Fix double rounding issues for multipleOf assertion (#71)

Resolves #70

Author: OptimumCode

src/commonMain/kotlin/io/github/optimumcode/json/schema/internal/factories/number/MultipleOfAssertionFactory.kt

@@ -7,10 +7,12 @@ import io.github.optimumcode.json.schema.internal.factories.number.util.NumberCo
 import io.github.optimumcode.json.schema.internal.factories.number.util.number
 import kotlinx.serialization.json.JsonElement
 import kotlinx.serialization.json.JsonPrimitive
+import kotlin.math.abs
 import kotlin.math.floor
 import kotlin.math.log10
 import kotlin.math.max
 import kotlin.math.pow
+import kotlin.math.round
 
 @Suppress("unused")
 internal object MultipleOfAssertionFactory : AbstractAssertionFactory("multipleOf") {
@@ -66,6 +68,8 @@ private fun isZero(first: Double): Boolean {
   return first == -0.0 || first == 0.0
 }
 
+private const val ROUND_THRESHOLD = 0.000000001
+
 private tailrec fun rem(
   first: Double,
   second: Double,
@@ -75,16 +79,38 @@ private tailrec fun rem(
     if (first < 1 && first > -1) {
       val newDegree = max(floor(log10(second)), degree)
       val newPow = 10.0.pow(-newDegree)
-      rem((first * newPow), (second * newPow))
+      rem(safeRound(first * newPow), safeRound(second * newPow))
     } else {
       val pow = 10.0.pow(-degree)
-      (first * pow) % (second * pow)
+      val newFirst = safeRound(first * pow)
+      val newSecond = safeRound(second * pow)
+
+      newFirst % newSecond
     }
   } else {
     first % second
   }
 }
 
+/**
+ * Rounds the [value] if an abs delta between original value and result of rounding
+ * is less than [ROUND_THRESHOLD].
+ * Otherwise, the original value is return.
+ *
+ * This method tries to solve the issue with double operation when not a precise result is returned.
+ * E.g. `19.99 * 100 = 1998.9999999999998` instead of `1999.0`
+ */
+private fun safeRound(value: Double): Double {
+  val rounded = round(value)
+  return if (abs(rounded - value) < ROUND_THRESHOLD) {
+    rounded
+  } else {
+    // we return the original value because the result was precise,
+    // and we don't need rounding to fix issue with double operations
+    value
+  }
+}
+
 private fun rem(
   first: Long,
   second: Double,

src/commonTest/kotlin/io/github/optimumcode/json/schema/assertions/number/JsonSchemaMultipleOfValidationTest.kt

@@ -4,6 +4,7 @@ import io.github.optimumcode.json.pointer.JsonPointer
 import io.github.optimumcode.json.schema.JsonSchema
 import io.github.optimumcode.json.schema.ValidationError
 import io.github.optimumcode.json.schema.base.KEY
+import io.kotest.assertions.assertSoftly
 import io.kotest.assertions.throwables.shouldThrow
 import io.kotest.core.spec.style.FunSpec
 import io.kotest.matchers.collections.shouldContainExactly
@@ -174,5 +175,22 @@ class JsonSchemaMultipleOfValidationTest : FunSpec() {
         }
       }
     }
+
+    test("BUG_70 rounding problem with some numbers because double does not behave as you expect") {
+      val schema =
+        JsonSchema.fromDefinition(
+          """
+          {
+            "multipleOf": 0.01
+          }
+          """.trimIndent(),
+        )
+      val errors = mutableListOf<ValidationError>()
+      val valid = schema.validate(JsonPrimitive(19.99), errors::add)
+      assertSoftly {
+        valid shouldBe true
+        errors shouldHaveSize 0
+      }
+    }
   }
 }