decimal: add fuzzing test

Fuzzing tests in Golang, see [1] and [2], requires Go 1.18+. However in
CI we use Go 1.13 that fails on running fuzzing tests. To avoid this
fuzzing test has been moved to a separate file an marked with build tag.

1. https://go.dev/doc/tutorial/fuzz
2. https://go.dev/doc/fuzz/

Part of #96

Author: ligurio

Makefile

@@ -51,6 +51,12 @@ test-decimal:
 	go clean -testcache
 	go test ./decimal/ -v -p 1
 
+.PHONY: test-fuzzing
+test-fuzzing:
+	@echo "Running fuzzing tests"
+	go clean -testcache
+	go test ./... -v -p 1 -tags fuzzing
+
 .PHONY: coverage
 coverage:
 	go clean -testcache

decimal/fuzzing_test.go

@@ -0,0 +1,34 @@
+//go:build fuzzing
+// +build fuzzing
+
+package decimal_test
+
+import (
+	"testing"
+
+	. "github.com/tarantool/go-tarantool/decimal"
+)
+
+func FuzzEncodeDecodeBCD(f *testing.F) {
+	samples := append(decimalBCDDecode, benchmarkSamples...)
+	for _, testcase := range samples {
+		if len(testcase.numString) > 0 {
+			f.Add(testcase.numString) // Use f.Add to provide a seed corpus.
+		}
+	}
+	f.Fuzz(func(t *testing.T, orig string) {
+		bcdBuf, err := EncodeStringToBCD(orig)
+		if err != nil {
+			return
+		}
+		var dec string
+		dec, err = DecodeStringFromBCD(bcdBuf)
+		if err != nil {
+			t.Fatalf("Failed to decode encoded value ('%s')", orig)
+		}
+
+		if dec != orig {
+			t.Fatalf("Decoded decimal (%s) is not equal to encoded one (%s)", dec, orig)
+		}
+	})
+}