tensor/internal/core/random_test.go

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

package core

import (
	"math"
	"strings"
	"testing"
)

func TestGeneratorDeterminism(t *testing.T) {
	g1 := NewGenerator(42)
	g2 := NewGenerator(42)
	f1, err := Floats(g1, 8)
	if err != nil {
		t.Fatalf("Floats: %v", err)
	}
	f2, err := Floats(g2, 8)
	if err != nil {
		t.Fatalf("Floats: %v", err)
	}
	if !Equal(f1, f2) {
		t.Fatalf("the same seed must give identical arrays")
	}

	g3 := NewGenerator(43)
	f3, _ := Floats(g3, 8)
	if Equal(f1, f3) {
		t.Fatalf("different seeds must give different arrays")
	}
}

func TestGeneratorFloats(t *testing.T) {
	g := NewGenerator(7)
	f, err := Floats(g, 1000)
	if err != nil {
		t.Fatalf("Floats: %v", err)
	}
	if f.Dtype() != Float || f.Shape()[0] != 1000 {
		t.Fatalf("Floats shape: %s", f)
	}
	for i := range f.Len() {
		v, _ := FloatAt(f, i)
		if v < 0 || v >= 1 {
			t.Fatalf("Floats out of [0,1): %v", v)
		}
	}
	// A thousand draws should cover both halves of the interval.
	m, _ := Mean(f)
	if m < 0.4 || m > 0.6 {
		t.Fatalf("Floats mean suggests bias: %v", m)
	}

	if _, err := Floats(g, -1); err == nil || !strings.Contains(err.Error(), "zero or greater") {
		t.Fatalf("Floats negative: %v", err)
	}
}

func TestGeneratorInts(t *testing.T) {
	g := NewGenerator(9)
	v, err := Ints(g, 600, 1, 4)
	if err != nil {
		t.Fatalf("Ints: %v", err)
	}
	if v.Dtype() != Int {
		t.Fatalf("Ints dtype: %s", v.Dtype())
	}
	seen := map[int64]bool{}
	for i := range v.Len() {
		x, _ := IntAt(v, i)
		if x < 1 || x >= 4 {
			t.Fatalf("Ints out of [1,4): %d", x)
		}
		seen[x] = true
	}
	// Six hundred draws over three values must hit all of them.
	if len(seen) != 3 {
		t.Fatalf("Ints coverage: %v", seen)
	}

	if _, err := Ints(g, 1, 5, 5); err == nil || !strings.Contains(err.Error(), "min must be less than max") {
		t.Fatalf("Ints range: %v", err)
	}
	if _, err := Ints(g, -1, 0, 1); err == nil || !strings.Contains(err.Error(), "zero or greater") {
		t.Fatalf("Ints negative: %v", err)
	}
}

// TestGeneratorBoundedUniform pins the two defects of the old rejection
// rule: it rejected the whole [t, n) residue band, starving the upper
// buckets (for spans above 2^63 some values never appeared), and a full
// [MinInt64, MaxInt64) span accepted only lo == 2^64-1, hanging the draw.
func TestGeneratorBoundedUniform(t *testing.T) {
	g := NewGenerator(11)
	const draws = 30_000
	counts := map[int64]int{}
	for range draws {
		v, err := Ints(g, 1, 0, 3)
		if err != nil {
			t.Fatalf("Ints: %v", err)
		}
		x, _ := IntAt(v, 0)
		counts[x]++
	}
	// The counts are binomial around draws/3; the old biased rule kept
	// bucket 2 a full draws/3 * (1/3) short, far outside this band.
	want := draws / 3
	tol := want / 10
	for b := range int64(3) {
		if diff := counts[b] - want; diff > tol || diff < -tol {
			t.Fatalf("bucket %d drawn %d times, want %d +/- %d", b, counts[b], want, tol)
		}
	}

	// Full-span draws must terminate promptly and spread over the range.
	g = NewGenerator(12)
	seen := map[int64]bool{}
	for range 64 {
		v, err := Ints(g, 1, math.MinInt64, math.MaxInt64)
		if err != nil {
			t.Fatalf("full-span Ints: %v", err)
		}
		x, _ := IntAt(v, 0)
		seen[x] = true
	}
	if len(seen) < 2 {
		t.Fatalf("full-span draws collapsed to %d distinct value(s)", len(seen))
	}
}

func TestGeneratorZeroSeed(t *testing.T) {
	// Seed zero must not degenerate into the all-zero state.
	g := NewGenerator(0)
	f, err := Floats(g, 4)
	if err != nil {
		t.Fatalf("Floats from seed 0: %v", err)
	}
	nonzero := false
	for i := range f.Len() {
		v, _ := FloatAt(f, i)
		if v != 0 {
			nonzero = true
		}
	}
	if !nonzero {
		t.Fatalf("seed 0 produced only zeros")
	}
}