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

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

web/src/lib/units.ts

@@ -12,19 +12,32 @@ const INCHES_PER_FOOT = 12
 export const MIN_DIMENSION_CM = 1
 export const MAX_DIMENSION_CM = 10_000
 
+// Below this, a garden-scale grid is fine enough to be worth questioning — it
+// usually means someone reached for plant spacing, which belongs to the bed, not
+// the garden. Soft: the form hints, it doesn't refuse.
+export const MIN_GARDEN_GRID_CM = 10
+
 /** Whether a centimeter dimension is within the server's accepted range. */
 export function isValidDimensionCm(cm: number): boolean {
   return Number.isFinite(cm) && cm >= MIN_DIMENSION_CM && cm <= MAX_DIMENSION_CM
 }
 
-/** Feet (+ optional inches) → whole centimeters. */
-export function cmFromFtIn(feet: number, inches = 0): number {
-  return Math.round((feet * INCHES_PER_FOOT + inches) * CM_PER_INCH)
+/** Round away IEEE-754 dust without rounding away precision: 12 × 2.54 is
+ *  30.479999999999997 in binary floating point, and the 1e-6 cm (10 nm) grain
+ *  here is far below anything a garden cares about. Every cm column is REAL, so
+ *  entry stays exact — this only stops 30.48 being stored as 30.479999999999997. */
+function roundCm(cm: number): number {
+  return Math.round(cm * 1e6) / 1e6
 }
 
-/** Meters → whole centimeters. */
+/** Feet (+ optional inches) → centimeters, exactly (1 ft → 30.48). */
+export function cmFromFtIn(feet: number, inches = 0): number {
+  return roundCm((feet * INCHES_PER_FOOT + inches) * CM_PER_INCH)
+}
+
+/** Meters → centimeters, exactly. */
 export function cmFromMeters(meters: number): number {
-  return Math.round(meters * CM_PER_METER)
+  return roundCm(meters * CM_PER_METER)
 }
 
 /** A value typed in the given unit (meters, or feet) → centimeters. */
@@ -79,9 +92,10 @@ export function formatSpacing(cm: number, unit: UnitPref): string {
   return `${Math.round(cm)} cm`
 }
 
-/** A spacing value typed in the given unit (cm, or inches) → whole centimeters. */
+/** A spacing value typed in the given unit (cm, or inches) → centimeters,
+ *  exactly (12 in → 30.48). */
 export function cmFromSpacing(value: number, unit: UnitPref): number {
-  return unit === 'imperial' ? Math.round(value * CM_PER_INCH) : Math.round(value)
+  return roundCm(unit === 'imperial' ? value * CM_PER_INCH : value)
 }
 
 /** Centimeters → a spacing number in the given unit, for prefilling an input.