tensor/grad/fuzz_test.go

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

package grad

import (
	"testing"

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

// Property fuzz targets over index-moving operations. `go test` runs
// the seed corpus on every commit; longer campaigns run under
// -fuzz=Fuzz<Name> when a shape-handling change lands.

// FuzzTransposeAxesRoundTrip drives random permutations through the
// axis move and its inverse: whatever valid permutation arrives, the
// double transpose must restore the exact element order.
func FuzzTransposeAxesRoundTrip(f *testing.F) {
	f.Add([]byte{0, 1}, 6)
	f.Add([]byte{1, 0}, 6)
	f.Add([]byte{2, 0, 1}, 8)

	f.Fuzz(func(t *testing.T, permBytes []byte, total int) {
		if total <= 0 || total > 4096 {
			t.Skip()
		}
		rank := len(permBytes)
		switch rank {
		case 2:
			total -= total % 2
		case 3:
			total -= total % 4
		default:
			t.Skip()
		}
		if total == 0 {
			t.Skip()
		}
		vals := make([]float64, total)
		for i := range vals {
			vals[i] = float64(i)
		}
		var shape []int
		if rank == 2 {
			shape = []int{total / 2, 2}
		} else {
			shape = []int{total / 4, 2, 2}
		}
		a, _ := core.FromFloats(vals, shape...)
		xt := FromArray(a, false)

		dims := make([]int, rank)
		for i, pb := range permBytes {
			dims[i] = int(pb) % rank
		}
		moved, err := xt.TransposeAxes(dims...)
		if err != nil {
			return // duplicate axes rejected by validation, fine
		}
		back, err := moved.TransposeAxes(inversePerm(dims)...)
		if err != nil {
			t.Fatalf("inverse of %v failed: %v", dims, err)
		}
		for i := range vals {
			if back.Data().FloatAt(i) != vals[i] {
				t.Fatalf("round trip lost element %d", i)
			}
		}
	})
}

// FuzzOneHotContracts checks both sides of the encoder contract for
// arbitrary code sets: in-range codes yield exactly one hot cell per
// row, any out-of-range code is a loud error. One input byte splits
// into a high bit forcing negativity plus a low-bit class selector.
func FuzzOneHotContracts(f *testing.F) {
	f.Add([]byte{0, 1, 2}, uint8(3))
	f.Add([]byte{5}, uint8(8))
	f.Add([]byte{200, 201}, uint8(3))

	f.Fuzz(func(t *testing.T, raw []byte, classByte uint8) {
		classes := int(classByte)%9 + 1
		codes := make([]int64, len(raw))
		valid := true
		for i, b := range raw {
			c := int64(b)
			if b >= 128 { // force some negative probes
				c = -int64(b - 127)
			} else {
				c %= int64(classes)
			}
			if c < 0 || c >= int64(classes) {
				valid = false
			}
			codes[i] = c
		}

		arr, _ := core.FromInts(codes, len(codes))
		hot, err := core.OneHot(arr, classes)
		if !valid {
			if err == nil {
				t.Fatalf("invalid codes accepted for %d classes", classes)
			}
			return
		}
		if err != nil {
			t.Fatalf("valid codes rejected: %v", err)
		}
		for i := range len(codes) {
			sum := 0.0
			for j := range classes {
				sum += float64(hot.FloatAt(i*classes + j))
			}
			if sum != 1 {
				t.Fatalf("row %d sums to %v, want one hot cell", i, sum)
			}
		}
	})
}

// FuzzConcatSplitGradientConserves mass: splitting a concatenated
// output's gradient must hand every element back to its own side with
// coefficient exactly one, for whatever layout the corpus invents.
func FuzzConcatSplitGradientConserves(f *testing.F) {
	f.Add([]byte{1, 2, 3, 4}, uint8(2))
	f.Add([]byte{9, 7, 5}, uint8(1))
	f.Add([]byte{10, 20, 30, 40, 50, 60, 70, 80}, uint8(0))
	f.Add([]byte{11, 21, 31, 41, 51, 61, 71, 81, 91, 101}, uint8(4))

	f.Fuzz(func(t *testing.T, raw []byte, rowsByte uint8) {
		rows := int(rowsByte)%7 + 1 // left matrix rows, 1..7
		leftLen := rows * 2
		if len(raw) <= leftLen {
			t.Skip()
		}
		rightRows := (len(raw) - leftLen) / 2

		leftVals := make([]float64, leftLen)
		for i := range leftVals {
			leftVals[i] = float64(raw[i])
		}
		rightVals := make([]float64, rightRows*2)
		for i := range rightVals {
			rightVals[i] = float64(raw[leftLen+i])
		}

		a, err := core.FromFloats(leftVals, rows, 2)
		if err != nil {
			t.Skip()
		}
		b, err := core.FromFloats(rightVals, rightRows, 2)
		if err != nil {
			t.Skip()
		}

		at := FromArray(a, true)
		bt := FromArray(b, true)
		joint, cerr := at.Concat(bt, 0)
		if cerr != nil {
			t.Fatal(cerr)
		}
		loss, serr := joint.Sum()
		if serr != nil {
			t.Fatal(serr)
		}
		if berr := loss.Backward(); berr != nil {
			t.Fatal(berr)
		}

		ga, gb := at.Grad(), bt.Grad()
		if ga.Len() != a.Len() || gb.Len() != b.Len() {
			t.Fatalf("gradient spans drifted: %d+%d vs %d+%d",
				ga.Len(), gb.Len(), a.Len(), b.Len())
		}
		for i := range ga.Len() {
			if ga.FloatAt(i) != 1 {
				t.Fatalf("left span slot %d = %v", i, ga.FloatAt(i))
			}
		}
		for i := range gb.Len() {
			if gb.FloatAt(i) != 1 {
				t.Fatalf("right span slot %d = %v", i, gb.FloatAt(i))
			}
		}
	})
}