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This is an algorithm to return a list of consecutive integer ranges from a list of integers.

The practical use of such an algorithm is to filter lists more efficiently, i.e. rather than check e.g. 1000 items (1..1000; x==1 || x==2 || ... || x==1000), it might be possible to check only 2 items (x >= 1 && x <= 1000).

Does this algorithm have any mistakes, can it be optimized, or are there any other improvements you can suggest?

(As a side note: I know this code does not follow the standard C# naming conventions. I do not like to follow that particular convention; I prefer "snake_case" as it is easier on my eyes.)

Sample output:

(x >= 0 && x <= 1690)
(x >= 13642 && x <= 15331)
(x >= 27283 && x <= 27296)
(x >= 27769 && x <= 27776)
(x >= 28249 && x <= 28256)
(x >= 28729 && x <= 28736)
(x >= 29209 && x <= 29222)

The algorithm (built Visual Studio 2017, C# 7.3, .NET 4.7.2):

public static List<(int from, int to)> get_consecutive_ranges(List<int> fids)

 if (fids == null 

Example use:

// slow:
body = body.Where(a => fids.Contains(a.fid)).ToList();

// fast:
body = body.Where(a => fids_fast.Any(x => a.fid >= x.from && a.fid <= x.to)).ToList();

As a pre-note, I had to translate everything to camelCase and PascalCase because I was having a very difficult time reading your code. No judgments, just understand I was running short on time and didn't translate it back on the conclusion.

Readability

Use meaningful names.

• 
  fids I changed to integers as from your description I gather it's the list of integers used to create the ranges.


• 
  fids_index seems very noisy. Standard convention is i, j, etc. When you reduce the iterator variable down to a single character it's much easier to focus on what actually matters.


• Booleans should be prefixed with Is or Has or Was etc. So in this case IsFirst and IsLast instead of first and last makes it easier to read as english. You could even consider using var isNotFirst = i != 0; as you only use !first in the algorithm.


• Last vs Previous. is_conseq_with_last is refering to previous, so I switched it to previous to avoid confusion with last.


• 
  start_index and end_index sound like they would be 0 and integers.Count. They're the start and end of the range you're tracking, let's name them accordingly: rangeStart and rangeEnd.

If-Statement Ordering & Flow

We've all been there, written an algorithm, tested it, and it works, all done right? Well, sometimes that algorithm can be restructured to express what we're actually doing much clearer.

 if (!first && fids[fids_index - 1] == fids[fids_index] - 1)
 
 is_conseq_with_last = true;
 
 else if (!first)
 
 is_conseq_with_last = false;
 

There's a common denominator here, !first let's work with that, clearly neither of these happen if it's true.

if (!first)

 if (fids[fids_index - 1] == fids[fids_index] - 1)
 
 is_conseq_with_last = true;
 
 else
 
 is_conseq_with_last = false;
 

Well, now it's obvious that inner if-statement can be reduced.

if (!first)

 is_conseq_with_last = fids[fids_index - 1] == fids[fids_index] - 1;

And if you wanted, a ternary operator.

is_conseq_with_last = !first && fids[fids_index - 1] == fids[fids_index] - 1;

And it's now also obvious that you're setting the variable with each iteration, so there's no need to declare it outside the loop.

Similar can be done with the bottom if-statement. Here's what I ended up with altogether.

My Version

I am running short on time so there might be a mistake or two and it's still in my coding style so don't take it for face value and try to apply the concepts I'm showing above yourself.

public static List<(int from, int to)> GetConsecutiveRanges(List<int> integers)

 if (integers == null 

Your code is really complicated. First, your code should be abstracted a little more. It is not specific to feature IDs, therefore the terminology should not use these words. The same algorithm can be used to select which pages to print from a document, therefore the variables should be just nums and ranges. To test your current code, I wrote:

[Test]
public void TestRanges()

 Assert.AreEqual("", Str(Ranges(new List<int>())));
 Assert.AreEqual("1", Str(Ranges(new List<int> 1 )));
 Assert.AreEqual("1-5", Str(Ranges(new List<int> 1, 2, 3, 4, 5 )));
 Assert.AreEqual("1-3, 5", Str(Ranges(new List<int> 1, 2, 3, 5 )));
 Assert.AreEqual("1, 3, 5-6", Str(Ranges(new List<int> 1, 3, 5, 6 )));

I wrote a helper function Str so that I don't have to construct a list of ranges for each test case:

public static string Str(List<(int from, int to)> ranges)

 var parts = new List<string>();

 foreach (var range in ranges) 
 if (range.from == range.to) 
 parts.Add(range.from.ToString());
 else 
 parts.Add(range.@from + "-" + range.to);
 
 

 return string.Join(", ", parts);

After renaming your function to Ranges, these tests ran successfully. So I was ready to refactor your code. I did not really do this since your code looked too complicated to start with. Instead, I remembered that I had successfully used the following pattern quite often:

var start = ...;
while (start < nums.Count) 
 var end = ...;
 while (end < nums.Count) 
 

With this knowledge I wrote the following code:

public static List<(int from, int to)> Ranges(List<int> nums)

 nums = nums.OrderBy(a => a).Distinct().ToList();

 var ranges = new List<(int from, int to)>();

 var start = 0;
 while (start < nums.Count) 
 var end = start + 1; // the range is from [start, end).

 while (end < nums.Count && nums[end - 1] + 1 == nums[end]) 
 end++;
 

 ranges.Add((nums[start], nums[end - 1]));
 start = end; // continue after the current range
 

 return ranges;

This code doesn't need any special cases for the last range, or anything else. A range either stops when the end of the numbers is reached, or when the following number is not consecutive. This sounds sensible, and this is exactly what the code is doing.

I removed the check for nums == null since it is not necessary. Collections should never be null, and if they are, the code immediately throws an exception, which is fine.

I also removed the special case for nums.Count == 0 since returning an empty list is much better than returning null. Again, expressions that have collection type should never be null. The test cases cover this case, so there's nothing to worry about.