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fix(user): replace Math.abs(hashCode()) with Math.floorMod in computeColor #490

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marcel merged 1 commits from fix/issue-471-floor-mod-hashcode into main 2026-05-09 15:49:01 +02:00
Conversation 7 Commits 1 Files Changed 2 +13 -1
2 changed files with 13 additions and 1 deletions
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3
backend/src/main/java/org/raddatz/familienarchiv/user/AppUser.java
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@@ -88,7 +88,8 @@ public class AppUser {
};
public static String computeColor(UUID id) {
return PALETTE[Math.abs(id.hashCode()) % PALETTE.length];
// Math.floorMod avoids the Integer.MIN_VALUE overflow trap in Math.abs(hashCode())
return PALETTE[Math.floorMod(id.hashCode(), PALETTE.length)];
}
@PrePersist

11
backend/src/test/java/org/raddatz/familienarchiv/user/AppUserTest.java
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@@ -35,4 +35,15 @@ class AppUserTest {
.count();
assertThat(distinct).isGreaterThan(1);
}
@Test
void computeColor_returnsValidPaletteColorForIntegerMinValueHash() {
// UUID "80000000-0000-0000-0000-000000000000" has hashCode() == Integer.MIN_VALUE.
// Math.abs(Integer.MIN_VALUE) overflows back to Integer.MIN_VALUE (negative), making
// Math.abs(hashCode()) % n unsafe for palette sizes that don't evenly divide MIN_VALUE.
// Math.floorMod eliminates this edge case entirely.
UUID minHashId = UUID.fromString("80000000-0000-0000-0000-000000000000");
assertThat(minHashId.hashCode()).isEqualTo(Integer.MIN_VALUE);
assertThat(EXPECTED_PALETTE).contains(AppUser.computeColor(minHashId));
}
}