1 files changed, 60 insertions, 0 deletions
diff --git a/render_test.go b/render_test.go
new file mode 100644
index 0000000..263c2da
--- /dev/null
+++ b/render_test.go
@@ -0,0 +1,60 @@
+package main
+
+import "testing"
+
+// TestAxisMap checks centering when content fits and clamped scrolling when it
+// does not.
+func TestAxisMap(t *testing.T) {
+	cases := []struct {
+		name                  string
+		pane, content, focus  int
+		wantBlank, wantScroll int
+	}{
+		{"fits centered", 100, 40, 0, 30, 0},
+		{"exact fit", 40, 40, 0, 0, 0},
+		{"scroll centered on focus", 40, 100, 50, 0, 30},
+		{"scroll clamped low", 40, 100, 0, 0, 0},
+		{"scroll clamped high", 40, 100, 999, 0, 60},
+	}
+	for _, c := range cases {
+		blank, scroll := axisMap(c.pane, c.content, c.focus)
+		if blank != c.wantBlank || scroll != c.wantScroll {
+			t.Errorf("%s: axisMap(%d,%d,%d) = (%d,%d), want (%d,%d)",
+				c.name, c.pane, c.content, c.focus, blank, scroll, c.wantBlank, c.wantScroll)
+		}
+	}
+}
+
+// TestPtToCell checks the point->cell mapping and that moving a point by one
+// stepPt lands exactly one cell away (the property the cursor/stamp nudges
+// rely on).
+func TestPtToCell(t *testing.T) {
+	// Page 600x800 pt rendered to a 120x80 px image (scaleX=0.2, scaleY=0.1),
+	// fitting within the pane (no scroll), centered with a 4-cell/2-px margin.
+	v := &view{
+		cols: 128, rows: 41,
+		scaleX: 0.2, scaleY: 0.1,
+		blankX: 4, blankY: 2,
+		scrollX: 0, scrollY: 0,
+		pageH: 800,
+	}
+
+	// Bottom-left of the page maps to the bottom-left of the image area.
+	col, row := v.ptToCell(0, 0)
+	if col != 4 || row != 41 {
+		t.Errorf("ptToCell(0,0) = (%d,%d), want (4,41)", col, row)
+	}
+
+	// One horizontal step moves exactly one column; one vertical step one row.
+	sx, sy := v.stepPt()
+	x, y := 300.0, 400.0
+	c0, r0 := v.ptToCell(x, y)
+	c1, _ := v.ptToCell(x+sx, y)
+	if c1-c0 != 1 {
+		t.Errorf("one stepPt in x moved %d cols, want 1", c1-c0)
+	}
+	_, r1 := v.ptToCell(x, y+sy)
+	if r0-r1 != 1 { // +y is up, which is a smaller row number
+		t.Errorf("one stepPt in y moved %d rows, want 1", r0-r1)
+	}
+}