Pad/internal/editor/state_api_fuzz_test.go

package editor

// Differential fuzzing of the production edit entry points (the layer that
// enforces rune granularity): HandleInsert, HandleBackspace, HandleDelete and
// HandleReplaceRange, including selection-aware variants, are driven with
// random operations against a shadow model. After every op we check:
//
//   - buffer content == model (no bytes lost/duplicated/reordered),
//   - content is still valid UTF-8 (rune granularity held),
//   - the chunk-size invariant (no chunk > 2x target),
//   - the LineIndex equals a full recomputation (oracle),
//   - the cursor stays within the file.
//
// This catches corruption that only appears at the API boundary: rune-width
// bugs (utf8BackspaceWidth/utf8AdvanceWidth), selection handling, and the
// IME rune->byte conversion, none of which the raw buffer fuzz sees.

import (
	"fmt"
	"math/rand"
	"testing"
	"unicode/utf8"

	"pad/internal/io/pool/types"
)

func fuzzStateAPI(t *testing.T, initial string, chunkSize, ops int) {
	t.Helper()
	st := newChunkedState(t, initial, chunkSize)
	cb := st.Editor.ChunkedBuffer
	cb.LineIndex = types.NewLineIndex(lineStartOffsets([]byte(initial)), 0, int64(len(initial)))

	model := initial
	var cursor int // every op path sets this before the invariant reads it
	rng := rand.New(rand.NewSource(4242))
	modelCap := 32 * 1024

	setSelection := func(a, b int) {
		st.Editor.SelectionAnchor = a
		st.Editor.SelectionStart = a
		st.Editor.SelectionEnd = b
	}

	for i := 0; i < ops; i++ {
		if len(model) > modelCap {
			p := runeStartPositions([]byte(model))
			a, b := p[len(p)/4], p[len(p)/2]
			setSelection(a, b)
			HandleBackspace() // deletes the selection
			model = model[:a] + model[b:]
			cursor = a
			st.Editor.CursorPosition = a
		}

		pos := func() int {
			p := runeStartPositions([]byte(model))
			return p[rng.Intn(len(p))]
		}
		switch rng.Intn(100) {
		case 0: // 28%: plain insert at a random position
			c := pos()
			text := fuzzRuneString(rng, fuzzInsertSize(rng, 300))
			st.Editor.CursorPosition = c
			HandleInsert(text)
			model = model[:c] + text + model[c:]
			cursor = c + len(text)
		case 1: // 12%: plain backspace
			c := pos()
			st.Editor.CursorPosition = c
			cursor = c
			HandleBackspace()
			if c > 0 {
				w := utf8BackspaceWidth(model[max0(c-4):c])
				model = model[:c-w] + model[c:]
				cursor = c - w
			}
		case 2: // 12%: plain delete (cursor stays put)
			c := pos()
			st.Editor.CursorPosition = c
			cursor = c
			HandleDelete()
			if c < len(model) {
				w := utf8AdvanceWidth(model[c:min4(c+4, len(model))])
				model = model[:c] + model[c+w:]
			}
		case 3: // 8%: backspace with live selection
			if len(model) == 0 {
				continue
			}
			a, b := randomSelection(rng, model)
			setSelection(a, b)
			st.Editor.CursorPosition = pos()
			HandleBackspace()
			model = model[:a] + model[b:]
			cursor = a
		case 4: // 8%: delete with live selection
			if len(model) == 0 {
				continue
			}
			a, b := randomSelection(rng, model)
			setSelection(a, b)
			st.Editor.CursorPosition = pos()
			HandleDelete()
			model = model[:a] + model[b:]
			cursor = a
		case 5: // 8%: insert with live selection (replace selection)
			if len(model) == 0 {
				continue
			}
			a, b := randomSelection(rng, model)
			text := fuzzRuneString(rng, fuzzInsertSize(rng, 100))
			setSelection(a, b)
			st.Editor.CursorPosition = pos()
			HandleInsert(text)
			model = model[:a] + text + model[b:]
			cursor = a + len(text)
		default: // 24%: IME replace at absolute rune indices (empty window
			// => the whole buffer is the window, as in newChunkedState).
			nr := utf8.RuneCountInString(model)
			ra := rng.Intn(nr + 1)
			rb := ra + rng.Intn(nr-ra+1)
			text := fuzzRuneString(rng, fuzzInsertSize(rng, 300))
			bs := runeStartToByteOracle([]byte(model), ra)
			be := runeStartToByteOracle([]byte(model), rb)
			HandleReplaceRange(ra, rb, text)
			model = model[:bs] + text + model[be:]
			cursor = bs + len(text)
		}

		// --- Invariants after every op --------------------------------
		full, err := cb.FullContent()
		if err != nil {
			t.Fatalf("op %d: FullContent: %v", i, err)
		}
		if full != model {
			at := 0
			for at < len(full) && at < len(model) && full[at] == model[at] {
				at++
			}
			t.Fatalf("op %d: buffer != model (got %d, want %d bytes, first diff at %d)",
				i, len(full), len(model), at)
		}
		if !utf8.ValidString(full) {
			t.Fatalf("op %d: buffer is not valid UTF-8", i)
		}
		assertMaxChunkSize(t, cb)
		assertLineIndexMatchesOracle(t, cb, fmt.Sprintf("op %d", i))
		if got := st.Editor.CursorPosition; got != cursor {
			t.Fatalf("op %d: cursor=%d, want %d (len %d)", i, got, cursor, len(model))
		}
	}
}

func TestStateAPI_Fuzz_EditOps(t *testing.T) {
	initial := fuzzRuneString(rand.New(rand.NewSource(7)), 300) +
		"\nsecond line\n第三行\n" + fuzzRuneString(rand.New(rand.NewSource(8)), 300)
	fuzzStateAPI(t, initial, 32, 2000)
}

func TestStateAPI_Fuzz_SmallFile(t *testing.T) {
	// Tiny file, tiny chunks: every edit is at or across a boundary.
	initial := "ab\ncd\n"
	fuzzStateAPI(t, initial, 3, 1500)
}

// randomSelection returns a rune-aligned [a,b) with a < b suitable for
// setting a live selection (selActive requires end > start). If the model is
// empty it returns (0, 0) and the caller must skip the selection op.
func randomSelection(rng *rand.Rand, model string) (int, int) {
	if len(model) == 0 {
		return 0, 0
	}
	p := runeStartPositions([]byte(model))
	if len(p) < 2 {
		return 0, len(model)
	}
	for {
		a := p[rng.Intn(len(p))]
		b := p[rng.Intn(len(p))]
		if a == b {
			continue
		}
		if a > b {
			a, b = b, a
		}
		return a, b
	}
}

func max0(n int) int {
	if n < 0 {
		return 0
	}
	return n
}

func min4(a, b int) int {
	if a < b {
		return a
	}
	return b
}