fix: clarify warnings to say distances are rounded, not truncated

Author: jvkersch

README.md

@@ -119,9 +119,9 @@ you want to have it added here, please open an issue.
 Technical Notes
 -------
 
-**Coordinate scaling:** Concorde computes distances using integer arithmetic.
-If your coordinates are small (e.g. in [0, 1]), distances between points will
-be rounded to 0, leading to incorrect results or crashes. Scale your
+**Coordinate scaling:** Concorde rounds all edge distances to the nearest
+integer. If your coordinates are small (e.g. in [0, 1]), distances between
+points will round to 0, leading to incorrect results or crashes. Scale your
 coordinates up before passing them to the solver — for example, multiply by
 1e6. Similarly, very large coordinates (above 1e7) may cause integer overflow.
 See `examples/truncation_demo.py` for a demonstration.

concorde/tests/test_concorde_datagroup.py

@@ -30,7 +30,7 @@ def test_from_data_warns_small_coordinates(self):
         ys = [0.2, 0.6, 0.8]
         with self.assertWarns(UserWarning) as ctx:
             TSPSolver.from_data(xs, ys, "EUC_2D")
-        self.assertIn("[-1, 1]", str(ctx.warning))
+        self.assertIn("rounds distances", str(ctx.warning))
 
     def test_from_data_warns_large_coordinates(self):
         # Large coordinates risk integer overflow in Concorde

concorde/tsp.py

@@ -52,17 +52,18 @@ def from_data(cls, xs, ys, norm, name=None):
 
         if max_abs <= 1.0:
             warnings.warn(
-                "All coordinates are in [-1, 1]. Concorde uses integer "
-                "distances internally, so small coordinates will be "
-                "truncated to 0. Consider scaling your coordinates "
-                "(e.g. multiply by 1e6).",
+                "All coordinates are in [-1, 1]. Concorde rounds "
+                "distances to the nearest integer, so distances "
+                "between nearby points will round to 0. Consider "
+                "scaling your coordinates (e.g. multiply by 1e6).",
                 UserWarning,
                 stacklevel=2,
             )
         if max_abs > 1e7:
             warnings.warn(
-                "Coordinates exceed 1e7. Large values may cause integer "
-                "overflow in Concorde, leading to incorrect results or "
+                "Coordinates exceed 1e7. Concorde rounds distances "
+                "to the nearest integer, and large values may cause "
+                "integer overflow, leading to incorrect results or "
                 "crashes. Consider scaling down.",
                 UserWarning,
                 stacklevel=2,