# find progressions

Dec 28th, 2021
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1. package ch.claude_martin;
2.
3. import java.util.ArrayList;
4. import java.util.Arrays;
5. import java.util.Set;
6. import java.util.TreeSet;
7.
8. class SomeClass {
9.
10.     static boolean test(int a, int b) {
11.         return a + 2 == b;
12.     }
13.
14.     public static void main(final String... args) {
15.         // Take the data from the arguments or use the example list:
16.         final var data = args.length > 0 ? Arrays.stream(args).mapToInt(Integer::parseInt).sorted().toArray()
17.                 : new int[] { 1, 2, 5, 7, 8, 9, 11, 12, 14 };
18.         System.out.println(Arrays.toString(data));// [5, 7, 9, 11, 12, 14]
19.         final var size = data.length;
20.         // three pointers are needed for this. Initially we look at the first two
21.         // elements of the data.
22.         // When third is used below to find longer progressions and add each sublist to
23.         // the result set.
24.         var p0 = 0;
25.         var p1 = 1;
26.         // The result set is a collection of sets. Each of those sets is a progression.
27.         var resultSet = new ArrayList<Set<Integer>>();
28.         // This loops through all possible pairs, but p1 is always < p1:
29.         while (p0 < size && p1 < size) {
30.             // The set contains at least 2 elements if it is part of the result set:
31.             var set = new TreeSet<Integer>();
32.             var a = data[p0];
33.             var b = data[p1];
34.             if (test(a, b)) {
38.                 // Now we look for longer progressions that start with the one we just found:
39.                 for (var p2 = p1 + 1; p2 < size; p2++) {
40.                     var c = data[p2];
41.                     if (test(b, c)) {
44.                     }
45.                 }
46.             }
47.             p1++; // increment pointer for "b":
48.             // But it we go out of bounds we instead increment p0:
49.             if (p1 == size) {
50.                 // And p1 points to the one next to the new p0:
51.                 p1 = ++p0 + 1;
52.             }
53.         }
54.         // Show the result set:
55.         System.out.println(resultSet);
56.     }
57. }