BitPackerTest.java

package com.example.bip39.bit;

import static org.junit.jupiter.api.Assertions.assertArrayEquals;
import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.junit.jupiter.api.Assertions.assertThrows;

import org.junit.jupiter.api.Test;

class BitPackerTest {

  @Test
  void readsBitsMostSignificantBitFirstFromSignedBytes() {
    byte[] source = {(byte) 0x80, (byte) 0xf1};

    assertEquals(1, BitPacker.readBits(source, 0, 1));
    assertEquals(0, BitPacker.readBits(source, 1, 7));
    assertEquals(0xf1, BitPacker.readBits(source, 8, 8));
    assertEquals(0x01, BitPacker.readBits(source, 12, 4));
  }

  @Test
  void writesBitsAndExtractsBytesWithoutLosingUnsignedValues() {
    byte[] target = new byte[3];

    BitPacker.writeBits(target, 0, 8, 0x80);
    BitPacker.writeBits(target, 8, 8, 0x01);
    BitPacker.writeBits(target, 16, 8, 0xff);

    assertArrayEquals(new byte[] {(byte) 0x80, 0x01, (byte) 0xff}, target);
    assertArrayEquals(target, BitPacker.extractBytes(target, 0, 24));
  }

  @Test
  void extractsBytesAcrossByteBoundaries() {
    byte[] source = {0x12, 0x34, 0x56};

    assertArrayEquals(new byte[] {0x23, 0x45}, BitPacker.extractBytes(source, 4, 16));
  }

  @Test
  void splitsKnownBitSequenceIntoElevenBitValues() {
    byte[] source = {0x12, 0x34, 0x56};

    assertArrayEquals(new int[] {145, 1301}, BitPacker.splitTo11BitValues(source, 22));
  }

  @Test
  void packsElevenBitValuesInMostSignificantBitOrder() {
    assertArrayEquals(
        new byte[] {0x12, 0x34, 0x54}, BitPacker.pack11BitValues(new int[] {145, 1301}));
  }

  @Test
  void elevenBitPackingAndSplittingRoundTrips() {
    int[] values = {0, 1, 1024, 2047, 42};

    byte[] packed = BitPacker.pack11BitValues(values);

    assertArrayEquals(values, BitPacker.splitTo11BitValues(packed, values.length * 11));
  }

  @Test
  void rejectsInvalidBitAccessAndOutOfRangeValues() {
    byte[] source = new byte[2];

    assertThrows(IllegalArgumentException.class, () -> BitPacker.readBits(source, -1, 1));
    assertThrows(IllegalArgumentException.class, () -> BitPacker.readBits(source, 0, 17));
    assertThrows(IllegalArgumentException.class, () -> BitPacker.writeBits(source, 0, 3, 0b1000));
    assertThrows(
        IllegalArgumentException.class, () -> BitPacker.splitTo11BitValues(source, Byte.SIZE));
    assertThrows(IllegalArgumentException.class, () -> BitPacker.pack11BitValues(new int[] {2048}));
    assertThrows(IllegalArgumentException.class, () -> BitPacker.extractBytes(source, 0, 7));
  }
}