// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: MIT

package stats

import (
	"testing"

	"sourcedock.dev/petrbalvin/tensor/internal/core"
)

// The pooled pair-slope buffer is sliced to the pair count before the
// median reads it: a reused buffer carries a longer tail from an earlier
// fit, and the median must never see it. The sequence below makes the
// long fit prime the pool and the short fits reuse it; a garbage
// collection between the calls would drop the entry and weaken the pin,
// so the fits run back to back.

func TestTheilSenPooledReuseKeepsExactSlope(t *testing.T) {
	// Both fixtures sit exactly on y = 3x - 2, so every pair slope is
	// exactly 3 and the answers are exact whatever the pair order.
	makeLine := func(n int) (*core.Array, *core.Array) {
		xs := make([]float64, n)
		ys := make([]float64, n)
		for i := range n {
			xs[i] = float64(i) + 0.5
			ys[i] = 3*xs[i] - 2
		}
		x, err := core.FromFloats(xs, n)
		if err != nil {
			t.Fatalf("FromFloats: %v", err)
		}
		y, err := core.FromFloats(ys, n)
		if err != nil {
			t.Fatalf("FromFloats: %v", err)
		}
		return x, y
	}
	bigX, bigY := makeLine(300)
	smallX, smallY := makeLine(40)
	if _, _, err := TheilSenRegression(bigX, bigY); err != nil {
		t.Fatalf("long fit: %v", err)
	}
	for round := range 3 {
		intercept, slope, err := TheilSenRegression(smallX, smallY)
		if err != nil {
			t.Fatalf("short fit %d: %v", round, err)
		}
		if slope != 3 {
			t.Fatalf("short fit %d: slope = %v, want exactly 3", round, slope)
		}
		if intercept != -2 {
			t.Fatalf("short fit %d: intercept = %v, want exactly -2", round, intercept)
		}
	}
}