write app plus touchup

1 files changed, 36 insertions, 0 deletions

diff --git a/src/utils/calculate_circumference.ts b/src/utils/calculate_circumference.ts
new file mode 100644
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+++ b/src/utils/calculate_circumference.ts
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+/**
+ * calculate_circumference.ts
+ *
+ * Pure function that computes the bicycle wheel circumference from rim and
+ * tire diameters (both in mm).  The formula uses 3.13772 instead of exact π
+ * to approximate the "roll-out" circumference (tire compressed under rider
+ * weight).
+ *
+ * Formula:  circumference_mm = 3.13772 × (2 × tire_diameter + rim_diameter)
+ */
+
+import type { CircumferenceResult } from "../types";
+
+/** The modified Pi constant that accounts for tire compression. */
+const ROLLOUT_PI = 3.13772;
+
+/**
+ * Returns circumference in multiple units, or null when inputs are invalid
+ * (non-positive or missing).
+ */
+export function calculate_circumference(
+  rim_mm: number,
+  tire_mm: number,
+): CircumferenceResult | null {
+  if (rim_mm <= 0 || tire_mm <= 0 || isNaN(rim_mm) || isNaN(tire_mm)) {
+    return null;
+  }
+
+  const circ_mm = ROLLOUT_PI * (2 * tire_mm + rim_mm);
+
+  return {
+    mm: Math.round(circ_mm),
+    cm: Math.round(circ_mm / 10),
+    inches: Math.round((10 * circ_mm) / 25.4) / 10,
+  };
+}