Correct negative half-tie rounding in symmetric shadowcasting

CHANGELOG.md

@@ -6,3 +6,6 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 This project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
+
+### Fixed
+- Symmetric shadowcasting rounds negative sector edges correctly, preserving visibility when a dungeon is rotated.

src/libtcod-fov/fov_symmetric_shadowcast.c

@@ -34,7 +34,6 @@
 
     Based on: https://www.albertford.com/shadowcasting/
  */
-#include <float.h>
 #include <math.h>
 #include <stdbool.h>
 
@@ -83,11 +82,11 @@ static float slope(int row_depth, int column) { return (2.0f * column - 1.0f) /
 /**
     Round half numbers towards infinity.
  */
-static int round_half_up(float n) { return (int)roundf(n * (1 + FLT_EPSILON)); }
+static int round_half_up(float n) { return (int)floorf(n + 0.5f); }
 /**
     Round half numbers towards negative infinity.
  */
-static int round_half_down(float n) { return (int)roundf(n * (1 - FLT_EPSILON)); }
+static int round_half_down(float n) { return (int)ceilf(n - 0.5f); }
 /**
     Scan a row and recursively scan all of its children.
 

tests/test_fov_symmetric.cpp

@@ -0,0 +1,50 @@
+#include <libtcod-fov/fov.h>
+
+#include <array>
+#include <catch2/catch_test_macros.hpp>
+#include <string_view>
+#include <utility>
+
+namespace {
+std::pair<int, int> rotate(int x, int y, int turns) {
+  for (int i = 0; i < turns; ++i) {
+    const int old_x = x;
+    x = y;
+    y = 6 - old_x;
+  }
+  return {x, y};
+}
+}  // namespace
+
+TEST_CASE("symmetric shadowcasting rounds negative sector edges", "[fov][symmetric]") {
+  const std::array<std::string_view, 7> dungeon = {
+      "#######", "###...#", "###.#.#", "#.....#", "#.#.#.#", "#.#..##", "#######"};
+  // Exact rational-slope reference, with the origin at (3, 1).
+  const std::array<std::string_view, 7> visible = {
+      "0011111", "0011111", "0011111", "0011100", "0011100", "0011000", "0011100"};
+  for (int turns = 0; turns < 4; ++turns) {
+    for (bool light_walls : {false, true}) {
+      CAPTURE(turns, light_walls);
+      TCODFOV_Map* map = TCODFOV_map_new(7, 7);
+      REQUIRE(map != nullptr);
+      for (int y = 0; y < 7; ++y) {
+        for (int x = 0; x < 7; ++x) {
+          const auto [rx, ry] = rotate(x, y, turns);
+          const bool floor = dungeon[y][x] == '.';
+          TCODFOV_map_set_properties(map, rx, ry, floor, floor);
+        }
+      }
+      const auto [pov_x, pov_y] = rotate(3, 1, turns);
+      REQUIRE(TCODFOV_map_compute_fov(map, pov_x, pov_y, 0, light_walls, TCODFOV_SYMMETRIC_SHADOWCAST) == TCODFOV_E_OK);
+      for (int y = 0; y < 7; ++y) {
+        for (int x = 0; x < 7; ++x) {
+          CAPTURE(x, y);
+          const auto [rx, ry] = rotate(x, y, turns);
+          const bool expected = visible[y][x] == '1' && (light_walls || dungeon[y][x] == '.');
+          CHECK(TCODFOV_map_is_in_fov(map, rx, ry) == expected);
+        }
+      }
+      TCODFOV_map_delete(map);
+    }
+  }
+}