Add Decoder option DisallowMissingColumns

Closes #33

decoder.go

@@ -18,6 +18,12 @@ type Decoder struct {
 	// options (Default: 'csv').
 	Tag string
 
+	// If true, Decoder will return a MissingColumnsError if it discovers
+	// that any of the columns are missing. This means that a CSV input
+	// will be required to contain all columns that were defined in the
+	// provided struct.
+	DisallowMissingColumns bool
+
 	// If not nil, Map is a function that is called for each field in the csv
 	// record before decoding the data. It allows mapping certain string values
 	// for specific columns or types to a known format. Decoder calls Map with
@@ -393,13 +399,18 @@ func (d *Decoder) fields(k typeKey) ([]decField, error) {
 		return d.cache, nil
 	}
 
-	fields := cachedFields(k)
-	decFields := make([]decField, 0, len(fields))
-	used := make([]bool, len(d.header))
-
+	var (
+		fields      = cachedFields(k)
+		decFields   = make([]decField, 0, len(fields))
+		used        = make([]bool, len(d.header))
+		missingCols []string
+	)
 	for _, f := range fields {
 		i, ok := d.hmap[f.name]
 		if !ok {
+			if d.DisallowMissingColumns {
+				missingCols = append(missingCols, f.name)
+			}
 			continue
 		}
 
@@ -427,6 +438,12 @@ func (d *Decoder) fields(k typeKey) ([]decField, error) {
 		used[i] = true
 	}
 
+	if len(missingCols) > 0 {
+		return nil, &MissingColumnsError{
+			Columns: missingCols,
+		}
+	}
+
 	d.unused = d.unused[:0]
 	for i, b := range used {
 		if !b {

decoder_test.go

@@ -1628,6 +1628,99 @@ string,"{""key"":""value""}"
 		}
 	})
 
+	t.Run("decode with disallow missing columns", func(t *testing.T) {
+		type Type struct {
+			String string
+			Int    int
+			Float  float64
+		}
+
+		t.Run("all present", func(t *testing.T) {
+			dec, err := NewDecoder(NewReader(
+				[]string{"String", "Int", "Float"},
+				[]string{"lol", "1", "2.0"},
+			))
+			if err != nil {
+				t.Fatal(err)
+			}
+			dec.DisallowMissingColumns = true
+
+			var tt Type
+			if err := dec.Decode(&tt); err != nil {
+				t.Fatalf("expected err to be nil; got %v", err)
+			}
+
+			if expected := (Type{"lol", 1, 2}); !reflect.DeepEqual(tt, expected) {
+				t.Errorf("want=%v; got %v", expected, tt)
+			}
+		})
+
+		fixtures := []struct {
+			desc        string
+			recs        [][]string
+			missingCols []string
+			msg         string
+		}{
+			{
+				desc: "one missing",
+				recs: [][]string{
+					{"String", "Int"},
+					{"lol", "1"},
+				},
+				missingCols: []string{"Float"},
+				msg:         `csvutil: missing columns: "Float"`,
+			},
+			{
+				desc: "two missing",
+				recs: [][]string{
+					{"String"},
+					{"lol"},
+				},
+				missingCols: []string{"Int", "Float"},
+				msg:         `csvutil: missing columns: "Int", "Float"`,
+			},
+			{
+				desc: "all missing",
+				recs: [][]string{
+					{"w00t"},
+					{"lol"},
+				},
+				missingCols: []string{"String", "Int", "Float"},
+				msg:         `csvutil: missing columns: "String", "Int", "Float"`,
+			},
+		}
+
+		for _, f := range fixtures {
+			t.Run(f.desc, func(t *testing.T) {
+				dec, err := NewDecoder(NewReader(f.recs...))
+				if err != nil {
+					t.Fatal(err)
+				}
+				dec.DisallowMissingColumns = true
+
+				var tt Type
+				err = dec.Decode(&tt)
+
+				if err == nil {
+					t.Fatal("expected err != nil")
+				}
+
+				mcerr, ok := err.(*MissingColumnsError)
+				if !ok {
+					t.Fatalf("expected err to be of *MissingColumnErr; got %[1]T (%[1]v)", err)
+				}
+
+				if !reflect.DeepEqual(mcerr.Columns, f.missingCols) {
+					t.Errorf("expected missing columns to be %v; got %v", f.missingCols, mcerr.Columns)
+				}
+
+				if err.Error() != f.msg {
+					t.Errorf("expected err message to be %q; got %q", f.msg, err.Error())
+				}
+			})
+		}
+	})
+
 	t.Run("invalid unmarshal tests", func(t *testing.T) {
 		var fixtures = []struct {
 			v        interface{}

error.go

@@ -1,6 +1,7 @@
 package csvutil
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
 	"reflect"
@@ -130,3 +131,21 @@ func (e *MarshalerError) Error() string {
 func errPtrUnexportedStruct(typ reflect.Type) error {
 	return fmt.Errorf("csvutil: cannot decode into a pointer to unexported struct: %s", typ)
 }
+
+// MissingColumnsError is returned by Decoder only when DisallowMissingColumns
+// option was set to true. It contains a list of all missing columns.
+type MissingColumnsError struct {
+	Columns []string
+}
+
+func (e *MissingColumnsError) Error() string {
+	var b bytes.Buffer
+	b.WriteString("csvutil: missing columns: ")
+	for i, c := range e.Columns {
+		if i > 0 {
+			b.WriteString(", ")
+		}
+		fmt.Fprintf(&b, "%q", c)
+	}
+	return b.String()
+}