first positive integer implementation

Author: jaebradley

src/main/java/problems/impl/FirstMissingPositiveIntegerIdentifierImpl.java

@@ -0,0 +1,48 @@
+package problems.impl;
+
+import problems.interfaces.FirstMissingPositiveIntegerIdentifier;
+
+public class FirstMissingPositiveIntegerIdentifierImpl implements FirstMissingPositiveIntegerIdentifier {
+
+  /**
+   * Given an array of integers, identify if any of these array values are between 1 and the
+   * length of the array, inclusive.
+   *
+   * If an array value is between 1 and the length of the array, then it is considered "seen" and
+   * it's boolean index (minus 1, since we start counting from 1) is set to true.
+   *
+   * @param values array of integers
+   * @return array of booleans that represents if any of the input values is between 1 and the
+   * length of the input array
+   */
+  @Override
+  public boolean[] identifyIfFirstIntegersHaveBeenSeen(final int[] values) {
+    final boolean[] seenValues = new boolean[values.length];
+    for (final int value : values) {
+      if (value > 0 && value <= values.length && !seenValues[value - 1]) {
+        seenValues[value - 1] = true;
+      }
+    }
+    return seenValues;
+  }
+
+  /**
+   * Identify the first positive integer not in the input integer array.
+   *
+   * If the input integer array is -1, -2 then the first positive integer is 1.
+   *
+   * @param values input integer array
+   * @return first positive integer not in input integer array
+   */
+  @Override
+  public int identifyFirstMissingPositiveInteger(final int[] values) {
+    final boolean[] seenIntegers = identifyIfFirstIntegersHaveBeenSeen(values);
+    for (int index = 0; index < seenIntegers.length; index++) {
+      if (!seenIntegers[index]) {
+        return index + 1;
+      }
+    }
+
+    return seenIntegers.length + 1;
+  }
+}

src/main/java/problems/interfaces/FirstMissingPositiveIntegerIdentifier.java

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+package problems.interfaces;
+
+public interface FirstMissingPositiveIntegerIdentifier {
+  int identifyFirstMissingPositiveInteger(int[] values) ;
+
+  boolean[] identifyIfFirstIntegersHaveBeenSeen(int[] values);
+}

src/test/java/problems/impl/FirstMissingPositiveIntegerIdentifierImplTest.java

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+package problems.impl;
+
+import org.junit.Assert;
+import org.junit.Test;
+import problems.exceptions.NoFirstMissingPositiveIntegerException;
+import problems.interfaces.FirstMissingPositiveIntegerIdentifier;
+
+import java.util.Arrays;
+
+public class FirstMissingPositiveIntegerIdentifierImplTest {
+  private final FirstMissingPositiveIntegerIdentifier firstMissingPositiveIntegerIdentifier = new FirstMissingPositiveIntegerIdentifierImpl();
+
+  @Test
+  public void itShouldReturnSeenValues() {
+    Assert.assertTrue(Arrays.equals(firstMissingPositiveIntegerIdentifier.identifyIfFirstIntegersHaveBeenSeen(new int[]{}), new boolean[]{}));
+    Assert.assertTrue(Arrays.equals(firstMissingPositiveIntegerIdentifier.identifyIfFirstIntegersHaveBeenSeen(new int[]{-1}), new boolean[]{false}));
+    Assert.assertTrue(Arrays.equals(firstMissingPositiveIntegerIdentifier.identifyIfFirstIntegersHaveBeenSeen(new int[]{0}), new boolean[]{false}));
+    Assert.assertTrue(Arrays.equals(firstMissingPositiveIntegerIdentifier.identifyIfFirstIntegersHaveBeenSeen(new int[]{1}), new boolean[]{true}));
+    Assert.assertTrue(Arrays.equals(firstMissingPositiveIntegerIdentifier.identifyIfFirstIntegersHaveBeenSeen(new int[]{2}), new boolean[]{false}));
+    Assert.assertTrue(Arrays.equals(firstMissingPositiveIntegerIdentifier.identifyIfFirstIntegersHaveBeenSeen(new int[]{2, 3}), new boolean[]{false, true}));
+    Assert.assertTrue(Arrays.equals(firstMissingPositiveIntegerIdentifier.identifyIfFirstIntegersHaveBeenSeen(new int[]{1, 3, 3, 1}), new boolean[]{true, false, true, false}));
+  }
+
+  @Test
+  public void itShouldIdentifyFirstMissingPositiveInteger() {
+    Assert.assertEquals(firstMissingPositiveIntegerIdentifier.identifyFirstMissingPositiveInteger(new int[]{}), 1);
+    Assert.assertEquals(firstMissingPositiveIntegerIdentifier.identifyFirstMissingPositiveInteger(new int[]{-1}), 1);
+    Assert.assertEquals(firstMissingPositiveIntegerIdentifier.identifyFirstMissingPositiveInteger(new int[]{0}), 1);
+    Assert.assertEquals(firstMissingPositiveIntegerIdentifier.identifyFirstMissingPositiveInteger(new int[]{1}), 2);
+    Assert.assertEquals(firstMissingPositiveIntegerIdentifier.identifyFirstMissingPositiveInteger(new int[]{2}), 1);
+    Assert.assertEquals(firstMissingPositiveIntegerIdentifier.identifyFirstMissingPositiveInteger(new int[]{2, 3}), 1);
+    Assert.assertEquals(firstMissingPositiveIntegerIdentifier.identifyFirstMissingPositiveInteger(new int[]{1, 3, 3, 1}), 2);
+  }
+
+}