Pad/internal/editor/tap_scroll_property_test.go

package editor

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

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

// This file proves the scroll-offset independence of tap-to-position.
//
// The invariant under test, for a no-wrap file where content line n occupies
// content-y [n*lh, (n+1)*lh):
//
//	For every scroll offset s >= 0 and every tap point ptY inside the visible
//	region, the cursor must land on the content line whose DRAWN range
//	contains the tap. Drawn range of window display line j is
//	[reg.Y - r + j*lh, reg.Y - r + (j+1)*lh), where r = s mod lh (the
//	renderer shifts the windowed layout up by exactly r:
//	`y := reg.Y - scrollOffset` in drawWrappedText, scrollOffset =
//	Mod(ScrollOffset, lineHeight)).
//
// Independent ground truth: content_y = (ptY - reg.Y) + s, so the tapped
// content line is floor(content_y / lh). That formula uses only "the content
// is shifted up by s" — it does NOT reuse tapLocalY.
//
// The code path under test is HandleTapAt -> tapLocalY -> SetCursorFromPoint
// -> textPosFromLocalPoint, with a GlyphLayout fabricated the way the
// windowed layout is (window display line j = content line k+j, baseline
// (j+1)*lh, window-relative byte offsets) and IMEWindowStartByte set to the
// same line k = floor(s/lh) that visibleByteRangePrecise picks
// (startLine := int(scrollOffset / lineHeight)).
//
// If the window-start coupling, the draw shift, or tapLocalY's sub-line
// remainder ever disagree, this test fails for some (s, ptY) pair.
func TestTapPosition_ScrollInvariant_Property(t *testing.T) {
	const (
		lines        = 500
		bytesPerLine = 5            // "L123\n"
		regionTop    = float64(62)  // editor region top, app dp (matches layoutFrame)
		viewportH    = float64(760) // app dp
	)
	lineHeight := float64(EditorLineHeight())
	// Build the file once.
	var file string
	for i := 0; i < lines; i++ {
		file += fmt.Sprintf("L%03d\n", i)
	}
	cb := NewChunkedBuffer("/prop.txt", DefaultChunkSize, nil, "")
	cb.SetContent([]byte(file))

	// Build the same line index the BuildLineIndex task produces.
	offsets := []int32{0}
	for i := 0; i < len(file); i++ {
		if file[i] == '\n' {
			offsets = append(offsets, int32(i+1))
		}
	}
	cb.LineIndex = types.NewLineIndex(offsets, 0, int64(len(file)))

	rng := rand.New(rand.NewSource(42))
	maxS := float64(lines) * lineHeight

	for tc := 0; tc < 4000; tc++ {
		// Random scroll offset: a mix of small, large, integer-line and
		// fractional-line values.
		var s float64
		switch tc % 4 {
		case 0:
			s = rng.Float64() * maxS // anywhere
		case 1:
			s = float64(int(rng.Float64() * maxS)) // exact integer dp
		case 2:
			s = float64(rng.Intn(lines)) * lineHeight // exact line boundary
		case 3:
			s = float64(rng.Intn(lines))*lineHeight + rng.Float64()*(lineHeight-1) // near boundary
		}
		sOff := ui.Dp(s)

		// What layoutFrame/visibleByteRangePrecise does: window starts at
		// content line k = floor(s / lh).
		start, _, _ := cb.VisibleByteRange(sOff, 0, ui.Dp(viewportH), EditorLineHeight(), false, ui.GlyphLayout{}, nil)
		if start%bytesPerLine != 0 {
			t.Fatalf("test setup: window start %d not on a line boundary", start)
		}
		k := start / bytesPerLine

		// Fabricate the windowed GlyphLayout exactly as the shaper would for
		// a no-wrap file: window display line j = content line k+j.
		nWin := int(math.Min(viewportH/lineHeight+2, float64(lines-k)))
		var gl ui.GlyphLayout
		for j := 0; j < nWin; j++ {
			gl.ByteOffsets = append(gl.ByteOffsets, j*bytesPerLine)
			gl.X = append(gl.X, 10)
			gl.Y = append(gl.Y, ui.Dp(float64(j+1)*lineHeight))
			gl.Advance = append(gl.Advance, 10)
		}

		TheState = NewState()
		TheState.Editor.ChunkedBuffer = cb
		TheState.Editor.GlyphLayout = gl
		TheState.Editor.IMEWindowStartByte = start
		TheState.ScrollOffset = sOff
		TheState.EditorRegion = ui.Region{X: 10, Y: ui.Dp(regionTop), W: 390, H: ui.Dp(viewportH)}

		// Random tap inside the visible region.
		a := rng.Float64() * viewportH // tap y relative to region top
		ptY := ui.Dp(regionTop + a)

		HandleTapAt(15, ptY)

		// Independent ground truth: content_y = a + s -> content line.
		wantLine := int(math.Floor((a + s) / lineHeight))
		if wantLine >= lines {
			wantLine = lines - 1
		}
		// Taps below the last shaped window line clamp to it (no glyphs
		// further down), matching textPosFromLocalPoint's documented clamp.
		if k+nWin-1 < wantLine {
			wantLine = k + nWin - 1
		}
		got := TheState.Editor.CursorPosition
		wantLo, wantHi := wantLine*bytesPerLine, wantLine*bytesPerLine+bytesPerLine
		if got < wantLo || got >= wantHi {
			t.Fatalf("s=%.2f (k=%d) tap a=%.2f: cursor=%d, want line %d [bytes %d,%d)",
				s, k, a, got, wantLine, wantLo, wantHi)
		}
	}
}