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

src/math/transform.cpp

@@ -0,0 +1,44 @@
+#include <plot/math/transform.hpp>
+
+namespace plot::math {
+
+namespace {
+
+/// Resolve the endpoints of one axis in transform space, widening a degenerate
+/// range by +/-0.5 so the mapping stays invertible.
+void axis_ends(Range r, bool log, f64& lo, f64& hi) noexcept {
+  lo = log ? Transform::log10_positive(r.min) : r.min;
+  hi = log ? Transform::log10_positive(r.max) : r.max;
+  if (!(hi > lo)) {
+    lo -= 0.5;
+    hi += 0.5;
+  }
+}
+
+}  // namespace
+
+f64 Transform::log10_positive(f64 v) noexcept {
+  constexpr f64 kMin = 1e-300;
+  return std::log10(v < kMin ? kMin : v);
+}
+
+Transform Transform::from_range(Range x, Range y, f64 px0, f64 px1, f64 py0, f64 py1, bool lx,
+                                bool ly) noexcept {
+  Transform t;
+  t.log_x = lx;
+  t.log_y = ly;
+
+  f64 x_lo = 0.0, x_hi = 0.0;
+  axis_ends(x, lx, x_lo, x_hi);
+  t.sx = (px1 - px0) / (x_hi - x_lo);
+  t.tx = px0 - x_lo * t.sx;
+
+  f64 y_lo = 0.0, y_hi = 0.0;
+  axis_ends(y, ly, y_lo, y_hi);
+  t.sy = (py1 - py0) / (y_hi - y_lo);
+  t.ty = py0 - y_lo * t.sy;
+
+  return t;
+}
+
+}  // namespace plot::math