🤖 Repo Assist — automated implementation

Summary

Implements the feature requested in #211 and approved by @tpetricek in 2014: a new Stats.interpolateLinearWith function with configurable extrapolation behavior for keys that fall outside the range of the source series.

New API

F# module function

Stats.interpolateLinearWith
    keys        : 'K seq
    keyDiff     : 'K -> 'K -> float
    extrapolation : Extrapolation
    series      : Series<'K, 'V>
    -> Series<'K, float>

Extrapolation discriminated union (new type)

C# extension method

series.InterpolateLinearWith(keys, keyDiff, Extrapolation.Linear)

Usage example

// series from key 2 to 6, slope = +5 per unit
let s = series [ 2 => 10.0; 4 => 20.0; 6 => 30.0 ]
let keyDiff a b = float (a - b)

s |> Stats.interpolateLinearWith [0;2;4;6;8] keyDiff Clamp
// key 0 → 10.0  (clamp to first value)
// key 8 → 30.0  (clamp to last value)

s |> Stats.interpolateLinearWith [0;2;4;6;8] keyDiff Missing
// key 0 → (missing)
// key 8 → (missing)

s |> Stats.interpolateLinearWith [0;2;4;6;8] keyDiff Linear
// key 0 → 0.0   (extrapolate: 10 − 5×2)
// key 8 → 40.0  (extrapolate: 30 + 5×2)

Implementation notes

• The existing interpolateLinear is unchanged — fully backwards-compatible.
• For Linear extrapolation the boundary slopes are pre-computed from the first/last two points of the series before the interpolation loop.
• For a single-point series (KeyCount < 2) Linear falls back to Clamp since no slope can be computed.
• Minimal change: 3 files touched (Stats.fs, SeriesStatsExtensions.fs, tests).

Test Status

• ✅ 4 new regression tests covering all three extrapolation modes and the in-range path
• ✅ All 578 tests pass (was 574 before this change)
• ✅ Build succeeded with 0 errors

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