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

include/plot/math/transform.hpp

@@ -0,0 +1,48 @@
+#pragma once
+#include <cmath>
+#include <plot/core/types.hpp>
+
+namespace plot::math {
+
+/// Affine data->screen mapping: `screen = data * s + t`, applied after `log10`
+/// when the corresponding log flag is set.
+struct Transform {
+  f64 sx = 1.0, tx = 0.0;
+  f64 sy = 1.0, ty = 0.0;
+  bool log_x = false, log_y = false;
+
+  f64 to_screen_x(f64 x) const noexcept {
+    const f64 v = log_x ? log10_positive(x) : x;
+    return v * sx + tx;
+  }
+
+  f64 to_screen_y(f64 y) const noexcept {
+    const f64 v = log_y ? log10_positive(y) : y;
+    return v * sy + ty;
+  }
+
+  f64 to_data_x(f64 screen_x) const noexcept {
+    const f64 v = (screen_x - tx) / sx;
+    return log_x ? std::pow(10.0, v) : v;
+  }
+
+  f64 to_data_y(f64 screen_y) const noexcept {
+    const f64 v = (screen_y - ty) / sy;
+    return log_y ? std::pow(10.0, v) : v;
+  }
+
+  /// Set scale/offset so `x` and `y` map onto `[px0,px1]` and `[py0,py1]`.
+  /// A degenerate range is widened by +/-0.5 in the (possibly logarithmic)
+  /// coordinate so the resulting mapping always stays invertible.
+  static Transform from_range(Range x, Range y, f64 px0, f64 px1, f64 py0, f64 py1, bool lx,
+                              bool ly) noexcept;
+
+  /// `log10` of `v`, with `v` clamped to at least 1e-300 first.
+  static f64 log10_positive(f64 v) noexcept;
+};
+
+}  // namespace plot::math
+
+namespace plot {
+using math::Transform;
+}