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

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

web/src/lib/units.test.ts

@@ -1,5 +1,6 @@
 import { describe, expect, it } from 'vitest'
 import {
+  cmFromDisplay,
   cmFromFtIn,
   cmFromMeters,
   cmFromSpacing,
@@ -11,15 +12,37 @@ import {
 } from './units'
 
 describe('cm conversions', () => {
-  it('feet+inches → whole cm', () => {
-    expect(cmFromFtIn(4)).toBe(122) // 4ft = 121.92 → 122
-    expect(cmFromFtIn(8)).toBe(244)
-    expect(cmFromFtIn(4, 6)).toBe(137) // 4'6" = 137.16 → 137
+  // Every cm column is REAL, so entry is exact — no whole-cm quantization. A 1 ft
+  // grid is 30.48 cm, not 30, or the error accumulates across the garden (#47).
+  it('feet+inches → exact cm', () => {
+    expect(cmFromFtIn(1)).toBe(30.48)
+    expect(cmFromFtIn(4)).toBe(121.92)
+    expect(cmFromFtIn(8)).toBe(243.84)
+    expect(cmFromFtIn(4, 6)).toBe(137.16)
   })
 
-  it('meters → whole cm', () => {
+  it('meters → exact cm', () => {
     expect(cmFromMeters(10)).toBe(1000)
     expect(cmFromMeters(1.22)).toBe(122)
+    expect(cmFromMeters(0.305)).toBe(30.5)
+  })
+
+  it('rounds away float dust, not precision', () => {
+    // 12 × 2.54 is 30.479999999999997 in IEEE-754; 1 ft must read as 30.48.
+    expect(cmFromFtIn(0, 12)).toBe(30.48)
+    expect(cmFromSpacing(12, 'imperial')).toBe(30.48)
+    expect(cmFromSpacing(1, 'imperial')).toBe(2.54)
+  })
+
+  it('round-trips display → cm → display at display precision', () => {
+    const imperial = [0.5, 1, 2.5, 4, 8, 12.5, 24]
+    for (const ft of imperial) {
+      expect(displayFromCm(cmFromDisplay(ft, 'imperial'), 'imperial')).toBe(ft)
+    }
+    const metric = [0.15, 1, 1.22, 3.05, 10, 24.5]
+    for (const m of metric) {
+      expect(displayFromCm(cmFromDisplay(m, 'metric'), 'metric')).toBe(m)
+    }
   })
 })
 
@@ -59,13 +82,14 @@ describe('plant spacing (small scale)', () => {
     expect(formatSpacing(30, 'imperial')).toBe('12″')
   })
 
-  it('parses an entered value back to whole cm', () => {
+  it('parses an entered value back to exact cm', () => {
     expect(cmFromSpacing(15, 'metric')).toBe(15)
-    expect(cmFromSpacing(6, 'imperial')).toBe(15) // 6in = 15.24 → 15
+    expect(cmFromSpacing(6, 'imperial')).toBe(15.24)
   })
 
   it('prefills an input from cm', () => {
     expect(spacingFromCm(15, 'metric')).toBe(15)
     expect(spacingFromCm(30, 'imperial')).toBe(11.8) // 30/2.54 = 11.81 → 11.8
+    expect(spacingFromCm(30.48, 'imperial')).toBe(12) // an exactly-12in grid reads back as 12
   })
 })