feat: add population and sample standard deviation calculations

[kazeburo]

PR Type

Enhancement, Tests, Documentation

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Description

• Add population and sample standard deviation methods.

• Improve numerical stability for extreme values.

• Test edge cases and document usage.

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Diagram Walkthrough

flowchart LR
  view["Sorted view"]
  methods["PopulationStdDev and SampleStdDev"]
  calculation["Centered, scaled calculation"]
  result["Standard deviation or error"]
  view --> methods
  methods --> calculation
  calculation --> result

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File Walkthrough

	Relevant files
Enhancement	sorted.go Add numerically stable standard deviation methods                sorted.go Add population and sample standard deviation methods. Use centered, scaled calculations with compensated summation. Handle empty inputs, insufficient samples, and infinities. +56/-0
Tests	sorted_test.go Cover standard deviation results and edge cases                    sorted_test.go Test standard deviation across varied magnitudes and special values. Verify small-input errors and frozen sorted views. +91/-0
Documentation	README.md Document standard deviation APIs and behavior                        README.md Document both methods, formulas, usage, and error behavior. Update English and Japanese feature descriptions. +38/-2

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

Benchmark Comparison

Base: main
PR: feat/stddev

BenchmarkSampdoAll results

goos: linux
goarch: amd64
pkg: github.com/monitoring-forge/sampdo
cpu: AMD EPYC 7763 64-Core Processor                
                                        │ base-bench.txt │            pr-bench.txt            │
                                        │     sec/op     │   sec/op     vs base               │
SampdoAll/sampdo/100000/duplicates-4         1.099m ± 7%   1.090m ± 2%       ~ (p=0.315 n=10)
SampdoAll/sampdo/100000/random-4             2.171m ± 2%   2.199m ± 1%       ~ (p=0.315 n=10)
SampdoAll/sampdo/100000/wide-4               2.246m ± 0%   2.242m ± 1%       ~ (p=0.436 n=10)
SampdoAll/sampdo/100000/sorted-4             165.0µ ± 3%   165.3µ ± 3%       ~ (p=0.353 n=10)
SampdoAll/sampdo/100000/response_time-4      1.720m ± 2%   1.714m ± 2%       ~ (p=0.971 n=10)
geomean                                      1.088m        1.088m       +0.01%

                                        │ base-bench.txt │             pr-bench.txt              │
                                        │      B/op      │     B/op      vs base                 │
SampdoAll/sampdo/100000/duplicates-4        1.532Mi ± 0%   1.532Mi ± 0%       ~ (p=0.726 n=10)
SampdoAll/sampdo/100000/random-4            1.532Mi ± 0%   1.532Mi ± 0%       ~ (p=0.588 n=10)
SampdoAll/sampdo/100000/wide-4              1.532Mi ± 0%   1.532Mi ± 0%       ~ (p=0.924 n=10)
SampdoAll/sampdo/100000/sorted-4            785.1Ki ± 0%   785.1Ki ± 0%       ~ (p=1.000 n=10) ¹
SampdoAll/sampdo/100000/response_time-4     1.532Mi ± 0%   1.532Mi ± 0%       ~ (p=0.895 n=10)
geomean                                     1.334Mi        1.334Mi       +0.00%
¹ all samples are equal

                                        │ base-bench.txt │            pr-bench.txt             │
                                        │   allocs/op    │ allocs/op   vs base                 │
SampdoAll/sampdo/100000/duplicates-4          5.000 ± 0%   5.000 ± 0%       ~ (p=1.000 n=10) ¹
SampdoAll/sampdo/100000/random-4              5.000 ± 0%   5.000 ± 0%       ~ (p=1.000 n=10) ¹
SampdoAll/sampdo/100000/wide-4                5.000 ± 0%   5.000 ± 0%       ~ (p=1.000 n=10) ¹
SampdoAll/sampdo/100000/sorted-4              4.000 ± 0%   4.000 ± 0%       ~ (p=1.000 n=10) ¹
SampdoAll/sampdo/100000/response_time-4       5.000 ± 0%   5.000 ± 0%       ~ (p=1.000 n=10) ¹
geomean                                       4.782        4.782       +0.00%
¹ all samples are equal

[github-actions]

PR Reviewer Guide 🔍

Here are some key observations to aid the review process:

⏱️ Estimated effort to review: 2 🔵🔵⚪⚪⚪

🧪 PR contains tests

🔒 No security concerns identified

⚡ No major issues detected

[github-actions]

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