feat: add eqv derivation with fields to compare (#415)

modules/cats/src/main/scala/derevo/cats/eqv.scala

@@ -1,9 +1,10 @@
 package derevo.cats
 
 import cats.Eq
+import derevo.{Derivation, NewTypeDerivation}
 import magnolia.{CaseClass, Magnolia, SealedTrait}
-import derevo.Derivation
-import derevo.NewTypeDerivation
+
+import scala.reflect.macros.blackbox
 
 object eqv extends Derivation[Eq] with NewTypeDerivation[Eq] {
   type Typeclass[T] = Eq[T]
@@ -25,6 +26,31 @@ object eqv extends Derivation[Eq] with NewTypeDerivation[Eq] {
 
   implicit def instance[T]: Eq[T] = macro Magnolia.gen[T]
 
+  def apply[T](eqFields: String*): Eq[T] = macro eqExtendedImpl[T]
+
+  def eqExtendedImpl[T: c.WeakTypeTag](c: blackbox.Context)(eqFields: c.Expr[String]*): c.Tree = {
+    import c.universe._
+
+    val T = weakTypeOf[T]
+    val eqFieldsSet = eqFields.map(_.tree).collect {
+      case Literal(Constant(field: String)) => field
+    }.toSet
+
+    val comparisons = T.decls.collect {
+      case m: MethodSymbol if m.isCaseAccessor && eqFieldsSet.contains(m.name.toString) =>
+        val name = m.name.toTermName
+        q"Eq[${m.typeSignature}].eqv(x.$name, y.$name)"
+    }
+
+    q"""
+        new Eq[$T] {
+          def eqv(x: $T, y: $T): Boolean = {
+            ..$comparisons
+          }
+        }
+      """
+  }
+
   object universal extends Derivation[Eq] {
     implicit def instance[T]: Eq[T] = Eq.fromUniversalEquals[T]
   }

modules/cats/src/test/scala/derevo/cats/EqSpec.scala

@@ -34,6 +34,15 @@ class EqSpec extends AnyFreeSpec {
         assert(Eq[Qux].eqv(Qux(1), Qux(1)))
         assert(Eq[Qux].neqv(Qux(1), Qux(-1)))
       }
+
+      "with ignoring fields" in {
+        @derive(eqv("bar"))
+        case class Foo(bar: Int, baz: Int)
+
+        assert(Eq[Foo].eqv(Foo(1, 2), Foo(1, 5)))
+        assert(Eq[Foo].neqv(Foo(2, 2), Foo(3, 2)))
+        assert(Eq[Foo].neqv(Foo(2, 2), Foo(3, 1)))
+      }
     }
   }
 }