Fix Counter.Bit64.Final.asBits calculation (#12)

Author: 05nelsonm

library/bits/build.gradle.kts

@@ -18,5 +18,13 @@ plugins {
 }
 
 kmpConfiguration {
-    configureShared(java9ModuleName = "org.kotlincrypto.bitops.bits", publish = true) {}
+    configureShared(java9ModuleName = "org.kotlincrypto.bitops.bits", publish = true) {
+        jvm {
+            sourceSetTest {
+                dependencies {
+                    implementation(project(":library:endian"))
+                }
+            }
+        }
+    }
 }

library/bits/src/commonMain/kotlin/org/kotlincrypto/bitops/bits/Counter.kt

@@ -278,7 +278,7 @@ public sealed class Counter private constructor() {
 
             public override fun asBits(): Final {
                 if (isBits) return this
-                return Final(lo shl 3, (hi shl 3) or (lo ushr 29), isBits = true)
+                return Final(lo shl 3, (hi shl 3) or (lo ushr 61), isBits = true)
             }
         }
 

library/bits/src/commonTest/kotlin/org/kotlincrypto/bitops/bits/CounterUnitTest.kt

@@ -209,21 +209,4 @@ class CounterUnitTest {
         val actual = ((bits.hi.toLong() and 0xffffffff) shl 32) or (bits.lo.toLong() and 0xffffffff)
         assertEquals(expected, actual)
     }
-
-    @Test
-    fun givenBit64Final_whenAsBits_thenConvertsAsExpected() {
-        val number = 5551889119L
-        val final = Counter.Bit64.Final(lo = number, hi = 1)
-        val bits = final.asBits()
-        assertNotEquals(final, bits)
-
-        // Should return same instance (already bits)
-        assertEquals(bits, bits.asBits())
-
-        var expected = Long.MAX_VALUE
-        expected += number + 1
-        expected *= Byte.SIZE_BITS
-        val actual = ((bits.hi and 0xffffffff) shl 32) or (bits.lo and 0xffffffff)
-        assertEquals(expected, actual)
-    }
 }

library/bits/src/jvmTest/kotlin/org/kotlincrypto/bitops/bits/CounterJvmUnitTest.kt

@@ -0,0 +1,113 @@
+/*
+ * Copyright (c) 2025 Matthew Nelson
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ **/
+package org.kotlincrypto.bitops.bits
+
+import org.kotlincrypto.bitops.endian.Endian.Big.beLongAt
+import java.math.BigInteger
+import kotlin.random.Random
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+/**
+ * Uses Java's [BigInteger] to verify [Counter.Bit64.Final.asBits] functions as expected.
+ * */
+class CounterJvmUnitTest {
+
+    @Test
+    fun givenBit64Final_whenAsMaxBits_thenConvertsAsExpected() {
+        val max = BigInteger("2").pow(128)
+        val maxBytes = max.divide(BigInteger("8"))
+        val maxBytesMinus1 = maxBytes.minus(BigInteger.ONE)
+        val expectedBits = maxBytesMinus1.times(BigInteger("8"))
+
+        val bMaxBytesMinus1 = maxBytesMinus1.toByteArray()
+        val bExpectedBits = expectedBits.toByteArray()
+
+        val bits = Counter.Bit64.Final(
+            lo = bMaxBytesMinus1.beLongAt(8),
+            hi = bMaxBytesMinus1.beLongAt(0),
+        ).asBits()
+
+        // BigInteger.toByteArray returns 17 bytes, prefixed with
+        // a leading 0 which is dropped.
+        val expectedLo = bExpectedBits.beLongAt(9)
+        val expectedHi = bExpectedBits.beLongAt(1)
+
+        assertEquals(-8L, expectedLo)
+        assertEquals(-1L, expectedHi)
+
+        assertEquals(expectedLo, bits.lo)
+        assertEquals(expectedHi, bits.hi)
+    }
+
+    @Test
+    fun givenBit64Final_whenAsBits_thenConvertsAsExpected() {
+        val pow67 = BigInteger("2").pow(67).minus(BigInteger.ONE)
+        val expectedBits = pow67.times(BigInteger("8"))
+
+        // BigInteger.toByteArray returns 9 bytes (most significant
+        // number stripped of leading zeros)
+        val bPow67 = pow67.toByteArray()
+        val bExpectedBits = expectedBits.toByteArray()
+
+        val bits = Counter.Bit64.Final(
+            lo = bPow67.beLongAt(1),
+            hi = bPow67[0].toLong(),
+        ).asBits()
+
+        val expectedLo = bExpectedBits.beLongAt(1)
+        val expectedHi = bExpectedBits[0].toLong()
+
+        assertEquals(-8, expectedLo)
+        assertEquals(63, expectedHi)
+
+        assertEquals(expectedLo, bits.lo)
+        assertEquals(expectedHi, bits.hi)
+    }
+
+    @Test
+    fun givenBit64Final_whenAsBitsFromRandomLargeNumber_thenConvertsAsExpected() {
+        repeat(500) {
+            val number = BigInteger("2").pow(88).minus(
+                BigInteger(
+                    Random.Default.nextLong(Long.MAX_VALUE - 500_000_000, Long.MAX_VALUE).toString()
+                )
+            )
+            val nBits = number.times(BigInteger("8"))
+
+            val bNumber = number.toByteArray().to16Bytes()
+            val bNBits = nBits.toByteArray().to16Bytes()
+
+            val bits = Counter.Bit64.Final(
+                lo = bNumber.beLongAt(8),
+                hi = bNumber.beLongAt(0),
+            ).asBits()
+
+            val expectedLo = bNBits.beLongAt(8)
+            val expectedHi = bNBits.beLongAt(0)
+
+            assertEquals(expectedLo, bits.lo)
+            assertEquals(expectedHi, bits.hi)
+        }
+    }
+
+    private fun ByteArray.to16Bytes(): ByteArray {
+        require(this.size <= 16) { "array.size is greater than 16 bytes..." }
+        val b = ByteArray(16)
+        copyInto(b, destinationOffset = 16 - size)
+        return b
+    }
+}