tensor/linalg/nonfinite_refusal_pins_test.go

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

package linalg

import (
	"math"
	"testing"

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

// Regression pins for non-finite and counting guards: the matrix
// exponential refuses a non-finite entry, the minimum-degree ordering
// matches its brute-force reference, the sparse LU non-zero counts
// follow the unit triangle, and the ILU intake refuses overflow.

// TestMatrixExpRejectsNonFinite pins the loud refusal for a
// non-finite entry, which the theta ladder used to read through an
// implementation-defined conversion into an all-NaN answer.
func TestMatrixExpRejectsNonFinite(t *testing.T) {
	a := mustF(t, []float64{math.Inf(1), 0, 0, 1}, 2, 2)
	if _, err := MatrixExp(a); err == nil {
		t.Fatal("expected an error for an Inf entry")
	}
	b := mustF(t, []float64{math.NaN(), 0, 0, 1}, 2, 2)
	if _, err := MatrixExp(b); err == nil {
		t.Fatal("expected an error for a NaN entry")
	}
	c, _ := core.FromComplexes([]complex128{complex(math.Inf(1), 0), 0, 0, 1}, 2, 2)
	if _, err := MatrixExp(c); err == nil {
		t.Fatal("expected an error for an Inf complex entry")
	}
}

// TestMinimumDegreeMixedPattern pins the ordering against a
// brute-force minimum-degree reference on a mixed-degree pattern,
// where the degree-sorted adjacency lists and the index-sorted set
// union used to disagree and corrupt the elimination. The sample is
// the measured failing case: the pre-fix code eliminated vertex 10
// before 1 and swapped the tail of the order.
func TestMinimumDegreeMixedPattern(t *testing.T) {
	rows := []int{0, 0, 0, 0, 1, 1, 1, 1, 2, 4, 5, 5, 6, 6, 7}
	cols := []int{1, 2, 5, 8, 3, 5, 6, 10, 6, 6, 9, 10, 7, 10, 10}
	const n = 11
	coo := edgesCOO(t, rows, cols, n)
	csc, err := CSCFromCOO(coo)
	if err != nil {
		t.Fatalf("CSCFromCOO: %v", err)
	}
	got, err := minimumDegree(csc)
	if err != nil {
		t.Fatalf("minimumDegree: %v", err)
	}
	// Brute-force reference: repeatedly eliminate the uneliminated
	// vertex with the fewest uneliminated neighbours (ties to the
	// smaller index), unioning neighbourhoods exactly.
	adj := map[int]map[int]bool{}
	addEdge := func(i, j int) {
		if adj[i] == nil {
			adj[i] = map[int]bool{}
		}
		if adj[j] == nil {
			adj[j] = map[int]bool{}
		}
		adj[i][j], adj[j][i] = true, true
	}
	for e := range rows {
		addEdge(rows[e], cols[e])
	}
	eliminated := map[int]bool{}
	var want []int
	for range n {
		best, bestDeg := -1, math.MaxInt
		for v := range n {
			if eliminated[v] {
				continue
			}
			d := 0
			for u := range adj[v] {
				if !eliminated[u] {
					d++
				}
			}
			if d < bestDeg {
				best, bestDeg = v, d
			}
		}
		want = append(want, best)
		eliminated[best] = true
		nb := map[int]bool{}
		for u := range adj[best] {
			if !eliminated[u] {
				nb[u] = true
			}
		}
		for u := range nb {
			for w := range nb {
				if u != w {
					adj[u][w] = true
				}
			}
			delete(adj[u], best)
		}
	}
	for i := range n {
		if got[i] != want[i] {
			t.Fatalf("order[%d] = %d, want %d (full %v vs %v)", i, got[i], want[i], got, want)
		}
	}
}

// TestSparseLUNNZCountsUTriangle pins that NNZ includes U's
// strict triangle, which the column walk used to miss.
func TestSparseLUNNZCountsUTriangle(t *testing.T) {
	// A tridiagonal matrix: L holds the subdiagonal, U the diagonal and
	// the superdiagonal, so the factor stores exactly 3n - 2 entries.
	const n = 8
	idx := make([]int64, 0, 6*n)
	vals := make([]float64, 0, 3*n)
	add := func(r, c int, v float64) {
		idx = append(idx, int64(r), int64(c))
		vals = append(vals, v)
	}
	for i := range n {
		add(i, i, 2)
		if i+1 < n {
			add(i, i+1, -1)
			add(i+1, i, -1)
		}
	}
	indices, err := core.FromInts(idx, len(vals), 2)
	if err != nil {
		t.Fatalf("FromInts: %v", err)
	}
	coo, err := core.NewSparseCOO(indices, floatsToArray(vals, []int{len(vals)}), []int{n, n})
	if err != nil {
		t.Fatalf("NewSparseCOO: %v", err)
	}
	f, err := NewSparseLU(coo)
	if err != nil {
		t.Fatalf("NewSparseLU: %v", err)
	}
	if want := 3*n - 2; f.NNZ() != want {
		t.Fatalf("NNZ = %d, want %d (L strict + U strict + diagonal)", f.NNZ(), want)
	}
}

// TestSparseILUOverflowRefused pins the overflow refusal on
// finite input, mirroring the LU and Cholesky guards.
func TestSparseILUOverflowRefused(t *testing.T) {
	idx := make([]int64, 0, 6)
	vals := make([]float64, 0, 3)
	add := func(r, c int, v float64) {
		idx = append(idx, int64(r), int64(c))
		vals = append(vals, v)
	}
	add(0, 0, 1e-200)
	add(0, 1, 1e100)
	add(1, 0, 1e100)
	add(1, 1, 1e200)
	indices, err := core.FromInts(idx, 4, 2)
	if err != nil {
		t.Fatalf("FromInts: %v", err)
	}
	coo, err := core.NewSparseCOO(indices, floatsToArray(vals, []int{4}), []int{2, 2})
	if err != nil {
		t.Fatalf("NewSparseCOO: %v", err)
	}
	if _, err := NewSparseILU(coo); err == nil {
		t.Fatal("expected an overflow error from the elimination")
	}
}

// edgesCOO builds a symmetric-pattern COO from edge lists.
func edgesCOO(t *testing.T, rows, cols []int, n int) *core.SparseCOO {
	t.Helper()
	idx := make([]int64, 0, 2*len(rows))
	vals := make([]float64, 0, 2*len(rows))
	add := func(r, c int) {
		idx = append(idx, int64(r), int64(c))
		vals = append(vals, 1)
	}
	for e := range rows {
		add(rows[e], cols[e])
		add(cols[e], rows[e])
	}
	indices, err := core.FromInts(idx, len(vals), 2)
	if err != nil {
		t.Fatalf("FromInts: %v", err)
	}
	coo, err := core.NewSparseCOO(indices, floatsToArray(vals, []int{len(vals)}), []int{n, n})
	if err != nil {
		t.Fatalf("NewSparseCOO: %v", err)
	}
	return coo
}