Units correctness: exact imperial entry, garden grid at dimension scale (#47) (#60)

Co-authored-by: Steve Dudenhoeffer <[email protected]>

web/src/lib/geometry.test.ts

@@ -6,6 +6,7 @@ import {
   snapLocalToBedGrid,
   snapPoint,
   snapValue,
+  visibleGridStepCm,
   worldToLocal,
   worldToScreen,
   zoomToFitRect,
@@ -128,3 +129,34 @@ describe('grid snapping', () => {
     expect(p.x).toBeGreaterThanOrEqual(-60)
   })
 })
+
+describe('visibleGridStepCm', () => {
+  it('draws the true grid when its cells are already legible', () => {
+    expect(visibleGridStepCm(30.48, 1, 6)).toBe(30.48)
+    expect(visibleGridStepCm(10, 0.6, 6)).toBe(10) // exactly at the minimum
+  })
+
+  it('coarsens to a multiple rather than fading out', () => {
+    // The #47 failure: a 3cm grid at 0.84 px/cm is 2.5px per cell. Instead of
+    // drawing nothing while snapping stays on, draw every 5th line (12.6px).
+    expect(visibleGridStepCm(3, 0.84, 6)).toBe(15)
+    expect(visibleGridStepCm(1, 0.84, 6)).toBe(10)
+  })
+
+  it('only ever returns a whole multiple of the true grid', () => {
+    for (const scale of [0.05, 0.3, 0.84, 1, 4]) {
+      const step = visibleGridStepCm(7, scale, 6)
+      expect(step).not.toBeNull()
+      expect(Math.round(step! / 7) * 7).toBeCloseTo(step!, 9)
+      expect(step! * scale).toBeGreaterThanOrEqual(6)
+    }
+  })
+
+  it('reports null for a degenerate grid rather than drawing nothing silently', () => {
+    expect(visibleGridStepCm(0, 1, 6)).toBeNull()
+    expect(visibleGridStepCm(-5, 1, 6)).toBeNull()
+    expect(visibleGridStepCm(NaN, 1, 6)).toBeNull()
+    expect(visibleGridStepCm(30, 0, 6)).toBeNull()
+    expect(visibleGridStepCm(1e-9, 1, 6)).toBeNull() // beyond the coarsening cap
+  })
+})