: csvheaders.ErrDuplicateHeader,
+			want:          nil,
+		},
+		"nil headers": {
+			headerMapping: nil,
+			expectedError: nil,
+			want:          [][]string{{"field 1", "field 2", "field 10"}, {"value 1", "field 2", "value 10"}},
+		},
+		"no header conflict": {
+			headerMapping: map[string]string{"Monday": "field 10"},
+			expectedError: nil,
+			want:          [][]string{{"field 1", "field 2", "Monday"}, {"value 1", "field 2", "value 10"}},
+		},
+	}
+
+	for name, testCase := range testCases {
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			r, err := csvheaders.NewReverseReplacer(csv.NewReader(bytes.NewBuffer(rawRows)), testCase.headerMapping)
+			if testCase.expectedError != nil {
+				if !errors.Is(err, testCase.expectedError) {
+					t.Fatalf("expected %s, got %s", testCase.expectedError, err)
+				}
+
+				return
+			}
+
+			got, err := r.ReadAll()
+			if err != nil {
+				t.Fatalf("unexpected error while reading: %s", err)
+			}
+
+			compareResults(t, testCase.want, got)
+		})
+	}
+}
+
+func compareResults(t *testing.T, want, got [][]string) {
 	if len(got) != len(want) {
 		t.Fatalf("got %d records, want %d", len(got), len(want))
 	}