Pad/internal/ui/caret_point_test.go

package ui

import "testing"

// TestCaretPoint_EmptyFirstLine is a regression test for the caret jumping
// to an adjacent line while scrolling: when the shaped window's first visual
// line is empty (no recorded glyphs), the old code anchored the case-B
// baseline on the smallest recorded Y — the first NON-empty line — so every
// boundary caret (line-end "\n", an empty line's lone byte, EOF) drew one
// line too low per leading empty line, and the caret visibly jumped a line
// when the window scrolled past the empty line.
func TestCaretPoint_EmptyFirstLine(t *testing.T) {
	const (
		lh     = Dp(16.8)
		ascent = Dp(13.3)
	)
	// Window text starting on an empty line: line 0 is a lone "\n" (byte 0,
	// no glyphs), line 1 is "abc" (bytes 1-3, "\n" at 4), line 2 is "def"
	// (bytes 5-7, no trailing newline). Baselines are the uniform shaper
	// grid: line i at ascent + i*lh; line 0 has no recorded glyphs.
	str := "\nabc\ndef"
	layout := GlyphLayout{
		LineHeight: lh,
		// VisualLineStarts: one entry per line start (0, 1, 5). The last
		// line has no trailing break; the entry for it is byte 5.
		VisualLineStarts: []int{0, 1, 5},
		ByteOffsets:      []int{1, 2, 3, 5, 6, 7},
		X:                []Dp{0, 10, 20, 0, 10, 20},
		Y:                []Dp{ascent + lh, ascent + lh, ascent + lh, ascent + 2*lh, ascent + 2*lh, ascent + 2*lh},
		Advance:          []Dp{10, 10, 10, 10, 10, 10},
	}

	near := func(got, want Dp) bool {
		d := float64(got - want)
		return d < 0.01 && d > -0.01
	}
	check := func(name string, byteOff int, wantX, wantY Dp) {
		t.Helper()
		x, y := CaretPoint(layout, str, byteOff, ascent, lh)
		if !near(x, wantX) || !near(y, wantY) {
			t.Errorf("%s: CaretPoint(%d) = (%v, %v); want (%v, %v)", name, byteOff, x, y, wantX, wantY)
		}
	}

	// Case A: a glyph-start byte is unaffected (always was) — "a" of line 1.
	check("glyph start (line 1)", 1, 0, ascent+lh)

	// Case B: the empty first line's lone insertion point sits on line 0.
	// Old code: minY = ascent+lh, line 0 -> y = ascent+lh (one line low).
	check("empty line 0 origin", 0, 0, ascent)

	// Case B: the terminating "\n" of line 1 sits at line 1's right edge.
	// Old code: minY = ascent+lh, line 1 -> y = ascent+2*lh (one line low).
	check("line 1 end (\\n)", 4, 30, ascent+lh)

	// Case B: EOF on the last line sits at line 2's right edge.
	// Old code: y = ascent+lh+2*lh (one line low).
	check("EOF (line 2 end)", 8, 30, ascent+2*lh)
}

// TestCaretPoint_EmptyFirstLines covers several leading empty lines: the
// anchor must walk back j full line heights, not one.
func TestCaretPoint_EmptyFirstLines(t *testing.T) {
	const (
		lh     = Dp(16.8)
		ascent = Dp(13.3)
	)
	// Two empty lines, then "abc".
	str := "\n\nabc"
	layout := GlyphLayout{
		LineHeight:       lh,
		VisualLineStarts: []int{0, 1, 2},
		ByteOffsets:      []int{2, 3, 4},
		X:                []Dp{0, 10, 20},
		Y:                []Dp{ascent + 2*lh, ascent + 2*lh, ascent + 2*lh},
		Advance:          []Dp{10, 10, 10},
	}
	near := func(got, want Dp) bool {
		d := float64(got - want)
		return d < 0.01 && d > -0.01
	}
	// End of the "abc" line (no trailing newline): right edge of line 2.
	x, y := CaretPoint(layout, str, 5, ascent, lh)
	wantX, wantY := Dp(30), ascent+2*lh
	if !near(x, wantX) || !near(y, wantY) {
		t.Errorf("EOF after two empty lines = (%v, %v); want (%v, %v)", x, y, wantX, wantY)
	}
	// The second empty line's origin: line 1.
	x, y = CaretPoint(layout, str, 1, ascent, lh)
	wantX, wantY = Dp(0), ascent+lh
	if !near(x, wantX) || !near(y, wantY) {
		t.Errorf("empty line 1 origin = (%v, %v); want (%v, %v)", x, y, wantX, wantY)
	}
}

// TestCaretPoint_NonEmptyFirstLine guards against regressions in the common
// case: with a glyph on the window's first line the anchor is the first
// recorded baseline (the old behaviour), unchanged.
func TestCaretPoint_NonEmptyFirstLine(t *testing.T) {
	const (
		lh     = Dp(16.8)
		ascent = Dp(13.3)
	)
	str := "abc\ndef"
	layout := GlyphLayout{
		LineHeight:       lh,
		VisualLineStarts: []int{0, 4},
		ByteOffsets:      []int{0, 1, 2, 4, 5, 6},
		X:                []Dp{0, 10, 20, 0, 10, 20},
		Y:                []Dp{ascent, ascent, ascent, ascent + lh, ascent + lh, ascent + lh},
		Advance:          []Dp{10, 10, 10, 10, 10, 10},
	}
	near := func(got, want Dp) bool {
		d := float64(got - want)
		return d < 0.01 && d > -0.01
	}
	// Line 0's "\n": line 0's right edge at line 0's baseline (ascent).
	x, y := CaretPoint(layout, str, 3, ascent, lh)
	wantX, wantY := Dp(30), ascent
	if !near(x, wantX) || !near(y, wantY) {
		t.Errorf("line 0 end = (%v, %v); want (%v, %v)", x, y, wantX, wantY)
	}
	// EOF: line 1's right edge.
	x, y = CaretPoint(layout, str, 7, ascent, lh)
	wantX, wantY = Dp(30), ascent+lh
	if !near(x, wantX) || !near(y, wantY) {
		t.Errorf("EOF = (%v, %v); want (%v, %v)", x, y, wantX, wantY)
	}
}

// TestCaretPoint_NoGlyphs covers a window of pure newlines: the ascent
// fallback anchor applies.
func TestCaretPoint_NoGlyphs(t *testing.T) {
	const (
		lh     = Dp(16.8)
		ascent = Dp(13.3)
	)
	str := "\n\n\n"
	layout := GlyphLayout{
		LineHeight:       lh,
		VisualLineStarts: []int{0, 1, 2},
	}
	near := func(got, want Dp) bool {
		d := float64(got - want)
		return d < 0.01 && d > -0.01
	}
	// The second empty line's origin: line 1.
	x, y := CaretPoint(layout, str, 1, ascent, lh)
	wantX, wantY := Dp(0), ascent+lh
	if !near(x, wantX) || !near(y, wantY) {
		t.Errorf("pure-newline window, empty line 1 = (%v, %v); want (%v, %v)", x, y, wantX, wantY)
	}
}