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maths: add O(log n) Fibonacci via matrix exponentiation#14557

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AKIB473:akib/fibonacci-fast
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maths: add O(log n) Fibonacci via matrix exponentiation#14557
AKIB473 wants to merge 2 commits intoTheAlgorithms:masterfrom
AKIB473:akib/fibonacci-fast

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@AKIB473 AKIB473 commented Apr 14, 2026

Describe your change:

Add a new implementation of the Fibonacci sequence using matrix exponentiation, achieving O(log n) time complexity compared to the O(n) iterative approach.

  • Add an algorithm?
  • Fix a bug or typo in an existing algorithm?
  • Add or change doctests?
  • Documentation change?

Checklist:

  • I have read CONTRIBUTING.md.
  • This pull request is all my own work -- I have not plagiarized.
  • I know that pull requests will not be merged if they fail the automated tests.
  • This PR only changes one algorithm file.
  • All new Python files are placed inside an existing directory.
  • All filenames are in all lowercase characters with no spaces or dashes.
  • All functions and variable names follow Python naming conventions.
  • All function parameters and return values are annotated with Python type hints.
  • All functions have doctests that pass the automated testing.
  • All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
  • If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword.

- Implements fibonacci(n) using fast matrix exponentiation
- Time complexity: O(log n), Space complexity: O(log n)
- Full docstring with Wikipedia reference
- Type hints on all parameters and return values
- Doctests for edge cases: fib(0), fib(1), fib(10), fib(20), fib(50)
- Raises ValueError for negative input
@algorithms-keeper algorithms-keeper bot added the require descriptive names This PR needs descriptive function and/or variable names label Apr 14, 2026
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def _mat_mul(
a: list[list[int]], b: list[list[int]]
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Please provide descriptive name for the parameter: a

Please provide descriptive name for the parameter: b

return result


def fibonacci(n: int) -> int:
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Please provide descriptive name for the parameter: n

@algorithms-keeper algorithms-keeper bot added the awaiting reviews This PR is ready to be reviewed label Apr 14, 2026
@AKIB473 AKIB473 force-pushed the akib/fibonacci-fast branch from 4fc96e8 to 66919bd Compare April 14, 2026 13:04
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Click here to look at the relevant links ⬇️

🔗 Relevant Links

Repository:

Python:

Automated review generated by algorithms-keeper. If there's any problem regarding this review, please open an issue about it.

algorithms-keeper commands and options

algorithms-keeper actions can be triggered by commenting on this PR:

  • @algorithms-keeper review to trigger the checks for only added pull request files
  • @algorithms-keeper review-all to trigger the checks for all the pull request files, including the modified files. As we cannot post review comments on lines not part of the diff, this command will post all the messages in one comment.

NOTE: Commands are in beta and so this feature is restricted only to a member or owner of the organization.

"""


def _mat_mul(a: list[list[int]], b: list[list[int]]) -> list[list[int]]:
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Please provide descriptive name for the parameter: a

Please provide descriptive name for the parameter: b

return result


def fibonacci(n: int) -> int:
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Please provide descriptive name for the parameter: n

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