Fix rounding to 0

hdyc-svelte/src/lib/engine.ts

@@ -10,7 +10,13 @@ export interface SolveResult {
 }
 
 function fmt(n: number): string {
-  return parseFloat(n.toFixed(6)).toString();
+  if (!Number.isFinite(n)) return n.toString();
+  if (n === 0) return '0';
+  if (Math.abs(n) < 1e-6) {
+    return n.toExponential(6);
+  }
+  const rounded = parseFloat(n.toFixed(6));
+  return rounded.toString();
 }
 
 function gcd(a: number, b: number): number {

tests/test_conversion_chart.py

@@ -0,0 +1,46 @@
+import math
+import re
+import unittest
+from pathlib import Path
+
+
+ROOT = Path(__file__).resolve().parents[1]
+CALCULATORS_TS = ROOT / "hdyc-svelte" / "src" / "lib" / "data" / "calculators.ts"
+NUMERIC_SAMPLES = [0.1, 0.5, 1, 2, 5, 10, 20, 50, 100]
+TARGET_SLUG = "angstroms-to-nautical-miles"
+
+
+def _parse_factor(slug: str) -> float:
+    text = CALCULATORS_TS.read_text(encoding="utf-8")
+    match = re.search(rf'"slug": "{slug}".*?"factor": ([0-9.eE+-]+)', text, flags=re.S)
+    if not match:
+        raise AssertionError(f"Could not find calculator definition for '{slug}'")
+    return float(match.group(1))
+
+
+def _js_fmt(value: float) -> str:
+    if not math.isfinite(value):
+        return str(value)
+    if value == 0:
+        return "0"
+    abs_val = abs(value)
+    if abs_val < 1e-6:
+        return f"{value:.6e}"
+    rounded = float(f"{value:.6f}")
+    if rounded.is_integer():
+        return str(int(rounded))
+    return repr(rounded)
+
+
+class QuickConversionChartRoundingTests(unittest.TestCase):
+    def test_angstroms_to_nautical_miles_chart_does_not_round_to_zero(self) -> None:
+        factor = _parse_factor(TARGET_SLUG)
+        outputs = [_js_fmt(sample * factor) for sample in NUMERIC_SAMPLES]
+        self.assertTrue(
+            any(output != "0" for output in outputs),
+            f"{TARGET_SLUG} quick conversion chart rounded every sample down to 0: {outputs}",
+        )
+
+
+if __name__ == "__main__":
+    unittest.main()