feat: project skeleton, core types and math with transform tests

tests/test_transform.cpp

@@ -0,0 +1,31 @@
+#include "harness.hpp"
+
+#include <plot/math/transform.hpp>
+
+using plot::Transform;
+
+PLOT_TEST(linear_round_trip) {
+  Transform t{2.0, 10.0, 3.0, -1.0, false, false};
+  PLOT_CHECK_NEAR(t.to_screen_x(0.0), 10.0, 1e-12);
+  PLOT_CHECK_NEAR(t.to_screen_x(5.0), 20.0, 1e-12);
+  PLOT_CHECK_NEAR(t.to_data_x(20.0), 5.0, 1e-12);
+  PLOT_CHECK_NEAR(t.to_data_y(t.to_screen_y(7.5)), 7.5, 1e-12);
+}
+
+PLOT_TEST(log_x_round_trip) {
+  Transform t{100.0, 0.0, 1.0, 0.0, true, false};
+  PLOT_CHECK_NEAR(t.to_screen_x(1.0), 0.0, 1e-12);
+  PLOT_CHECK_NEAR(t.to_screen_x(10.0), 100.0, 1e-12);
+  PLOT_CHECK_NEAR(t.to_data_x(100.0), 10.0, 1e-9);
+}
+
+PLOT_TEST(zoom_anchor_is_invariant) {
+  // Anchoring a zoom at a data coordinate must leave that coordinate's screen
+  // position unchanged.
+  Transform t{1.0, 0.0, 1.0, 0.0, false, false};
+  const double anchor_data = 3.25;
+  const double before = t.to_screen_x(anchor_data);
+  t.sx *= 4.0;
+  t.tx = before - anchor_data * t.sx;
+  PLOT_CHECK_NEAR(t.to_screen_x(anchor_data), before, 1e-12);
+}